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Occasionally someone reports that they've installed tyre X on bike Y and have "no clearance issues". Others who have bike Y then wonder whether this might be a good idea.

The desire for this perceived upgrade seems to be coming from a culture of bigger-is-better. Even a racer might prefer 700x28, and wider MTB tyres provide more traction.

In my mind one should more carefully follow manufacturer recommendations. Even if 700cx28 fit when 700x25 were the supposed maximum, if 29"x2.6" fit in a place meant for 2.4", or if 26"x4.6" fit in a place meant for 26"x4.0", this extra clearance is there by design to leave space for sand, pebbles, and mud or snow respectively.

Is this about right? What's to stop someone from installing the largest tyre that'll fit?

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  • The good things is that as soon as a certain model has a new, wider size, the old stocks of narrower size got almost instantaneously a rebate. Why stop people subventioning you buying right size tires :D ?
    – EarlGrey
    Commented Feb 1, 2023 at 13:09
  • "wider MTB tyres provide more traction" ...in some contexts, but not all. Tread pattern, rubber compound are arguably more important.
    – Paul H
    Commented Feb 3, 2023 at 18:52

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Nothing stops. IF the tyre indeed fits (not just one paper through some uncertain number like 28 mm or 2.4", but by actual trying the clearance), there is no reason not to use it if the rider has a desire to use larger tyres.

The bigger is better is nonsense though. There are various tyres fitting various kinds of riding and riding surfaces. However, if for the rider's use case the large tyre is indeed desirable, there is nothing to stop them.

"this extra clearance is there by design to leave space for sand, pebbles, and mud or snow respectively." Sure, and this is what the rider must consider when actual trying the fit and considering the actual type of riding and riding surfaces they will use. There is no manufacturer's hard rule like "a 2 mm gap must be kept for mud". The rider is responsible to decide which tyre is the best fit and how large gap they need for their purpose.

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Whether a tyre will 'fit' or not is somewhat subjective. And the same tyre may fit differently on the same frame depending on what rims it is installed on.

A couple of examples:

  • A road bike might be listed as having clearance for 28mm tyres. That limit could come from either the height of the tyre (clearance to seat tube or fork crown) or width of the tyre (clearance to chainstays). It will be based on the stock wheels and assuming the bike gets ridden in wet conditions on less than perfect roads (tyres pick up grit/gravel in the wet). If the limit is on the height of the tyre, then a user riding wider rims on perfect roads in dry conditions only might be able to fit a significantly bigger tyre in there.
  • An XC MTB might be listed as having clearance for 2.4" tyres. However being an XC bike manufactured by an imaginary company in Spain the bike comes with low profile semi slick tyres as standard. However the same bike being ridden in Scotland in thick mud with a much more aggressive tyre might realistically only fit a 2.2" tyre.

Tyre clearances from a manufacturer are generally guidelines that they feel comfortable to provide warranty support for. There's absolutely nothing stopping a user applying some common sense to pick the tyre that works best for them.

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There are a few factors.

  • There is a possibility of tyres being too wide for the rim (Sheldon has a table) - but perhaps you include that in "fit".

*On the other hand, tyres come up different sizes depending on the rims, so if you change the rims for a different width, the frame's apparent clearance changes.

  • Often, the biggest tyre that fits the frame means no mudguards, or far less good ones. This is probably less of an issue for MTBs than for other bikes.

  • There comes a point where wide tyres, with their necessarily lower pressures, increase the rolling resistance.

  • If (near) slicks are what you want (for grip and rolling resistance on road), you have very limited choice in wider sizes. Even the Big Apple has some tread, and maxes out at 2.35".

  • Eventually you get to a point where wider tyres are less aerodynamic.

  • Mud clearance is an issue. I have 2.1" on the rear of my MTB. I could maybe just clear 2.2" at the front derailleur but even with 2.1" the gap has been known to fill up so much with mud that the wheel stopped turning (a bridleway crossing a ploughed field; I had to walk the 2nd half until I could find a stick to remove the mud).

  • Soft MTB tyres bulge out, and not always how you'd expect on the stand. If you get to essentially zero clearance at your preferred pressure then land a jump, there's a chance of the tyres touching the chainstays.

  • Similarly any play in the wheel could use up a fraction of a mm on cornering.

These last two points only really apply if you go extremely close; I'd expect mud to be an issue sooner.

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  • Wide tyres don't “necessarily” mean lower pressure, though that is for sure the usual correlation. For a counterexample, it makes sense to use higher pressure for slopestyle than for cyclo-cross, though the tyres are much wider. Nor is lower pressure the main reason for higher rolling resistance. A XC tyre will typically have lower rolling resistance than a downhill one, even one that is both narrower and pumped to higher pressure. (I know, you probably know all this, but the second point in the answer does imply otherwise.)
    – leftaroundabout
    Commented Feb 1, 2023 at 23:05
  • @leftaroundabout While you can find outliers, taking a range of tyres actually narrow to wide, the max pressure will decrease as the width increases. The rolling resistance is dependent on many more factors, but eventually trends in the same direction as you're deforming more rubber by a larger amount. But I'm thinking of a frame/fork that can take a huge range, like gravel bikes or some hybrids For example the max pressure on my wide gravel tyres (that almost fit my tourer) is less than I choose to run on the rugged off-road touring tyres I use on that bike, far lower than the max pressure.
    – Chris H
    Commented Feb 2, 2023 at 9:00

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