# Rotational Dynamics: Tires

#### Polyrix

Hi guys,

I have a question. I have lived life in the apparent belief that if a car is moving at a fast enough speed, turning it's wheel becomes increasingly difficult. Could somebody explain that concept to me? I assume it has something to do with the angular momentum of the wheel to make the rotation?

Thank you. Also, please offer any advice on how I can quantify the information... I am doing an analysis of a car.

Thank you.

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#### TVP45

It's much simpler if you look at a motorcycle to start. There, you must lean left to turn left. In a car, there's a whole bunch of stuff to consider. The car may be set up to understeer or oversteer for example. And, you have to look at the camber angle, etc, etc. Start with motorcycles.

#### Polyrix

Unfortunately, its the assignment. I'm getting rediculous values.. around 700 for moment of inertia, around 500 for angular acceleration, resulting in a torque that's... really, really large. I'm assuming those are probably wrong? I'm doing this all on qualitative observation of a tape.

#### DannoXYZ

The momentum of the auto acts on the caster angle to straighten the wheel out. Heavier and faster moving autos will have a stronger self-straightening torque on the steering axis.

Do you have diagram of auto's front-suspension with steering arm? The offset of the steering arm from steering-axis will give you leverage. The steering-axis inclination will also result in another offset called scrub-radius that affects the force required to turn the wheel as well.

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#### rcgldr

Homework Helper
In addition to the caster, at higher cornering forces, you also have contact patch deformation related to slip angle that also generates an opposing torque to steering inputs.

#### Prologue

What's interesting is that if there where no caster (and if it is at low speeds with no contact patch ;) ) then the wheels will not torque back, which seems counter to experience. But in everyday experience there is always a caster, no matter how small.

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