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Standards for the mobility requirements of military vehicles
Journal ofTerramechanics, Vol. 25, No. 3, pp. 171-189, 1988. Printed in Great Britain.
STANDARDS
0022-4898/8853.00+0.00 Pergamon Press plc. © 1988 ISTVS
FOR THE MOBILITY REQUIREMENTS MILITARY VEHICLES
OF
J. C. LARMINIE*
Summary--This article introduces a series of criteria which can be used to specify the cross-country performance of military vehicles, according to their type, and the priority to the user for mobility. The standards have been taken from existing vehicles, so are practical. The article also explains how the criteria were derived.
INTRODUCTION
DESCRIPTIONS of vehicles by designers and manufacturers often omit many important facets of performance, in particular for wheeled vehicles a statement of ground pressure. It is thus often difficult for the user to ensure a vehicle will meet a military requirement. Over recent years a series of criteria and assessment methods have been evolved which allow performances to be stated with some precision. The various criteria, and assessments of their implications in terms of performance, have been presented and justified in Refs [1-5]. The need to adopt, and take into use such criteria and the methods of assessing their implications in terms of performance was explained by the author in "Predicting The Performance of Fast Cross Country Vehicles", Ref. [6]. Further background material for the subject is in Refs [7-13]. This article puts forward a series of lists of values of each criterion, suitable for a range of vehicles filling various roles, and to meet various levels of importance of mobility in the priorities of the user. Also included for completeness is a description of the Mean Maximum Pressure (MMP) system for quantifying soft-ground performance, since some of the bibliography is not readily available to readers. These standards were originally presented in the form of a paper to the 4th Annual British Conference, ISTVS, Sutton Bonnington, in September 1986. However, it has been possible to make recent additions to some of the tables, and therefore these have been up-dated, and they now make the paper obsolete. MOBILITY STANDARDS
The recommended standards for various types of vehicles are given in Tables I to 3. These standards have been taken from current vehicles. Thus they are pragmatic values that make allowances for cost restrictions on buying and running the vehicles, and the engineering and operational constraints on designers. The standards are met by some current vehicles, and thus are a realistic level. Also, current vehicles have most definite limits to their mobility, which in relatively untaxing circumstances will prove a restriction on the operating flexibility of the user. Thus they are a realistic minimum requirement. *Major J. C. Larminie, Tithe Barn, Lytchett Matravers, Poole, Dorset BHI6 6BJ, U.K. 171
172
J. C. LARMINIE TABI.E I. MOBII.ITY STANDARDS:HEAVY AFV* Priority Criteria
Desirable
Essential
Less than 230 More than 19(25)
Less than 280 More than 10.5(14)
Remarks
Major factors: Ground pressure M M P in kPa
Gross power to weight ratio: k W / t (hp/t)
Secondary factors: Ground clearance: minimum, (mm)
480
Turning circle Length to breadth (L/C) limits Fording depth: (m) Pitch ratio, m a x i m u m Vertical step height Width limit (m) Height, m a x i m u m (m) Suspension periods (s): Bounce, m i n i m u m Pitch, m i n i m u m Damping: % critical Available b u m p deflection from static, m i n i m u m (ram) Stability tilt angle, m i n i m u m
400 Pivot turn.
1.2-1.8 1.1-1,9 -1.5 1.5 2.0 Not specified. 3.5 -3.0 4.0 0.75 1.5 30-45
0.65 1.0 25-50
260
200
40 °
35 °
See Notc
*These standards have been collected from various sources, and reflect current capability. The desirable limits are met by the more mobile vehicles. Note: The essential height limit is for bridge clearance. The desirable limit is tactical.
POWER
The usefulness of the installed power depends on the effectiveness with which the transmission allows the engine to apply its torque. Variations in this are shown pictorially in Fig. 1. Typical values are listed in Table 4. SOFT G R O U N D P E R F O R M A N C E
A vehicle must have a reasonably light ground pressure. This will limit sinkage into moderately soft ground, and thus minimize resistance to motion, so making the most of the available power for speed and manoeuvrability. In worse going, the light ground pressure will avoid immobilization, when resistance exceeds traction. In Table 5 are the desirable ground pressures needed to operate on various types of ground, given in the MMP system. These ground pressures assume the vehicle is running
173
MOBILITY REQUIREMENTS OF MILITARY VEHICLES TABLE2. MOBILITYSTANDARDS:LIGHTAFV (Weight less than 30 tonne) Criteria
Combat vehicles Desirable Essential
Armoured Carriers
Less than 140 More than 22.5(30)
Less than 230 More than 15(20)
Less than 450 More than 9(12)
450 Pivot turn
350 As HMLC
300 As IMMLC
Float 1.5 As HMLC 2.9 --
1.25 1.5 2.0 As IMMLC 3.5 2.2
0.75 1.5 -As IMMLC 2.9 2.9
0.75 1.5 30-45
0.65 1.0 25-50
0.65 1.0 25-50
210
130
130
38°
33°
30°
Major factors: Ground pressure MMP in kPa Gross power to weight ratio: kW/t (hp/t)
Secondary factors: Ground clearance: minimum, (mm) Turning circle Fording depth: (m). Unprepared Prepared Pitch ratio (for tracks) max. Angles of approach etc, (for wheels) Width limit (m) Height, maximum (m) Suspension periods (s): Bounce, minimum Pitch, minimum Damping: % critical Available bump deflection from static, minimum (mm) Stability tilt angle, minimum
Notes: (1) For wheeled AFVs, refer to HMLC or IMMLC in Table 3(a) if gross weight less than 6 t, otherwise Table 3(b). (2) Armoured Carriers, as distinct from true Combat Vehicles, are simple vehicles with a limited role. (3) These standards have been collected from various sources, and reflect current capability.
straight. A vehicle with skid-steering will need a g r o u n d pressure M M P some 20% lower t h a n one with n o r m a l a c k e r m a n or articulated steering. It will be seen that these desirable values generally call for g r o u n d pressures less t h a n those given in Tables 1-3, thus e m p h a s i s i n g that those listed s t a n d a r d s are n o t asking for too generous a performance. In T a b l e 6 are the g r o u n d pressures actually achieved by v a r i o u s vehicles. It will be seen that generally g r o u n d pressures are heavier t h a n the desirable ones in T a b l e 5, a n d h a r d l y meet the m i n i m u m s t a n d a r d s listed in Tables 1-3. I n T a b l e 7 are given relationships of g r o u n d pressure to resistance to m o t i o n caused by sinkage, a n d related to soil strength. This resistance is a d d i t i o n a l to the o t h e r n o r m a l c o m p o n e n t s of resistance to m o t i o n : hard surface rolling resistance, gradient, a n d air resistances.
J. C. L A R M I N I E
174
TABLE 3. MOBILIIY STANDARDS: B VEtttCLES Mobility classes Criteria
HMLC
IMMLC
MMLC
ILMLC
LMLC
(a) Utilities and light trucks (Payload less than 4 tonne)
Major factors: Gro und pressure MMP in kPa Gross power to weight ratio: k W / t (hp/t)
Less than 280 More than 22(30)
280 to 350 More than 19(25)
350 to 550 More than 12(16)
5511 to 700 More than 12(16)
Greater than 700 Less than 12(16)
400 Under 11
260 Under 12
180 Under 13
150 Over 13
115 Over 13
0.76 1.50
0.60 1.10
0.50 1.00
0.50 --
1t.50 --
45 ° 40 ° 130°
40 ° 38 ° 155°
40 ° 38 ° 155° Usually not critical.
35 ° 311° 155<,
_ _ --
0.75 1.5 30-45
0.65 1.0 25-45
0.6 0.5 25-50
0.6 0.5 20-50
0.6 0.5 20-50
200
130
125
125
100
35 °
33 °
30 °
28 °
28 °
Less than 350 More than 11(151
350 to 450 More than 9(121
450 to 600 More than 7.51101
600 to 700 More than 6(8)
Greater than 700 More than 4.4(6)
400
300
300
3011
200
17.5(16)
19(17.5)
20.5(19)
27(25)
27(25)
1.25 1.50
0.75 1.50
0.75 1.50
0.75 1.50
0.50 --
45 ° 40 ° 130° 2.9 2.6
40 ° 38 ° 155° 2.9 3.0
40 ° 38 ° 155 ° 2.5 3.0
35 ° 30 ° 155 ° 2.5 --
---2.5 --
0.75 1.5 30-45
0.65 1.0 25-45
(1.6 0.5 25-50
0.6 0.5 20-50
0.6 0.5 20-50
Secondary factors: G r o u n d clearance: minimum, (ram) Turning circle between kerbs (m) Fording depth (m) Unprepared Prepared Clearance angles: Approach, Min: Departure, Min: Under veh. Max: Width limit Suspension periods (s): Bounce, minimum Pitch, minimum Damping: % critical Available bump deflection (mm), Min Stability tilt angle, minimum
(b) Medium trucks (Payload 4 to 8 tonne)
Major factors: Ground pressure MMP in kPa Gross power to weight ratio: k W / t (hp/t)
Secondary factors: Ground clearance: minimum, (mm) Turning circle max. walls (kerbs) (m) Fording depth (m) Unprepared Prepared Clearance angles: Approach, Min: Departure, Min: Under veh. Max: Width limit Cab height (m) max. buspension periods (s): Bounce, minimum Pitch, minimum Damping: % critical
MOBILITY R E Q U I R E M E N T S O F M I L I T A R Y VEHICLES
Available b u m p deflection (mm), Min Stability tilt angle, m i n i m u m (load C G 0.66 m above floor)
175
200
130
125
125
100
33°
30 °
28 °
26 °
26 °
Less than 350 More than 11(15)
350 to 450 More than 9(12)
450 to 600 More than 7.5(10)
600 to 700 More than 6(8)
Greater than 700 More than 4.4(6)
400
350
300
300
200
19.5(18)
21.5(20)
24(22.5)
27(25)
--
1.25 1.50
0.75 1.50
0.75 1.50
0.75 1.50
0.50 --
45 ° 40 ° 1300 2.9 3.3 --
40 ° 38 ° 155 ° 2.9 3.4 --
40 ° 38 ° 1550 2.5 3.4 4
35 ° 30 ° 155 ° 2.5 -4
--2.5 -4
0.75 1.5 30-45
0.65 1.0 25-45
0.6 0.5 25-50
0.6 0.5 20-50
0.6 0.5 20-50
200
130
125
125
100
33 °
30 °
28 °
26 °
26 °
(c) Heavy trucks (Payload heavier than 8 tonne)
Majorfactors: G r o u n d pressure M M P in kPa Gross power to weight ratio: k W / t (hp/t)
Secondaryfactors: G r o u n d clearance: minimum, (mm) Turning circle Max. walls (kerbs) (m) Fording depth (m) Unprepared Prepared Clearance angles: Approach, Min: Departure, Min: Under veh. Max: Width limit (m) Cab height, (m) Max. Ht. with container (m) Suspension periods (s): Bounce, m i n i m u m Pitch, m i n i m u m Damping: % critical Available b u m p deflection (mm), Min Stability tilt angle, m i n i m u m (load CG 0.66 m above floor)
Notes: Table 3a: These standards have been used by M O D as design guidelines given to manufacturers for such vehicles. Table 3b: A new standard recommended is the turning circle within walls. Also the previous frontal area and volume requirements have been replaced by width and cab-height. The L M L C power is the legal minimum. Otherwise, these standards are those in Ref. [1], which have been used by M O D as design guidelines given to manufacturers for such vehicles. Table 3c: (1) These standards have been derived from those well established for 4-8 tonners [Table 3(b)], but with larger turning circles and height, and ground clearance for IMMLC. (2) Container height: M M L C 2.44 m (8ft); LMLC 2.59 m (8ft 6in).
176
J . C . LARMINIE
POWER
SPECff::IED
260kW
(350 H P )
ENGtNE 1
SPARK I G N ! T I O N PETROL
ENGINE 2 :
TURBO CHARGED DIESEL
ENGINE OPERATING RANGES
C2 2 3 - 1 6 ( 1 0 5 ) 105 F .... kW
16 -*
: I (..z. (2 2)
C2 - - !
18 F'- - - - - - r
HP
C
23 12~
C~ 5 6 - 2 4 1 5 ~ - 1 8 1 :
23(32)
56 5~ ~ -4
Nm
,,250L
T2 300 -
35O
1000-
250-
750200
-
-250 ;
~
TI 500-
15C-
100-
L 10
1~5
I 20
! 25
} 3T0 35 ENGINE SPEED
1 I Z,0 ':-5 RPM • 100
' 5'0
l 55
i 60
. 6~5
N
FIG. 1.1-1.5. Effective power: engine flexibility. 2. Effective power: flexible engine, few gears. 3. Poor effective power: narrow engine range and few gears. 4. Effective power: narrow engine range but more gears. 5. Effective power as average over whole speed range.
MOBILITY REQUIREMENTS OF MILITARY VEHICLES kW
HP
[-3so
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ROAD SPEED
FIc~. 1.4. EXAMPLE : 1&t kW
VEHICLE
125 N P l t ~
l~,.SkWtt
HP "350
REQUIRED POWER TO MEET USER REQUIREMENT
250-
NET POWER -300
200-
MESHANICAL TRANSMISSION
Prg 1/,
E : 91%
F~g 1.2
E : 86('/,
F i g 13
E :
81°/,
260 kW
EF~ICrENCY
93°1(,
:- 2&2 kW
DIESEL/ B GEARS V ~ MAX
PETROL / 5 GEARS
I
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--253
DIESEL I $ GEARS
,~ MAX
o~ £L 1 5 0 - -200
USER SPEED REQUIRED ESSENTIAL 5 !
4
10 20
30
15 L-O
c~ • --~
20
6J0
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DESIRABLE
1
25
7
T
•
80
90
m/s
30
I
100
r
~10
ROA0 SPEED FIG. I . i NOTES I
DIESEL/ 5 GEARS UNSATISFACTORY DUE TO EXCESSIVE
2
HYDROSTATIC TRANSMISS;ON EFFECTIVE POWER 203 kW
ENGINE LUGGING / LABOURING (SEE Fig I 3)
km/h
179
MOBILITY REQUIREMENTS OF MILITARY VEHICLES TABLE 4. TRANSMISSIONEFFICIENCYAND EFFECTIVENESS Typical Values for MBT About 15%. Engine installation losses: Transmission efficiency losses: Epicyclic Gearbox. Spur gears gears 7% 10% Final drive. About 3,%. Gearbox effectiveness (e.g. Fig. 1.5): 4 Gears 6 Gears 8 Gears Friction clutch. Torque converter. Friction clutch.
81% 83% 82%
85% 88% 86%
91% ---
CVT 22%
CVT 100% -100%
Max speed km/h 70 70 55
Notes: (1) The Continuously Variable Transmission (CVT) is assumed to be hydrostatic. (2) The engine is assumed to be a turbo-charged diesel. TABLE 5. DESIRABLEGROUNDPRESSUREMMP VALUES Vehicle design aims: MMP Required for Satisfactory Performance (Presented in Ref. [2]) MMP Levels for performance priority Condition
Temperate climate, wet, fine-grain (eg clay) soil Tropical, wet fine-grain soil European bogs Muskeg Over-snow
Ideal
Satisfactory
Maximum acceptable
150
200
300
90 5 30 10
140 10 50 25-30
240 15 60 40
Trafficability limits: For fine-grain (e.g. clay) soils, wet. for one pass: RCI = 0.096 MMP [Rating Cone Index of soil strength in lbf/in2; MMP in kPa]. For multiple passes: Number of passes: ! 2 5 10 25 50 Multiply One-pass RCI by: 1 1.2 1.53 1.85 2.35 2.8 Resistance from soil sinkage:
R = Resistance (kN). W = Weight (kN) (mass x 9.81). M =
M M P (kPa).
C = Soil Cone Index in kPa. Take CI = RCI + 0.8 1 lbf/in 2 = 6.895 kPA. The formulae for MMP for tracked and wheeled vehicles are given in Appendix A.
235 220 236 164 121 210 145 363 279 243 256 257 178 259 168
Leopard 1 Leopard 2
USA M60 Sheridan M551 M113
Abrams M1 Bradley M2/M3
White/Int Half-track M3 (WW2) Merkava T55 T62 T72 BMP2
AMX30 AMXI0P
-14.5
7.65 60 36 38 -14.6
51.4 22.36
48.1 15.8 11.6
44.4 55.2
62 53.6 51.6 52.4 8.26 12.7 15.28 41.0 25.3 37
Weight tonne
-NTA
1.0 0.95e 0.95 0.95e -0.97e
0.98e 0.98e
0.98 NTA 0.98
0.95 0.96e
0.94 0.93 0.93 NTA 0.87 0.87 NTA 0.94 0.96 0.92e
Factor C
437
276 576 392
Panhard M l l VBL (4 x 4) VAB: VTT (4 × 4) VAB: VTT (6 × 6) AMX]0RC (Skid steer 6 × 6)
185 355 524 439 272 421 427 428 320
278 525 692 620 479
Land Rover ~t Truck 4t Bedford MK Truck 8t Bedford TM Foden FH70 Limber DROPS Prototype IMMLC (8 x 6) Twister XMS08 (artic) Cadillac Gage V150 (4 × 4) FM Canada Grizzly (6 × 6) GM Canada LAV(USMC)(8 × 8) M998 H u m m e r S-D-P. Pandur (6 × 6) Luchs (8 × 8) Engesa Urutu EEl (6 × 6) BTR 60P
540 351 519 455 403 349 710
MMP kPa
Saxon AT105 Ferret Scout Car Mark 2 Daimler Armoured Car (WW2) Saladin Stalwart Fox M2 Amphibious Rig (4 × 4)
Wheeled vehicles
15.8
3.54 13 14.2
--11.79 11.95 3.87 11.2 19.5 14 --
2.1 9.5 16.3 26.6 30
11.66 4.5 7.5 11.6 14.5 6. l 21.5
Weight tonne
14 × 20
9.00 × 16 14 x 20 14 × 20
--l l . 0 × 16 11.0 × 16 36 x 12.5-16.5 12.5 × 20 14 x 20 13 × 20 --
6.50 × 16 12 × 20 15.5/80R20 16 × 20 20.5 × 25
14.75/80R20 9.00 × 16 10.5 x 20 12 x 20 14 × 20 11 × 20 16 × 20
Tyre size
Factor C is thc track link footprint as a proportion of (pb); NTA means (' not taken into account; (e) means estimated. Where weight is blank (--), MMP is from unconfirmed data. For MMP of wheeled vebicles, tyres are assumed to be inflated to pressure fo~ roads, with 18c~ deflection. Tyres from different manufacturers have significant differences in their dimensions.
282 274 276 288 106 135 205 235 198 290
MMP kPa
Challenger Chieftain Mark 5 Centurion Mk 10 (steel track) Centurion (rubber track) Scorpion Stormer FV432 Vickers Mark 3(1) Warrior (MCV80) Tank S (Sweden)
Tracked vehicles
TABLE 6. MEAN MAXIMUMPRESSUREFOR VARIOUSVEHICLe'S
Z m
181
MOBILITY REQUIREMENTS OF MILITARY VEHICLES TABLE 7. RELATIONSHIPS:RESISTANCE,GROUNDPRESSUREAND SOIL STRENGTH SoIL strengths: RCI
Ground pressure MMP kPo
25 172
IlO
I bf / in z kPo
35 241
310
414
70 485
105 724
7.6
3.9
2.4
1.4
1.0
0.5
Very light
140
r2.0
6.3
3.8
2.2
1.6
0.7
Light tracked AFV
185
21 .0
10.8
66
3.8
2,8
1.3
230
32.0
16.5
I0, I
5.8
4.3
1.9
Light MBT
I
I
AFV
I
285
48.0
25. I
15.4
8.8
6.5
2.9
Heavy MBT, Motor car
3i0
57.0
29.5
18. I
10.3
7.6
:5.5
Good wheeled ArM
350
72.0
:57.4
22.9
13, I
9.7
4.4
Normal wheeled AFV
45O
118.0
61 . I
37.4
21.4
15.8
7.2
I m
475
67.9
41.6
23.7
17.6
8.0
5O0
75.0
45.9
26.2
19.4
8.8
525
82.5
50.5
28.8
21.4
9.7
550
90.3
55.3
31.6
25.4
10.6
575
98.5
60.3
34.4
25.5
I 1.6
600
107.0
65.6
37.4
27.7
12.6 15.6
MILitary Load carriers
625
71.0
40.5
50.0
650
76.6
43.7
32.4
14.7
675
82.5
47. I
34.9
15.8
700
88.6
50.5
37.4
17.0
800
115.0
65.6
48.5
22.0
Forming
--Increasing
Weo ther
-
-
CiviLian heavy truck For total reslstonce: odd hard ground resistance eg : -
time since Lost ploughed
Decreosin, amount of recent rain
Soil
ALmost boggy
Very soft
Soft
Medium
Good
Wotking
Sticky. Boots sucked off
Heavy
Tiring. Mud works up L e g s
Easy, but boots needed
Normal shoes
Firm
Car 13% Truck or I 7-28% WheeLed AFV Tracked AFV 4,1%
NOTES
I
For wheeled vehicles with deflated tyres, hard ground raring resistance wiLL be increased by Lyre deformation, but resistance due to sinkage Less.
2.
AvaiLabLetraction is some : 3 0 - 7 0 % cross country depending on salt strength. This avaiLabLe for overcoming: ( a ) Sinkage resistance above. (b) Hard rood rolling resistance. (c) Extra Lyre deformation resistance If Lyres deflated. (d) Gradients (soy 5 - 1 0 % typicaLLy).
5.
ALso some traction is needed in hand for monoeuvrlng (10%).
4.
Heavy Line denotes practical mobility Limit.
182
J.C. LARMINIE OBSTACLE CROSSING
The standards in Tables 1-3 include obstacle crossing clearance angles for B Vehicles. These are defined pictorially in Fig. 2.
1 -T ....................
r .............
d
\ "\
l FI(;. 2. Obstaclecrossing clearance angles. THE GROUND
Peace and war The nature of the soil, and its roughness, vary greatly between peacetime training and test areas, and likely battlefields. As a generality, in peace the ground must stand much use, and tends to be the drier, firmer soils, particularly those less fertile, and so not wanted for agriculture. Thus there are the sandy heathland training areas of Europe, and similar examples elsewhere. Because such areas are farmed but little, yet much cut up by tracked vehicles, the surface tends to be rougher than fields smoothed by tilling. Allowance must be made for these differences between peacetime and war terrain, in the appraisal of trial results, and user comments on performances in service. The peacetime soils tend to give less liability to bogging, but are rougher, with more wear and tear, and make speed more often limited by the capacity of the suspension and the judgement of the driver. Trafficability limits The user needs guidance as to the extent to which soils of limiting strengths will be met. Some sources quote the performance available to the user in terms of the percentage of area non-trafficable. This is not helpful, since it does not allow the user to interpret the implications for operations of any particular percentage non-trafficable. The fallacy of the concept is shown pictorially in Fig. 3. Statistical terrain classification The classification of terrain into statistical patterns can give misleading results when analysing vehicle operation. It is the prime duty of the vehicle commander and driver to choose a route which avoids all the worse going and hazards.
MOBILITY REQUIREMENTS OF MILITARY VEHICLES
D
183
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_zm
o
Fi2ili !ii i i i i i i!i i i i iiii!iiiij !i iiili iili!iliFi FIG. 3. Varying possibilities if 20% of area non-trafficable. Vehicle routes
The p e r f o r m a n c e must be judged with reference to realistic routes which represent typical use in service. These m u s t be reconnoitred, analysed, and the data on features along them recorded.
184
J . C . LARMIN1E ASSESSING PERFORMANCE
General limitations The user needs to know the implications in service of various levels of mobility. In practical terms, the effect of mobility on tactics must be well known to all, so that full use of the vehicle potential can be made, and yet crisis be avoided should too much be asked of it. In long term planning, the user must be able to justify demands for certain standards of ability. At present it is hard for the designer or user to be able to express performance and its deepest implications in precise and clear terms. Neither real trials or simulations can provide a full answer due to lack of realism. Thus the dependence on the vehicle criteria, as in Tables 1-3 for stating the characteristics required. Major influences The assessment of performance is limited in scope by two aspects on which there is a major lack of information, and from which the influence on actual performance in service is very great. (a) The driver has a great influence on performance, and ability varies over a wide range. There is very little firm data on the range of driver ability, and its effect on vehicle speeds and approaches to hazards. (b) The softness of the soil varies widely from place to place, and season to season. Some examples of the magnitude of the effect of such changes on Armored Fighting Vehicle (AFV) performance when moving tactically over open ground in north-western Europe are given in Fig. 4, by courtesy of EASAMS Ltd. Tactical speed limits The merits of the mobility of a vehicle will only be tested to the limit, or their cost justified in full when the vehicle is driven as fast as possible. In theory, an AFV is driven as fast as possible from one tactical bound to another. But in m a n y situations this is not so, and speed will be held in check. This might be to keep station with another vehicle, or when moving through some place of restricted visibility, such as orchards or vineyards, or near buildings as in the outskirts of villages or in urban areas. Then there are the forests, which despite the difficulty, will have their share of combat. Open country, referred to as " g o o d tank going", covers well under half the area of many parts of Europe, and of many other parts of the world of interest to tank-owning countries. Peacetime use The mobility of a military vehicle is justified by the needs for war, But a vehicle must be used in peace for training, hopefully for m a n y years. It must thus cope well with peacetime conditions, and so this needs to be part of the assessment of its suitability for service. Generally, peacetime use is more harsh than war in mobility terms, due to the rough ground, the abrasive soil, and the training of raw learners. Reliability is an important facet of mobility. POSTSCRIPT
The series of standards published in this article are set at very modest levels. They thus represent the lowest standard the user should be asked to accept. They have been driven already by cost saving to their present low values, and there is no case for this excuse to be used for further lowering of standards.
MOBILITY REQUIREMENTS OF MILITARY VEHICLES
60-
185
CROSS COUNTRY SURFACES
-15 HARD Rr /-=/i RCI N/A FIRM Rr 70/, Re! 105 50-
MEDIUM Rr 13"/= RCI 65
SOFT Rr 20=/= RCI /.5 40-
-10 VERY SOFT Rr 30*/= RCI 35 u~
30-
GROUND PRESSURE MMP OF 285 kPa
E
..-¢
20-
AVERAGE SPEEDS FOR SINGLE TANKS DRIVEN AS FAST AS POSSIBLE ON BROAD SAMPLE TACTICAL ROUTES OF ROADS, TRACKS AND CROSS COUNTRY
-5
( WHOLE ROUTE ' 76 km/ 10-
15
29
i J
'
kW/tonne
I
POWER / WEIGHT
AVERAGE
SPEEDS
"
BROAD UNDER
FIG. 4.
S A M P L E OF R E A L I S T I C VARIOUS CONDITIONS
TACTICAL
ROUTES
Average speeds for net outputs.
It will be seen that ground pressure standards are set at reduced figures for the lighter vehicles. This is for two reasons. Light vehicles can reasonably have lower ground pressure, and do so, because of the effect of scale. They also need it, since the light vehicle will be small, and to it any obstacle will be relatively big, or soft top layers of soil deep. The fact that many current vehicles have poor mobility does not mean that there are frequent boggings, since the crews soon learn their limits, and keep clear of risk. The effect of poor mobility will be restricted tactics, and thus the end product will be greater cost in terms of battle casualties.
186
J.C. LARMINIE
The published work of ISTVS reflects the activities of its members. Many are connected with research. However, the end product of such work must ultimately have some practical application, otherwise the research will be sterile. These mobility standards are based on the research of several members of ISTVS, and offer such a practical application. It is hoped that future work will help to refine knowledge of how such criteria, and these levels for them, appear to work in practice. This article is published to make existing experience available more widely, and in the hope that it will bring forth more information of a strongly practical value. REFERENCES [11 J . C . LARMINIE, A study of mobility requirements, Volume 1. Report for MOD(LSOR) classified Restricted. EASAMS Ltd. Ref. 2721/R/2 (1981). [2] J . C . LARMINIE, A forecast of speeds across country. Report for M O D [RARDE(CH)] classified Restricted. EASAMS Ltd. Ref. 2744/R/1 (1982). [3] V.G. CLEARE, Factors affecting the performance of high speed track layers. Proc. L Mech. Engrs (AD) (1963). [4] D. ROWLAND,Tracked vehicle ground pressure and its effect on soft ground performance. Proe. LS'TVS 4th Int. Conf., Stockholm (1972). [5] D. ROWLAND, A review of vehicle design for soft ground operation. Proc. ISTVS 5th Int. Conf., Detroit (1975). [61 J. C. LARMINIE, Predicting the performance of fast cross country vehicles. Proc. ISTVS 8th Int. Con/~, Cambridge, Vol. 3 p. 1005 (1984). [7] D. ROWLAND and J. W. PEEL, Soft ground performance prediction and assessment for wheeled and tracked vehicles. Proc. L Mech. Engrs 205/75. [8] Various Authors, AII-WheelDrive International Conf., 1986. I. Mech. Eng. Conference Publication 1986-1 (1986). [9] Various Authors, Army Staff Course Vehicles and Mobility Handbook, M O D classified Restricted, RMCS Shrivenham, Date as revised. [10] R. M. OGORKIEWICZ, Tracked and wheeled light armoured vehicles. International Defi'n~e Revie,. lnteravia Publishing Group, Geneva (1986). [11] E. B. MACLAURIN, The effects of tread pattern on the field performance of tyres. Proc. ISTVS 7th Int. ConL, Calgary (1981). [12] D O A E ( M O D ) , Analysis of soil conditions of boggings in World War 2. A O R G Report 13/51 (1951). [13] T. L. H. BUTTERFIELD, Design and development of fighting tanks. Proc. 1. Mech. Engrs. VoL 180 Pt. 2A No. 5 (1965-66)
APPENDIX: T H E MEAN M A X I M U M PRESSURE SYSTEM FOR VEHICLE G R O U N D PRESSURE
Introduction to ground pressure measurement System requirement. A term is needed to give an indication of likely vehicle sinkage into the soil, and thus a lead to the resistance to motion. It should also give a guide to the vehicle grip for traction, and thus allow a forecast of the likely ground conditions when resistance will exceed traction, and the vehicle will fail. The system must be easily understood by the layman, and readily applied. Systems available. For many years Nominal G r o u n d Pressure (NGP) has been used. It assumes even weight distribution over the planform. For tracks, it takes simply the length and breadth of that part of the tracks on the ground. T h u s it does not take into account the uneven load on the tracks, which varies with the number and size of the wheels, and the pitch length of each individual link. With a combination of few small wheels, and a very fine pitch, the track is almost unloaded half-way between wheels. For wheeled vehicles there are two forms of NGP. Tyre manufacturers use the contact patch of a tyre on a hard surface. This is suitable for comparing tyre Ioadings, but not the loading of soft ground, nor will it give any comparability with tracks. A more realistic wheels N G P was developed by the School of T a n k Technology, which gave an approximation to the tyre planform at sinkage deep enough to be likely to cause immobilisation, but this was limited in realism. The US Army has developed the Vehicle Cone Index (VCI) system. It is applicable to tracks or wheels, and gives a reasonable yardstick. However, it is entirely empirical and contains unlikely combinations of expressions, and it is cumbersome to use. Mean M a x i m u m Pressure (MMP) derives its name from a study of actual tank pressure measurements. There are peaks of m a x i m u m pressure under each wheel. Each wheel bears a different load. However, provided the vehicle design is not a freak, it
MOBILITY REQUIREMENTS OF MILITARY VEHICLES
187
has been found satisfactory to treat the pressure as the mean of all the peaks. A relationship was prepared by the fighting vehicle research and development establishment at Chobham, RARDE(CH), giving MMP in terms of track and wheel parameters. Later development produced an expression to give a comparable equivalent MMP for wheeled vehicles. The wheeled MMP is based on work on Soils Numerics done by the US Army Waterways Experiment Station (WES), which had also produced VCI. The terms of this wheeled vehicle numeric agree with the common appreciation of the factors that are important, and with the results of other research work, and so it appears a basically sound term. A full comparison of NGP, VCI, and MMP is given in Ref [1], and the subject is also covered in Ref [9]. The MMP system was published and justified in Refs [4] and [5]. It should be noted that the original RARDE(CH) report, No. 72031 of July 1972 was superseded by those Refs [4] and [5], and has out-of-date formulae. Thus it should be used with caution. MMP is expressed in Kilo-Pascals (kPa) (Newtons per square metre). They are not quoted with conversions to other units, such as lb/in 2, since MMP is a new expression, and so there are no old measures to compare it with.
Mean maximum pressure expressions The MMP formulae and associated expressions are given in Table A1. It will be seen that there are different wheeled MMP formulae for both clayey, cohesive fine-grained soils, and for sandy, frictional coarse-grained ones. The development and application of MMP has so far been based almost wholely on that for clayey soils. The formula for sand is only recently derived, and only partly developed. Clay soils are a greater mobility problem than sand, and thus it is the clay MMP that is normally quoted, and is more important. The wheels formula for clay allows comparison with the tracked MMP. This is not so for the sand wheeled MMP, which should only be used for comparing vehicles of fairly similar characteristics.
TABLE A 1. MEAN MAXIMUMPRESSUREEXPRESSIONS
Principal MMP formulae (a)
MMP Formula for tracked vehicles: 1.26W
MMP =
kPa
2m c b (pd)°'5 W m d b p c (b)
= = = = = =
vehicle weight (kN) number of axles wheel diameter (m) track width (m) track link pitch (m) track link profile factor: area/pb.
MMP Formula, for wheels on fine-grained (cohesive) (clayey) soil: MMP = W m d b
= = = = 6/h = K =
KW 2m b °85 d I'ls (3/h) °'5
kPa
vehicle weight (kN) number of axles tyre diameter tyre breadth unladen (m) tyre deflection on hard ground % factor from table below. Factor K:
Number of axles
Proportion of axles driven 1
3/4
2/3
3/5
1/2
1/3
1/4
2 3 4 5 6
3.65 3.9 4.1 4.32 4.6
--4.44 ---
-4.35 --5.15
---4.97 --
4.4 -4.95 -5.55
-5.25 --6.2
--6.05 ---
188
J . C . LARMINIE
Differential Locks. If differential locks are in use, the equivalent MMP is improved: 4 × 2 vehicles; MMP × 0.98. 4 × 4 or 6 × 6; MMP × 0.97.
Secondary M M P formulae (a) MMP Formula, for wheels on dry coarse-grained (frictional) sand soil: S TW
MMP -
2m b I 5 dl.5 ,Wh
kPa
S = constant for proportionality (see below). T = tyre tread factor: 1 for smooth tyre; 1,4 road tyre, 2.8 road/cc tyre; 3.3 earth mover. Otherwise, factors are as for fine-grained soils (Para l.b). Constant S: The proper relationship of MMP for wheels on dry sand with that for fine-grained soil has yet to be fully researched. For the present take: S = 0.60. (b)
MMP Formula for belt tracks with pneumatic tyres: MMP --
0.50 W 2m b (d 6) 0.5
kPa; 6 = tyre deflection on hard ground (m).
Otherwise, as for wheeled MMP. For belt tracks on solid rimmed wheels, the tracked MMP is used, with the pitch (p) being taken as the length of the steel reinforcement at the tread. (c)
MMP
=
MMP Formula, for wheels of different sizes on different axles (fine-grained soils): K --
2m
Wi
[
b~~8~ d'~ ~5 (6/h)¢~ 5
Wi
+ .... +
b '85~ d,1'5 (6/h)l '5
where suffix (1) refers to the first axle, and so on to the ith axle, and all other factors are the same as in the normal MMP formula for wheels.
Soundness o f term The MMP system appears to be the most sound of the available terms. It accords to common sense, it has given usable and repeatable results, and has given reliable guidance. It is relatively simple to use. Its limitations appear less than those of other systems. It has now been used to quite a wide extent. Its weaknesses are: (a) It assumes the weight is evenly spread on all axles. (b) It assumes the soil is deep and homogeneous. Thus it has no allowance for the size of the vehicle in relation to the depth to firmer underlayers. It has no factor for the aggressiveness of the track or tyre tread, and thus the ability to grip firmer underlayers, particularly when on grass. (c) It makes no allowance for steering effects. It can be said that MMP is the least bad of the systems available, it can be used with confidence as a guide. For the purposes of military vehicle mobility standards, some 20 kPa is the minimum difference in MMP that should be considered significant.
MOBILITY R E Q U I R E M E N T S OF M I L I T A R Y VEHICLES
189
Special applications of MMP Forecasts oftrafficability. Because the M M P formula for wheels is derived from the WES soils numerics, it is possible also to relate it to that establishment's work on trafficability, and the soil strength Rating Cone Index (RCI). Since the wheeled M M P formula has been specifically prepared to be comparable to that for tracks, a family of relationships of M M P and RCI exists. The limiting soil strengths to allow one vehicle to pass or for several passes, are given in terms of M M P and RCI in Table 5. The relationship of M M P to resistance on various soils is in Table 7. This allows calculations of speed to be made once the soil's strength is known. Reference [5] gives guidance on the use of M M P as a yardstick for vehicle design. For instance, for tracked vehicles, the factor by which M M P exceeds N G P gives a measure of the efficiency of the layout, where M M P - N G P = Q, a factor of merit. Typical ranges of Q are from 1"5 for slow vehicles such as caterpillar tractors, to the bad figure of 3.8 for some designs with emphasis on s m o o t h running at high speed, and thus a short pitch track. The Panther tank, with large overlapping wheels achieved Q of 1.71, which was reflected in a good M M P of 150 kPa. Wheeled vehicle trains. It is satisfactory to apply the wheels M M P formula to wheeled vehicles which do not conform to the normal simple rigid layout of two or three or four axles with tyres of similar size on them all. Thus a vehicle with trailer can be likened to a rigid single one of the same n u m b e r of axles, with the appropriate n u m b e r of them driven. If the tyres are of different sizes, this can be taken into account in the calculations, with appropriate allowance for the weight distribution to each axle. The M M P for a half-track can be calculated by taking the front axle as a single axled wheel vehicle, using the value K = 3'32. The rear suspension is treated as a tracked vehicle; the weight is applied to the two suspensions according to the laden weight distribution of the vehicle. The whole vehicle M M P is then found by taking an average, with weighting for that weight distribution. Twinned rear wheels. When running straight on soft ground, twinned wheels on the rear axles are of limited benefit, since the twinned rears will overlap the ruts of the single wheels on the front axle. However, limitations in mobility usually are evident when steering, and then the axles will not follow in the same ruts. Practice does show that twin wheels are some benefit on soft ground. It is therefore realistic to use a compromise formula: Replace (2 m) by -
(2m + w) 2
where: w = the total number of wheels, m = the number of axles as before.
STANDARDS
0022-4898/8853.00+0.00 Pergamon Press plc. © 1988 ISTVS
FOR THE MOBILITY REQUIREMENTS MILITARY VEHICLES
OF
J. C. LARMINIE*
Summary--This article introduces a series of criteria which can be used to specify the cross-country performance of military vehicles, according to their type, and the priority to the user for mobility. The standards have been taken from existing vehicles, so are practical. The article also explains how the criteria were derived.
INTRODUCTION
DESCRIPTIONS of vehicles by designers and manufacturers often omit many important facets of performance, in particular for wheeled vehicles a statement of ground pressure. It is thus often difficult for the user to ensure a vehicle will meet a military requirement. Over recent years a series of criteria and assessment methods have been evolved which allow performances to be stated with some precision. The various criteria, and assessments of their implications in terms of performance, have been presented and justified in Refs [1-5]. The need to adopt, and take into use such criteria and the methods of assessing their implications in terms of performance was explained by the author in "Predicting The Performance of Fast Cross Country Vehicles", Ref. [6]. Further background material for the subject is in Refs [7-13]. This article puts forward a series of lists of values of each criterion, suitable for a range of vehicles filling various roles, and to meet various levels of importance of mobility in the priorities of the user. Also included for completeness is a description of the Mean Maximum Pressure (MMP) system for quantifying soft-ground performance, since some of the bibliography is not readily available to readers. These standards were originally presented in the form of a paper to the 4th Annual British Conference, ISTVS, Sutton Bonnington, in September 1986. However, it has been possible to make recent additions to some of the tables, and therefore these have been up-dated, and they now make the paper obsolete. MOBILITY STANDARDS
The recommended standards for various types of vehicles are given in Tables I to 3. These standards have been taken from current vehicles. Thus they are pragmatic values that make allowances for cost restrictions on buying and running the vehicles, and the engineering and operational constraints on designers. The standards are met by some current vehicles, and thus are a realistic level. Also, current vehicles have most definite limits to their mobility, which in relatively untaxing circumstances will prove a restriction on the operating flexibility of the user. Thus they are a realistic minimum requirement. *Major J. C. Larminie, Tithe Barn, Lytchett Matravers, Poole, Dorset BHI6 6BJ, U.K. 171
172
J. C. LARMINIE TABI.E I. MOBII.ITY STANDARDS:HEAVY AFV* Priority Criteria
Desirable
Essential
Less than 230 More than 19(25)
Less than 280 More than 10.5(14)
Remarks
Major factors: Ground pressure M M P in kPa
Gross power to weight ratio: k W / t (hp/t)
Secondary factors: Ground clearance: minimum, (mm)
480
Turning circle Length to breadth (L/C) limits Fording depth: (m) Pitch ratio, m a x i m u m Vertical step height Width limit (m) Height, m a x i m u m (m) Suspension periods (s): Bounce, m i n i m u m Pitch, m i n i m u m Damping: % critical Available b u m p deflection from static, m i n i m u m (ram) Stability tilt angle, m i n i m u m
400 Pivot turn.
1.2-1.8 1.1-1,9 -1.5 1.5 2.0 Not specified. 3.5 -3.0 4.0 0.75 1.5 30-45
0.65 1.0 25-50
260
200
40 °
35 °
See Notc
*These standards have been collected from various sources, and reflect current capability. The desirable limits are met by the more mobile vehicles. Note: The essential height limit is for bridge clearance. The desirable limit is tactical.
POWER
The usefulness of the installed power depends on the effectiveness with which the transmission allows the engine to apply its torque. Variations in this are shown pictorially in Fig. 1. Typical values are listed in Table 4. SOFT G R O U N D P E R F O R M A N C E
A vehicle must have a reasonably light ground pressure. This will limit sinkage into moderately soft ground, and thus minimize resistance to motion, so making the most of the available power for speed and manoeuvrability. In worse going, the light ground pressure will avoid immobilization, when resistance exceeds traction. In Table 5 are the desirable ground pressures needed to operate on various types of ground, given in the MMP system. These ground pressures assume the vehicle is running
173
MOBILITY REQUIREMENTS OF MILITARY VEHICLES TABLE2. MOBILITYSTANDARDS:LIGHTAFV (Weight less than 30 tonne) Criteria
Combat vehicles Desirable Essential
Armoured Carriers
Less than 140 More than 22.5(30)
Less than 230 More than 15(20)
Less than 450 More than 9(12)
450 Pivot turn
350 As HMLC
300 As IMMLC
Float 1.5 As HMLC 2.9 --
1.25 1.5 2.0 As IMMLC 3.5 2.2
0.75 1.5 -As IMMLC 2.9 2.9
0.75 1.5 30-45
0.65 1.0 25-50
0.65 1.0 25-50
210
130
130
38°
33°
30°
Major factors: Ground pressure MMP in kPa Gross power to weight ratio: kW/t (hp/t)
Secondary factors: Ground clearance: minimum, (mm) Turning circle Fording depth: (m). Unprepared Prepared Pitch ratio (for tracks) max. Angles of approach etc, (for wheels) Width limit (m) Height, maximum (m) Suspension periods (s): Bounce, minimum Pitch, minimum Damping: % critical Available bump deflection from static, minimum (mm) Stability tilt angle, minimum
Notes: (1) For wheeled AFVs, refer to HMLC or IMMLC in Table 3(a) if gross weight less than 6 t, otherwise Table 3(b). (2) Armoured Carriers, as distinct from true Combat Vehicles, are simple vehicles with a limited role. (3) These standards have been collected from various sources, and reflect current capability.
straight. A vehicle with skid-steering will need a g r o u n d pressure M M P some 20% lower t h a n one with n o r m a l a c k e r m a n or articulated steering. It will be seen that these desirable values generally call for g r o u n d pressures less t h a n those given in Tables 1-3, thus e m p h a s i s i n g that those listed s t a n d a r d s are n o t asking for too generous a performance. In T a b l e 6 are the g r o u n d pressures actually achieved by v a r i o u s vehicles. It will be seen that generally g r o u n d pressures are heavier t h a n the desirable ones in T a b l e 5, a n d h a r d l y meet the m i n i m u m s t a n d a r d s listed in Tables 1-3. I n T a b l e 7 are given relationships of g r o u n d pressure to resistance to m o t i o n caused by sinkage, a n d related to soil strength. This resistance is a d d i t i o n a l to the o t h e r n o r m a l c o m p o n e n t s of resistance to m o t i o n : hard surface rolling resistance, gradient, a n d air resistances.
J. C. L A R M I N I E
174
TABLE 3. MOBILIIY STANDARDS: B VEtttCLES Mobility classes Criteria
HMLC
IMMLC
MMLC
ILMLC
LMLC
(a) Utilities and light trucks (Payload less than 4 tonne)
Major factors: Gro und pressure MMP in kPa Gross power to weight ratio: k W / t (hp/t)
Less than 280 More than 22(30)
280 to 350 More than 19(25)
350 to 550 More than 12(16)
5511 to 700 More than 12(16)
Greater than 700 Less than 12(16)
400 Under 11
260 Under 12
180 Under 13
150 Over 13
115 Over 13
0.76 1.50
0.60 1.10
0.50 1.00
0.50 --
1t.50 --
45 ° 40 ° 130°
40 ° 38 ° 155°
40 ° 38 ° 155° Usually not critical.
35 ° 311° 155<,
_ _ --
0.75 1.5 30-45
0.65 1.0 25-45
0.6 0.5 25-50
0.6 0.5 20-50
0.6 0.5 20-50
200
130
125
125
100
35 °
33 °
30 °
28 °
28 °
Less than 350 More than 11(151
350 to 450 More than 9(121
450 to 600 More than 7.51101
600 to 700 More than 6(8)
Greater than 700 More than 4.4(6)
400
300
300
3011
200
17.5(16)
19(17.5)
20.5(19)
27(25)
27(25)
1.25 1.50
0.75 1.50
0.75 1.50
0.75 1.50
0.50 --
45 ° 40 ° 130° 2.9 2.6
40 ° 38 ° 155° 2.9 3.0
40 ° 38 ° 155 ° 2.5 3.0
35 ° 30 ° 155 ° 2.5 --
---2.5 --
0.75 1.5 30-45
0.65 1.0 25-45
(1.6 0.5 25-50
0.6 0.5 20-50
0.6 0.5 20-50
Secondary factors: G r o u n d clearance: minimum, (ram) Turning circle between kerbs (m) Fording depth (m) Unprepared Prepared Clearance angles: Approach, Min: Departure, Min: Under veh. Max: Width limit Suspension periods (s): Bounce, minimum Pitch, minimum Damping: % critical Available bump deflection (mm), Min Stability tilt angle, minimum
(b) Medium trucks (Payload 4 to 8 tonne)
Major factors: Ground pressure MMP in kPa Gross power to weight ratio: k W / t (hp/t)
Secondary factors: Ground clearance: minimum, (mm) Turning circle max. walls (kerbs) (m) Fording depth (m) Unprepared Prepared Clearance angles: Approach, Min: Departure, Min: Under veh. Max: Width limit Cab height (m) max. buspension periods (s): Bounce, minimum Pitch, minimum Damping: % critical
MOBILITY R E Q U I R E M E N T S O F M I L I T A R Y VEHICLES
Available b u m p deflection (mm), Min Stability tilt angle, m i n i m u m (load C G 0.66 m above floor)
175
200
130
125
125
100
33°
30 °
28 °
26 °
26 °
Less than 350 More than 11(15)
350 to 450 More than 9(12)
450 to 600 More than 7.5(10)
600 to 700 More than 6(8)
Greater than 700 More than 4.4(6)
400
350
300
300
200
19.5(18)
21.5(20)
24(22.5)
27(25)
--
1.25 1.50
0.75 1.50
0.75 1.50
0.75 1.50
0.50 --
45 ° 40 ° 1300 2.9 3.3 --
40 ° 38 ° 155 ° 2.9 3.4 --
40 ° 38 ° 1550 2.5 3.4 4
35 ° 30 ° 155 ° 2.5 -4
--2.5 -4
0.75 1.5 30-45
0.65 1.0 25-45
0.6 0.5 25-50
0.6 0.5 20-50
0.6 0.5 20-50
200
130
125
125
100
33 °
30 °
28 °
26 °
26 °
(c) Heavy trucks (Payload heavier than 8 tonne)
Majorfactors: G r o u n d pressure M M P in kPa Gross power to weight ratio: k W / t (hp/t)
Secondaryfactors: G r o u n d clearance: minimum, (mm) Turning circle Max. walls (kerbs) (m) Fording depth (m) Unprepared Prepared Clearance angles: Approach, Min: Departure, Min: Under veh. Max: Width limit (m) Cab height, (m) Max. Ht. with container (m) Suspension periods (s): Bounce, m i n i m u m Pitch, m i n i m u m Damping: % critical Available b u m p deflection (mm), Min Stability tilt angle, m i n i m u m (load CG 0.66 m above floor)
Notes: Table 3a: These standards have been used by M O D as design guidelines given to manufacturers for such vehicles. Table 3b: A new standard recommended is the turning circle within walls. Also the previous frontal area and volume requirements have been replaced by width and cab-height. The L M L C power is the legal minimum. Otherwise, these standards are those in Ref. [1], which have been used by M O D as design guidelines given to manufacturers for such vehicles. Table 3c: (1) These standards have been derived from those well established for 4-8 tonners [Table 3(b)], but with larger turning circles and height, and ground clearance for IMMLC. (2) Container height: M M L C 2.44 m (8ft); LMLC 2.59 m (8ft 6in).
176
J . C . LARMINIE
POWER
SPECff::IED
260kW
(350 H P )
ENGtNE 1
SPARK I G N ! T I O N PETROL
ENGINE 2 :
TURBO CHARGED DIESEL
ENGINE OPERATING RANGES
C2 2 3 - 1 6 ( 1 0 5 ) 105 F .... kW
16 -*
: I (..z. (2 2)
C2 - - !
18 F'- - - - - - r
HP
C
23 12~
C~ 5 6 - 2 4 1 5 ~ - 1 8 1 :
23(32)
56 5~ ~ -4
Nm
,,250L
T2 300 -
35O
1000-
250-
750200
-
-250 ;
~
TI 500-
15C-
100-
L 10
1~5
I 20
! 25
} 3T0 35 ENGINE SPEED
1 I Z,0 ':-5 RPM • 100
' 5'0
l 55
i 60
. 6~5
N
FIG. 1.1-1.5. Effective power: engine flexibility. 2. Effective power: flexible engine, few gears. 3. Poor effective power: narrow engine range and few gears. 4. Effective power: narrow engine range but more gears. 5. Effective power as average over whole speed range.
MOBILITY REQUIREMENTS OF MILITARY VEHICLES kW
HP
[-3so
250"-1
177
/
/ /
:
: z~i
-5600
ilii*,
:
i~
....
- 3oi J
:'~:~!
i;i:i
::iiii'% ::::'"
-5000/6000
/
ii!ii:iiiii!
/
-~000
3SO0 x 3000 150--
0
~w z
I
!
I
! t
/
I
/
/
I I
I
100-
I
/
I l~e q l ~
i
/
/
i
t]
10
I
20
=2000
o/" RESISTANCE
10
5
kW
/
EFFECTIVE POWER IS AREA UNDER GRAPH
i
I
-2500
/
/
I
30
/
15 I
I
40
50
/•83 86 o/=
;
20
I
25
I
80
90
100
,
-1500
,1 I
60 ROAD SPEED
70
I
3O
I I
m/$
]i
110
km/h
FIG. 1.2.
HP
/
/
-2300
/
-2000
/ /
/
I1 o o. 150-
00
/ Z o. n- 1500 Lu z
z
u3
!
z
I
/
l i0
/
I
i
-1200
/
/
/
I
'
E'L ROAD TANCE
i
- 1000
rgI"EFFECTIVE POWER IS AREA I UNDER GRAPH : 8 1 % I
I
I0
5 ,
1()
I I
20
i
30
I
15 i
40 /.0
w z w
/
I
I
-150
/
i
I
20 iI
50 60 ROAD SPEED FIG. ].3.
I I 70
I I i 80
25 I 90
30 l 100
m/s
I w
110
km 1 h
WW
HP
-350 250-
_-i¸i:
-2300
/
-2000
/ / /
200-
I I
I
/
/
/
o 150~
11
I
t
/
I
I I
I
I
I
!200
f
I
I 100-
!
I
I
I':',
!
/
I
200 I
/
/
I
c~ 1500 Lu z L9 z uJ
/
!
(
/
I
-1000
•/ ~ T A N C E rII~EFFECTIVE POWER IS AREA I
UNDER
GRAPH
f 10
91a/°
:
5
I
10
20
' ~ 30
15 '1 ~0
'~ 50
20 l 60
] 70
25
30
m/s
'' 80
90
100
1t0
km/h
ROAD SPEED
FIc~. 1.4. EXAMPLE : 1&t kW
VEHICLE
125 N P l t ~
l~,.SkWtt
HP "350
REQUIRED POWER TO MEET USER REQUIREMENT
250-
NET POWER -300
200-
MESHANICAL TRANSMISSION
Prg 1/,
E : 91%
F~g 1.2
E : 86('/,
F i g 13
E :
81°/,
260 kW
EF~ICrENCY
93°1(,
:- 2&2 kW
DIESEL/ B GEARS V ~ MAX
PETROL / 5 GEARS
I
v ~'.,x V
--253
DIESEL I $ GEARS
,~ MAX
o~ £L 1 5 0 - -200
USER SPEED REQUIRED ESSENTIAL 5 !
4
10 20
30
15 L-O
c~ • --~
20
6J0
~
70
DESIRABLE
1
25
7
T
•
80
90
m/s
30
I
100
r
~10
ROA0 SPEED FIG. I . i NOTES I
DIESEL/ 5 GEARS UNSATISFACTORY DUE TO EXCESSIVE
2
HYDROSTATIC TRANSMISS;ON EFFECTIVE POWER 203 kW
ENGINE LUGGING / LABOURING (SEE Fig I 3)
km/h
179
MOBILITY REQUIREMENTS OF MILITARY VEHICLES TABLE 4. TRANSMISSIONEFFICIENCYAND EFFECTIVENESS Typical Values for MBT About 15%. Engine installation losses: Transmission efficiency losses: Epicyclic Gearbox. Spur gears gears 7% 10% Final drive. About 3,%. Gearbox effectiveness (e.g. Fig. 1.5): 4 Gears 6 Gears 8 Gears Friction clutch. Torque converter. Friction clutch.
81% 83% 82%
85% 88% 86%
91% ---
CVT 22%
CVT 100% -100%
Max speed km/h 70 70 55
Notes: (1) The Continuously Variable Transmission (CVT) is assumed to be hydrostatic. (2) The engine is assumed to be a turbo-charged diesel. TABLE 5. DESIRABLEGROUNDPRESSUREMMP VALUES Vehicle design aims: MMP Required for Satisfactory Performance (Presented in Ref. [2]) MMP Levels for performance priority Condition
Temperate climate, wet, fine-grain (eg clay) soil Tropical, wet fine-grain soil European bogs Muskeg Over-snow
Ideal
Satisfactory
Maximum acceptable
150
200
300
90 5 30 10
140 10 50 25-30
240 15 60 40
Trafficability limits: For fine-grain (e.g. clay) soils, wet. for one pass: RCI = 0.096 MMP [Rating Cone Index of soil strength in lbf/in2; MMP in kPa]. For multiple passes: Number of passes: ! 2 5 10 25 50 Multiply One-pass RCI by: 1 1.2 1.53 1.85 2.35 2.8 Resistance from soil sinkage:
R = Resistance (kN). W = Weight (kN) (mass x 9.81). M =
M M P (kPa).
C = Soil Cone Index in kPa. Take CI = RCI + 0.8 1 lbf/in 2 = 6.895 kPA. The formulae for MMP for tracked and wheeled vehicles are given in Appendix A.
235 220 236 164 121 210 145 363 279 243 256 257 178 259 168
Leopard 1 Leopard 2
USA M60 Sheridan M551 M113
Abrams M1 Bradley M2/M3
White/Int Half-track M3 (WW2) Merkava T55 T62 T72 BMP2
AMX30 AMXI0P
-14.5
7.65 60 36 38 -14.6
51.4 22.36
48.1 15.8 11.6
44.4 55.2
62 53.6 51.6 52.4 8.26 12.7 15.28 41.0 25.3 37
Weight tonne
-NTA
1.0 0.95e 0.95 0.95e -0.97e
0.98e 0.98e
0.98 NTA 0.98
0.95 0.96e
0.94 0.93 0.93 NTA 0.87 0.87 NTA 0.94 0.96 0.92e
Factor C
437
276 576 392
Panhard M l l VBL (4 x 4) VAB: VTT (4 × 4) VAB: VTT (6 × 6) AMX]0RC (Skid steer 6 × 6)
185 355 524 439 272 421 427 428 320
278 525 692 620 479
Land Rover ~t Truck 4t Bedford MK Truck 8t Bedford TM Foden FH70 Limber DROPS Prototype IMMLC (8 x 6) Twister XMS08 (artic) Cadillac Gage V150 (4 × 4) FM Canada Grizzly (6 × 6) GM Canada LAV(USMC)(8 × 8) M998 H u m m e r S-D-P. Pandur (6 × 6) Luchs (8 × 8) Engesa Urutu EEl (6 × 6) BTR 60P
540 351 519 455 403 349 710
MMP kPa
Saxon AT105 Ferret Scout Car Mark 2 Daimler Armoured Car (WW2) Saladin Stalwart Fox M2 Amphibious Rig (4 × 4)
Wheeled vehicles
15.8
3.54 13 14.2
--11.79 11.95 3.87 11.2 19.5 14 --
2.1 9.5 16.3 26.6 30
11.66 4.5 7.5 11.6 14.5 6. l 21.5
Weight tonne
14 × 20
9.00 × 16 14 x 20 14 × 20
--l l . 0 × 16 11.0 × 16 36 x 12.5-16.5 12.5 × 20 14 x 20 13 × 20 --
6.50 × 16 12 × 20 15.5/80R20 16 × 20 20.5 × 25
14.75/80R20 9.00 × 16 10.5 x 20 12 x 20 14 × 20 11 × 20 16 × 20
Tyre size
Factor C is thc track link footprint as a proportion of (pb); NTA means (' not taken into account; (e) means estimated. Where weight is blank (--), MMP is from unconfirmed data. For MMP of wheeled vebicles, tyres are assumed to be inflated to pressure fo~ roads, with 18c~ deflection. Tyres from different manufacturers have significant differences in their dimensions.
282 274 276 288 106 135 205 235 198 290
MMP kPa
Challenger Chieftain Mark 5 Centurion Mk 10 (steel track) Centurion (rubber track) Scorpion Stormer FV432 Vickers Mark 3(1) Warrior (MCV80) Tank S (Sweden)
Tracked vehicles
TABLE 6. MEAN MAXIMUMPRESSUREFOR VARIOUSVEHICLe'S
Z m
181
MOBILITY REQUIREMENTS OF MILITARY VEHICLES TABLE 7. RELATIONSHIPS:RESISTANCE,GROUNDPRESSUREAND SOIL STRENGTH SoIL strengths: RCI
Ground pressure MMP kPo
25 172
IlO
I bf / in z kPo
35 241
310
414
70 485
105 724
7.6
3.9
2.4
1.4
1.0
0.5
Very light
140
r2.0
6.3
3.8
2.2
1.6
0.7
Light tracked AFV
185
21 .0
10.8
66
3.8
2,8
1.3
230
32.0
16.5
I0, I
5.8
4.3
1.9
Light MBT
I
I
AFV
I
285
48.0
25. I
15.4
8.8
6.5
2.9
Heavy MBT, Motor car
3i0
57.0
29.5
18. I
10.3
7.6
:5.5
Good wheeled ArM
350
72.0
:57.4
22.9
13, I
9.7
4.4
Normal wheeled AFV
45O
118.0
61 . I
37.4
21.4
15.8
7.2
I m
475
67.9
41.6
23.7
17.6
8.0
5O0
75.0
45.9
26.2
19.4
8.8
525
82.5
50.5
28.8
21.4
9.7
550
90.3
55.3
31.6
25.4
10.6
575
98.5
60.3
34.4
25.5
I 1.6
600
107.0
65.6
37.4
27.7
12.6 15.6
MILitary Load carriers
625
71.0
40.5
50.0
650
76.6
43.7
32.4
14.7
675
82.5
47. I
34.9
15.8
700
88.6
50.5
37.4
17.0
800
115.0
65.6
48.5
22.0
Forming
--Increasing
Weo ther
-
-
CiviLian heavy truck For total reslstonce: odd hard ground resistance eg : -
time since Lost ploughed
Decreosin, amount of recent rain
Soil
ALmost boggy
Very soft
Soft
Medium
Good
Wotking
Sticky. Boots sucked off
Heavy
Tiring. Mud works up L e g s
Easy, but boots needed
Normal shoes
Firm
Car 13% Truck or I 7-28% WheeLed AFV Tracked AFV 4,1%
NOTES
I
For wheeled vehicles with deflated tyres, hard ground raring resistance wiLL be increased by Lyre deformation, but resistance due to sinkage Less.
2.
AvaiLabLetraction is some : 3 0 - 7 0 % cross country depending on salt strength. This avaiLabLe for overcoming: ( a ) Sinkage resistance above. (b) Hard rood rolling resistance. (c) Extra Lyre deformation resistance If Lyres deflated. (d) Gradients (soy 5 - 1 0 % typicaLLy).
5.
ALso some traction is needed in hand for monoeuvrlng (10%).
4.
Heavy Line denotes practical mobility Limit.
182
J.C. LARMINIE OBSTACLE CROSSING
The standards in Tables 1-3 include obstacle crossing clearance angles for B Vehicles. These are defined pictorially in Fig. 2.
1 -T ....................
r .............
d
\ "\
l FI(;. 2. Obstaclecrossing clearance angles. THE GROUND
Peace and war The nature of the soil, and its roughness, vary greatly between peacetime training and test areas, and likely battlefields. As a generality, in peace the ground must stand much use, and tends to be the drier, firmer soils, particularly those less fertile, and so not wanted for agriculture. Thus there are the sandy heathland training areas of Europe, and similar examples elsewhere. Because such areas are farmed but little, yet much cut up by tracked vehicles, the surface tends to be rougher than fields smoothed by tilling. Allowance must be made for these differences between peacetime and war terrain, in the appraisal of trial results, and user comments on performances in service. The peacetime soils tend to give less liability to bogging, but are rougher, with more wear and tear, and make speed more often limited by the capacity of the suspension and the judgement of the driver. Trafficability limits The user needs guidance as to the extent to which soils of limiting strengths will be met. Some sources quote the performance available to the user in terms of the percentage of area non-trafficable. This is not helpful, since it does not allow the user to interpret the implications for operations of any particular percentage non-trafficable. The fallacy of the concept is shown pictorially in Fig. 3. Statistical terrain classification The classification of terrain into statistical patterns can give misleading results when analysing vehicle operation. It is the prime duty of the vehicle commander and driver to choose a route which avoids all the worse going and hazards.
MOBILITY REQUIREMENTS OF MILITARY VEHICLES
D
183
i:i:i:i:i:
,.........,-........ :,x.:.:
I!ii!iiiiii
~:::~:~:~ E .I¢
I il
I~
~
-~"~''
i:??:!:!
t~
z ~ OO
~
o
~0
_zm
o
Fi2ili !ii i i i i i i!i i i i iiii!iiiij !i iiili iili!iliFi FIG. 3. Varying possibilities if 20% of area non-trafficable. Vehicle routes
The p e r f o r m a n c e must be judged with reference to realistic routes which represent typical use in service. These m u s t be reconnoitred, analysed, and the data on features along them recorded.
184
J . C . LARMIN1E ASSESSING PERFORMANCE
General limitations The user needs to know the implications in service of various levels of mobility. In practical terms, the effect of mobility on tactics must be well known to all, so that full use of the vehicle potential can be made, and yet crisis be avoided should too much be asked of it. In long term planning, the user must be able to justify demands for certain standards of ability. At present it is hard for the designer or user to be able to express performance and its deepest implications in precise and clear terms. Neither real trials or simulations can provide a full answer due to lack of realism. Thus the dependence on the vehicle criteria, as in Tables 1-3 for stating the characteristics required. Major influences The assessment of performance is limited in scope by two aspects on which there is a major lack of information, and from which the influence on actual performance in service is very great. (a) The driver has a great influence on performance, and ability varies over a wide range. There is very little firm data on the range of driver ability, and its effect on vehicle speeds and approaches to hazards. (b) The softness of the soil varies widely from place to place, and season to season. Some examples of the magnitude of the effect of such changes on Armored Fighting Vehicle (AFV) performance when moving tactically over open ground in north-western Europe are given in Fig. 4, by courtesy of EASAMS Ltd. Tactical speed limits The merits of the mobility of a vehicle will only be tested to the limit, or their cost justified in full when the vehicle is driven as fast as possible. In theory, an AFV is driven as fast as possible from one tactical bound to another. But in m a n y situations this is not so, and speed will be held in check. This might be to keep station with another vehicle, or when moving through some place of restricted visibility, such as orchards or vineyards, or near buildings as in the outskirts of villages or in urban areas. Then there are the forests, which despite the difficulty, will have their share of combat. Open country, referred to as " g o o d tank going", covers well under half the area of many parts of Europe, and of many other parts of the world of interest to tank-owning countries. Peacetime use The mobility of a military vehicle is justified by the needs for war, But a vehicle must be used in peace for training, hopefully for m a n y years. It must thus cope well with peacetime conditions, and so this needs to be part of the assessment of its suitability for service. Generally, peacetime use is more harsh than war in mobility terms, due to the rough ground, the abrasive soil, and the training of raw learners. Reliability is an important facet of mobility. POSTSCRIPT
The series of standards published in this article are set at very modest levels. They thus represent the lowest standard the user should be asked to accept. They have been driven already by cost saving to their present low values, and there is no case for this excuse to be used for further lowering of standards.
MOBILITY REQUIREMENTS OF MILITARY VEHICLES
60-
185
CROSS COUNTRY SURFACES
-15 HARD Rr /-=/i RCI N/A FIRM Rr 70/, Re! 105 50-
MEDIUM Rr 13"/= RCI 65
SOFT Rr 20=/= RCI /.5 40-
-10 VERY SOFT Rr 30*/= RCI 35 u~
30-
GROUND PRESSURE MMP OF 285 kPa
E
..-¢
20-
AVERAGE SPEEDS FOR SINGLE TANKS DRIVEN AS FAST AS POSSIBLE ON BROAD SAMPLE TACTICAL ROUTES OF ROADS, TRACKS AND CROSS COUNTRY
-5
( WHOLE ROUTE ' 76 km/ 10-
15
29
i J
'
kW/tonne
I
POWER / WEIGHT
AVERAGE
SPEEDS
"
BROAD UNDER
FIG. 4.
S A M P L E OF R E A L I S T I C VARIOUS CONDITIONS
TACTICAL
ROUTES
Average speeds for net outputs.
It will be seen that ground pressure standards are set at reduced figures for the lighter vehicles. This is for two reasons. Light vehicles can reasonably have lower ground pressure, and do so, because of the effect of scale. They also need it, since the light vehicle will be small, and to it any obstacle will be relatively big, or soft top layers of soil deep. The fact that many current vehicles have poor mobility does not mean that there are frequent boggings, since the crews soon learn their limits, and keep clear of risk. The effect of poor mobility will be restricted tactics, and thus the end product will be greater cost in terms of battle casualties.
186
J.C. LARMINIE
The published work of ISTVS reflects the activities of its members. Many are connected with research. However, the end product of such work must ultimately have some practical application, otherwise the research will be sterile. These mobility standards are based on the research of several members of ISTVS, and offer such a practical application. It is hoped that future work will help to refine knowledge of how such criteria, and these levels for them, appear to work in practice. This article is published to make existing experience available more widely, and in the hope that it will bring forth more information of a strongly practical value. REFERENCES [11 J . C . LARMINIE, A study of mobility requirements, Volume 1. Report for MOD(LSOR) classified Restricted. EASAMS Ltd. Ref. 2721/R/2 (1981). [2] J . C . LARMINIE, A forecast of speeds across country. Report for M O D [RARDE(CH)] classified Restricted. EASAMS Ltd. Ref. 2744/R/1 (1982). [3] V.G. CLEARE, Factors affecting the performance of high speed track layers. Proc. L Mech. Engrs (AD) (1963). [4] D. ROWLAND,Tracked vehicle ground pressure and its effect on soft ground performance. Proe. LS'TVS 4th Int. Conf., Stockholm (1972). [5] D. ROWLAND, A review of vehicle design for soft ground operation. Proc. ISTVS 5th Int. Conf., Detroit (1975). [61 J. C. LARMINIE, Predicting the performance of fast cross country vehicles. Proc. ISTVS 8th Int. Con/~, Cambridge, Vol. 3 p. 1005 (1984). [7] D. ROWLAND and J. W. PEEL, Soft ground performance prediction and assessment for wheeled and tracked vehicles. Proc. L Mech. Engrs 205/75. [8] Various Authors, AII-WheelDrive International Conf., 1986. I. Mech. Eng. Conference Publication 1986-1 (1986). [9] Various Authors, Army Staff Course Vehicles and Mobility Handbook, M O D classified Restricted, RMCS Shrivenham, Date as revised. [10] R. M. OGORKIEWICZ, Tracked and wheeled light armoured vehicles. International Defi'n~e Revie,. lnteravia Publishing Group, Geneva (1986). [11] E. B. MACLAURIN, The effects of tread pattern on the field performance of tyres. Proc. ISTVS 7th Int. ConL, Calgary (1981). [12] D O A E ( M O D ) , Analysis of soil conditions of boggings in World War 2. A O R G Report 13/51 (1951). [13] T. L. H. BUTTERFIELD, Design and development of fighting tanks. Proc. 1. Mech. Engrs. VoL 180 Pt. 2A No. 5 (1965-66)
APPENDIX: T H E MEAN M A X I M U M PRESSURE SYSTEM FOR VEHICLE G R O U N D PRESSURE
Introduction to ground pressure measurement System requirement. A term is needed to give an indication of likely vehicle sinkage into the soil, and thus a lead to the resistance to motion. It should also give a guide to the vehicle grip for traction, and thus allow a forecast of the likely ground conditions when resistance will exceed traction, and the vehicle will fail. The system must be easily understood by the layman, and readily applied. Systems available. For many years Nominal G r o u n d Pressure (NGP) has been used. It assumes even weight distribution over the planform. For tracks, it takes simply the length and breadth of that part of the tracks on the ground. T h u s it does not take into account the uneven load on the tracks, which varies with the number and size of the wheels, and the pitch length of each individual link. With a combination of few small wheels, and a very fine pitch, the track is almost unloaded half-way between wheels. For wheeled vehicles there are two forms of NGP. Tyre manufacturers use the contact patch of a tyre on a hard surface. This is suitable for comparing tyre Ioadings, but not the loading of soft ground, nor will it give any comparability with tracks. A more realistic wheels N G P was developed by the School of T a n k Technology, which gave an approximation to the tyre planform at sinkage deep enough to be likely to cause immobilisation, but this was limited in realism. The US Army has developed the Vehicle Cone Index (VCI) system. It is applicable to tracks or wheels, and gives a reasonable yardstick. However, it is entirely empirical and contains unlikely combinations of expressions, and it is cumbersome to use. Mean M a x i m u m Pressure (MMP) derives its name from a study of actual tank pressure measurements. There are peaks of m a x i m u m pressure under each wheel. Each wheel bears a different load. However, provided the vehicle design is not a freak, it
MOBILITY REQUIREMENTS OF MILITARY VEHICLES
187
has been found satisfactory to treat the pressure as the mean of all the peaks. A relationship was prepared by the fighting vehicle research and development establishment at Chobham, RARDE(CH), giving MMP in terms of track and wheel parameters. Later development produced an expression to give a comparable equivalent MMP for wheeled vehicles. The wheeled MMP is based on work on Soils Numerics done by the US Army Waterways Experiment Station (WES), which had also produced VCI. The terms of this wheeled vehicle numeric agree with the common appreciation of the factors that are important, and with the results of other research work, and so it appears a basically sound term. A full comparison of NGP, VCI, and MMP is given in Ref [1], and the subject is also covered in Ref [9]. The MMP system was published and justified in Refs [4] and [5]. It should be noted that the original RARDE(CH) report, No. 72031 of July 1972 was superseded by those Refs [4] and [5], and has out-of-date formulae. Thus it should be used with caution. MMP is expressed in Kilo-Pascals (kPa) (Newtons per square metre). They are not quoted with conversions to other units, such as lb/in 2, since MMP is a new expression, and so there are no old measures to compare it with.
Mean maximum pressure expressions The MMP formulae and associated expressions are given in Table A1. It will be seen that there are different wheeled MMP formulae for both clayey, cohesive fine-grained soils, and for sandy, frictional coarse-grained ones. The development and application of MMP has so far been based almost wholely on that for clayey soils. The formula for sand is only recently derived, and only partly developed. Clay soils are a greater mobility problem than sand, and thus it is the clay MMP that is normally quoted, and is more important. The wheels formula for clay allows comparison with the tracked MMP. This is not so for the sand wheeled MMP, which should only be used for comparing vehicles of fairly similar characteristics.
TABLE A 1. MEAN MAXIMUMPRESSUREEXPRESSIONS
Principal MMP formulae (a)
MMP Formula for tracked vehicles: 1.26W
MMP =
kPa
2m c b (pd)°'5 W m d b p c (b)
= = = = = =
vehicle weight (kN) number of axles wheel diameter (m) track width (m) track link pitch (m) track link profile factor: area/pb.
MMP Formula, for wheels on fine-grained (cohesive) (clayey) soil: MMP = W m d b
= = = = 6/h = K =
KW 2m b °85 d I'ls (3/h) °'5
kPa
vehicle weight (kN) number of axles tyre diameter tyre breadth unladen (m) tyre deflection on hard ground % factor from table below. Factor K:
Number of axles
Proportion of axles driven 1
3/4
2/3
3/5
1/2
1/3
1/4
2 3 4 5 6
3.65 3.9 4.1 4.32 4.6
--4.44 ---
-4.35 --5.15
---4.97 --
4.4 -4.95 -5.55
-5.25 --6.2
--6.05 ---
188
J . C . LARMINIE
Differential Locks. If differential locks are in use, the equivalent MMP is improved: 4 × 2 vehicles; MMP × 0.98. 4 × 4 or 6 × 6; MMP × 0.97.
Secondary M M P formulae (a) MMP Formula, for wheels on dry coarse-grained (frictional) sand soil: S TW
MMP -
2m b I 5 dl.5 ,Wh
kPa
S = constant for proportionality (see below). T = tyre tread factor: 1 for smooth tyre; 1,4 road tyre, 2.8 road/cc tyre; 3.3 earth mover. Otherwise, factors are as for fine-grained soils (Para l.b). Constant S: The proper relationship of MMP for wheels on dry sand with that for fine-grained soil has yet to be fully researched. For the present take: S = 0.60. (b)
MMP Formula for belt tracks with pneumatic tyres: MMP --
0.50 W 2m b (d 6) 0.5
kPa; 6 = tyre deflection on hard ground (m).
Otherwise, as for wheeled MMP. For belt tracks on solid rimmed wheels, the tracked MMP is used, with the pitch (p) being taken as the length of the steel reinforcement at the tread. (c)
MMP
=
MMP Formula, for wheels of different sizes on different axles (fine-grained soils): K --
2m
Wi
[
b~~8~ d'~ ~5 (6/h)¢~ 5
Wi
+ .... +
b '85~ d,1'5 (6/h)l '5
where suffix (1) refers to the first axle, and so on to the ith axle, and all other factors are the same as in the normal MMP formula for wheels.
Soundness o f term The MMP system appears to be the most sound of the available terms. It accords to common sense, it has given usable and repeatable results, and has given reliable guidance. It is relatively simple to use. Its limitations appear less than those of other systems. It has now been used to quite a wide extent. Its weaknesses are: (a) It assumes the weight is evenly spread on all axles. (b) It assumes the soil is deep and homogeneous. Thus it has no allowance for the size of the vehicle in relation to the depth to firmer underlayers. It has no factor for the aggressiveness of the track or tyre tread, and thus the ability to grip firmer underlayers, particularly when on grass. (c) It makes no allowance for steering effects. It can be said that MMP is the least bad of the systems available, it can be used with confidence as a guide. For the purposes of military vehicle mobility standards, some 20 kPa is the minimum difference in MMP that should be considered significant.
MOBILITY R E Q U I R E M E N T S OF M I L I T A R Y VEHICLES
189
Special applications of MMP Forecasts oftrafficability. Because the M M P formula for wheels is derived from the WES soils numerics, it is possible also to relate it to that establishment's work on trafficability, and the soil strength Rating Cone Index (RCI). Since the wheeled M M P formula has been specifically prepared to be comparable to that for tracks, a family of relationships of M M P and RCI exists. The limiting soil strengths to allow one vehicle to pass or for several passes, are given in terms of M M P and RCI in Table 5. The relationship of M M P to resistance on various soils is in Table 7. This allows calculations of speed to be made once the soil's strength is known. Reference [5] gives guidance on the use of M M P as a yardstick for vehicle design. For instance, for tracked vehicles, the factor by which M M P exceeds N G P gives a measure of the efficiency of the layout, where M M P - N G P = Q, a factor of merit. Typical ranges of Q are from 1"5 for slow vehicles such as caterpillar tractors, to the bad figure of 3.8 for some designs with emphasis on s m o o t h running at high speed, and thus a short pitch track. The Panther tank, with large overlapping wheels achieved Q of 1.71, which was reflected in a good M M P of 150 kPa. Wheeled vehicle trains. It is satisfactory to apply the wheels M M P formula to wheeled vehicles which do not conform to the normal simple rigid layout of two or three or four axles with tyres of similar size on them all. Thus a vehicle with trailer can be likened to a rigid single one of the same n u m b e r of axles, with the appropriate n u m b e r of them driven. If the tyres are of different sizes, this can be taken into account in the calculations, with appropriate allowance for the weight distribution to each axle. The M M P for a half-track can be calculated by taking the front axle as a single axled wheel vehicle, using the value K = 3'32. The rear suspension is treated as a tracked vehicle; the weight is applied to the two suspensions according to the laden weight distribution of the vehicle. The whole vehicle M M P is then found by taking an average, with weighting for that weight distribution. Twinned rear wheels. When running straight on soft ground, twinned wheels on the rear axles are of limited benefit, since the twinned rears will overlap the ruts of the single wheels on the front axle. However, limitations in mobility usually are evident when steering, and then the axles will not follow in the same ruts. Practice does show that twin wheels are some benefit on soft ground. It is therefore realistic to use a compromise formula: Replace (2 m) by -
(2m + w) 2
where: w = the total number of wheels, m = the number of axles as before.