rotational inertia

[Strebor04]

"The moment of inertia of an object about a given axis describes how difficult it is to change its angular motion about that axis. For example, consider two discs (A and B) of the same mass. Disc A has a larger radius than disc B. Assuming that there is uniform thickness and mass distribution, it requires more effort to accelerate disc A (change its angular velocity) because its mass is distributed further from its axis of rotation: mass that is further out from that axis must, for a given angular velocity, move more quickly than mass closer in. In this case, disc A has a larger moment of inertia than disc B."

Now that i have your attention. In your own words based on experiences or you ability to perform more difficult scientific equations than I...please elaborate on what will happen if i go from a size 18x8 wheel weighing, lets say 26 lbs to a 19x8 wheel weighing, lets say 21 lbs. Both weights are sans tires.

Will the extra inch in diameter counterbalance/counteract the reduction in weight? i.e. will my car be quicker or slower with the larger rims? Steering more responsive or less? More nimble or sluggish?

Thank you in advance.

[barkerd427]

In my experience you wont notice a difference in anything with just the rim change like that. Just make sure you have some good wide, stiff tires and you won't notice a change. With good tires you may even see a slight improvement, but not much. You will also want to try to get the outer diameter of the old and new tire to be the same.

[swoardrider]

please elaborate on what will happen if i go from a size 18x8 wheel weighing, lets say 26 lbs to a 19x8 wheel weighing, lets say 21 lbs. Both weights are sans tires.

Will the extra inch in diameter counterbalance/counteract the reduction in weight? i.e. will my car be quicker or slower with the larger rims? Steering more responsive or less? More nimble or sluggish?

Thank you in advance.

If your looking for a scientific explanation on effects of unsprung weight on your little equation, I can't answer that.

But if you want a real world answer, then I can tell you that the 19" set up in your equation will make your car a little faster off the line. Assuming both size tires are the same diameter, the 19" tire will have less rubber and weigh about a pound or 2 less. So, your 19" set up is about 6 lbs per wheel lighter than the 18" set up. I've been able to feel a 5 lbs per corner wheel weight difference off the line around town. On the track, I can really feel the lighter weight of my 18lbs track rims while accelerating out of turns.

Most people on the forum here will tell you that their car felt a little quicker when they upgraded from stock cast 18" rims which weighs about 29lbs, to an aftermarket 19" forged rim in the 23lbs range.

[Grishbok]

all the research ive done on the subject of wheel size has ultimately pointed toward 18" diameter wheels being the best for performance driving, however 19" isnt too far off either. I can assure you, our street legal cars will never reach the point where 18's are greater than 19's etc etc. unless you are professionally racing (or drag racing)

Even then its a constant trade on the benefits of the 18 over the 19. The performance GTI often swaps down to 18" wheels in order to acquire an extra 1" drop, this tactic has worked for them very well. So theres other factors in play when it comes to wheel size. Street driving... entirely subjective.

[Zebman]

Here's a thought: how about going from a 26lb 18" wheel, to an 18lb. 17" wheel? If you really are concerned about performance.