Zulu

Ok, I got into a discussion with one of my co-workers about rotational inertia. One of us is wrong I am trying to find out which one. Here is my view on it. Tire height and width being the same, and wheel weight being the same, I think that a car with a 15" rim will be faster then a car with a 17" rims. The reason I said that the smaller rim would be faster is because the weight of the rim would be closer to the center allowing the engine to turn it faster and thereby accelerate faster. He says that the weight distribution wouldn't matter and that the only thing that would hinder acceleration would be tire width, since a wider tire would provide more traction and it would require more power turn wider tire. What do you guys think. I need some scientific data on this, but I am having no luck finding any. Maybe we're both off base, but hopefully it's not me hehe. Thanks in advance.

Fabrik8

Damn you really have my brain working on this. I think the inertia is the same between the two though. I don't think that where the weight is located has anything to do with it. I can argue both ways though, this sucks.
I need to find an answer, this is gonna bug me until I do.
The wind-up gyroscopes with the little string keep popping into my head, they have all their rotational mass on the outer diameter, which I'm assuming is to maximize inertia for a given mass at whatever diameter. I'd almost argue the whole case on that point alone. I seem to remember something about inertia and the rotational mass vs. distance from the center of whatever is rotating, like the closer to the center, the less inertia a given mass carries (stores?)..

I'm not a physics student, so don't anyone rip my argument or vocab to shreds, I know it's probably not correct.

ToyotaTechGeek

ok, F1 cars use 15" wheels. why? i happen to think that metal weighs more than rubber, so unsprung weight might have some bearing. but you said if the wheel/tire combos weighed the same and had the same overall diameter. friction could be a reason that you might think that a wider footprint might slow a car down. and larger diameter wheels might be more stable at higher speeds, but remember that it's probably due to their greater weight (i use German cars for this argument) and less deflection of the tire's sidewall. i realize that i've gone off on a tangient in respect to your original post. maybe these theorum/questions/factors will help you in your argument/debates. are we talking straightline speed or overall?

99blackcivicsi

Small light wheel is going to improve acceleration

big heavy wheel is going to hurt acceleration

small heavy wheel is going to hurt accerlaration

big light wheel is going to hurt acceleration

In import drag racing there is a reason people run a 13" wheel that wieghs 5 pounds because the 15" wheel that wieghs 9 pounds is going to slow them down because it is bigger and heavier.

to add to that the taller the tire the slower the car will be on acceleration but it will have more top end, the smaller the tire hieght it will accerlate faster but have less top end.

The slower all motor cars run a 13" wheel (light and small) w/ 20" tall slick, the fast turbo street cars run a 13" wheel with a 25" slick (why 25? no one makes a 26" for a 13) if you want a 26" slick you have to step up to a 15" wheel which in fact makes the car slower off the line and all acceleration. also normally the size of wheel you run determines the size of tire your going to run on the street.

in conslusion a car running a 15" wheel on street tires runs a 14.5 and the same car runs a 17" wheel on street tires it runs say a 14.8 that is slower no matter how you look at it. your right hes wrong tell him to get over it

PS: if i rambeled a little bit sorry this is just how thoughts go through my head.

sil80er

then why do jgtc cars use like 18" or 19" wheels

99blackcivicsi

are they consered with what they do from a stop? they are just after top end and handeling (correct me if i'm wrong) they dont' do 1/4 mile springs if they do on a back stretch or something they do 90-200+ which a 13 or 15 " wheel just won't do the gearing would be too high if they did use a 13'15" wheel the sidewall on the tire would be totally rediculous looking and wouldn't pull through a corner well.

Zulu

Yes I am talking straight line acceleration from a stop. I don't kow that the size of the wheel would have much of an effect at speed (say a rolling start on a freeway run). The thing about the import drag racing is one of the points I brought up, citing that they use small wheels with tall tires, and not "Dubs" with tall tires. Keep the chatter up!! This is some good stuff!!

MIGHTYMOUSE

it depends completely on the composition of the wheel, not just size, you can most certainly have an 18" wheel with less rotational inertia than a 15" wheel. material, width and tire hight are just as important as in racing applications a tire can easily outweigh a wheel


you can do a rough calculation with this http://www.ogura-clutch.com/calculator/part1.cfm

Fabrik8

Ok, I got the facts from my engineer dad. He says the farther you get from the center, the more rotational intertia a given mass will have. The farther out the mass, the more momentum it will have.

So the answer to the original question is that from a purely rotational inertia/angular momentum standpoint, the smaller wheel will have less angular momentum and should accellerate faster. This assumes the same wheel weight, tire weight, overall tire diameter, etc. For the accelleration/"which is faster" question, you have to factor in what everyone else said, like tire grip, sidewall flex, contact patch, width, etc.. Kinda cool to think that a 19 that weighs the same as a 15 and uses the same overall time diameter will have a lot more rotational inertia than the 15..

Seriously though, unless all you care about is dragging, who cares? I'd rather have a bigger wheel than smaller. Especially if I had the $$.. =)

OnyxEros

to clear those nasty brakes
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