Tracks vs wheels

Image by Georgina Kerridge

Bringing it on

The age-old tracks versus wheels debate is still going strong, with both militaries and OEMs focused on finding the optimal solution for mobility requirements. Have recent shifts in focus given one an advantage over the other?

Peter Donaldson

You need tracks for the heaviest vehicles and the toughest terrain, but you need wheels to go far and fast over easier ground. This fundamental truth still holds, but a closer look at these still-evolving traction technologies reveals significant developments that are improving the mobility, load-carrying capacity and durability of both.

Needs have changed since the wars in Afghanistan and Iraq created a boom in demand for MRAP vehicles. Mark Van Drie, VP sales and marketing at Hutchinson, noted that modern requirements for both wheeled and tracked vehicles emphasise speed and mobility achieved through reducing the weight of components, even to the point of significantly reducing and, in some cases, even eliminating armour protection.

‘A premium is placed on the ability to insert or extract quickly from the battlefield and retain enhanced mobility while in theatre,’ he said, adding that the focus seems to have shifted from survivability through heavier armouring and MRAP-style V-hull designs to survivability through agility and superior threat detection and/or mitigation systems.

 

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As an example, he cited the US Army’s wheeled Infantry Squad Vehicle competition recently won by General Motors and Ricardo, in which the decision criteria emphasised mobility and weight. ‘Low weight enables air transportability and vertical bypass of enemy positions. Superior cross-country mobility enables vehicles to avoid roads where mines, IEDs, ambushes and enemy kill zones will be focused.’

Decision criteria

The main considerations when deciding between tracked and wheeled vehicles are the intended mission and the level of support that the operator is willing to dedicate. Tracked vehicles cost more and require a larger logistics footprint to support operations, but can carry the heaviest armour while still maintaining mobility, whereas wheeled vehicles have superior speed and stealth with lower cost and a much better ‘tooth-to-tail’ ratio, Van Drie emphasised.

Damien Delannoy, sales director for the defence business unit of vehicle drivetrain and suspension specialist Texelis, noted that warfare is changing rapidly and that one of the most noticeable trends is towards both urban and long-range movement.

These requirements have a significant impact on whether to go down the tracked or wheeled route, he said, adding that the key questions to ask during the decision process centre on the nature of the ground, how far the vehicle will have to travel between actions and the kind of combat in which it will have to participate.

‘Travelling over long distances is possible in tracked vehicles; however, for the crew and passengers it can frequently lead to an unpleasant experience, whereas wheeled makes for a more comfortable ride and hence the passengers are able to emerge fit to fight,’ he explained.

The shift in focus to urban operations gives wheels a major advantage because they allow greater agility, said Peter Simson, director at Tyron Runflat, a company which specialises in inserts that keep vehicles moving despite punctures or more severe damage that deflates tyres.

 

🜂 Needs have changed since the wars in Afghanistan and Iraq created a boom in demand for MRAP vehicles and militaries are now less focused on heavier armouring. (Photo: US Army)

 

Fitness for purpose is the first key question, he noted, along with mobility and serviceability in the field, plus the economies of running wheeled rather than tracked vehicles are all key parts of the decision-making process in meeting a particular requirement.

In some roles, size and weight have to some extent dictated the choice between these two means of locomotion, with tracks lending themselves to heavy weaponry and broad load-base vehicles, such as bridge layers, for example, but there is change in evidence here, he added.

Blurred lines

‘We think the line between the two has blurred with the significant growth in the ultra-manoeuvrable 8×8 wheeled platforms featuring in defence forces. In addition, the concept of operations sees deployments over many kilometres, and it is here that wheels score definitively, for fuel efficiency in addition to crew comfort.’

He also noted that demand for 8×8 vehicles has grown in roles that would traditionally have been filled by tracked platforms, citing the need for manoeuvrability, flexibility and lighter weight for urban operating environments driving users towards wheeled vehicles. ‘However, typically the need for firepower and protection levels in certain roles – particularly in very difficult terrain – will be a major deciding factor,’ he emphasised.

Perhaps surprisingly, given the larger number of drivetrain parts in, to provide an extreme example, an eight-wheeled vehicle with all-wheel drive and all-wheel steering (8x8x8) compared with a similar tracked vehicle, weight limits imposed by requirements such as transportability by air do not necessarily have much influence on the decision, according to Delannoy, with other factors being more important.

 

🜂 Demand for 8x8 wheeled platforms – such as these Mowag Piranha III 8x8 APCs – has blurred the line between tracks and wheels. (Photo: US Army)

 

‘The difference in weight for an infantry fighting vehicle, whether tracked or wheeled, with similar armour is relatively small and does not have a bearing on the discussion. The important issue is cross-country mobility,’ he said. ‘Vehicles operating in soft ground conditions and over hundreds of kilometres with high levels of armour or firepower will probably need to be tracked; those operating over thousands of kilometres on hard ground will probably best be wheeled solutions.’

However, Van Drie said that there is a middle ground between lighter-weight vehicles where wheels make the most sense and heavy vehicles where track is required.

Equality of mobility

‘That middle ground is defined not so much by a number, but as a capability – the point where modern wheel, tyre and suspension technology provides equivalent all-terrain mobility to tracks,’ he said. ‘In terms of vehicle weight, that point progressively goes up.’

He added that tyre technology continues to improve, with better rubber compounds and designs that provide higher traction, durability and load capacity. He also pointed to central tyre inflation systems (CTIS), which enable vehicles to increase or decrease air pressure to optimise the tyre’s footprint on the terrain.

As a result, he said, the spots on the map where a wheeled vehicle cannot go are shrinking rapidly. ‘Wheeled combat vehicles now routinely exceed 60,000lb and some, such as the Eitan and Boxer, are approaching 80,000lb combat-loaded,’ he pointed out. ‘These vehicles can move cross-country at speed; something that was unheard of 20 years ago.’

 

🜂 The two-piece aluminium alloy wheels for the Joint Light Tactical Vehicle are significantly larger than the legacy HMMWV steel wheels but carry 72% more load for the same weight. (Photo: USMC)

 

As an example, he cited the USMC’s programme to replace its tracked Amphibious Assault Vehicle fleet with the eight-wheeled Amphibious Combat Vehicle. This, he noted, was ‘sparked at least in part by a demonstration in which eight-wheeled combat vehicle cross-country mobility compared favourably with that of the M1 tank.’

Less weight, more independence

With wheeled vehicles in particular, Simson emphasised, the importance of weight reduction in the wheels themselves, components attached to the hub such as brake discs and calipers and, in some cases, final reduction gearing has increased. These components are underneath the spring and together make up the vehicle’s unsprung mass, and minimising this is a key focus in the effort to improve vehicle handling and reduce wear and tear.

Hutchinson offers aluminium wheels that reduce weight by more than 50% compared with conventional steel wheels, according to Van Drie. The two-piece alloy wheels for the Joint Light Tactical Vehicle, for example, are significantly larger than the legacy HMMWV steel wheels but carry 72% more load for the same weight. A further benefit is the integration of CTIS valves and accessory mounts directly into the wheels, he noted, adding functionality while reducing weight and complexity.

In recent years, one of the most important improvements in what is essentially a very mature technology is the development of independent suspension, Delannoy said. Texelis has partnered with Timoney to develop such systems for a range of 4×4, 6×6 and 8×8 vehicles. This means they can carry more weight than using traditional arrangements such as live and beam axles, providing greater payload capacity for protection and firepower and greater mobility.

Runflat revolution

Van Drie added that runflat capability for tyres has greatly added to the resiliency of wheeled vehicles in combat situations because tyre vulnerability to battle damage was one of the key drawbacks to using them in combat. Now available for all sizes and weights of wheeled vehicle, runflats have advanced past providing a simple limp-home capability to where the vehicle can continue its mission at normal speeds and then return to base, he said.

Tyron’s Simson concurred: ‘The driver is able to retain control of the vehicle and continue at speed with full manoeuvrability for up to 150km, even with all wheels flat.’ This is a major plus for modern vehicles compared with their tracked counterparts because a severed track stops the vehicle and must be repaired before it can continue, which can be difficult or impossible under fire.

 

🜂 Composite rubber tracks used on vehicles such as the BvS10 armoured all-terrain vehicle are smoother and quieter than older types of steel track. (Photo: USMC)

 

Most fielded runflat inserts have to be installed and repaired at a field base, while some even require complete tyre and wheel assemblies to be flown to a logistics hub to have damaged components replaced. With its multi-piece runflat, Tyron offers a system that allows maintenance and repair in-theatre without the need for specialist tools such as hydraulic presses.

Runflats made of carbon-fibre reinforced plastic represent a more advanced option that can reduce unsprung mass by around 40%, Simson said, enabling significant increases in payload, armour or performance. ‘Used on lightweight reconnaissance vehicles, for example, improved acceleration, deceleration, ride comfort and handling are all of great benefit.’

Smoother, quieter, stronger

For tracked vehicles, Simson said, the advances have come in the areas of weight, durability and sound reduction. He explained that there have been many improvements in design and materials used for road wheels and tracks that can decrease total vehicle weight, reducing fuel consumption and increasing mobility while lengthening the mean time between failures.

Composite rubber band track is smoother and quieter than older types of steel track. However, he pointed out that there have also been advances in conventional track that have reduced noise and vibration, such as refinement of block sprocket engagement techniques and the application of high-durability elastomers to roadwheels, support rollers, sprockets, idlers and track pads.

 

🜂 Independent suspension systems can carry more weight than traditional arrangements such as live and beam axles, providing greater payload capacity for protection and firepower and greater mobility. (Image: Texelis)

 

Continuous band track is very challenging to the logistics system and time-intensive for repair, cautioned Van Drie, but segmented band track can be replaced one section at a time.

It is important to remember that tracked vehicles also have wheels, and rethinking them can yield dividends, he noted. Welded hollow wheel structures made from high-strength materials improve stiffness and load capacity while reducing weight. They can also be shaped to eliminate areas that collect mud and other abrasive debris in service, he added, while sacrificial spray-on metal anti-abrasion coatings are simpler and lighter than bolt-on wear plates and easy to refurbish.

Furthermore, the durability of elastomers used in ground pads, backing pads and road wheels has also improved. For example, ground pad life expectancy has grown by a factor of two or three, while road wheel rubber lasts between two or four times longer than with legacy compounds, according to Hutchinson’s own testing.

Design integration

Future developments will certainly include greater use of materials such as carbon composites, and Delannoy reported that it will be some way into the future before they become commonplace.

In terms of emerging technological developments, Van Drie believes autonomous CTIS for wheeled vehicles will improve their mobility still further. However, non-pneumatic tyres should be considered for high-load applications, although the lack of ability to adjust the tyre footprint to the terrain is a drawback that will have to be overcome. Turning to tracked vehicles, he said that designing tracks and roadwheels together as a package is the way to go.

Far from one achieving final victory over the other, these two fundamental ways of putting power to the ground look set to continue evolving together.