Does anybody know of any site that has some kind of 'calculator' for figuring out how much rotational mass rims would be, to compare same-weight but different-size rims and stuff like that? I don't know if this even makes any sense but maybe someone can help?

 

I think you'd just have to use physics to figure it out. You need to figure out moment of inertia. After that, I believe calculus is needed as the wheel accelerates. A calculator would be nice, but it might not be accurate.

Rim design varies greatly. If there was more weight on the outside of the rim versus the inside, the numbers would change. The type/design of the tires also play a role in the result.

Then again, I think that there are so many variables, that it wouldn't really be accurate unless you took a physical test on a specific wheel/tire combination. A very interesting idea though.

 

Eh being in 10th grade the only physics I know is what I learned on honda-tech, and I'm only in pre-cal so no calculus for me either That's basically the answer I was expecting (because of the different variables) but thanks Anybody have anything else to add?

 

as far as the physics of a wheel in motion--it gets a little hairy.

but even comparing the moments of inertia of certain wheel sizes would be useful. (MoI is like "mass" in an angular/rotating system).

it's pretty simple to figure out a wheel's moment of inertia--using a couple of assumptions--since the majority of a wheel's mass is located on the "rim" of the wheel (vs. the mass of the spokes) so for our calculations, we can assume that the wheel is a ring. It's been found that the MoI of a ring is mR^2, where m=mass of the ring, R is the radius of the ring. (in case you're ever interested, there's a more precise method of calculating the MoI of an object that involves a definite integral, but you'll probably learn that much later on--hell, i never even really use that method for MoI calcs in my engineering courses)

note that the radius of the ring is squared, so a rim's size has a profound impact on the MoI of a wheel.

so, here's a sample calc to put it all together.

given:
10kg 16" (converted to mm = 406.4) wheel

req:
MoI (or simply "I" for short)

m=10 kg
R=203.2mm
I=mR^2=4.13*10^5 kg-mm^2

now, comparing this with a 17" wheel of the same mass:

m=10kg
r=215.9mm
I=4.66*10^5 kg-mm^2

(4.66-4.13/4.66)=11.4% difference in I, which is pretty significant in purely numerical terms.

anyway, hope that helps. keep in mind that once you get a figure for I, understand the dynamic system that is a moving car is complicated enough that just knowing I won't give you the ability to project how much faster or slower a car will be with a x wheel vs. y.

 

^^^

Very interesting

 

also, keep in mind that a larger rim/lower profile tire further exacerbates the difference in I between rim sizes. since low profile tires tend to have as much if not more mass than their taller brethren, that's even more mass further away from the rotational center.