[Closed] why do fat tyres feel harder at the same pressure when compared to narrower ones

force = pressure x area (fat tyres have more area)

I'd say it was the other way around...40psi feels rock solid on a 1.95 tyre but still feels 'squidgy' on a 2.3...

More likely its to do with thicker/stiffer casings and inner tubes.

The area under your thumb which you use to press the tyre remains constant, so if the tyre is at the same pressure, it should require the same force to deform it by the same amount.

So change in force for a unit squash ~ change in area (which was the question). Possibly a fat tyre will expand its footprint both longitudinally and laterally, so this marginal change will be greater. And if you have a fat tyre at the same pressure you would put in a thinner one, then it would not be optimally squashed and might not be using the full width of the treads until squashed a bit. You might not notice the effect as much at pressures that were optimal for the fat tyre, too low for the thin one.

Though surely the area you'd be looking at in this though case would be the end of your prodding finger not the whole tyre?

Edit: too slow

tyres of the same structure but different sizes should feel similar but the fat tyre slightly softer if anything. As you push your thumb into the tyre you are effectively reducing the tyre's volume. Since pressure and volume and inversely proportional as you push your thumb in the pressure will ramp up slightly - making it slightly harder to push in further. the bigger the volume of a tyre the smaller the increase in pressure will be....so should feel slightly softer.

But in reality this effect is tiny - they should feel about the same

One reason why it may feel like a smaller tyre is softer than a larger tyre is that the smaller tyre has to deform less before you hit the rim - just because its smaller. Tyres of equal pressure deform the same for a given impact but the larger volume tyre may not bottom out.

Volume change from poking the tyre with your finger is negligible; what you're feeling is the effect of the tension in the tyre casing caused by the pressure inside.

This is mostly as a result of the hoop stress in the tyre:

[hoop stress] = [pressure] x [cross section diameter] / ( 2 x [wall thickness] )

(Cross section diameter roughly corresponds to tyre width)

Some pictures [url= http://flocycling.blogspot.co.uk/2014/11/flo-cycling-why-do-you-use-less-tire.html ]here[/url]

There's a component that depends on wheel diameter, too, but that page ignores it.

There's some remarkably wrong anecdotal evidence and poor science in this thread, apart from tillydog's post.

Big tyres feel harder at the same pressure because when you push on the casing a larger area is pushing back against your thumb. If the casing was infinitely flexible and stretchy then this would not be the case but as it is you increase the tension between the beads and the air pressure pushes back against this tension. The bigger the tyre, the larger an area which is being acted upon by the air pressure.

A truely fat tyre at 7psi feels as hard as a normal tyre at 20+ psi.

Ah you're talking about thumb-poking not riding. Doh. I stand by my post in relation to riding feel though (ok if we're being pedantic, riding on flat surfaces). What matters is the rate of change of contact patch area with distance of squash. Hoop stress might be relevant to the feeling of a small (in relation to the size of the contact patch) lump in the trail, I am not sure. There would be reductions in the force contributed by other parts of the contact patch to take account of.

Although that is not relevant to the thumb-poke test, hoop stress is not the only factor there. The angle at which the tyre forms a well around your thumb will also affect the force your thumb feels and that will be different for a given movement with a fat tyre. I would tentatively suggest that this effect will mitigate somewhat the effect of increased hoop stress.

Then again... the hoop stress would change as you deformed the tyre (lower I think) which would make the narrow tyre even softerer.

tillydog - Member
Volume change from poking the tyre with your finger is negligible; what you're feeling is the effect of the tension in the tyre casing caused by the pressure inside.

This is mostly as a result of the hoop stress in the tyre:

[hoop stress] = [pressure] x [cross section diameter] / ( 2 x [wall thickness] )

(Cross section diameter roughly corresponds to tyre width)

Some pictures here

There's a component that depends on wheel diameter, too, but that page ignores it.

I was thinking this, and it corresponds with the observation in the following post that you're pushing a larger area around your thumb in when feeling a fat tyre.

More likely its to do with thicker/stiffer casings

I would go with this being what you are feeling.

Except it isn't. A fat tyre feels harder than a downhill tyre at twice the pressure, even though they weigh about the same (so the DH tyre has a much thicker stiffer casing).