Forces acting on a rolling wheel

[ozyris]

Hi,

I have been trying to find some information on the forces acting on the wheel when the vehicle is moving but I am a little bit confused. I would be grateful if someone explain me the basic principles.

What I understood so far is;
(assuming front wheel drive)

* when the car is stationery; there is a torque applied to the wheel and this is less than the "static friction force".

* When a torque equal to the static friction is applied the wheel starts rolling. And at that moment static friction disappears and a "dynamic friction" comes to stage and this is usually smaller than the static friction. As long as the torque is equal to the resisting forces then the wheel rolls at a constant speed.

* And if a bigger force is applied the car accelerates at a rate proportional to the difference between the "torque" and "dynamic friction+air resistance".


The things I don't understand are;

- I still don't know if the drag force and rolling resistance and dynamic friction are all same thing?

- "hysteresis" is also another grey area for me. I understand that it is a force couple caused by the difference in pressure between the front and the rear contact lines of the front wheel. This is generating a moment resisting to the torque applied. Is this the force I called "dynamic friction" or an additional resistance? Because if it is the only resisting force between the wheel and the surface I find it very difficult to understand how this "couple" can provide the movement at a very low speed (say 1 second after the car starts moving). Because at a low speed the distance between the couple will be very very small?

Any help appreciated. Thanks in advance.

 

[jack action]

- I still don't know if the drag force and rolling resistance and dynamic friction are all same thing?

Read the Theory-->Longitudinal acceleration-->Forces involved section at the bottom of this web page for detailed info on the forces involved.

- "hysteresis" is also another grey area for me. I understand that it is a force couple caused by the difference in pressure between the front and the rear contact lines of the front wheel. This is generating a moment resisting to the torque applied. Is this the force I called "dynamic friction" or an additional resistance? Because if it is the only resisting force between the wheel and the surface I find it very difficult to understand how this "couple" can provide the movement at a very low speed (say 1 second after the car starts moving). Because at a low speed the distance between the couple will be very very small?

The resistance when you slide a block on a surface is friction, the one when you roll a wheel is rolling resistance and they are 2 different things.

There is static and dynamic friction, but we usually refer to only one type of rolling resistance (which would be equivalent to the dynamic friction). It's logical since there can't be "rolling" resistance without "rolling".

The rolling resistance is always due to the deformation of the wheel that causes a misalignment between the vertical weight acting on the axle and its reacting force acting on the ground. So contrary to the friction (which is an actual force), rolling resistance is not a new force that is created, it is a torque caused by a distance created between 2 already existing forces.

But just like friction, rolling resistance depends on the weight supported by the wheel (since it is the force that contributes to the torque called "rolling resistance"). All other characteristics are nicely included in a coefficient which depends on the type of wheel and ground; just like friction.

Rolling resistance does not create motion, it resists motion. In rolling, a horizontal friction force between the wheel and the ground creates motion.

 

[Danger]

Although it isn't part of the original question, I feel obliged to point out something else. Resistance in wheel bearings and brake pucks (if disc brakes are in use) also must be factored in. If you are considering the driving wheels, then it's somewhat arbitrary as to how far back along the drivetrain you want to go.