Simple question....is a larger rim going to slow a car down? Why or why not?

All of the answers I've found are unclear or offered by 15 year old ricers. We seem to have some physics-heads in here, so I figured I'd ask.

 

Larger rim, It all depends, you can get an 18 inch rim, with a 30 series tire on it that is way smaller, then a 14 inch tire with a 75 series tire on it...

 

I assume you are talking about acceleration, not total top speed. I guess it depends on what is limiting that particular vehicle in the first place. A larger outside diameter tire will give a larger contact patch. However, a larger rim and tire combo may weight more than a smaller one, meaning it's harder to overcome enertia and start it moving. Also there is the issue of needing more tourque at the driveshaft to transmit the same force to the road at the tire edge.
I've heard of some educated ricers using smaller tires when racing to in effect give them more tourque.

A great question to make one think.

 

From a purely physical standpoint and assuming that when you say bigger wheels you mean that the overall diameter of the tire/wheel combo stays the same, you're just switching in a larger wheel/lower profile tire set...

My gut feeling is, it will slow down accelleration and braking but shouldn't really have much of an effect on overall speed. The reason is... as seventyseven eluded to, is the greater rotational mass, further away from the center axis. It will take more torque to get them moving and conversly to slow them down again. Now, weight also comes into play. If the larger wheels are also heavier than the previous set, you now have more weight to try to spin up/down and that too will mean slower changes

 

OK...let me add a few things. It's all hypothetical but will tighten up the original question a bit I think:

Let's say a Mustang GT, out of the showroom with factory rims, as compared to one with 18"+ or whatever is bigger than factory for a new Mustang.

 

Ok, here's the simplified version... it's all a matter of inertia, you know "objects at rest tend to stay at rest, objects in motion tend to stay in motion."

Any of these things will effect the inertia of a tire/wheel combo and you have to take them all into account when figuring the effects.

If you make the tire/wheel combo heavier it has greater inertia, so it will be harder to start rotating, and will also be harder to slow down once it is.

If you move the concentration of weight away from the axis of rotation you increase inertia, again, harder to start, harder to stop.

In addition, if you make the overall circumference of the tire larger and don't adjust the rearend gears, you've effectivly made the overall gear ratio smaller, so it's harder to get off the line, but you'll have more top end. Conversely if you make the tire smaller in circumference, you'll be quicker off the line, but have less top end.

All of these things need to be figured into the equation when trying to determine what your change will do to the car.

 

when getting bigger rims, you should upgrade your brakes

 

Taller tires give you more top end at the same rpm,gear. Slight difference in acceralation most of the time. (wont notice the difference, unless it a big difference)

Brakes will not heat up as fast on a taller tire, at 60 mph the wheel is turning slower than a smaller tire.

Will try and later post sites that have a calculator that shows rpm, mph, wih different tire size. TCIAUTO.COM go to racing calculator, sroll near bottom.

Tire size (height) is what will make your final ratio different, not rim size.

 

All depends on the weight of the two tire/wheel combos being compaired as well as the overall diameter of the combo.

A larger diameter setup will change performance characteristics - decrease braking power (more "leverage" on brakes) and typcially these have more weight to them as they are larger (depends). The larger diameter tires will also change the final drive ratio and have a similar effect to changing differential gears.

 

If you want to see how weight and diameter effect rotation, here's what to do.

Find an office type chair or barstool or something that freely rotates 360° and have a seat, grab a couple ten pound weights and hold one in each hand. Hold the weights close to you and have somebody spin you around until you get some speed. Make note of how fast you are going, then extend your arms straight out and see how you slow down. Pull your arms back in and you speed back up.

 

There's also the sprung/unsprung weight issue as it relates to handling characteristics/suspension performance. The inertial loads of a heavier or lighter wheel/tire combination can greatly effect the way your shocks and springs react.

 

Confused on braking action here. Larger tires will make fewer revolutions than a smaller tire, at same speed. Braking action will be better on larger tires, may require more pedal pressure. Dont think you will really notice.

Wieght difference probably will be close in aftermarkets.

All in all, my thoughts are larger (taller height) tires more speed (mph), smaller tires get you up to speed quicker, mph will be lower at same rpm.

If you are looking into the 1/10000 differences, don't forget airdrag on wider tires.

 

I've seen in several reputable places where they mentoin the larger diameter tire acts as a longer bar in leverage against the brake causing it to have a decrease in stopping ability.


In the real scheme of things about this whole topic - we're not talking about a big difference here...not enough to really worry about. It's hard to say what the real performance changes will be without testing.

This would be more of a discussion if we were talking about big stuff - like 35s vs stock tires where we're talking 80lbs vs 30 lbs.

 

On 5.0 Mustangs, I've seen a few tests where the car lost about 5-10hp at the rear wheels when switching from a 15" drag race wheel/tire to a 17" road race wheel/tire. Effectively, the larger diameter wheel/tire becomes a "heavier" flywheel.

I've been told by wheel/tire shops that installing these huge 20"+ wheels on the SUVs is dropping the brake pad life down to 10-20K miles. That's purely anecdotal obviously but based on the dyno numbers I've seen with the Mustangs, it does not seem unreasonable.

 

Bigger rims tend to slow your speedometer and your milage down on your car or truck and smaller rims tend to speed it up. You also might want to invest in some better brakes because the bigger the rim the heavier it is. Another thing you might want to consider when upgrading rims to a bigger size they tend to eat up ball joints quick.