# Effect of wheel diameter in a car

1. Jul 17, 2014

### rambharath94

Hello guys

I want to know how the wheel diameter of car affects the car's performance parameters(physical significance) such as
1)speed
2)torque
3)handling.

I would also appreciate if some equations are used to substantiate the physical understanding of these.

2. Jul 17, 2014

### jack action

Just to make sure, are you referring to the wheel diameter or tire diameter ?

3. Jul 17, 2014

### Ranger Mike

Steering wheel or wheel for the tires?

4. Jul 18, 2014

### rambharath94

Thanks for replying . i actually mean the rim diameter

5. Jul 18, 2014

### rambharath94

wheel for the tyres.

6. Jul 18, 2014

### rcgldr

Assuming the wheel material is denser than the rubber in a tire, then a larger wheel increases the combined weight of wheel and tire. Assuming a fixed outer diameter of a tire, the side walls can be either too tall or too short for optimal handling, depending on how stiff the sidewalls are. As an example, in the case of a car like a Porsche 911, a wheel diameter around 18 inches is usually optimum for racing tires, while a Formula 1 car uses a smaller wheel, and relatively taller and stiffer sidewalls.

7. Jul 18, 2014

### jack action

The following assumes the tire diameter stays constant.

Inertia:

By modifying the wheel diameter, you will modify the total inertia of the wheel/tire assembly, thus affecting the acceleration of the vehicle.

It may increase or decrease, depending on the shape and design of the wheel. Here is a calculator to get a crude evaluation for rotational inertia of wheel/tire.

Tire stiffness:

With a small wheel diameter, the vertical stiffness of the tire usually increases. This means that the tire radius will vary more under vertical load (such as weight transfer) (deflection = load / stiffness). This affects the force and speed measured at the contact patch (force = torque / radius; speed = rpm * radius).

The preceding is also true for braking, which will affect front/rear brake bias.

Having a higher tire vertical stiffness also means that the tire can loose contact more easily with the road - if it is relatively too rough - hence affecting handling.

Similarly, the longitudinal stiffness will most likely decrease with smaller wheel diameter. This means that a very large acceleration/braking force can be absorbed, giving a smoother force transfer to the ground, like on drag tire for example:

Tire rigidity:

Having a smaller wheel diameter means a less rigid tire. This means that it is easier to remove the tire from the wheel under a lateral force, i.e. under lateral acceleration; A beadlock is then necessary.

A more rigid tire usually also means lower rolling resistance.

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Also, as the wheel diameter increases (no matter the tire diameter), the force between the rim and the tire increases, which can lead to slipping; Again, a beadlock may be necessary.

Finally, a larger wheel usually means more space for a larger brake system which can absorb more heat.

8. Jul 18, 2014

### Ranger Mike

we had a great discussion on mech engineering post

Alloy wheels and fuel economy
by Dr Lots-o'watts
use search above to check it out..