question about rim size affecting speed

Ok me and my friend have been discussing this for a while. Im not sure who is right we both arent very sure to tell you the truth. When you have custom rims that are bigger than stock does this speed you up or slow you down. I say its all about the weight of the rims. But does size really play a big difference say going from 17s to 18s?

his car is slower now that he went from 15s to his custom 17s and we know its because of weight, but if they weighed the exact same do you think this would make a difference in speed?? just food for thought and im kinda curious myself.

[AFBeefcake]

Are you talking the speedometer being off or the performance?

actuall performance, i was just kinda curious after it came up in discussion, i know that the spedometer will be off

[svxistentialist]

Well, as you understand that the speedo can be off, you will understand what causes it.

If you take the standard rim and tyre size to be ground zero, then any tyre or rim change that changes the rolling diameter of the wheel will change performance.

If the new wheel is bigger[taller] than standard, your speedo will read slow, and your overall gearing will go up. This means the engine is pushing a higher gear than standard, and will not easily reach the same top speed in top gear. Acceleration will be slower too.

If the new wheel is smaller than standard, you are lowering the final drive. The car will accelerate better, and the speedo will be reading fast. With the lower final drive it could actually attain a higher top speed than standard, depending on how much power is available in your highest gear.

An additional and less welcome change, if you go for wider boots as they say in England, a wider profile in the tyres, then the footprint goes up. This means your tyres are dragging more tarmac all the time, and your car will be a lot slower, or at least use up more power.

All of this applies to a situation where the car has "adequate" power, and no more. If your motor is heavily tuned and putting out enough horsepower to break traction, then bigger, i.e. taller, wheels will mean a lot more speed.

For the average car, taller wheel/tyre combination plus wider rubber usually means a serious drop in performance.

Joe

[svxistentialist]

Originally Posted by Jaundice76

his car is slower now that he went from 15s to his custom 17s and we know its because of weight, but if they weighed the exact same do you think this would make a difference in speed?? just food for thought and im kinda curious myself.

It's not really because of the weight. Extra heavy wheel/tyre combination adds to what is called the unsprung weight of the car. It makes it very difficult for the dampers to control the wheel bounce frequency, giving you less grip and disimproved braking.

If it's slower, it's because of the rolling diameter going up and sapping up your power, and probably also more drag from wider tyres.

Now that he has the bigger wheels, tell him to stick in a turbo!!

Joe

[drivemusicnow]

I'm not sure if Joe answered your question or not... however I'll throw in a brief little explanation of the aspects.

There are two seperate components to think about, the wheel, and the tire. When you consider your friends car, he went from a 15 inch wheel with a specific tire size to a 17 inch wheel with a different tire size.

Like Joe said, if these tires are wider, they will add more drag, and therefor parasitic loss. If these tires are larger or smaller in their rolling radius, they will affect performance similar to a gear change. No "loss" in power to the ground, however the difference is in the end drive ratio between the motor and the ground. In this manner, your speedo will be incorrect.

The other thing to consider is the difference between rolling inertias between the 15 inch wheel/tire combo, and the 17 inch wheel tire combo. As physics shows us, it is easier to turn something if the weight is closer to the center of a "circle" rather than the outside. when you put on larger wheels the RIM of the wheel is further from the center. this makes it "harder" to turn that wheel. Also to note are the differences between lower profile tires and "normal" tires. Typically, a low profile tire will weigh in at equal or MORE than a normal tire. This weight is also further from the center of the wheel, thus making it even harder to turn.

This goes with the unsprung rotating mass parasitic losses

Again, like Joe said, wider tires, while adding drag, improve grip. So if the car breaks traction with a normal sized tire, a wider tire will improve accel times, But the parasitic losses are still there.

this answers my question exactly, and it does make sense that the farther something is from the center of a circle the harder it would be to turn. Im glad i researched this because i was almost thinking of moving up to 18s on the svx, but i have recently decided it makes more sense to just get the stock rims refinished