For drag racing purposes, is this true? I read it on Supraforums (sticky in the Mk III forum)... so dropping from 25lb wheels to 15 lb wheels= 10 * 4 = 40lbs of rotational mass, which is then equivalent to dropping 400lbs off the car? And conventional wisdom says 100lbs = .1 in the 1/4 mile. I'm just skeptical of all this logic.

What made me think of this is that my buddy has the convertible rx7 wheels on his 88 turbo... they are 15's and weigh 13lbs each, while my aftermarket wheels are 17's and are 23lbs.

Anybody care to explain this?

5/8/2006 11:12:50 AM

going from 25 pound rims to 15 pound rims is a huge difference.

of course this logic isnt infallable, its just a good convention to use when throwing numbers around. Go the strip and try it.

5/8/2006 11:26:19 AM

I know in our drag car doing in the low 6 range, 100lbs was indeed worth about a .1 in the 1/8th, in a slower car, I imagine 100lbs probably = .1 in the quarter.

5/8/2006 11:28:05 AM

it sounds to me like you are trying to compare 100lbs of static mass to 400lbs of rotational mass - go take a physics class maybe

5/8/2006 11:32:32 AM

Methinks you should take a reading comprehension class first.

5/8/2006 11:41:42 AM

Quote :

"I know in our drag car doing in the low 6 range, 100lbs was indeed worth about a .1 in the 1/8th, in a slower car, I imagine 100lbs probably = .1 in the quarter."

i never understood saying 100lb is .1 in the 1/4..

if you have an old steel boat like a nova or cutlass (what, 4000#?) or something and drop 100#, i dont see the improvement being as significant as dropping 100# off of a car that weighs, say, 1800#..

may be a general rule of thumb, but people seem to act like that's a law of nature or something.

[Edited on May 8, 2006 at 11:52 AM. Reason : ...]

5/8/2006 11:52:25 AM

Think about the circles you always hear this "law of nature" talked about...most of the cars there are all similar weight.

5/8/2006 12:22:11 PM

true...

5/8/2006 12:27:04 PM

Quote :

"it sounds to me like you are trying to compare 100lbs of static mass to 400lbs of rotational mass - go take a physics class maybe"

i have taken a physics class (took 211 and 212), and in the context of this knowledge the comparison doesn't make much sense to me. I wasn't going to call bullshit though because my knowledge in these matters is limited and maybe somebody could give some proof for why this claim could be true.

^^^ of course. Obviously 100 lbs = .1 in the 1/4 mile can't be too accurate across a broad range of cars. 100 lbs off a miata is a lot more percentage-wise than 100lbs off an fbody...

[Edited on May 8, 2006 at 1:51 PM. Reason : .]

5/8/2006 1:49:28 PM

Quote :

"For drag racing purposes, is this true? I read it on Supraforums (sticky in the Mk III forum)... so dropping from 25lb wheels to 15 lb wheels= 10 * 4 = 40lbs of rotational mass, which is then equivalent to dropping 400lbs off the car? And conventional wisdom says 100lbs = .1 in the 1/4 mile. I'm just skeptical of all this logic"

WHile the wheels do rotate on a car, they are not what people are talking about with rotational mass, or it shouldn;t be. Thats more on the motor internals; rods,crankshaft, cams, flywheel, etc.

Wheels, also tires and suspension pcs, fall into the whole sprung vs unsprung weight. Yes there is a magnification when you remove unsprung weight vs removing sprung weight. But there are various estimates for the multiplier, seems to vary from 1lb unsprung = 5-10lbs sprung.

5/8/2006 2:55:41 PM

Quote :

"they are 15's and weigh 13lbs each, while my aftermarket wheels are 17's and are 23lbs"

Think about the weight of the wheel/tire combo instead of just the rims. His rims might be lighter, but his tires are probably heavier since there is more rubber to make up the bigger sidewall. So, the difference might not be as much as you think.

5/8/2006 3:11:53 PM

I've always though sprung/unsprung only effected handling.

I know the wheels would be magnified by a different "coefficient" than the pulleys, crank,flywheel clutch etc, because it moves at a different speed. then there is the drive shaft that spins at transmission out put rpm and then the wheels that spin at trans*differential gear ratio

i think and since the acceleration of the wheel is many many times that of the car the wheels do make a huge difference

i think it was Sports car revolution that dynoed an rsx before and after big brakes and even though the brakes were lighter the moment of inertia was much greater on the big brakes and the rotational acceleration of the larger brakes caused a 5 whp drop from stock. (same day same temp same dyno similar humidity)

so i'm sure wheels would have a greater effect then that but in the opposite direction.

5/8/2006 3:53:10 PM

i remember the project on sports car revolution losing something like 4hp after a big break kit...

5/8/2006 6:06:55 PM

its not hard to calculate and prove/disprove, just break out that physics book, cause i am done for this semester and i ain't throwing around numbers till august bitches

5/8/2006 11:06:52 PM

yeah i have dynamics sitting right next to me... but i don't feel like dealing with I values and the like

5/8/2006 11:32:10 PM