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      04-22-2012, 06:45 AM   #10
somonster
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Drives: honda, bmw
Join Date: Mar 2011
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bigger rims can affect your car in terms of quickness, handling, and if you change your tire outside diameter slightly it can create speedo error.

The FIRST issue is the RIM size and weight. It's not just the weight of a rim; it's how the weight is distributed in radial distance from the center spindle. Larger diameter rims distribute more metal out near the periphery of the wheel, which creates a greater inertial load that requires more energy to get rolling quickly. Given two wheels of similar weight, the smaller diameter one will rotate more easily and thus be the "faster" of the two.
Consider this: Even if a larger rim is a little lighter than a smaller rim, just having the the metal at a farther radius from the hub can make it "slower" to rotate than the smaller but heavier rim.

The SECOND issue is TIRE size. Sidewall height usually decreases as you increase rim size (need to watch out for slight overall diameter changes, as this will lead to speedometer error). Short sidewalls are stiff (RFTs even more so) and help the contact patch track steering inputs more immediately. The down side of short stiff sidewalls is a deterioration in ride smoothness and handling over pavement irregularities esp in corners.
Consider this. If you get rid of your runflats and pack a limited-service spare in the trunk floor, you're lugging around extra sprung weight.

So how does this come together when you're picking a new set of wheels?
Most up-sized rims exist for BLING's sake. Finding the right rim / tire ratio to "look good" is easy - just get the big honking painted rims with rubberband tires. You'll get very responsive steering but if you have stiff short sidewalls, but your ride quality and handling over bumps and cracks will suffer to some degree. Putting 22" rims on your car isn't a good way to make it faster.

If you want a rim / tire combo to "go fast," you want to use a smaller and lighter rim to avoid placing heavy metal way out near the periphery of your wheel. You need to balance the rim/tire ratio to control slop due to side-wall flex, what you can safetly tolerate depends on how hard you drive your car. Run-flats have stiffer sidewalls and are OEM equipment on my X6, but I've never used them on my track cars.
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