# Why do smaller wheels on a car ?

1. Nov 13, 2008

### midget200

why do smaller wheels on a car....?

I recently had to do a science lab for school, and my guiding question was 'Does the size of a car’s wheel affect the acceleration rate?'

when i completed this lab, my results were that a smaller wheel affects the acceleration rate more then a bigger wheel. can anyone tell me why?

2. Nov 13, 2008

### fluidistic

Re: why do smaller wheels on a car....?

Strangely I think that in reality the size of the wheels doesn't affect the acceleration rate of the car. I believe that for a given power of the motor, the car will make rotates the wheels according to the power of the motor and not the size of the wheels. In other words if you put smaller wheels they will rotates faster than big ones but the car will accelerate at the same rate as if you would have put big wheels. I might be wrong though.
So it remains to you to find out why did you get these results. Maybe systematic errors... you're the only one that know how you did the lab.

3. Nov 13, 2008

### Danger

Re: why do smaller wheels on a car....?

Welcome to PF, Midget.
The main difference is that smaller wheels give a higher effective gear ratio for the power train. That improves acceleration, but decreases the top speed.

4. Nov 13, 2008

### Topher925

Re: why do smaller wheels on a car....?

True, but you can also consider the greater moment of inertia that comes along with bigger wheels as well. If you consider turning as acceleration, which it technically is, the size of the wheels has a very significant affect on the performance of the vehicle. Thy dynamics of which I won't go into unless requested as it gets rather complicated.

In application, wheels are made as small and as light as possible for a given amount of required traction. That is unless looks are important (which they are from a business stand point) then its just what ever size looks cool.

5. Nov 13, 2008

### schroder

Re: why do smaller wheels on a car....?

If the engine and drive system is capable of delivering the required torque, 18-inch wheels will give you better acceleration than 15-inch. But there is a big initial jolt to the drive system in turning those 18-inch wheels along with a greater tendency for the wheels to hop on the track. As I recall the real pros used smaller diameter wheels that were wider and a bit under inflated or racing slicks to get best grip and smoothest acceleration. However, they would use larger diameter front wheels (for rear-wheel drive) to shift more weight on to the rear drive wheels to keep them firmly on the track.

6. Nov 13, 2008

### Danger

Re: why do smaller wheels on a car....?

The slicks used on diggers give the best compromise, although you can't use them off the track. They're short and fat to start with at low rpm's, then grow to be quite tall and narrow as speed increases. It's like having a constantly variable axle ratio.

7. Nov 13, 2008

### HallsofIvy

Staff Emeritus
Re: why do smaller wheels on a car....?

WHAT affect did the smaller wheels have? Did they help or hurt acceleration rate?

8. Nov 13, 2008

### Naty1

Re: why do smaller wheels on a car....?

Danger got it absolutely right:

The basic reason is a typical gasoline engine needs to get revolutions up to mid range to begin to develop significant torque....a faster turning wheel (smaller circumference hence smaller diameter) allows that to happen fastest....

9. Nov 13, 2008

### rcgldr

Re: why do smaller wheels on a car....?

Smaller wheels or smaller radius tires? Smaller wheels with the same diameter tires produces a minor effect assuming the larger wheel and tire combo weigh more than the smaller wheel and tire.

A smaller radius tire would increase the effective gear ratio as stated by Danger.

The force at the rear tire equals the rear wheel torque divided by the radius. With the same rear wheel torque, if the radius is smaller, the force is larger. However the engine speed versus car speed is higher with a smaller radius, so you reach red-line rpms in the engine at a lower speed in each gear with a smaller radius.