[415roushJAYR]

any brand of wheels that are light weight in the 18' inch size? staggered! did not want to put japanese wheels on my car such as roja and rayswheels. but i will if i cant find any others

[dakine858]

Originally Posted by 415roushJAYR

any brand of wheels that are light weight in the 18' inch size? staggered! did not want to put japanese wheels on my car such as roja and rayswheels. but i will if i cant find any others

Here is some enlightening info for those people that think they'll get accelleration improvement from removing a pound from the wheels compared removing a pound from the chassis

Some, but not all, of the work in getting the car to 60 is giving it translational kinetic energy (TKE). This is the energy of the whole car plunging straight along at 60 ignoring anything rotating, say like it was skidding on frictionless ice with the engine off. Some more of the work, but not all the rest, is giving rotational kinetic energy (RKE) to the wheels and tires. (I’m also going to do these calculations for engine flywheels and post on the performance forum.) Since work = power x time, if you reduce the work required by reducing these energies, you get to 60 faster.

First, I’ll compare 1 lb on the car with 1 lb on a wheel:

The TKE = 1/2 x the mass x (velocity squared). For a 1 pound mass (lbm) on the car going 60 mph (= 88 ft/sec), the required TKE = [1/2] x 1 lbm x (88 x 88) ft ft / sec sec = 3872 lbm ft ft / sec sec

The RKE = 1/2 x the rotational inertia x angular velocity squared. The rotional inertia (RI) = mass x (radius squared). The angular velocity (w) is in radians = rev/sec x 2 pi radians/rev.

Assuming that the wheel and tire mass is lumped at a diameter of 18” (= radius of .75 ft), a 1 lbm on the wheel has an RI = 1 x (.75 squared) = .562 lbm ft ft.

With a tire circumference of 76” (= 6.33 ft), 88 ft/sec is 13.9 rev/sec. w = 2 x pi x 13.9 = 87.3 rad/sec

So RKE = 1/2 x .562 lbm ft ft x 87.3 x 87.3 / sec sec = 2143 lbm ft ft / sec sec.

Taking 1 pound off a wheel saves both TKE and RKE. But the ratio compared to taking a pound off the chassis is only 1.6 = (3872 + 2143) / 3872. That is, taking 1 pound off a wheel is the same as taking 1.6 pounds off the chassis FOR THE PURPOSE OF ACCELLERATION. Sorry, but guys hoping for 5 sec. 0-60 times by lightening the wheels and tires just aren’t gonna make it...

As a sanity check, let’s see how this works with Garfield’s lightweight wheel test.

Assuming a car weight of 2800 lb and a weight of 200 lb for his set of 17" s-lites and runflats, the calculations above give that the wheel and tire RKE is an additional 4% above the TKE to get the car to 60. (For just doing a ratio, you can use the weight in pounds instead of the pounds in mass. The pounds weight ratio is 430,000/10,842,000. And no cracks from the Newton-kilogram-meters/sec guys)

Garfield saved 11 lb per wheel and, I’m guessing, 2 lb per tire, for a total savings of 52 lb. This reduces the TKE required by 52/2800 = .019 = 1.9% He also saved 52/200 of the 4% wheel and tire RKE, for another 1% savings. With a total savings of 2.9%, the reduction from his 'stock' 6.9 sec time should be about .029 x 6.9 sec = .2 sec.

Plus, you're going to lose some traction and handling ability with lightweight wheels.