Rolling resistance is the force applied on the tire by the ground that opposes the tire motion.
Is rolling resistance is friction ? No ,rolling resistance is not friction.Check the below details to get your answer.
We are going to see two cases
1. When tire is stationary.
2. When tire is rotating.
CASE 1:- Tire is stationary
When tire is stationary then the inflation pressure inside the tire is equal to the contact pressure between the ground and tire treads,you can see the figure below.
When the tire is stationary ,it is still standing on the ground ,then the bottom treads at the contact patch deforms symmetrically. From the center they deform maximum and from the sides they deform least. So because of this we get the symmetric distribution of the forces by the treads on the ground along the center line of tire.Now to maintain equilibrium equal and opposite reaction force acts from the ground on the tire i.e W. You can see the distribution in the below figure.
Note:- Above is the section of tire i.e one layer ,So this distribution is only in One dimension.
CASE 2:- Tire is Rotating
When tire is rotating then the force distribution by the treads changes.Let the tire is rotating in anticlockwise direction.This rotation will cause the unsymmetrical distribution of the force along the center line.As tire rotates in anticlockwise fashion ,it will cause the left tread to get loaded first and then the further treads on the right of that treads will get loaded. As tires are the viscoelastic material,so because of which when treads get loaded and unloaded while rotating,it will cause the hysteresis loss in tire treads and loading and unloading lines will not be same.Below curve of viscoelastic material shows that on particular strain their are two stress ,one on the loading line and another on the unloading line.You can see that the loading stress is more than the unloading stress. So, as the tire rotates in anticlockwise direction it will cause the left side bottom tread of the tire to get load first and then that tread will get unloaded on rotation.
Left side of tire is loading and right side is unloading,as loading stress is more than unloading stress it will cause the curve to shift in left direction.See the figure below.
Note:- If tire is rotating in clockwise direction then curve will shift to right of center line and it will cause right to get loaded and left to get unloaded.
In this case we are considering the anticlockwise rotation. So, to balance the distributed forces by the treads the equal and opposite force is applied on the tire shown by W at left side of center line.
If i want to shift this W in the above figure to the center of the tire ,i have to apply the equal and opposite forces at the center line(these forces are shown by red color , check the below figure).Now the above red color force and the lower W force will form couple and will produce a opposite torque on the tire shown by M. As tire is rotating in anticlockwise direction it will push the ground to left and ground will apply the force to the right on the tire which is shown in green color i.e Traction. Similarly this opposite torque is shown by the Red color on the ground which is called Rolling resistance shown by Fr.
Where, Fr= fr * W
fr - Rolling resistance coefficient
Conclusion
Rolling resistance is not friction.
It opposes the motion of vehicle.
It is because of the viscoelastic property of tire.
Fr= fr * W
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