Car Wheels Automatically Recenter

[EFuzzy]

Hello Everyone,

My friend and I were having a discussion of why car wheels would naturally return back to a forward position if one doesn't hold on to the steering wheel. It could be that its just the car turning to match the direction of the wheel, but I think the wheels turn more to match the direction of the car (it could be an illusion).

From what I understand, turning works because the wheel microscopically slips on the pavement in the direction of the current car movement, but since the wheel is spinning, the kinetic friction only acts normal to the wheel, causing a circular acceleration. This model doesn't explain where the torque comes from that causes the wheel to turn back to forward however. Could someone explain where this (possibly non-existent) phenomenon comes from?

Thanks,
Efuzzy

 

[rcgldr]

It's called caster. The contact patch is behind where the pivot axis would intercept the pavement. Any side force at the contact patch results in a torque force that steers the tire into the force. So if a car is turning right, there's a side force to the right, causing the front tires to steer left. It's easy to see this effect with the wheels of a shopping cart.

 

[EFuzzy]

Oh thanks, that explains it. So if the axle was pointed straight up instead of at an angle, would the caster disappear?

 

[mgb_phys]

Yes, the effect is much larger and more important with two wheels
http://en.wikipedia.org/wiki/Bicycle_physics