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      08-02-2008, 02:55 AM   #1
room335
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Red face Bigger or lighter wheels better?

For functional purposes, not looks, would it be better to go to a 17 inch wheel that weighs 16.8 lbs or an 18" wheels which weighs 25 lbs? Conventional wisdom is that less unsprung weight is better. Does a 17 inch tire weigh more than a comparable 18 inch tire? Does an 18 inch wheel/tire handle any better than a 17 inch wheel/tire? I am more interested in handling for road, track and autocross than looks.

Anyone know what an 18 inch A C Schnitzer wheel weighs?
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      08-02-2008, 01:24 PM   #2
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For all the reasons you stated, go with the 17" wheels. The 18s will offer you nothing more than looks / styling.
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      08-02-2008, 01:31 PM   #3
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I autox with 18's. But that's what my car came with from the factory (w/sport pkg), so when I got my 2nd set for racing, I also got 18's.
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      08-02-2008, 01:44 PM   #4
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You should take a look at Axis wheels, theirs 18's are only 16 pounds each. Thats surreal for how little they cost.
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      08-02-2008, 02:28 PM   #5
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I'd go bigger and lighter

If you plan to upgrade the brakes, bigger wheels give you more flexibility.
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      08-02-2008, 03:13 PM   #6
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Wider and lighter 17's if handling is what you want. Yes, 18in tires are heavier and more expensive than 17's. Comparable 18's tires are about a pound heavier based on TireRack info.
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      08-02-2008, 03:49 PM   #7
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This is what size and weight of rims does to a car's handling:

You've already touched upon unsprung weight and its effect. Unsprung weight is the enemy of performance. Higher unsprung weight means slower acceleration, longer braking distance, less responsive steering, and more force required to change the direction of the vehicle thus affecting the ubiquitous "handling. Unsprung weight, placed further from the center of rotation, also has a higher rotational inertia, so even if you're comparing a 17" rim and an 18" rim with the same weight, the 18" will affect the car's dynamics like a heavier 17" rim. How 18" vs 17" affects the dynamics of the car is mainly in acceleration and deceleration, since it's going to require more force to speed up the wheel and slow it down.

Where larger rims are beneficial to a handling of the car is in two folds. First, larger rims can clear larger brake systems, so larger rims affect the dynamics in that it allows for better brake torque and more brake fade resistance. Larger rims also inadvertently have a shorter sidewall if you keep the overall radius/diameter of the rim/tire combo the same, therefore allowing the sidewall to deflect less during cornering, resulting in better turn-in and less "squirm".

What size rim combo really depends on a lot of factors. First, you want to put the smallest size brakes that can effectively stop your car and also provide adequate fade resistance. That means the lighter the car, the more you can get away with a smaller diameter brake system, the heavier the car, the larger "heat sink" you will need. Ideally, you should run the SMALLEST and LIGHTEST rim that will fit and clear your brakes. On a car with 14" brembo kit that means 18" rims. On a Z4 2.5 with 12" rotors, a 16" rim will suffice.

Next step, if you'd like the side benefit of sharper turn-in from a larger rim, run a tire with a stiffer sidewall instead of going to a larger rim. It is easier to find tires to suit your needs than rims that are light and small. For example, in the top end summer tire category you have Bridgestone RE-01 and Yokohama Advan Neova's having near R-comp like sidewall stiffness or the all-around great performer AND a relatively comfortable and flexible sidewall of the Micheline PS2s. You do not want to add a combination that has an incredibly stiff sidewall and a very short sidewall, because some sidewall deflection will aid in contact patch deformation to help acceleration, braking, and cornering grip. So match the tire with the right sidewall stiffness to the rims. For R-Comps your choices are limited, but R-Comps are meant for track use only and typically comes with much stiffer sidewalls than street tires, and the compound is significantly grippier than the best of summer tires.

Ultimately, what size rims you will need to run also depends on what size tire you need to run. Again, that depends on a multitude of factors. A lighter car may need to run a slightly skinnier tire, a 275mm width tire may not be worked hard enough to generate enough heat to reach optimum grip on a 2,000 lbs car for example, while that same 275mm tire may easily be overwhelmed and overheated on a 4,500 behemoth. The width of the tire you need to run will determine the width of the rim and the offset you'll need.

This is, of course, given that you need "performance" rather than "style." Obviously, if style is a concern, I'd get the lightest possible 18" or 19" rims and slap the widest tires I can find to fit under the wheel well and performance be damned. Or if budget is a concern, then I'd find the cheapest 18" or 19" rim REPLICAS and weight and performance be damned, since my car seems to make 330hp on a good day.

At the end of the day you will need to decide to find a happy medium based on what you expect out of your vehicle.
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      08-31-2008, 06:26 AM   #8
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