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Summary: A tyre that is 2.00 inches, or more, in thickness, that is designed to have a maximum allowed pressure or 7 or 8 bars, or 100-120 PSI -> does such a thing exist?

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I've been obsessed for years about combining road bikes and mountain bikes into a single ideal bicycle.

I know about the options that already exist as compromises between road bikes and mountain bikes: hybrid bicycles (flat bar bicycles with tyres of intermediate thickness) and gravel bikes (road bikes that can go offroad as long as the terrain is not too technical).

But none of those options are good enough for me. I have imagined my own solutions.

It would take too much time to write all the details of the design that I have thought about here right now. I might write the entire design on this forum, but not right at this moment. For the time being there is one particular component that I think is the most crucial that I wish to ask about: tyres.

My design is not about compromise but about changing between "modes": a mtb mode and a road mode. The important detail is that I want a bicycle that is very good in those modes, not a compromise. So as an MTB I want to be a very good MTB. There's a lot of properties and components that affect the quality of a mountain bike, but for now, I'm focusing on tyres, so the more versatile the MTB the thicker the tyres are.

A thicker tyre is also slower on the road, especially if run at the low pressure that off-road-ing demands. So to create this swiss-knife of a bicycle, I have thought about this: a 2.00 or 2.20 or 2.30 inch tyre that I would use at 7-8 bars (100-120 PSI) on the road and 1-2 bars in the forest (15-30 psi). Good idea in theory, but does such a tyre even EXIST? That is my question.

Alternative related questions: Can a MTB tyre be modified to resist 7-8 bars of pressure?

Can a tyre less than 2.00 inches (such as a gravel bike tyre for example) be made o be puncture resistant and not slide of the rim at small presure of 1-2 bars?

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  • 11
    Are you aware that on road bikes, it’s now known that higher pressure often leads to higher rolling resistance? 120 psi is definitely not regarded as ideal. Also, a huge part of what makes an MTB an optimal MTB is the suspension, which would slow a road or gravel bike down.
    – Weiwen Ng
    Apr 3 at 20:46
  • 2
    following up on what @WeiwenNg said about tire pressure with a link to something Jan Heine wrote on the subject with some data renehersecycles.com/myth-16-higher-tire-pressure-is-faster
    – GageMartin
    Apr 3 at 21:07
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    The “higher pressure is slower” argument only applies to supple road tires. MTB tires on the road will lose too much energy due to casing flex.
    – MaplePanda
    Apr 3 at 21:16
  • 6
    The fallacy here is that you're expecting there is one single ideal bike that can be good/best at everything. A MTB and a road bike have areas where leaning one way is advantageous on the MTB and disadvantageous on a road bike. Like handlebar width. So your "ideal" bike would be average between, ending up narrow for a MTB and still wide for a road bike. A "do everything" bike is a compromise and therefore is great at nothing.
    – Criggie
    Apr 4 at 4:42
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    Aargh. That Jan Heine link again. The explanation kind of makes sense, but he just can't explain his measurement methods or even include one commonly available tire in the comparison with his favourites, there's no way to tell what the numbers mean or whether they are completely made up.
    – ojs
    Apr 4 at 8:58
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2.00 or 2.20 or 2.30 inch tyre that I would use at 7-8 bars (100-120 PSI)

Does such a tire exist?

No.

A little math...

The surface area of a torus is 4 x pi^2 x R x r, where R is the large radius of the wheel and r is the radius of the tire here.

Let's put a 2.00 tire on a 29er rim...

4 x pi^2
  x 15" (approx radius of wheel at center of tire cross section)
  x  1" (radius of tire's surface)

= 592 in^2

Assume the tire surface area is only about 2/3 of that, or about 400 square inches. (It's probably more...)

Pump that tire up to 100 psi and each of those 400 square inches of tire surface has 100 pounds of pressure pushing on it.

That's a total of 40,000 pounds of force trying to push that tire off the rim.

Even if the tire can hold, the bead of the tire will be pushing HARD against the rim, trying to tear it apart.

40,000 pounds of force for a 2.00 tire at 100 psi. Pump a 2.40 up to 120 PSI and it's more like 50,000 pounds of force.

There's a reason why tires have maximum pressure ratings, and larger tires have lower maximum rated pressures, and it's not because higher pressure starts to increase rolling resistance.

It's because high pressures in a large tire are downright dangerous.

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    You are right that big high pressure tires don't work, but you give the wrong reason. The real reason has nothing to do with surface area, and everything to do with tire curvature: It's the curvature of the tire that translates the pressure of the air into tension of the tire. A slim road tire has a lot of curvature, so it translates the high pressure into a low tension. A MTB tire would translate the same pressure into a much higher tension due to its low curvature. And it would tear at the rim with that force. Most likely it won't be the tire that fails, but rather the rim. Apr 4 at 17:42
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    @cmaster-reinstatemonica can you do the math or give a link to explanation? Example of tire-like geometric form with high curvature and high surface area would be nice too.
    – ojs
    Apr 5 at 8:53
  • @cmaster-reinstatemonica The real reason has nothing to do with surface area, and everything to do with tire curvature: It's the curvature of the tire that translates the pressure of the air into tension of the tire. The only way for something to have a lot of curvature in a circular form is to be small, which means a small surface area. Apr 5 at 11:55
  • @ojs It's a simple physics argument: A flat rubber sheet cannot exert a normal force because the forces within the rubber are all within the rubber plane. You need to give the rubber a curvature in order for each rubber particle to see a normal force due to its neighbours being slighty out-of-plane. Mathematically, the resulting pressure grows linearly with the second deviation of place along the rubber surface (which will also produce the correct direction of the force). Apr 5 at 11:56
  • @cmaster-reinstatemonica So to get the total force you integrate the normal force vectors from the tire pressure over the surface of the tire... Apr 5 at 11:58
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No. It makes no sense to run a large tire at high pressures. As indicated in the comments, modern cycling has now accepted that higher tire pressure does not equate to lower rolling resistance in the real world.

Further, the ideal MTB tire (presuming there is such a thing, which there is not) requires tread to bite into soft surfaces. What tread pattern and depth changes depending on the surface - rock, mud, sand, hard pack to name a very few. Surfaces change from moment to moment, so choosing a mountain bike tire is far more complicated and important than a road bike. Most MTB competitive bikers change tires for the conditions of the day, and many recreational riders swap between tires regularly, this despite the manufacturers spending decades looking to the ideal all purpose tire.

A better approach to the ideal bike is lower the cost of bikes to allow for a bigger N and S. (Refer https://www.velominati.com/ rule 12)

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    Picking the right bike for the job is fine up to a point but there are plenty of times you can only have one bike though maybe 2 sets of tyres - no storage, on holiday with only room for one bike, a single ride or tour on mixed surfaces. In the last case you'll probably need to compromise
    – Chris H
    Apr 4 at 13:52
  • Totally agree that one bike is often all you have for various reasons. (I am currently down to one). The trick is accepting that with the specialization in the industry (e.g. Gravel bikes) manufacturers have actually made it possible for an rider to find the ideal compromise for them. The problem now is almost too much choice.
    – mattnz
    Apr 4 at 19:21
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    I seem to be in the position of having 3 - but trying to make them as versatile as possible anyway (rugged tourer, hardtail, hybrid with carrying capacity) so for gravel/dry dirt I could use any, and the MTB and tourer are surprisingly close in pace over some local tracks, at least until they get muddy.
    – Chris H
    Apr 4 at 19:27
  • @mattnz "requires tread to bite into soft surfaces" The theory I'm going by, at least currently, since I am revising this idea constantly in my head, is to compensate lack of thread (well, smaller knobs, or semi-slick) with a tyre as wide as possible and pressure as low as possible, in order to maintain grip off-road. Initially I thought about gravel bike tyres, but because of off road grip I came up with this idea: wide tyres and change pressure depending if i'm going on the road or off-road. But wider tyres means more rolling resistance on road. I want to reduce that with more pressure. Apr 5 at 12:22
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Even though pumping up your MTB tires harder will help with reducing rolling resistance on the road, 100-120psi is far beyond what you need. 50 to 60psi is already getting too high, and most tires can handle that. However, not all rims can handle that kind of pressure, so you’ll want to be aware of that. Perhaps select wide road bike rims if you really need to go that high.

Ultimately, the thick tread, thick/stiff casing, and soft rubber used on MTB tires are guaranteed to roll slowly. At some point, you might be faster carrying a set of road tires to swap on when you need to ride on the road.

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    Yes. I routinely ride my hardtail mtb on the road to get to the trails. 45-50 psi makes for a much easier ride (2.1 - 2.3" tyres) but it's still not going to be as quick as my hybrid running marathon plus (and they're slow tyres). Depending on your off-road riding conditions, you can choose tyres that aren't too bad. In the dry I run wtb nano (more of a gravel tyre,with a thin smooth centre strip) but in mud it's nobby nic / rapid rob which are considerably slower. If the first part of a ride is long and on road,it's worth starting pumped up,and letting down,but not worth stopping to pump up
    – Chris H
    Apr 4 at 13:50
  • Brilliant! Thank you. I thought about the idea that maybe it SHOULDN't be pressure that i;m focusing on. "the thick tread, thick/stiff casing, and soft rubber used on MTB tires" - I felt faster on schwalbe marathon mondial compared to maxxis crossmark which were my previous tyres. I rode those at 4.2 bars and the mondials at 5 and I thought the pressure difference was the reason i enjoyed the ride more. But I had a suspicion there was something else at play. Thank you for confirming my suspicion. So I should get stiff 2 inch or above tyre for the combo I'm trying to create here. Apr 5 at 12:24
  • SChwalbe marathon mondials are touring tyres not mtb tyres so they are deisgned for other uses. I have made a mistake last summer of keeping the pressure up irrespective of terrain. That was some bumpy trail riding, I'll tell you that. So i havent had the opportunity to test this pressure changing idea. I am assuming even the stiffest tyres WILL bend and create more off road grip if the pressure is low enough, especially since i weigh 100 kg. It remains to be seen what my experience will be, but stiffer 2 inch tyres are lot easier (well... not sci-fi) to get than 8 bar 2 inch tyres. Apr 5 at 12:27
  • @Biketouringfan No, for your use case, you want the lightest-duty MTB tires you still find acceptable on the trail. XC racing tires have sidewalls comparable to road tires, so most of the losses are in the wider tread and knobs. You therefore want to find the least knobby tire you can still happily ride MTB with. Wide, aggressive gravel tires are an option too.
    – MaplePanda
    Apr 5 at 17:24
  • @Biketouringfan Stiff casings are mostly found on enduro and downhill tires, which are most definitely going to roll too slowly on the road to be acceptable. Too much soft tread.
    – MaplePanda
    Apr 5 at 17:26
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You can't inflate wide MTB tires to road tire pressures. And it's not just that there are no tires that support that, it's just as much that there are no rims that support it.

The air pressure within your tire puts the tire under tension. How much tension is determined by both the pressure and the curvature of the tire. The slimmer the tire, the higher the curvature, and the lower the tension for the same pressure. As such, a 2 inch tire at 50 psi has pretty much the same tension as a 1 inch tire at 100 psi. All the pressure ratings for tires of different widths typically produce just about the same tension of the tire material.

Now, the tire tension translates directly into the force that the tire applies to the sides of the rim. Again, all tires within their respective rated pressures put just about the same amount of force on your rims, independent of whether they are road tires, or balloon tires. If you create a tire that allows twice the pressure as normal tires of the same width, that tire will put much more force on the rims than they are designed to hold. Most likely, you'd get a failing rim before you get a failing tire!

As such, no tire company wants to manufacture such tires, it would require them to also supply heavy duty rims, and explain to their customers that they must not use their tire with any rim that is not specially rated for the use of such a tire.

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    +1 for the explanation of tension, but I'd bet that good downhill- or dirtjump-targeted rims would have little problem withstanding 2" tyres pumped to 7 bar. Those rims already need to be quite overbuilt in order to survive the occasional “case” on jump landings, which involve much higher forces than the tension of even such a tyre/pressure combination. In fact, many dirt jumpers do run wide tyres at very high pressure. Wide tyres are needed to prevent cutting into the ground too much, but puncture protection and rolling resistance are more important than grip, hence high pressure. Apr 4 at 22:22
  • ...what may not withstand such pressures is the tyre beads – especially foldable kevlar ones slightly stretch so the tyre may blow off the rim. Apr 4 at 22:24
  • In your opinion, what is the long term outcome? for what I've done last summer: inflate my tyres at night to 5 bars, when it's cold outside and then use them as such during my ride in the summer sun. I have schwalbe maraton mondials, rated for max 5 bars. They are touring tyres that are designed to be indistructible. My rim is a 26 inch mountain bike rim. I have yet to visibily see any destruction of the rim, but then again I am no mechanic. Apr 5 at 12:30
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    @Biketouringfan the rim probably looks fine until it's in two pieces.
    – Brad
    Apr 5 at 14:11
  • @Brad So I should perhaps pump i to 5 bars during the day or more generally at a warm temperature? Or do you mean 5 bars is enough to destroy the rim? Apr 5 at 16:12
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The other two answers noted that large tires can't be pumped up to high pressures.

Let me explain a practical solution to your problem:

  • Buy a gravel bike or hybrid bike with ample tire clearance (depending on if you want drop bar or flat bar)
  • Build two sets of 622x19C rim wheels
  • Put knobby wide tires on one wheel set (with these rims you can go up to 44mm)
  • Put narrow high pressure road tires on the other wheel set (with these rims you can go down to 28mm)

If you want puncture resistance, there's a solution: tire armor made of foam. My recommendation would be not to use this on the road bike wheel set, but on the MTB wheel set you may want to use it.

The only problem with this plan is that you have to decide which wheel set to install before going to ride. Thus, most gravel bikes have tires of intermediate width that are a compromise between road and MTB uses, simply because one has to ride quite a lot on a road before arriving at the MTB trail.

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  • I use this solution with smooth tread tires on one set (higher pressure) and knobby tires on the other set. I use identical gearing so the changeover is pretty trivial if the rim width is the same on both. If the rims are different you may need to adjust the brakes.
    – Buffy
    Apr 4 at 18:45
  • Bike manufacturers would go far wider than 44mm on a 19mm rim (hence my MTB having narrower rims than my tourer) though I think they push it too far (hence my MTB squirming around at the back on hard stuff at mud pressure).
    – Chris H
    Apr 4 at 19:29
  • I am well aware of gravel bikes. They're not good enough for me. It was late at night when I posted this, hence why i said i didn't have time to go into details about the rest of bycicle. Well, the idea is, I want to ride a TT MTB. So MTB with aerobars. That would be the bike I would switch modes: a road mode (TT mode) and a mtb mode. I have an mtb that I converted into a touring bike, focus on comfort. This summer I will converted a 2nd time, this time into a TT MTB. If I like it, I will buy a new bike, a 29inch mtb in place of this 26inch mtb i have now and convert that into a TT MTB. Apr 5 at 12:33
  • If I dont like it, I may be force to accept that it can't get any better than a gravel bike, or i go back to my original passion which has been to do bike touring at a lousy 20-25 kph. Either way my bike is old and I want to replace it. I have thought about this idea of mixing properties of road bikes and mtb bikes as a matter of interest into technology. But now that my bike is over 10 y old I now want to apply some of that. Or to get over it. Either way, the thing that prompted all this is I need to buy a new bike. Should it be another mtb or a gravel bike or a touring bike or a hybrid, idk Apr 5 at 12:36

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