friction on bicycle wheels

[tony lee]

hello , i am learning about forces . there is a digraph in the text book showing the friction on the wheels of a bicycle . i want to ask why the back wheel with the petas and the front wheel has a diffeernt direction of friction ??

thank you

[Fuzzwood]

Can you provide an image?

[John Cuthber]

One wheel pushes the bike. The bike pushes the other wheel.

[studiot]

 

One wheel pushes the bike. The bike pushes the other wheel.

 

 

 

I like this simple way to put it.

 

I hope those red blooded physicists note that no maths was needed and no mathematicians were harmed in the production of this post.

 

 

+1

Edited October 18, 2014 by studiot

[tony lee]

but two wheels are moving at the same direction , why will have different direction of friction ??

[Nicholas Kang]

It depends on the source of motion.

 

The rear wheel is powered by the force transmitted from the pedal to the rear wheel, so the rear wheel travels in a clockwise motion when touching the ground, resulting in an opposite direction(forward) motion of friction.

 

From the front wheel perspective, nothing propels the front wheel, it moves forward (not any clockwise/anticlockwise/angular movement) but net movement is forward. So, the friction is acted backwardly.

 

Combining both explanation, the rear wheel`s friction motion is forward while the front wheel`s friction is backward, thus opposite.

Refer: http://www.quora.com/When-a-bicycle-is-not-in-motion-the-force-of-friction-exerted-by-the-ground-on-the-two-wheels-acts

[John Cuthber]

but two wheels are moving at the same direction , why will have different direction of friction ??

If there was no friction between the back wheel and the ground the bike wouldn't move.

 

 

However for the front wheel the friction force is the only thing making it turn so the force must act the other way.

[Tim the plumber]

I would expect the frictional forces to be in the same direction but of course that's just the friction.

 

The force of the chain pulling the cog on the back wheel will be in a forward direction.

 

Are you sure that all the forces are frictional?

[studiot]

Gosh bicycles in particular seem to cause much confusion, so here is a more formal explanation for those not satisfied by John Cuthber's one.

 

The confusion arises primarily because of the unnecessary complication of the rotational dynamics of the wheels.

 

Consider the bicycle as a free body and apply Newton's First Law.

It does not matter whether wheels spin, arms waggle, springs compress or the Lord Mayor blows his trumpet whilst sitting on the bicycle. All is irrelevant to the basic mechanics.

 

The free body has two points of contact with the outside universe. It is only possible to apply external forces there.

 

In the horizontal direction the only forces the external world applies are the frictional forces at each wheel.

For a fee body in constant motion (cycling along at constant speed) they must be equal and opposite by Newton's First Law.

 

In maths we can say

 

If the bicycle is travelling from left to right, so the front wheel is at the right hand side and The friction forces are F_f for the front and F_r for the rear

 

If |F_r| > |F_f| the bicycle is accelerating

 

If|F_r| =|F_f| The bicycle is travelling at constant speed

 

If |F_r| < |F_f| The bicycle is decelerating or (braking or freewheeling)

 

Note I have used the moduli of the frictional forces since one must be negative.

Can you work out which one?