Why do pneumatic tyres 'bump'?

[2Btoo]

Chaps,

Here's one that may be a stupid question, but I will ask it anyway. Why do pneumatic tyres 'bump'?

Let me explain ... if you ride a bike (or any other machine with inflatable tyres) over a bump you can feel that 'bump' in the handlebars. I'm thinking about short sharp bumps - like up or down a kerb.

My question is why do you feel that bump?

The outer surface of the tyre is only connected to the hub (and hence to the rest of the bike) by the sidewalls and the air in the tyre. This outer surface will clearly dent in when the bump is hit but what happens then? The air inside will be compressed slightly but this increase in pressure will be felt all around the rim of the wheel, not just on the bit adjacent to the bump. Is there a degree of momentum in that the air by the tyre is pushed fast towards the rim of the wheel and this pushes it up? Surely this must be a miniscule force.

The sidewall will be squashed inwards too, but sidewalls are thin and floppy and (I'm guessing) can't transmit much compressive force. Hardly any, given how thin they are on bike tyres (little more than a couple of layers of canvas).

So, why do you feel the 'bump' through the handlebars of the bike? Is the answer simply a combination of the above two factors, each of which is small but enough to make itself felt? Or am I missing something?

Thanks!

[carlpea]

Newton's Third Law.

The tyre doesn't absorb the entire force created when going over the bump so has to go somewhere.

[2Btoo]

Carlpea,

Yes, Newton's third law needs to be obeyed. And it is - you can feel the 'thump' as the bike moves.

However why does this happen? How is the force transmitted from the surface of the tyre to the rim of the wheel?

[Monty Python]

The sidewall of the tyre can only flex by a certain amount, dictated by the construction and the air pressure in the tyre. If the height of the obstruction in greater than this then the entire wheel has to move vertically, or the wheel would stop abruptly and you'd end up on the floor.

[2Btoo]

MP,

Yes - exactly so. But you can feel bumps that are smaller in amplitude than the height of the sidewall.

[motco]

The force that compresses the sidewall is resisted by an equal and opposite force from the wheel rim. Because that force is momentarily greater than a second earlier the downward reaction has to be greater to resist it which it cannot be without accelerating the rim upwards.

[Alias218]

When you cycle over an obstacle, let's say a small pebble, the tyre doesn't get displaced by the full volume of that pebble (due to resistances caused by rubber elasticity, sidewall firmness, air pressure and so on) resulting in the tyre rolling over a portion of the pebble and lifting off the ground slightly. A portion of that pebble will be enveloped briefly by the tyre, but the remainder that doesn't will result in the tyre and ergo the wheel and the rest of the bike lifting upwards - your bump.

Given that a properly inflated tyre is not exactly soft, even small disturbances will be felt perhaps not as bumps but as vibrations. Pneumatic tyres only have so much give.

If you want a silky smooth ride, have a look at airless tyres. They still won't damp out every lump and bump but they look pretty good.

[dave_s13]

Same reason planes fly and get knocked about by turbulence and why objects travelling really fast into the earths atmosphere explode in a ball of fire.

Try riding a bike with a rigid rim and compare that to a pneumatic tyre. Then come back and tell us the combination of high pressure air and rubber doesn't dissipate a lot of force.

The physics of it are probably really boring and complicated. I wouldn't worry about it.

After you've picked your fillings back up and had your spine corrected, be calm in the knowledge that it's magic and people with bigger brains than you and me are happy at their work keeping such things in check.

Edited by dave_s13 on Wednesday 11th October 18:22

[Huff]

Also, pV=nRT.

The tyre at the point it hits the bump is a constant-volume device: while a small amount of energy compresses the air in the tyre/exchanges volume for a small raise in local air temp; essentially - depending on timescale, and a huge messy train of partial differentials - much of the force will just be carried on through that 'cushion': much of the whole air volume (and tyre enclosing it) is effectively moved - not just the little bit deflected by the rim.

Edited by Huff on Thursday 12th October 20:54

[Gandahar]

New S class puts foam in the tyres.

To reduce bump?

No, to reduce tyre noise.

Question is, do foam filled tyres reduce sound levels and bump or not?