Effect of wheel size on car speed

[Dangermouse]

I read this on one of the forums

"Rear tire size will effect final drive too; smaller diameter tires will result in a quicker car, larger diameter tires will give more top speed."

So by quicker car I assume quicker acceleration. Assuming the poster was correct.

So assuming all other things are = e.g. gear ratio motor rev... If am guessing if I race a car with 15mm wheels it will be a bit quicker out of the corners but will be passed on a long straight by a car with 17mm wheels on it.

My question is:

"Why is this so?" (my fellow Aussies will get that)

DM

[waaytoomuchintothis]

To make it even harder to make a choice, the torque of the motor has a great deal of bearing on this. A low torque motor needs smaller wheels to get decent acceleration, whereas a torque-y motor can handle big honkin' wheels and match the acceleration of the cars with smaller wheels.

[legrandnormand]

It all depends on the engine RPM and the rear end ratio; if you have a high revolution engine, a high rear end ratio, you'll need large wheels other wise, you'll burn your regular tires. On the bad side, you won't get a high speed but a great acceleration.

[chappyman66]

Everything else being equal, a car with a larger diameter wheel (and hence tire) has a larger circumference and therefore travels a greater distance for each revolution of the wheel. The number of revolutions is dictated by the gearing, and the literal RPMs depend on the motor.

Granted, everything else is not equal and so there are trade-offs as folks have suggested. Note that larger wheels also raise the rear end of the car, potentially raising the CG far enough that you find the car doesn't handle as well. So then you end up raising the rear axle to lower the CG.....and now all things aren't equal again.....

[Nor Cal Mike]

It is the outside diameter of the tire that is the true determinator of final wheel speed and how many feet per second a car will travel. Most modern tires fitted to larger diameter wheels are lower profile. If you put a classic 15mm wheel and high profile tire next to a 17mm wheel fitted with a low profile tire such as used on a modern GT or LMP car, you most likely will find that they are the same overall diameter.

[chappy]

David, if all things are equal,, ie gear ratio and sufficient torque to turn the larger tires and the smaller tires , you will get a higher top speed, acceleration shouldnt be a large difference if the motor has the torque for the bigger wheels and the same motor for the smaller wheels.
A quick way to check is called rollout.
First put a mark on the sidewall and roll the tire from a marked point on a piece of paper.
then when the mark comes backmto the bottom again mark the paper and measure, do it to both wheels and you will, see the larger diameter wheel, tire combo has traveled further.
The next simple test is called pinion roll.
you do the same as above, but instead of stopping when the mark on the sidewall hits bottom again, roll it untill the pinion has rotated 1 complete revolution, then measure the distance again.
This way you will know the distance travelled by 1 revolution of the motor, and one revolution of the tire.
You can do it on a smaller scale by only moving the pinion one tooth and measuring the distance aswell.
if the motor is capable of turning the bigger wheels you will go faster at the top, but will cover more distance per motor revolution.
With todays offset gears etc you can compensate for the change in height of the wheel by using various offset options to maintain the low centre of gravity, or infact if using a pod, loosen the rear pod screws and shim the pod upward into the body by using spacers to lower the body down over the wheels. Of course you may be limited by inside wheel well clearance.
Bob

[Modlerbob]

[Florida_Slotter]

[legrandnormand]

[Nor Cal Mike]

[Modlerbob]

[balsaworks]

[Jesla]

[dr fabio]

What you want is something called "rollout". It takes into account the diameter of the tyre and the gear ratio.

example, gear ratio is 9/27, diamter is 22mm

R/O = (9 x pi x 22) / 27 = 23.02mm distance travelled per motor revolution.

gear ratio 10/27, diameter is 19.8mm

R/O = (10 x pi x 19.8) / 27 = 23.02 mm

So the two overall ratios are exactly the same.

[dr fabio]

Just adding a bit more to this topic. If your tyres are traction limited, that is you are getting wheel spin as you punch out of the corner, then going to a taller ratio (numerically lower) may actually give you better acceleration and more top end at the same time.