Why is it difficult to ride a bicycle with a flat tyre?

[nandu]

Hi all

Why is it difficult to ride a cycle with flat tyre? Friction, I guess, must be the cause, but friction doesnt depend on the area of contact, only on the mass.

 

[Kral]

Re: flat tyre: physics

nandu,
With a flat tire, you are constantly flexing the rubber, which requires force.

 

[Prashant_Gupta]

Re: flat tyre: physics

Hello nandu,

From the way I perceive it, friction plays a minor role in answering the question you put up. Another effect, namely the 'Toppling effect', comes into play in this situation. The first question then to be asked is, why does a body topple? For this, imagine a block of wood kept on a table. In the situation where no force is applied, Normal reaction, N=mg. When we start applying a force tangential to the upper surface of the block, some force, or rather torque, is needed to counter this torque, (about the centre of gravity of the block). Therefore, N starts shifting towards the other end of the block, so as to provide the necessary torque to overcome the force. Hence the block does not topple. But at one stage force applied reaches a certain level for which the normal reaction reaches the end of the block, and if the force is increased any further, N goes beyond the body and the body topples.

So this is what applies to your question. If we have a fully inflated tyre, the distance up to which N can shift so as to counter the force applied is very less, hence for even a small force, the tyre will topple, i.e. it will start to rotate. But if the tyre is flat, N can travel much more and hence a greater force is needed to make it rotate.