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Off-Road Mobility: Problems & Solutions

This article is published in Miltech

“An army without mobility is but a corpse”, stated Sir Basil Liddell-Hart, and these words are still very much valid to underline the key importance of mobility for military purposes. Indeed, mobility is a primary requirement for all land forces, virtually irrespective of their size and main missions.

by Fulvio Bianchi - Tactical mobility, which is the subject of the present article, can be described as the range of characteristics and features that enable military vehicles to transport a given payload over different types of terrain. Military vehicles intended to offer more than token off-road capabilities demand an ad-hoc design or, if based on commercial models, the introduction of appropriate modifications and adaptations.

Needless to say, not all military vehicles must necessarily be able to move cross-country, this being rather a function of both their intended main missions, and the geographical characteristics of the operating theatre in which these missions must be performed. Thus, a certain capability to move cargo and personnel away from the road networks will probably be useful even in highly-developed areas, and such becomes indispensable when operating in regions with a poor or non-existent infrastructure.

Military vehicles, including both combat and logistic types, are normally classified according to both their payload/weight (Military Load Class, MLC) as well as their level of mobility. Thus, the US Army uses the categories (in order of decreasing performance) of tactical, logistic and commercial vehicles, and this is closely mirrored by the Italian Army’s tactical, tacticallogistic and logistic (commercial) models; the British Army classifies its vehicles as High Mobility (HM), Improved Medium Mobility (IMM), Medium Mobility (MM) and Low Mobility (LM); the Bundeswehr has adopted the simple designations of A, B, C and D classes for its vehicles; and the French Army’s classification system (this being arguably the most accurate) distinguishes between tout terrain (all terrain), tout chemin (all paths) and banalisé (modified commercial types) vehicles.

The SX series is described by MAN as “probably the most mobile and reliable truck on Earth”, and is claimed to outperform all other high-mobility military trucks. (Photo: MAN)

In order to properly assess the various problems linked with off-road mobility in a military framework and the different solutions available, it is necessary to appreciate that the success of any transport mission depends on three factors, namely:

• The characteristics of the terrain to be crossed,
• The characteristics and performance of the vehicle, and
• The skill level of the driver.

These three factors are closely interdependent. The characteristics of the terrain to be crossed not only have a direct bearing on the required vehicular performance, but also demand that the driver should possess a particular talent to enable him/her to select the most appropriate path to overcome or circumvent the obstacles being encountered, and to take the necessary action at the vehicle’s controls. This interaction involves very many parameters, as we will see in what follows.

Environment and Terrain

A first and not insignificant influencing parameter is represented by the geographical location of the operational theatre, with all that this implies in terms of average temperatures, rain/ snow, etc. However, much more significant are the nature and conditions of the terrain over which the vehicles must move to perform their missions.

The Renault SHERPA range of tactical trucks insures the transport of loads from 2t to 20t even in the most hostile environmental conditions. (Photo: Renault)

Military transport missions can be carried out at least in part by using the existing infrastructure of primary and secondary paved roads, macadam roads or tracks offering adequate strength to provide sufficient flotation and traction, and in these cases there are no special difficulties. Things change abruptly, however, when the assigned mission requires the vehicle to leave the proverbial beaten path, and venture into the broad variety of topographical conditions that are encountered in military operations worldwide, and which include a full range of obstacles and physical structures to impair the movement of wheeled vehicles.

Prominent amongst the factors that can impair/slow down a vehicle moving crosscountry or even immobilise it, including on hard soil, are the surface features of the terrain – form, type, position and distribution of the obstacles. In particular, negotiating slopes is a rather tricky business that can go awry very quickly with the onset of lack of friction or insufficient traction phenomena. A good crosscountry vehicle, including heavy types must thus be able to climb slopes of at least 60% (some 31°) and preferably even higher. In technical terms, this translates into a requirement for a traction/weight ratio in excess of 0.6 while on all-wheel drive. By the same token, driving off-road will rather often require moving along the terrain’s contour lines with side slopes of up to 30% or even 40% – which translates into an important requirement for excellent lateral stability through a wide track and low centre of gravity to reduce the risk of overturning.

The FIAT IVECO LMV Light Multirole Vehicle (a.k.a. LINCE in Italian Army service, a.k.a. PANTHER in British Army service) negotiating a difficult cross-country path. (Photo: FIAT IVECO)

There are virtually countless forms of natural or artificial obstacles to be encountered when moving off-road, and in addition to the friction/traction and stability problems as briefly mentioned above, these obstacles can create serious movement difficulties because of the vehicle’s underside coming into contact with the ground. This applies for example to hills, boulders, steps, ditches with rectangular or trapezoidal section, ravines, potholes, stumps and so on. When such obstacles cannot be directly overcome, the vehicle must circumvent them, which unavoidably causes a very significant lowering of the average speed and may well create a whole series of new problems; for instance, trying to manoeuvre a vehicle between two obstacles that are less than twovehicle lengths apart from each other is liable to result in steering problems.

Roughness or unevenness of the route is a peculiar type of obstacle, this term being applied here to indicate random undulations with a width of between a few centimetres and some tens of centimetres and a peak-to-peak distance of between some tens of centimetres and a few metres. These undulations induce strong vibrations in the running gear and suspension, which are then transmitted to the vehicle’s body and can reach such amplitudes as to require the vehicle’s speed to be drastically reduced.

Thick vegetation can severely limit or totally prevent vehicle’s movement, and even when movement remains possible it creates additional resistance and restrict forward visibility. And then, of course, there is water. Rivers, ponds, swamps and lakes are encountered in most military missions apart from very specific regions. Fully amphibious vehicles are less and less in demand, and attention is rather being focussed on fording capabilities (up to some 1.5m maximum) which require the introduction of a number of appropriate features involving most particularly the intake/exhaust systems and electric circuitry. Even adequate fording characteristics do not completely eliminate the mobility obstacle represented by water, however, in that the potential dangers of very soft soil at the bottom of the ford and/or the presence of underwater obstacles must still be considered.

Although totally amphibious vehicles are less and less in demand for military applications, adequate fording capabilities remain an important requirement. (Photo: US Army)

All the above is based on the optimistic assumption that the soil as such is hard enough not to present an obstacle to mobility in itself. Unfortunately, however, more often than not military off-road movement must take place across soft soil. Soft soil is arguably the single main pain in the neck for cross-country movement (particularly of heavy vehicles), in that under the joint action of normal (vehicle weight) and tangential (traction force) loads it is liable to deform itself with important modifications in the conditions of the upper layer. The interaction between soft soil and a moving vehicle generally results in wheels sinking, which translates into increased drag and motion resistance and, thus, more and more traction force being required to keep the vehicle moving – until the available traction force becomes insufficient, and the vehicle is effectively stuck. Furthermore, while the single or group obstacles that were described above are normally localised and can often be circumnavigated, the many iterations of dangerously soft soil (mud, sand, ploughed terrain, marshes, etc.) tend to cover rather wide areas and may literally stretch from horizon to horizon.

It is rather difficult to properly describe a soil in terms of its composition and physical properties. Two extremes can be defined: purely plastic/cohesive soil (clay), and purely frictional/non-cohesive soil (sand). In general terms, clay soils are internally cohesive and thus present shear resistance to tangential loads even when no normal loads are applied; in non-cohesive soils, however, the sand particles can only transmit tangential forces when normal loads are applied onto them by the vehicle’s wheels. It will be easily understood that this diverging behaviour has a fundamental impact on the design of tyres optimised for either soil (more about this later). Between these two extremes, there is an enormous variety of conditions as regards granularity, the possible presence of organic materials, water content and many other parameters, which result in a corresponding variety of mechanical characteristics affecting mobility.

The Tatra original chassis design with central backbone tube and swinging half-axles. While each wheel follows the profile of the terrain, the rigid backbone tube eliminates any twisting or bending of the chassis. (Photo: Tatra)

These mechanical characteristics are expressed through the so-called Cone Index (CI). The name is due to the fact that the soil’s strength is measured in terms of the force (lb/sq.in) that is required to push a coneshaped metal penetrator of standardised dimensions into the soil, to progressively increasing depths. Given that the soil’s mechanical strength is liable to be progressively reduced as a consequence of the passage of one or more vehicles, the Remoulding Index (RI) is then introduced, the expression CI x RI finally giving the Rating Cone Index (RCI). Very soft or wet soils will have a RCI of around 25, while good hard soil is in excess of 100.

In order to provide a standard classification benchmark to assess the respective off-road mobility performance of different vehicles, the US Army Waterways Experiment Station (which was also responsible for the formulation of the CI-RCI criteria mentioned above) developed the VCI (Vehicle Cone Index). The VCI is calculated through a fairly complex non-dimensional formula, and it can be put in direct relation with RCI whereby if the vehicle’s VCI is lower than the soil’s RCI then we are in “GO” conditions, while for VCI>RCI we have a “NO GO”. The larger the RCI-VCI difference, the higher the expected mobility of the vehicle.

A cutaway model of a run-flat tyre for military uses. (Photo: RunFlat Ltd)

A different mobility index, designated MMP (Mean Maximum Pressure) has been formulated by the British Army Engineer Corps. This formula is based on the vehicle’s mass, number of axles, tyre size and drive configuration; it is simpler to calculate than VCI and it is more suitable for a direct comparison between different vehicles, but it cannot be directly correlated to the terrain characteristics.

To conclude these notes, climatic conditions and altitude do not have a direct impact on mobility. However, the former factor has an influence on the design of the engine’s cooling plant, the air conditioning for the cab and more in general all materials and structures needing to be compatible with both high (normal +35°-42°C, maximum +50°C) and low temperatures (normal -19°-31°C, minimum -49°C). More significantly, most not so say all engines will derate at very high temperatures. By the same token, high altitudes have a negative influence on both the engine’s power output (although this is less of a problem with today’s supercharged engines) and the efficiency of the cooling plant, as a result of air being less dense.

Technological Issues for Off-Road Vehicles

General Considerations

Intuitively enough, the overall design of a vehicle, its configuration and dimension have a direct and significant impact on its off-road mobility characteristics. The factors influencing these characteristics include, for example, the number of axles, static and dynamic wheel loads, wheel base, track, and others.

Frame torsion (left) improves a vehicle’s ability to adapt itself to the terrain being negotiated. (Drawing: Author)

The power train should be based on a sufficiently powerful engine (for a total power to weight ratio of some 10-15kW/t for medium/heavy vehicles and up to 25kW/t or more for lighter vehicles) offering adequate torque even at lower rotation speeds, and coupled to a transmission (gearbox + transfer box) with appropriate gear spacing over the entire speed range from 3-4km/h to 90-100km/h. All-wheel drive is a must, and it should further feature the possibility to lock both the longitudinal as well as the transverse differential(s).

Geometric characteristics are the key factor affecting the vehicle’s ability to cope with the geometry of the terrain’s surface. Particularly important in this regard are the angles of attack and departure, the ramp angle, and the ground clearance (particularly with reference to the axles). High ground clearance, ideally up to one half the vehicle’s wheel diameter is very useful in avoiding stopping dead against obstacles or “bellying down” in deep mud. It is interesting in this context to recall the USA Army’s M520 GOER 4x4 8t transport vehicle, which featured very large diameter wheels (18.00-33) and no suspension in order to obtain a perfectly flat bottom not unlike a barge. The GOER proved very useful in Vietnam’s mud paths and soft/ wet terrain conditions, which effectively prevented operations of conventional tactical trucks.

The Tatra T815-series 8x8 highmobility heavy duty tactical truck. (Photo: Tatra)

Articulated vehicles are another interesting approach to the problem. The yaw, roll and pitch articulation between the two elements of the vehicle allow it to better relate to the geometry of the terrain being negotiated, while at the same time equalising wheel loading. In addition to the already mentioned GOER, this solution was adopted for the GAMA GOAT 6x6 1.5t and other similar designs, but cost considerations prevented it from gaining widespread acceptance and the only current example is the Oshkosh Mk48 8x8 heavy tactical truck in service with the US Marine Corps.

Another significant contribution to a vehicle’s capability to adapt itself to uneven terrain is provided by the ladder frame, which as regards trucks normally takes the form of two side rails with transverse cross-members.

Two main design solutions are available:

• Torsion-resistant frame, which facilitates the installation of rigid bodies but results in the suspension being the only way to provide the required wheel travel in “twist” situations;
• So-called torsion-friendly frame, which behaves more or less like a torsion spring by allowing angular travel between the vehicle’s sections in correspondence to the axles (up to 10° with long wheel bases). This improves the vehicle’s ability to negotiate twisting conditions, but it necessarily require specific solutions for the installation of the body.

Suspension

Suspension is a very important element to absorb the shocks and loads being transmitted to the vehicle by uneven terrain. The choice of an appropriate suspension scheme and design is a fundamental factor towards increasing the maximum permissible cross-country speed while limiting the physical “beating” to the passengers and payload. Too hard suspension necessitates the vehicle’s speed being restricted to what the vehicle’s mechanical components and even more so the passengers can tolerate.

A light tactical truck equipped with rigid axles demonstrates its twisting ability. (Photo: Author)

In very simple terms, suspension is a system to absorb the shocks and reduce the vertical acceleration being imparted to the vehicle, as well as the acceptable displacement amplitude. The more flexible the elastic elements of the suspension (leaf or coil springs, torsion bars, hydropneumatic struts), the more comfortable the conditions while driving off-road – but also more serious the stability problems, particularly as regards roll when turning or negotiating a side slope.

Suspension can be basically sub-divided into the two broad categories of independent and non-independent designs, each of these having their respective advantages and shortcomings. Apart from the lower capacity classes of military transport vehicles (payloads of less than 2t) which feature a generalised use of independent suspension schemes, rigid axles tend to be the preferred solution for heavier vehicles. With rigid axles having a relatively large mass, however, more energy will be transmitted to the vehicle, resulting in more severe acceleration and vibration phenomena to affect passengers and cargo. On the other hand, non-independent suspension with rigid axles is inherently cheaper due to its relatively simple construction, and furthermore it can provide better performance under “twist” conditions in that the difference between the loads that are applied to the wheels of the axles twisting in opposite directions is lower than with independent suspension.

Independent suspension offers the capital advantage of allowing a higher speed on uneven terrain, all other conditions being the same. The wheels, with a lower unsprung mass, roll over the obstacles independently of each other; lower energy and accelerations are thus transmitted to the vehicle, resulting in better dynamic comfort conditions. Furthermore, independent suspension also provides better behaviour when turning; in addition to lower roll, this is due to the possibility of placing the body in a lower position (thus, lower centre of gravity) in that the axles with their differentials are no longer there. This is a particularly important aspect for wheeled AFVs, in that it allows for total height being reduced by some 400mm.

Modern technologies are being introduced to improve tactical vehicles’ overall effectiveness in off-road missions through the use of more sophisticated suspension. These involve, for example, shock absorbers with variable damping action (so-called semi-active suspension) or active suspension able to continuously modulate reaction forces.

Tyres

Centralised Tyre Inflation Systems (CITS) allows for footprint and specific ground pressure to be optimally adjusted to match the ground conditions. (Photo: Mercedes Benz)

Tyres are a most fundamental element for a vehicle’s ability (or lack thereof) to move across country. All other things being equal, the vehicle’s performance is ultimately dependent on its tyres’ capability to transmit an adequate traction force to the ground, cross soft soil without too much wheel sinking, and provide sufficient grip to overcome vertical obstacles.

Given the extreme variety of soil conditions and physical/mechanical characteristics that are to be encountered when moving off-road, as described above, so-called universal tyres have since been replaced by specialised models, whose structure and tread pattern are optimised for specific conditions while at the same time also providing acceptable (although admittedly not optimal) performance for road travel. For instance, vehicles operating in regions with a prevalence of dry/noncohesive soil (sand) will receive tyres with a “soft” tread pattern that does not damage the soil’s surface structure, and in some cases so-called ribbed tyres (semi-slick design with only two circular grooves) are mounted on the non-steering wheels. At the opposite end of the spectrum, cohesive soils containing a more or less high percentage of water dictate the adoption of very aggressive tread patterns.

In any case, the generalised requirement for low specific weight (=lower loads being transmitted to the terrain) is satisfied through the adoption of large-diameter, large-width tyres, with single tyres on all axles. Nominal inflation pressures for road travel with no speed limits are in the region of 3-4 bar, i.e. roughly half the figure for standard commercial vehicles.

The issue of tyre vulnerability should also not be ignored. A puncture, be it caused by sharp stones or other causes, may well result in the vehicle being immobilised. For this reason, the presence of an emergency running system (run flat) is virtually mandatory to allow the vehicle to keep moving with a punctured tyre, both on and off-road, over significant distances and with a sufficient level of safety.

It is also significant that the specific structure of cross-country tyres allows for their use at inflation pressures much lower than the nominal figures (down to less than 1 bar in an emergency), although this of course implies reducing speed. Centralised tyre inflation systems (CTIS) are increasingly adopted to adapt tyre inflation values to the conditions of the terrain being encountered, quickly and without the driver having to leave the cab.

Cost Factor

As a final note, it must be underlined that achieving even a very high degree of crosscountry mobility for tactical vehicles does not imply having to overcome really significant technical problems. Rather, the true problem is cost. Designing, developing and producing a high-mobility tactical vehicle is a very expensive endeavour, this being further compounded by the relatively small series production runs for military customers.

The Driver

The main task of the driver of an off-road tactical vehicle consists in selecting the optimal driving path, as a function of the mission to be performed and the characteristics and performance of the vehicle.

The new German Army MUNGO protected carrier does not need to be told how to move cross-country. (Photo: Krauss-Maffei Wegmann)

The main factors to be considered in this regard are as follows:

• Adequate visibility in both the horizontal and vertical planes. From this particular point of view, front engine bonnet vehicles are less ergonomically efficient than COE (Cab Over Engine) configurations;
• Ride tolerance limits to vibrations and shocks. In addition to the vehicle’s suspension and tyres, the presence of elastic mounts for the whole cab and/or the driver’s seat provide a dramatic improvement in this regard. Modern trucks so equipped are capable of truly impressive performance, including e.g. driving over secondary roads at an average speed of 70km/h for eight hours without exceeding fatigue limits;
• Adequate reaction time. Driving a tactical truck off-road requires constant attention to a variety of internal and external inputs and immediate corresponding actions on the clutch, brakes, gearbox, differential lock, tyre pressure and so on;
• Climate control and ergonomics;
• Adequate training.