# 044 Circular Motion of Cars and Motorcycles

For cars to turn, the wheels must be turned into the bend so that the frictional force between the road and the tyre has a component perpendicular to the current direction of travel of the car. This component of friction provides the required centripetal force for circular motion.

This centripetal force, unfortunately, has a “side effect”. It produces a torque about the center of mass of the car which tilts the car away from the bend. Why does the car not flip over? Well, if the car turns too sharply, it does flip over. But most of the time, as the car leans outward, the contact force on the inner wheel (which tends to flip the car) drops, and the contact force on the outer wheel (which opposes the flip) increases. This restores rotational equilibrium to the car so it does not slant any further.

This “self-correction” mechanism is however not available to motorbikes since they have only one set of wheels. Instead, motorbike riders must learn to lean into the bend. The shifting of the centre of mass allows them to achieve rotational equilibrium.

## 2 thoughts on “044 Circular Motion of Cars and Motorcycles”

1. Mahesh Shenoy says:

I like to add that, the torque about the CoM is balanced.. But if you consider the torque about the point of contact, then there is only one torque which is due to gravity and then the system is not in equilibrium.. there are two ways to look at it

a) from the inertial frame (ground).. we can see that this torque causes the cyclist to precess ..(like how a bicycle wheel precesses due to rotation) in which case the bicycle is not in equilibrium

b) from the non inertial rotating frame (as a pillion rider maybe), then we need to add the centrifugal force.. this centrifugal force also acts at CoM and this will balance out the torque due to gravity and thus now in this rotational frame.. the cycle is in equilibrium