Flat vs inflated bike tire - friction

doubleyou

Hey there

I'm wondering why it's remarkably more exhausting to ride a bike with a flat tire, compared to a bike with a hard inflated tire.

The friction force "killing my efforts" so to speak, should be the same;
the normal force is the same (my mass doesn't change), and the friction coefficient should be the same aswell (the material of the tire is obviously the same, and the surface shape is the same), so the friction coeffecient should very well be the same if you ask me.

The only thing that changes in the situation, is the area of contact between tire and road, but that DOESN'T matter - friction is independant of the area it's working on.

So WHY is it more exhausting to ride a bike with a flat tire????


PS:
I know the argument why race cars use wide tires - that's because the tire is more soft, and therefore the tire needs to be big to be stable. So the friction is better because of different friction coefficient and not because of the width of the tires...
But in this case with the bikes, the coefficient doesn't change, and the normal force is also the same - so the friction should be the same....

NobodySpecial

The static friction of the bike tire and the road doesn't change - but you don't care about that, the part that is fixed in contact with the road isn't doing any work.

What does change is the energy needed to deform the new bit of tire as it comes into contact with the road (to create the flat contact patch) this energy is then mostly lost as the tire springs back. Since work is force * distance it is the amount you have to compress the tire that matters.

In a solid (or high pressure) tire there is very little change in shape - and what change there is 'springs back' as the tire rotates out of the contact patch putting some energy back into your forward motion.

doubleyou

So I guess it's the same explanation to why it's more exhausting to ride a mountainbike with wide tires; there's more rubber to deform than on the small tires on a city bike for example = more work lost

rcgldr

During deformation and restoration of the rubber tire near the contact patch, energy is lost to heat. If you compress or stretch a block of rubber, then relax so that the block returns to its original shape, the force on the return path will be less than the force on the compression or stretching path, with the net loss in energy becoming heat. This is called elastic hysteresis:

http://en.wikipedia.org/wiki/Hystere...tic_hysteresis

doubleyou

Thank you - I know this already :) . I just wanted to make sure that this is what makes riding a mountainbike with wide tires more exhausting. And obviously it is.

rcgldr

This also depends on tire pressure, and the thickness of the tread. If you could get wide tires to handle 120 or so psi pressure, and if the tread depth was very thin, similar to a road bike tire, then the main issue would be weight and not rolling resistance, since the amount of deformation of the tire would be less.

NobodySpecial

Remember that pressure and tire area aren't independant.
The pressure * contact patch area = weight on the wheel
So if you put more pressure in larger tires you just end up with a smaller patch in contact with the road.

Nothign really to do with the above - just interesting

rcgldr

This ignores the fact that the tread surface itself is compressable, and the tread's compressability is a function of tire contruction, not pressure.

jack action

Yes, the friction coefficient stays the same, but not the rolling resistance coefficient!

Furthermore, the friction coefficient have nothing to do with the effort you have to give. The friction only reacts to how much force you put at the tire contact patch. The friction coefficient represents the maximum force you CAN put on the road.

This is not the case with rolling resistance coefficient which represents the actual force you have to fight to keep moving forward.

Find more about the subject on this link (It uses car to explain the theory, but everything applies to bicycle as well). Check the theory tab at the bottom of the page.