Which is lighter - bigger or smaller wheel for same diameter tire?

[Arrowhead]

My guess it's not really worth worrying about but I am curious. Say you have a 28" tall tire, which assembly of tire and wheel is lighter, an 18" wheel or a 20" wheel? You have to make up the difference in metal or rubber. I'm sure there are way too many variables to have a consistent answer and you would need some specs on everything to make a real comparison and even then the wheel material / design is probably the biggest factor. But is there a general consensus about it?


Well think I answered my question. Spent some time on tire rack and they have weights listed for tires and wheels so just a matter of picking the combinations to get the weights. Like I figured too many variables for a blank statement I think.

[parsonsj]

Wheels are far heavier than tires -- so you can discount the tire part, and worry only about wheels (and the wheel/tire combination tends to have the same diameter, so again, you can discount the tire). The physics about wheel weights is called moment of inertia, or MOI.

The basic equation for MOI for a cylinder rotating around a center axis is I=M*R(squared). M = mass, R is the radius. So while weight matters, radius (diameter) matters a whole lot more.

Most of a wheel's mass is in in the hoop, so the approximation of a wheel as a cylinder is appropriate.

Using your 18" vs 20" example:

MOI of 18" wheel (let's say it weighs 25 lbs): = 25 lbs * 9(squared) = 2025 lbs square inches.
MOI of 20" wheel (let's say it weighs 28 lbs): = 28 * 10(squared) = 2800 lbs square inches.

It weighs 10% more, but its MOI is 38% more. In other words, the 20" wheels require 38% more energy to accelerate/decelerate than do the 18" wheels.

A rule of thumb from Mazda Racing is that each 1" of wheel diameter effectively adds 50 lbs of weight to your vehicle. Again, using the 18 vs 20 wheel example, that's 2" * 4 wheels * 50 lbs = 400 lbs. If you put 20" wheels on your car, it will "feel" as if it weighs 400 lbs more than if you used 18" wheels.

Hope that helps!

jp

[Buryingthesun]

^ Seem's to have nailed it, but I would also like to add if your are hell bent on 20's spend the money on forged wheels being they are much lighter then cast (in most cases)

[Arrowhead]

WOW great insight, thanks!

I felt kind of stupid asking but I'm glad I did. No, not bent on 20's just used as an example. But I also read increasing tire OD is a more effective way of increasing contact patch than a wider tire. Don't know about that.

[TheJDMan]

WOW great insight, thanks!

I felt kind of stupid asking but I'm glad I did. No, not bent on 20's just used as an example. But I also read increasing tire OD is a more effective way of increasing contact patch than a wider tire. Don't know about that.

Increasing tire diameter is one way of increasing the tire contact patch which is exactly why farm tractors have taller rear tires with the added benefit of increased ground clearance. But tall tires and high ground clearance don't handle well which is one reason road racers use short wide tires.