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Thanks Stan, awesome post! ... (archive)

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Posted by Austin on November 21, 2000 at 12:57:37:

In Reply to: LONG post.....but interesting (m) posted by Stan on November 20, 2000 at 19:33:07:

I think it's safe to say the ol' butt dyno registers a 2 or 3 tenths of a second increase to 60 with my '98 M3 wheels installed vs. the stockers. A set of lighter wheels, in the 17lb. range, would help - that's about 7 lbs. less per wheel. Also, I'd lose the spacers and longer lug bolts with a correctly offset wheel - more weight shed. I bet with this setup, the loss could be cut in half, to maybe .125 sec. to 60. Of course this is all extremely unscientific, but one thing's for sure: Lighter is better.

Below is a response to an email that I received recently. How much lighter? Well unfortunatly it's not something a scale can measure since it depends where the mass is located. The rim vicinity (as opposed to the center section) and the tires themselves have a disproportionately large effect. You can measure the effect on the street or on a chassis dyno though.

<< I can't imagine that the rotating mass is much farther out on the them either. Would the necessary spacers have much of an effect? >>

WRT wheel mass and inertia, remember that there are two main things going on. Unsprung weight and moment of inertia abouth the axis of rotation.

For unsprung weight only the grand total of the wheel and tire mass matters. A 17 inch and a 15 inch wheel and tire with the same weight have the same effect on unsprung weight. Things like ride, and bumpy road cornering are affected. Pretty intuitive.

For the effect of rotating the wheel and tire things get a little trickier to understand. Think of the wheels and tires as flywheels. At a given speed, a vehicle requires the same amount of energy to keep going whether the wheels and tires are heavy or light. But when you try to speed up or slow down then the inertia effects matter.

The moment of inertia about the axis of rotation depends largely upon where the mass is distributed. With the mass near the axis having a low effect on the MOI and the mass further out having an effect related to the SQUARE of the radius. If you double the radius that a given amount of mass is located from the axis the effect on MOI quadruples. This is why when a flywheel is lightened, they try to get rid of as much mass from the outer rim as possible.

If you compared a 15 inch and a 17 inch version of the same wheel, the weight of the 17 would go up mostly because the larger circumference of the rim would need more metal if it was the same cross section as the smaller wheel. And the extra weight is further out from the axis of rotation so the MOI effect is amplified due to the radius squared factor. If we assume that most of the 15 incher's rim section's weight is around 7.5 inches from the center and 8.5 inches away for the 17 then the effect on MOI due to radius would be a 28 + % increase for the 17s. And since the mass of the rim section is more or less proportional to the circumference which goes up with diameter then 17/15 = about a 13% increase from the 17s so together we have a 44% higher MOI from the 17s if we look only at the rim section.

Therefore a 17 inch wheel needs to be a good bit lighter than a 15 inch setup to have the same effect on acceleration. Of course as the rim diameter goes up you need less sidewall height, so we need to consider this too. This is a smaller effect than the effect from the wheels though. Although it looks like the reduced sidewall height doesn't change the tire weight much and of course the reamining weight is all further out from the CL so the MOI effect goes up. Some tire companies try to cut the weight in the tread vicinity via lighter belts and such. Pirelli tried to do this with the new P zeroes.

Spacers are so low in diameter that their main effect is to slightly increase unsprung weight.

Dyno tests I have seen show a broadband drop of a few HP when large diameter wheels and tires are added. One 2 wheel chassis dyno test involving a civic, 14s versus 17s found that the car acted like it had lost 6 HP...and that was measuring only two of the wheels!! The quicker you try to speed up, the more the "tax" you have to pay due to the heavy wheels. So the problem is greater at low speeds in the lower gears. A few 0-60 tests I have seen show increase of a few tenths of a second with the big stuff on small motored cars. Some feel that lighter rollers are noiceably easier to turn since you have to pivot all of that stuff and there is a gyroscope effect.


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