Direction of friction on bicycle wheels

[iitjee10]

When a bicycle is moving on a road, what is the direction on each of its wheels.
I think backward on both wheels because the condition of pure rolling is there on the wheels.
But then there won't be the necessary forward force that propels the bicycle forward. So what is the exact direction of friction? Is it different on both wheels. A general clarification with FBD's , forces and torques would be appreciated

 

[bp_psy]

I think backward on both wheels because the condition of pure rolling is there on the wheels.

Could you explain what do you mean by that?

Keep in mind that friction acts in the direction opposing the direction of the motion.Now think what is the direction of motion of a point on a wheel at the point of contact with the road and what would be the direction of the friction force acting on that point.

 

[koolraj09]

Consider a bicycle first. Pedaling the bicycle (accelerates and) rotates the back wheel in clockwise sense. The velocity of point of contact is therefore backwards. Hence, friction must act forward on the rear wheel.

This frictional force causes a translational acceleration of the entire bicycle, including the front wheel. The velocity of point of contact tends to develop in the forward direction. So friction acts in the backward direction.