Running w/ 17s

Road race, Auto X, or the Drag Strip

Postby Plex » Mon Sep 13, 2004 10:58 pm

Sorry for getting it wrong Scott, i was under the impression that you had them mold them for you too, my bad.
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Postby PreGrid » Tue Sep 14, 2004 2:49 am

No problem :)

I just didn't want anyone getting disappointed after going in there looking for a sure fit. You know...

Anyway, Best,


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Postby alltracman78 » Tue Sep 14, 2004 10:51 pm

2of81 wrote:Acording to my Snap-on freon scale:

Stock ST185 15's----42 lbs
TTE 17's--------------41 lbs
Volk SE37A 18's-----40 lbs

All of these were done with tires

But, a larger wheel will move the heavier part (wheel) futher out, which will make it harder to spin.
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Postby 2of81 » Wed Sep 15, 2004 1:27 am

I remember reading in a old issue of Mini-Truckin that 1 lb rotating mass taken off equals 8 lbs unsprung mass. I switched to my stock wheels and felt a differance in braking, I'm probablly wrong on my stuff so take it however you want.
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Postby the911s » Wed Sep 15, 2004 5:52 pm

here is a pic of my ST165 with 17s: would have worked fine, except I went with 225/45 tires (wanted the Azenis bad). 215s would have fixed my problem but i didn't want to spend any more money on a car i knew i was gonna have to sell


And... yes, if you add weight to wheels and tyres, it will make more of a difference to performance than if it was something that didn't rotate. Nothing complicated here. you don't have to rely on what you hear in magazines. here is the deal, more or less. i don't feel like busting out my physics book or looking anything up, so this may be slightly less than accurate but i am sure adrian or someone can correct me.

the masses on your car can be broken up into a few categories: sprung, unsprung, rotating, maybe a few more that don't apply here. in each case we are dealing with inertia, which is the resistance to being accelerated. however, rotating inertia has a more complicated formula than the other two.

sprung weight is weight that is carried by the suspension. better to add sprung weight than unsprung weight (i.e., brakes, wheels& tires). if you add unsprung weight, the ability of the suspension to keep the wheels on the road at all times goes down (if you have an oscillating lever and put weight at the end, the frequency goes down. this is what is happening with your suspension. you are making it harder for the shocks to control all that weight at the end of this lever).

however, if you add weight to your wheels and tires, you will not only be adding unsprung weight but rotating mass. this increases rotational inertia according to the equation 1/12 Mass X Radius^2 . Of course, that's not exactly what it is for a wheel because it's not solid and it has spokes and stuff, but you can see that you can't really say adding weight to wheels makes 8 times as much difference as adding weight to the rest of the car. It depends on where on the wheel you add it, the length of your suspension arms, and many other variables. However, you can see that you are adding both rotational inertia (making it harder to accelerate and decelerate the wheels) and unsprung mass (degrading suspension function), so this is something to be avoided.
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