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Primer: CTIS & tyre choice

Guns, missiles and exotic armour technologies steal the headlines and make for exciting marketing materials, but far more often than people perhaps realise it is mobility that is a key decider in combat. Central Tyre Inflation Systems (CTIS) are a way to level up wheeled AFVs and significantly increase their mobility.

What is CTIS?

CTIS (very uncommonly also called Tyre Pressure Control System (TPCS), mainly in the US civilian market) is a system that gives you the ability to vary the pressure in your tyres dynamically from controls in the cab. Just like most modern cars have tyre pressure monitoring, most modern off road tactical trucks and AFVs have the ability to also adjust the pressures on demand via a compressor system plumbed into all the wheels.

The goals of CTI systems are to constantly maintain optimum pressures, allow the pressures to be dynamically adjusted to optimise to the current terrain conditions, and provide (limited) enduring capability in the event of accidental or combat damage to the tyres.

Typical controls for a CTI system.

The controls for CTIS are usually a relatively simple and intuitive set of pressure level buttons mounted somewhere in the cab so that the driver can manipulate them without having to exit the vehicle. These typically consist of around five modes:

  • Highway/Road: Standard high pressure setting for travelling on roads, just like a normal civilian truck. The pressure ensures good fuel economy (within the bounds of heavy truck or AFV norms) and handling on smooth, hard surfaces.

  • Cross Country/Off-Road: Drops the pressure a little to make the tyres more 'squishy' so they have a greater surface area (or "contact length"), reducing ground pressure and making it easier to move in soft and wet ground. The cost is increased wear, and if the vehicle goes on the road on this setting the road handling and wear will be significantly compromised - for a typical medium tactical truck a 15% under inflation can mean 25% reduction in the tyre's life. A lesser benefit is that lower pressures and 'squishier' tyres means less incidences of damage when the tyres interact with sharp or abrasive things like rocks and foliage.

  • Sand/Snow/Mud: Very soft surfaces like sand, snow and deep mud are very challenging for all vehicles, so an enhanced mode drops the pressures to a bare minimum to try and create maximal surface area to try and get the vehicle to almost float on the surface and not dig in. The same drawbacks and concerns as cross-country endure, but more so.

  • Emergency: Emergency mode is intended to be used only where a vehicle is stuck and is trying to self extract. It puts the pressure so low as to be high risk of the tyre slipping the rims, and may well see tyres rip or otherwise get damaged due to extremely low pressures being set.

  • Run Flat: If the tyre is punctured either by incidental damage or the more unique threat for a military vehicle of having holes shot in the tyres, a run flat mode puts the compressor on permanently and tries to keep it inflated in spite of the hole(s) in it. Most military CTIS can sustain a basic inflation level (enough for the vehicle to move but not at normal pressure) with two or three 12.7 mm (.50") bullet holes in it.

What specific impact does all this have? Test data shows CTIS can increase drawbar pull forces in 8x8s (meaning how good they are at pulling things, including themselves, across the ground) by 43-132%, and towed motion resistances (the forces slowing the vehicle down and trying to stop it from progressing) decrease by 11-18%. Potential speed also increases 20-36%.

Chart showing drawbar pull increasing when using a low pressure mode.

CTIS works really well. It allows you to optimise your vehicle to the terrain you're on and maximise contact area when it is needed. Tyre contact area has slightly less benefit to tractive performance in harder surfaces, in soft terrain it is critical.

Why does tyre pressure matter so much?

Tyres are the primary interface between the vehicle and the ground, which means they are how the vehicle distributes its weight, transfers tractive force to move the vehicle, and effects a steering action to turn the vehicle, or resist turning.

Take a representative 8x8 - Boxer. The Boxer has a GVW of c.38 tonnes, and has eight wheels. That means that if evenly balanced, there is up to c.4,750 kg bearing through each wheel (note: in reality the weight is not evenly balanced and the wheels have to allow for load spikes as the vehicle moves over uneven terrain, but let's keep it simple and even for the purposes of this example).

It should be obvious that to stop yourself sinking into soft ground like mud you want the maximum surface area you can get. Lowering the pressure increases the surface area of the tyre, which in return reduces the ground pressure being exerted by the vehicle and making it easier (or indeed possible at all) to move across soft ground.

As a much smaller secondary factor, CTIS also means that the vehicle can monitor and maintain optimal tyre pressure all the time, which means much longer practical tyre life and less time spent doing tyre maintenance and pressure top ups.

It's not all that simple though. Lowering pressures has a few drawbacks. The biggest immediate risk is that you are making the fit of the tyre to the rim very loose, and the risk that it will twist under torque or be forced sideways and detach from the rim completely. That's pretty terminal, as there is now no tyre at all, the load spikes on the remaining wheels and the vehicle can rapidly get stuck.

A Russian Pantsir S1 that has lost a tyre and become stuck in Ukraine in 2022.

Getting the tyre back on, assuming it didn't tear or break up in the process of coming off, can be a tough job and may need the vehicle to be lifted by a winch or crane, given it is more than likely sunk into the mud by this point.

So you want to be very prescient about when you use it, and how low you take the pressures. The temptation for a driver will be to drop it as low as possible, but the potential for shedding a tyre is higher. Good training and smart systems need to be combined to mitigate the inherent risks of fitting this tool.

Does CTIS allow 8x8s to match tracked mobility?

No it doesn't. This is a claim that comes around from time to time, and on the basis of simple physics and a dash of terramechanics it just cannot work. At a simplistic level, it's the fact that it doesn't matter how much you reduce the pressure or increase the size of the tyres, you will never get the same surface area that you can achieve with a tracked vehicle.

Comparable tracked and wheeled vehicles will never achieve the same contact areas.

The chart below shows some more specific comparative data, with broadly equal size and weight vehicles, one an 8x8 and one tracked compared to one another. The y-axis gives % of the drawbar pull of a tracked equivalent - it never gets beyond 86%, and that is in the most advantageous comparative conditions and only in sand which for some odd but interesting reasons can favour wheels somewhat. Self-evidently unless it can reach 100% it has not matched a tracked equivalent, and 14% shy is still quite significant when vehicles are operated in extreme conditions where any advantage or shortfall can be ruthlessly exploited. Assuming very low pressures in the extreme soft to emergency levels, it averages around 60% of a tracked equivalent in the best scenario.

8x8 AFV drawbar pull versus a tracked equivalent.

More significant and irrespective of the tracks versus wheels argument is that in high moisture content (HMC) clay, the wheeled vehicle passes below 0% mobility, meaning it is immobilised. Even in MMC (medium moisture clay) it is very close to 0% at higher pressures.

Wheeled vehicles see higher, and more irregular, shear forces being transferred into the ground.

Why does that matter? Eastern Europe is a relatively firm environment compared to other areas, but also has high silt levels reflecting high density of rivers, lakes, swamps and similar soft and mobility hampering terrain (see European Soil Data Centre (ESDAC) maps of European soil composition below).

Claims around 8x8 vs tracks are also always made in something of a Top Trumps comparison, comparing them in isolation as a 1v1 assessment. Those willing to concede that wheels will never match tracks say that the difference is narrow and not significant.

But that ignores reality - a vehicle won't be travelling alone. As a group of vehicles move over a area the multiple passes mean the terrain degrades for each vehicle that drives over it. The higher peak shearing pressures of the wheeled vehicles mean they not only are less able to cross this terrain as it degrades, but they degrade it faster than tracked vehicles would.

A French VBCI stuck in soft soil

Drive a company of 8x8s over a piece of soft ground, even if it is not especially tough going to start with, and there is a real chance that the later elements will struggle to progress at all as the repeated passes turn a damp field into a muddy swamp. Not only are tracked vehicles less damaging to the ground, but they also have a greater margin to be able to tolerate the damage too.

The result is that in any terrain tracks will have better tractive performance for an equivalent size and weight vehicle, but particularly in wet and soft conditions. That gap means tracked things can go places wheeled vehicles cannot, can be loaded more heavily, be smaller in size or otherwise have capability gap exploited without becoming comparatively less mobile. (To be clear, there are plenty of big drawbacks to using tracks, but we're sticking to the soft terrain mobility comparison for now).

Looking at the bigger picture, opponents can model your terrain accessibility if they know basic specifications of your vehicles, and if you have wheeled vehicles then they may be able to more easily outmanoeuvre you or plan their own fires and manoeuvres against your movements than if you were more mobile.

Systems like NATO Reference Mobility Model (NRMM) can output handy charts readily based on reference profiles that are maintained for all allied and opposition vehicles.

Example of how mobility can be modelled into terrain access assessments.

So choosing 8x8 over tracks (from purely tactical mobility perspective) means you choose at least one of: reduced & more predictable avenues of manoeuvre, reduced payloads and equipment fits, and higher instances of bogging/mobility issues.

That doesn't mean wheeled vehicles are bad, it's just the nature of the tracks versus wheels trade-off, you choose to sacrifice the above to gain more strategic wins around speed and ease of strategic and operational deployment, crew comfort, ease of support and all those substantial benefits. But at the tactical level the 8x8s mobility is always inferior.

Does tyre choice make a difference?

Yes, absolutely. Not all tyres are equal, that's why an F1 car and a Dakar Rally truck have tyres that don't look remotely the same. But even in the world of knobbly off-road tyres, there are huge differences too.

There are a lot of tyre suppliers around, so for the sake of a consistent discussion I am going to use the Michelin military tyres catalogue to discuss this. The concepts are just as applicable to anything else, so don't over focus on the specific products here.

So, the Michelin catalogue for military off road vehicles is based around the X-Force tyre range, broken into four core tyre families (note families, because the range of sizes required to accommodate all the potential usage cases is vast and means varied capability within each family.

Any tyre that gives you additional performance in one terrain type tends to sacrifice performance in another. Broadly you get on- or off-road oriented grip - the more aggressive the bumps, which are called cleats, the better it will grip in mud/snow/sand but the less it can grip a road or hard surface. Deep cleats means more area to grip mud, but larger tops to the cleats means more area for gripping the road.

Generic orientation of the differing tyres in the X-Force range.

You can see that these range from relatively smooth to very aggressive in profile, and while there are further design elements at play, broadly you can see the more gaps and grooves, the more surface area for soft terrain off-road, but less hard/smooth surface contact on-road. Mud & snow chains can mitigate where a tyre is not aggressive enough.

You can see this when you look through the tyres. The XZL 2 is the road oriented tyre - shallow cleats, big flat contact area (comparatively speaking), and as such is going to be really good on long road marches and hard ground. There is a XZL that came before the newer XZL 2, and its a relatively all-round tyre that does a bit of everything but is not optimised for anything either, though it leans to the road user more than the off-road, marginally.

XS is the middle of the road option, a bit of everything but most relevant to sand, so very much targeted at the MENA region and anyone operating big vehicles in sandy places.

XML is the 'proper' off road tyre, it looks like a tractor tyre with huge cleats and deep edges to really grip into the mud. The sidewalls are reinforced to take a beating from rocks and debris, and its generally a solid military tyre. But conversely, it has limited contact area for roads, so it will handle poorly (in comparison to a XZL 2) and the wear on hard abrasive tarmac will be horrendous.

Beyond the profile and handling is another really important factor - the load index, which is the maximum weight a tyre can support. Going back to the three tyres above, even where they are like for like in terms of size, it doesn't mean they all have the same weight capacity. Broadly speaking the more off road optimised the tyre becomes, the lower its weight capacity gets.

Equivalent tyres for a Boxer vary from around 5,600 kg per tyre for XZL road tyres to around 4,500 kg for an XML mud tyre. That's a differential of 8,800 kg of weight on the vehicle taking it from up to 44.8 tonnes to a maximum of 36 tonnes, just by changing the tyres!

Bear in mind too that these are the max permissible weight, not the basic GVW ÷ number of wheels I did above. You have to work out what the peak load is on any one tyre in anticipated usage and set that as the maximum. The actual capacity may be significantly lower as a result - i.e. you may have 5,600 kg load index tyres, but the static weight on each tyre is only 4,200 kg to allow the load to spike to 5,600 kg under specific operating conditions.

This is where some interesting considerations come in. Trent Telenko wrote a great Twitter thread talking about tyre oriented logistics and why this might have been hurting the Russians in Ukraine.

One aspect that wasn't covered was that a lot of Russian vehicles are fitted with cheap Chinese copies of off-road tyres which may be a significant contributor to the problems Russia is having and exacerbating the poor maintenance Trent talks about.

In the case of the Pantsir S1 discussed in the example (pictured further up this post), the vehicle is fitted with what look like Michelin XZL tyres, but are in actuality Yellow Sea YS20, a bad copy of the Michelin and only vaguely suitable for buses and road going trucks, not heavy duty military off-road applications.

Their load capacity is only a maximum of 3,350 kg, compared to 4,500 kg to 5,600 kg for the Michelins they emulate. Across an 8x8, that's a max GVW of 28,000 kg versus 36,000 kg to 44,800 kg or to look at it the other way, if you have a c.36 tonne GVW vehicle and fitted these tyres thinking they are equivalent items, you're now 8 tonnes overweight and the tyres are going to tear themselves to pieces very quickly, rather like they have done in Ukraine, vastly exacerbated by woeful maintenance and substandard materials that probably also degrade faster than a 'premium' OEM tyre.

So tyre choice can be an enormous factor in your mobility, even without the addition of CTIS for flexible and adaptable tyre pressures.

Anything novel or exciting to look at?

There isn't a huge range of options for making wheeled things more tracked, because when you want that you tend to get something tracked instead, but more recently there have been some novel developments oriented towards agriculture and forestry that are now commercially mature and could be explored.

'Overtracks' are a increasingly popular in forestry where they can be rapidly fitted to a vehicle and consequently ramp up soft sol performance significantly. They are not, nor are they seeking to be, 'true' tracks. The construction is primarily polymer, and is strictly for slower speed off-road usage. Using at speed and particularly on paved surfaces is likely to rapidly damage them.

But they are a means to gain a very simple boost in soft soil performance, and the protective lips around the edge help to defend the sidewalls of the tyres from sharp things in the soil that might otherwise tear or chip them.

Fitting them to dedicated forestry machinery is easy, as they have very exposed wheels - things like a Ponsse Scorpion King harvester. Fitting them to an AFV is a harder proposition, as there needs to be completely clear area around and between both wheels being connected by track. With most modern 8x8 having steering on the front two axles, we are talking about tracking the rear two and making a modern half track. Though many AFV have obstructions between each axle, most of these are not structural and could be removed as a modification to the design.

An obvious first candidate for overtracks is the SUPERAV (in service as the ACV with the USMC). It has fairly a exposed wheel well area, and importantly is intended to operate in the surf zone and heavy sand beach areas where mobility is a challenge. Maybe they won't work due to some of the specificities of AFVs, but I'd love to see someone like MCOTEA do a trial with them and show us.

Direct replacement tracks, whereby a track unit can be fitted directly in place of a wheel without further modification to the vehicle, are an increasingly popular option in agriculture and have a few niche civilian options out there (including the novel approach bottom right below where you just drive onto the tracks and then lock them to the wheels, rather than replace the wheel).

The issue with all of these tends to be the load capacity required by a modern AFV, and the aggressive usage profile, both of which are well beyond the capabilities of most systems like this. But conceptually there could be some value in investigating these technologies to allow a swap to something a little more soft soil oriented when your force is likely to encounter especially grim terrain. A bit like overtracks, being able to swap out the rear axle(s) for tracks could be the difference needed to operate effectively when things get really muddy.

Closing comments.

So to try and reach a satisfying conclusion from the start point - is CTIS as a technology a good addition to wheeled AFVs? Absolutely yes. It significantly enhances mobility, it extends effective tyre life (both actively and through allowing you to cycle the tyres to maintain sidewall flexibility and stop the rubber going brittle), and it's generally a brilliant bit of tech.

Does it make 8x8 AFV mobility peer to that of an equivalent tracked vehicle? Not at all, the basic physics block you from ever doing that in a useful way, and advances in track technology like composite rubber tracks are only widening the gap, not narrowing it.

Does that mean a tracked vehicle is inherently better than a wheeled one? No. Everything is a compromise, and everything has its place. Tracked vehicles are very heavy and the maintenance burden of them is hideous. They are unrivalled in soft terrain, that's precisely why they exist, but they have substantial drawbacks elsewhere. Ideally you would have a mix of capabilities to draw on.

If you are looking for a straight tracks versus wheels fight, the answer will be entirely dependent on the specific needs, anticipated usage profiles, and a good dose of how big your resources are. Wheels have their place, and so do tracks.

Note: Also look out for my related post coming in the next day or so (will update this post with link when its live) on why road marching wheeled AFV thousands of miles is pretty crazy as a proposal, or at least logistically onerous. It was originally part of this post but things were getting unwieldy and it sits as a short side piece quite well.


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