Why are mountain bike tires rated for so much lower pressure than road bikes?

Question

Road bikes with their sleek, thin and flimsy looking tires are designed to take extremely high pressures, going up to 130 PSI (900 kPa). Mountain bikes have much more solid looking, broader tires. And their recommended tire pressure is much lower (my mountain bike has a max rating of 45 PSI / 310 kPa).

I thought that the much larger, more solid looking tire of a mountain bike would be able to handle 90 PSI (620 kPa) for sure if the flimsy road bike counterpart can handle 130 PSI (900 kPa). And besides, I'd be riding the mountain bike in the city on flat roads, just like a road bike. I found out that this logic isn't sound and that the mountain bike tire can even explode if I go that much above the recommendation.

But, I don't understand where my logic breaks down. Shouldn't the tube material of the road and mountain bikes be roughly the same? So why is it that the road bike tube can easily handle 130 PSI (900 kPa) while for a mountain bike, 90 PSI (620 kPa) is dangerous?

Answer 1

There are three aspects to it:

1. Rated max pressure doesn't necessarily have much to do with how much pressure the tyre can really withstand safely. All ratings on products are at least a bit below what the product can actually handle, but how big the safety factor is can vary a lot, for various reasons.
2. Wider tyres do have an inherently harder job containing high pressure. This is essentially the point Criggie made in a comment, though I find it requires some more explanation.
3. Of course intended use case, what L.Dutch's answer focuses on, also has something to do with it. MTB tyres are tough, but tough in different ways from road tyres since both are designed with different challenges in mind. However, this point is often exaggerated.

Let's first address the "inherent physics" point.

See, a tyre doesn't really "withstand the pressure" in the way e.g. an object in a hydraulic press withstands the pressure. In that case, the forces are directed on a solid surface, which distributes them to the bulk material underneath.

A tyre actually does something more demanding: it contains the pressurized air purely through surface tension. That effect is reliant on a curvature radius. The whole reason a tyre is able to withstand pressure at all is that the surface is curved, so that the purely tensile stress in the plies causes an inwards-pointing normal force against the air inside. The stronger the curvature, the less force in the plies is needed to contain the same amount of pressure. That's why even flimsy road tyres can withstand a lot of pressure.

So MTB tyres would already need to be stronger to withstand the same pressures. Now, they are stronger than road tyres in some senses – but mostly in the rubber part, the plies aren't much different. And the rubber is actually little more than a by-stander when it comes to the tensile stresses: rubber is just too flexible (which is why an inner tube by itself can hardly with stand any pressure at all).

However, all of this would still be somewhat irrelevant if the tyres were tubular, because ply rupture isn't the typical failure mode with over-inflated MTB tyres. But the tyres are clinchers, and the typical failure mode is the tyre bead slipping off the rim. And here we have an additional combination of disadvantages:

1. Though MTB tyres are much wider than road tyres, they don't run on proportionally wider rims. Accordingly, the ply forces are not only stronger (due to the weaker curvature) but they also point in the direction most prone to pulling the bead off.
2. The standard design of MTB tyre nowadays has a foldable polymer bead, which is more stretchable than the traditional steel beads, which also makes it easier to rip off.
3. Tubeless means that the pressure can fully "follow" the bead and push it over the edge.

So all of this means that a typical MTB tyre will indeed pop off the rim if you try to inflate it to 90 psi, and certainly none could handle 130 psi.

But 45 psi, really that low?

No. Actually, even such a tyre can typically be run at significantly higher pressure, especially when using inner tubes. The question becomes, why would you?

Well, you've asked about one reason: road riding. And yeah, that is a valid reason, don't let anybody tell you that you shouldn't ride an MTB on the road. (In particular not someone who drives their MTB to the trails in a car, talk about efficiency...)

And indeed using a higher-than-normal pressure helps reducing the inefficiency of the wider tyres, going to the 45 psi limit certainly makes sense. At that pressure the tyres of course won't have as much grip and small-bump damping off-road as with a more typical 20-30 psi, but that's not a concern on the road. On the plus side, they roll easier. Basically, you prevent much of that sidewall-flexing that's so useful off-road (and can even preserve energy because the wheels don't bounce as much) but only seeps energy on-road.

But what about even higher pressures then? Well, you can go higher, and it may still improve rolling somewhat, but here we get into diminishing returns: even a hypothetical MTB tyre at 130 psi would obviously not roll as well as a road tyre, due to the knobs and aero. In fact it might well roll worse than at 60 psi, because it can't even take up the vibrations by the knobs and road surface anymore. Such pressures only really make sense for slick tyres on a velodrome's wooden planks.

So there's not really any reason why manufacturers of MTB tyres would rate them for more than 45 psi: all it would likely accomplish is that the people who misunderstand rated pressure as the pressure you're supposed to run would be even more dissatisfied with the off-road performance. And the occasional case where a tubeless pops off at already 50 psi (can happen due to tolerances) doesn't help either.

The one application where going significantly higher than 45 psi can make sense for MTB tyres are extremely hard jumps, freeriding. You know, the kind of thing like jumping down 5 metres and landing on a stair set. But that is a) dangerous at any pressure, and the people who do this sort of thing know that anything going wrong is their own responsibility and "rated" values are irrelevant b) even then tyre inserts are probably the better option for preventing pinch flats, instead of going to 80 psi or so.

Answer 2

Of course it would be possible to build mountain bike tyres that could take the same sort of pressure as road bike tyres. But they'd be very heavy, expensive (with all the extra material in them) and give a horrible ride. They'd also kill your rims. The force acting outwards on the rims doesn't just depend on the tyre pressure but the area that pressure is acting on, which would be larger in rough proportion to the increase in width. Only the skinniest road tyres are run much over 100psi (690 kPa) (and only when running with tubes) so to run the same pressure in MTB tyres you'd be putting something like 2.5 times the force onto your rims.

If you want to ride your MTB exclusively on roads, you can, and you can get tyres to suit. There are plenty of tyres in the 37-50 mm range that will give decent performance on road, at a decent pressure. Some will run really rather hard (the wider touring tyres, for example, that are often fitted to tandems as well as heavily laden bikes). Tyres of this width will fit the majority of MTB rims with no problem - indeed my MTB has narrower rims than one of the wheelsets for my tourer. If you like to ride on roads to the trails as I do, just pump them up to the max before leaving home, let them down after the road section - and see how it feels riding back at trail pressure (I prefer the handling on tarmac with a little more air, but can't normally be bothered).

Answer 3

Here are some rough numbers to illustrate the forces involved with larger tires at higher pressures, using imperial measurements (inches, PSI, etc).

Just so you can get an idea of the magnitude of the forces involved.

First, area of a torus is 4 x pi^2 * R * r.

Start with a 29er with a 2" tire. Assume a 14" radius R and a 1" radius r:

4 x pi^2 * 14 * 1 = 552.698

That's over 550 square inches. At 100 PSI, that's a total of over 55,000 lb of force. A 3" tire would be over 75,000 lb of force at 100 PSI, and a 4" tire would be over 100,000 lb of force.

Now, all of that force isn't going to be trying to force the bead through the rim - maybe only 1/3 to 1/2 of it, and that's split over two rims, so it's only 10+ thousand pounds of force trying to push the tire bead through the rim. For a mere 2" tire at 100 PSI.

Only.

Large tires at high pressures are downright dangerous. Large tires that need to run at high pressure, such as large truck tires that can go up to 150 PSI or even higher, are usually inflated in a tire cage for safety reasons.

Answer 4

Another consideration is the rim that these tires are mounted on. With modern double-wall rims, the outer wall needs to withstand the tire pressure. Considering that most MTBs are using less than 40psi or so, it would be entirely unnecessary to make rims strong enough (read: heavy enough) to hold up against 130psi. Also, the rim sidewalls need to be strong enough to withstand the immense outwards force. Although yes, modern MTB rims are trending towards thicker and stronger sidewalls to better withstand impacts, the entire structure of the rim also needs to be strong enough to support the increased load--having super thick sidewalls won't help if the rest of the rim fails first.

Answer 5

I'd be riding the mountain bike in the city on flat roads, just like a road bike.

you are using the bike where it is not purposefully designed to go. That aside, a lower tire pressure:

• allows the tire to deform and better adapt to the ground on which it is rolling. Not a big deal on hard paved roads, but when you go on loose ground you want to increase your contact area to ensure proper grip. A colleague of mine does bike racing on sand: they use tires with pressures which anybody else would say are deflated.
• allows a better damping of the ground asperities. If you have ever been on pave with a road bike, you know that high pressure means a lot of bumps on your arms. Again, on poor ground like the ones where a mountain bike is supposed to go, this is an advantage.

In order to satisfy the above points, the tire itself is built in such a way that makes it less resistant to high pressures: stiffer side walls make a worse damping/flexing while would better hold the pressure load.

Answer 6

You don’t want to ride a MTB tyre at road bike tyre pressures. The recommended or maximum pressure on most tyres is usually much higher than you’d ever want to ride for optimum comfort and rolling resistance on real roads, unless you are very heavy or riding with lots of luggage.

Answer 7

My understanding is that tire pressure correlates directly to contact area.

Contact force from the tire must exactly counteract the weight. A tire having no weight on it is curved, with near zero contact. Placing weight onto the tire deforms it to touch the ground. Equilibrium is reached when air pressure applied across the contact area balances out the weight. Therefore:

Contact Area * Air Pressure = Wheel Load (weight).

For a 200 pound (90.7 kg) rider and bike, each wheel might carry around 100 pounds.

• At a typical 22-35 psi MTB tire pressure, this would give a contact patch from 2.8 to 4.5 square inches.
• However at 65-115 psi road bike tire pressures, the contact patch would be much smaller at 0.9 to 1.5 square inches.
• Also to note -- tire pressure also correlates inversely with vertical compliance to absorb bumps & retain traction.

For MTB riding, larger contact patches help traction on dirt or mud; and retain traction on loose surfaces (gravel) as some pieces slip. Greater compliance helps keep tread in grip, but at the cost of friction as tire movement requires work be performed against the surface.

For road riding, a smaller contact patch and stiffer (less compliant) tire reduce friction losses and the amount of work the tire does. However the cost is less traction, decreased adherence to bumpy surfaces, and almost zero offroad ability.

Given the different different pressures to be contained, the designs of road-bike and MTB tires therefore differ. Road tires have thicker rubber & more plies to contain the pressures. Off-road tires use thinner rubber given the lower air pressures required.

If you are riding on-road on your MTB, recommendations are to run 30-50 psi in your tires. One of the joys of an MTB however is the wide dynamic range; trying lower pressures will help you hop on/ off kerbs, navigate parks or walking trails, and generally open a wider vista of riding experience.

Answer 8

Adding to the answer from L.Dutch.

Wider/high volume tires will generally provide a firmer road/trail feel at lower pressures. 40 PSI in a MTB tire will feel show minimal deflection, while 40 PSI in a narrow/low volume tire will deflect more easily. As such, larger tires simply don't require pressures as high as narrow ones to reduce deflection.

In fact, when inflated to higher pressures, bigger tires become very bouncy - think of a basketball. So instead of deforming around small bumps, the tire will bounce off them. This is counterproductive, and makes them ineffective at absorbing bumps while riding.

Fortunately, maintaining relatively lower pressure in larger tires allows for an effective balance between rolling efficiency and absorption of bumps. The contact patch of a wider tire is smaller than a narrower tire at the same pressure. But since they cannot be run at the same pressures as narrower tires, much larger tires generally become less efficient.

Lastly, optimal tire pressure is very situation dependant. Most people riding mountain bikes do so on rougher terrain. Running them at lower pressure provides better traction and more comfort. But if you're primarily running them on the road, you could certainly keep them at a pressure closer to their maximum rating (e.g. 50 PSI).

That said, I'd avoid running them at max rated pressures, as they would likely feel too bouncy over bumps - this actually makes them less efficient than running them slightly lower and allowing them to deflect more. Ultimately, the terrain and the combined weight of you, your bike, and anything you carry will determine the optimal tire pressure.