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Old 09-02-2009, 08:09 AM   #53
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Join Date: May 2009
Location: Lincoln, NE
Posts: 159
My Ride: has no C pillars
Originally Posted by phlfly View Post
I'm not really getting your point.
Let's think about wheel(metal part) and tires(rubber part).
So when you're going a next size wheel, you should go next lower size tire(Aspect Ratio or height)?! Right?
So your wheel+tire diameter is staying the same, so your speed indication didn't change or in other words you don't need update your speed reading ratio(conversion).
This is all correct. As a general rule, you should size your wheels and tires such that the tires' outer diameter is as close to OE as possible.

Originally Posted by phlfly View Post
BTW the speed of rotation would be same at any point of the wheel/tire.
Yes and no. The angular speed is the same at any point of the wheel/tire, but the linear speed is not. Regardless, my previous posts address how increased mass moment of inertia affects acceleration and braking... not steady speed.

Originally Posted by phlfly View Post
So you're believing , that is bigger wheel will move mass rotation. I don't see how. The change(+_ 3-4 lb) would not significant, to compare to mass of the vehicle itself. Even more, the mass should almost compensating, by less rubber would use with bigger wheels.
If you keep the same tire OD and width then you are correct that the mass moment of inertia of the tires would decrease, but not enough to compensate for the increase in mass moment of a heavier (or even equally heavy, but larger diameter) wheel. As far as it's significance to the bigger picture, that depends on several other factors. A lighter car, a less powerful engine, a lower drag coefficient, or less rolling friction would all increase the percentage change in acceleration/braking due to a change in mass moment of inertia. As I said before, the effects would be more noticable on a 318 over an M3. Ultimately, it's up to the driver, if he/she notices or cares about any performance degradation. What's insignifcant to you, might be significant to someone else.

For the record, I can't wait to retire my 17" M68s for 18" TR Motorsports MT1s. Just waiting for the funds.
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