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Warfare is speeding up. For combat vehicles, tracked and wheeled, that means being agile and fast, able to cope with various terrain and extreme driving conditions, whatever the tactical circumstances. Excellent ground mobility will be a defining factor for all future conflicts.

Whether deployed to hotspots or in any of the varied terrains of an increasingly uncertain European theatre, wheeled and tracked vehicles on future battlefields need to display the best possible ground mobility that their designers, engineers, latest technology and, of course, budget, can deliver. This article looks at what is meant by ground mobility, the many factors impacting it and shares some of the thoughts from industry.

Military vehicles on the battlefield of tomorrow will need to perform with optimum efficiency in all scenarios in which they are designed to operate and in the face of the widest possible range of both natural and man-made obstacles. They must be fast across country and on road in all weathers if they are to complete their missions and survive. Survivability on tomorrow’s battlefield will have as much to do with effective ground mobility, as it will with other technical aspects of a vehicle design, such as armour protection and weaponry. Optimum mobility is now one of the highest priorities militaries demand from their vehicles, and one of the chief attributes manufacturers must confer on their latest designs.

RBSL Managing Director, Peter Hardisty, commenting on the importance of mobility for the future battlefield, underlined the aforementioned sentiments to ESD, saying, “Our Armed Forces are in constant evolution. They need to be flexible, adaptable and capable, and we seek to facilitate that by providing armoured vehicles with superior levels of mobility.” In the context of a latest defence industry development he added, “The British Army is preparing for the future battlefield, especially its ability to reorganise for a range of operations at increased readiness. Its recent procurement of the BOXER 8×8, wheeled vehicle is just one example of how this is being realised and how advanced mobility remains, and will remain, a top priority.”

Mobility Variously Defined

The mobility of a military vehicle, one typically transporting troops and weaponry, combines not only its ability to move, or be moved, freely and easily, but also its capability to move over any specified terrain, even when affected by adverse environmental conditions, such as the widest range of weather. The optimum mobility of a vehicle also factors in its effectiveness in being able to achieve various tactical objectives in different conditions and in varied terrains. Engine and transmission type, horsepower, suspension, resulting speed capabilities, acceleration, turning circle and vibration are just some of the factors contributing to the mobility of a military vehicle. Other technical attributes conferred on a final vehicle design will also play their part in determining the final ground mobility of a vehicle and if everything, technologically, that can be included to deliver optimum ground mobility has been incorporated within the design for the available budget, then an optimum ground mobility can be considered as having been realised. But just as beauty is in the eyes of the beholder, so, too, is optimum mobility a subjective realisation.

Assessing Mobility – Theory and Practice

Assessing the mobility of a new military vehicle is typically carried out under different environmental conditions and over a wide range of surfaces and terrains prior to any final contractual commitments and operational deployment; during vehicle development phases, mathematical modelling will be used extensively to determine the ultimate mobility of a design. Studies by the US Army’s Engineer Research and Development Center (ERDC), for example, treat mobility in its mathematical modelling as a function of “trafficability”, itself defined as the ability of a given vehicle to traverse a specified terrain. Other
complex mathematical models look at the effect of soil moisture on mobility, as well as tyre-soil interaction for wheeled vehicles and track-soil interaction for tracked vehicles. Different terrain types, deformable ground versus hard ground, adverse environmental conditions, gradients, and other parameters can all be factored into various mathematical models, although the creation of one single generic mobility model, applicable to all vehicles for all scenarios, remains a challenge for both academia and industry. This challenge is made all the more fluid as technologies, (new transmission types and latest suspension developments), employed to optimise the mobility of new tracked and wheeled military vehicles, improve and evolve. So, while the mobility of a vehicle is an extremely important attribute, all the variables mentioned above make the ultimate, optimum mobility of any vehicle both difficult to define, as well as subjective to the satisfied end-user and maker – “it’s performing just as we’d hoped… for the procurement budget we had available”. Even then, however, under rapidly-changing operational scenarios in different terrain, predicting absolute mobility is highly unpredictable.

RAND Corporation’s report showed that several technologies affect the performance of both tracks and wheels, though tracks still retain their advantage over wheels in off-road scenarios. Pictured is a LEOPARD 2A6. The continuous upgrading of the LEOPARD 2 A4 to the current A7V variant retains mobility as one of the vehicle’s key combat value criteria alongside firepower and protection. (Photo: KMW)

On-Road/Off-Road – A Basic Distinction

Assessing the mobility of a new military vehicle is typically carried out under different environmental conditions and over a wide range of surfaces and terrains. Depicted is a FUCHS CBRN 6×6 in Jordan. (Photo:. UK MoD Crown Copyright)

That said, with the actual mobility of any military vehicle determined by the eventual terrain over which it operates, notwithstanding vehicle attributes for mobility – wheels, tracks, transmission, horsepower, suspension, weight and so on – one general terrain-type distinction in classifying mobility can be made: that of ‘on-road’ and ‘off-road’ mobility.
On-road mobility primarily depends on the type of vehicle used, though it can be assumed in most situations that any required speed will be achieved more easily on-road using wheeled vehicles rather than tracked. On-road, wheeled vehicles do have better mobility and displayed dashboard speeds are typically a function of horsepower and weight; greater horsepower improves mobility, increased weight decreases mobility.

Tracks confer greater mobility to a vehicle in snowy conditions due to the surface area of the track, much like a snow shoe. Depicted is a VIKING in deep snow.(Photo: UK MoD Crown Copyright)

The friction of tyres on a road surface is another factor to consider. This will vary with tyre type and size and the surface area in contact with the road. Environmental conditions will also play their part – wet conditions decrease friction, dry conditions increase friction. That said, in some environmental conditions – wet, icy and snowy scenarios – the on-road mobility of tracked vehicles will often surpass that of wheeled vehicles. Prolonged on-road driving for tracks, however, can lead to mechanical problems due to issues such as excessive vibration and such use can impact mobility at some stage. Military operations, however, will, invariably, require intensive degrees of off-road mobility for all vehicles, thus making both good off-road and on-road mobility performance, essential.

Engine and transmission type, horsepower, suspension, resulting speed capabilities, acceleration, turning circle and vibration are just some of the factors contributing to the mobility of a military vehicle (Photo: BAE Systems)

However, when it comes to quantifying off-road mobility this is very complex to define and calculate; it depends on a number of factors, many of which are hard to measure. Vehicle weight in the off-road scenario is the key attribute impacting its ground mobility, although the resistance of the surface also plays a vital role. Complexities arise, however, when trying to make a direct correlation between weight, surface type and mobility, because surface type and ground pressures – the ratio of gross vehicle weight to the surface area of contact the vehicle tracks or wheels have with the ground – vary so widely, they make resistance extremely hard to predict.

One term frequently used when discussing ground mobility and the relationship between soil strength and vehicle ground pressure, is the Vehicle Cone Index, or VCI, which has been further defined as the minimum soil strength necessary for a self-propelled vehicle to consistently make a prescribed number of passes, in track, without becoming immobilised.
A threshold ground pressure, which varies among vehicle types, once reached will result in a vehicle becoming increasingly bogged down. This is why tracks are preferred for off-road movement over wheels, as tracks provide greater surface area over which to disperse the weight of the vehicle and, consequently, deliver less ground pressure. In general, the off-road mobility of a vehicle is optimised by having a higher horsepower-to-weight ratio, low VCI, low ground pressure, and an advanced suspension system for the vehicle.

In addition to mathematical modelling for determining ground mobility, simulation models are also used to understand the performance of a vehicle in different terrains and other conditions. One of the most popular simulation models for analysing mobility is the NATO Reference Mobility Model (NRMM), which comprises three modules: vehicle dynamic module, obstacle crossing performance module, and primary prediction module. NRMM can predict the mobility of a combat vehicle for both on-road and off-road operations, with mobility typically predicted as achieving an effective maximum speed balanced against differing attributes. However, NRMM has its limitations, not least of which is being unable to determine mobility in complex terrains; as a result, adaptations to this simulation model have been proposed so that it can help predict the ground mobility of wheeled vehicles under different terrain types – flat and rigid, or deformable – and when facing different obstacles.

A Manufacturer’s Thoughts on Mobility

With RBSL’s Peter Hardisty quoted earlier underlying the importance of ground mobility and stressing that “advanced mobility remains, and will remain, a top priority”, ESD sought some further industry insights from the company in this regard.

Emphasising some of the earlier sentiments in this article, an RBSL spokesperson said that ground mobility is generally defined by a vehicle’s ability to access different types of terrain and its speed in traversing that terrain. As such, “optimum mobility” is difficult to define, as a vehicle’s mobility can always be improved upon. However, it was stressed that in the defence industry, with each customer requiring value for money, an optimum balance has to be achieved between cost versus performance versus weight, etc.

There have also been a number of key programmes looking at the future mobility of armoured vehicles, such as DARPA’s Ground X-Vehicle Technologies and Dstl’s UK Future Ground Combat Programme to name just two. At the same time, RBSL has also been performing R&D into future mobility, focusing on innovation in suspension systems, electrification of vehicles, autonomy, advanced cooling systems, as well as future fuels, all of which will have some impact on the overall ground mobility of associated vehicles.

Wheels v. Tracks

As to whether there is a trend towards the use of wheels for heavy armoured vehicles for mobility reasons as opposed to tracks, the spokesperson said that there have been a number of studies performed to analyse the “wheels-versus-tracks” question. The general conclusion from these studies is that a tracked vehicle provides better traction and lower ground pressure than an equivalent wheeled vehicle, which, as discussed above, can be of considerable advantage in off-road scenarios. However, the difference in performance is narrowing with improvements in the technology of wheeled vehicles. Additionally, wheeled vehicles also offer significantly lower running costs and reduced maintenance activities, so, the choice depends on the operational environment in which a vehicle will be expected to perform at optimum levels, with tracked vehicles preferred for significant cross-country operation and wheeled vehicles preferred where roads are the primary operating scenario.
Indeed, in regards to wheeled vehicles and the impact of tyre types on mobility, RBSL’s spokesperson said that the company generally selects tyres on the basis of their worst-case usage. That means either mud-pattern tyres for vehicle types to be used mainly in temperate climates, with either sand or all-terrain tyres fitted to vehicles used in hot climates. In addition, most of any wheeled fleet will be fitted with run-flat inserts to remain mobile under some sort of failure condition. RBSL has worked with the majority of the biggest OEM tyre suppliers over the years, and these are selected either through a competitive procedure/tender, or as a result of a customer’s specific wishes.

The right side track of a Soucy CRT which had been in the trials and shows clear wear, vs. the left side track which is, effectively, new and did not go through the trials. (Photo: Tim Guest)

On the subject of the use of traditional tracks versus Composite Rubber Track, CRT, and the potential impact of CRT on overall vehicle ground mobility, RBSL said that in trials it had conducted of CRT on both WARRIOR and CVR(T), the track performed extremely well under the majority of conditions giving significant reduction in noise and vibration signatures. The trial RBSL WARRIOR was actually on display on the Soucy (CRT maker) stand at DSEi. The tracks, which weigh up to 50% less than equivalent traditional tracks, went through extensive tests with UK specialist company, NPrime, during the summer and the results showed vibration to be 40% less than with traditional tracks, as well as a 25% saving on fuel. In addition, the life of electronics and other vehicle components improved by some 70%. It was also shown that by adopting the CRT, 1.5 metric tonnes that could now be used for improving armour protection or carrying other systems and armament were freed up. Crucially, it was clearly shown that using CRT will impact the total cost of ownership of a vehicle platform, which, as stated, can have implications for mobility decisions. The particular vehicle on display at DSEi and one of its tracks had been part of the UK’s 10-week ATDU tests at Bovington involving various units of the UK armed forces putting the tracks through 10 x 500km battlefield missions. By the end of those, the tracks showed a degree of wear, though when compared with new track at DSEi, still appeared to have thousands more kilometres left in them. Asked if RBSL intends to adopt CRT for future armoured tracked vehicles the company said they view CRT as a developing technology that started with CVR(T) on light vehicles, though used by other armies across the globe (for example, the Norwegians on CV90). As armoured vehicles become heavier, more development is required if CRT is to be more widely used and adopted. They added that to use CRT on new RBSL platforms, a full evaluation would be conducted for each class of vehicle. An optimal track would then be selected based on customer requirements.

The Vehicle Cone Index (VCI) has been defined as the minimum soil strength necessary for a self-propelled vehicle to consistently make a prescribed number of passes, in track, without becoming immobilised. (Photo: UK MoD Crown Copyright)

In addition, it’s also not only about the maturity of a technology, but also about the logistical support needed in the field to maintain the track’s capability, whether CRT or metal tracks. Further from a mobility standpoint, it was said that CRT provides a number of pros and cons to a conventional track system depending upon the type of vehicle and the customer requirements. One example cited in suggesting that CRT doesn’t currently meet all of the needs of armoured vehicles was that it is not aggressive enough in gripping on ground going up hills and when lifting/towing. It is understood Soucy CRT is currently pitched for the Warrior Automotive Improvement Programme.

An important difference between military trucks and their civilian equivalents lies in mobility: logistic support must be able to stay close to its combat- ready clients (Photo: UK MoD Crown Copyright)
Vehicle weight in the off-road scenario is the key attribute impacting its ground mobility. Depicted is a CVR(T) on desert terrain in Afghanistan. (Photo: UK MoD Crown Copyright)

On the subject of suspension and transmission developments in relation to ground mobility improvements for future vehicle platforms, the RBSL spokesperson indicated that the latest suspension developments for future armoured vehicles are aimed at providing increased articulation, so they flex more readily, delivering a smoother, better ride – and incorporating increased ride and damping control through active and semi-active systems. Of transmission types, it’s understood RBSL is actively exploring various different transmission options, including hybrid electric and hydromechanical systems. The company said that electric drive systems offer the potential of enhanced silent watch, silent manoeuvre, as well as improved economy. The optimum transmission is a compromise between performance, usability and cost.

A Final Thought

A final thought from RBSL on mobility, was that its position alongside other vehicle attributes such as armour/protection on the list of priorities for a given design and customer is that its importance will vary from vehicle to vehicle and, interestingly, sometimes from country to country. Taking the Boxer programme as an example of this and looking at France, Germany and the UK, mobility has been prioritized over protection by France and Germany, who opted for firepower and mobility in their Boxer vehicles as top priorities, whereas the UK has prioritised firepower and armour over mobility. 

Acknowledgements:
Rheinmetall BAE Systems Land (RBSL). ‘A Fuzzy Simulation Model for Military
Vehicle Mobility Assessment’ by George, Singh, Dattathreya and Meitzler.
RAND Corporation – “Assessing Tracked and Wheeled Vehicles for Australian Mounted Close Combat Operations”.

Tim Guest is a freelance journalist, UK Correspondent for ESD and former officer in the UK Royal Artillery.