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Thank you for doing this, Crossbow! Mad props!
 

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lots of good links, but how about some bottom line results: Audiworld had a good derivation regarding acceleration: http://www.audiworld.com/tech/wheel13.shtml


Assume you have oem 17"rim/215 tires at about 44 lbs each. You find new tire/rim set of 4 with exact same rolling radius, but 10 lbs lighter per corner. They have less static weight, and less rotating weight.

AT BEST, if all the weight loss was using magic foam rubber at the tire OD, then per corner, you save 10 lbs static weight and 10 lbs rotating weight (with an implied factor of static + inertia of 2.0). This means, assuming same road interface efficency, new setup will run as if you dropped 2.0 x 10 = 20 lbs off each corner, or 80 lbs off the total scale car weight, for a set of tires/wheels that is 40 lbs lighter.

The 2.0 inertia factor is an extreme limit. A more accurate average factor is 1.7 or so. So dropping 10 lbs at tire/wheel per corner has a net effect of dropping 17 lbs per corner, including inertia effect..
 

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this was a really good writeup on the mechanics/physics...

The effects of wheel weights will be different depending upon the type of driving. For skidpad and racetrack performance, you won't notice too much of a difference between light & heavy wheels. But try some canyon carving up Mulholland (Malibu) or Old San Marcos (S.B.) and you will definitely see an obvious difference. Wheels & tires are unsprung weight, meaning they aren't controlled by the suspension and will follow the contours of the road.

When you hit a 6" bump, that wheel & tire will move up 6". The energy this imparts through the suspension into the body will depend upon the mass of the wheels & tires. A 55-lb combo will hit the body harder and launch it off the bump more than a 40-lb wheel & tire. The more energy pushing up on the body, the higher it will move the body and the more the body will pull on the suspension as it launches off the back side of the bump, lossing traction momentarily.

Similar thing when you drive over a drop. Gravity will pull the wheel down with a fixed force. The springs then pushes down another amount based upon the spring rate. A heavier wheel & tire will get pushed down slower with the same spring-rate. Thus there's more 'hang' time over the bump, meaning you have wait longer to regain traction. This longer drop time also causes the body of the car to fall more into the hole than with a ligher wheel. Dropping more into the hole also means a bigger launch coming out of it.

So overall, on bumpy terrain, light wheels & tires allow the suspension to keep the body more level with a constant load on all the tires for best grip. Heavier wheels & tires tend to make the body bounce around more over the same terrain and this jostling causes uneven load on the tires and gives less grip.

The unsprung weigh of the rear axle comes into play here as well. Take a rough, bumpy mountain road all out with a solid rear axle. Then try it again at the same speed with an IRS rear end and you'll notice a dramatic improvement in cornering and drive traction.
 

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QUOTE (Toadster @ Aug 26 2008, 02:50 AM) index.php?act=findpost&pid=1420999
this was a really good writeup on the mechanics/physics...

.... So overall, on bumpy terrain, light wheels & tires allow the suspension to keep the body more level with a constant load on all the tires for best grip. Heavier wheels & tires tend to make the body bounce around more over the same terrain and this jostling causes uneven load on the tires and gives less grip.

The unsprung weigh of the rear axle comes into play here as well. Take a rough, bumpy mountain road all out with a solid rear axle. Then try it again at the same speed with an IRS rear end and you'll notice a dramatic improvement in cornering and drive traction.
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Yup. Main benefit of light wheels and tires is handling and smooth ride, not 0-60 times.
 

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