If you are holding the wheel at the same angle in a car, why does speeding up or slowing down change the radius of your turn?

Question

I generally feel like I can figure things out, but this has just always kept me stumped. A good example is going up or down one of those screw-shaped parts of some parking garages. You can find the correct angle for your speed, but if you slow down or speed up without changing the wheel, the turn becomes sharper or wider.

Why?

In: Physics

Answer 1

Because tires don’t grip perfectly. There’s some scrub which is exaggerated with more speed.

Answer 2

In addition to what others have said, as speed increases more force is applied to the ‘outside’ tire in the turn, which is going to tend to follow a wider path. 

Answer 3

The mass of the car is always trying to go forward and the wheels are trying to “pull” the car to turn (this is why if you let go of the wheel it will recenter itself). So the faster you go, the more the car wants to go straight, the more the car will put its weight on the outside wheel which will make the inner wheel slip more easily due to loss of traction so overall the car will slip a bit more. Also, the more the weight on the exterior of the tire, the bigger the turning radius. Like turn your wheels at an angle, the inside of the tire makes a smaller radius than the outside of the tire. So the more you have to rely on the outside of the tire, the larger the radius. Couple this with added slipping, and you get a bigger radius when going fast. Note that if your tires are inflated properly, that your car has a good suspension, etc… all this effect will be minimized, but badly inflated tires, old worn suspension, this effect will be greater.

Answer 4

In addition to what others have noted your suspension geometry can also change when you accelerate or decelerate due to the weight transferring momentarily to the front axel or rear axel. Put simply when the front of your car lifts up or drops down the actual angle of the tires can change even though you aren’t turning the steering wheel. This is only temporary and it will return to normal not long after you reach a steady speed because that up/down force comes from actively speeding up and slowing down.