Rotational mass, come on in.

[TurboDSMJ]

Hey all,

I'm looking for a set of wheels for next track season, and I've narrowed it down between two sets.

Weld Prostars, bigs and littles.
-3.5 inch wheel up front, and a 275/50/15 MT Street in the rear.

Chrome ZR1's.
-17x9.5 up front with my current front tires, and a 17x11 in the rear with a 315/35/17 MT Street.

I figure it'd be cheaper to get the ZR1's, but by how much do you think the car will be slowed down do to having a 45lb. wheel/tire in the rear, compared to a 20lb. wheel/tire in the rear? I talked to a few people, seems like maybe .3 is possible. I'd like to make the car as subtle looking as possible, and I think the Pro Stars might capture some unwanted attention, not that the noise already doesn't do that. But I like the idea of a factory look.

Who can fill me in, let's talk.

-Matt.

[ITSTOCK]

Expect a couple tenths difference.

BUT, skinnies suck for daily driving, or just driving on the street compared to normal radials. If you drive the car at all, and want to continue enjoying driving the car on the street, stick with the ZR1's. It's well worth the car being .2 (MAYBE .3) tenths slower.

[cmr076]

ideally isnt it every 100 lbs is 1/10 of a second?

[SovXietday]

Originally Posted by cmr076

ideally isnt it every 100 lbs is 1/10 of a second?

That's a chassis weight rule, this is rotational weight. IE, weight that the engine needs to physically turn.

[cmr076]

but wouldnt it still apply? i mean weight is weight right..

[SovXietday]

Originally Posted by cmr076

but wouldnt it still apply? i mean weight is weight right..

It works two fold on wheels.

Not only are you adding overall weight, but you're making the engine work harder. You'll "free up" more power by using lighter wheels because the engine won't need to use that power to spin the wheels in the first place.

AKA, drivetrain loss. The weight/friction of the drivetrain makes you lose power.

[BradC]

Originally Posted by cmr076

but wouldnt it still apply? i mean weight is weight right..

Rotational weight is worse than chassis weight. I forget the exact relationship, but every pound of rotational weight is akin to (IIRC) two pounds of static weight.

Think of a stick held straight out, and rotate that up 90 deg with your wrist. Now do that with a few pounds on the end of it, you can see the difference immediately.

[TurboDSMJ]

It's not a daily driver, it's out maybe once or twice a week on short trips, and racing.

[cmr076]

get bogart wheels! there the sickness.

[TurboDSMJ]

$1,200 is the most I'll spend on wheels/tires, since my fuel system and the nitrous set up are already costing me a bunch, not to mention the suspension being put on now is a killer.

I can get all four ZR1's new, for $600, and buy rear tires at $200 a piece, and use my factory fronts. So the ZR1 set up will cost me about $1,000.

I can do Pro Star's and tires for $1,200 out the door.

[Wiisass]

well a realistic weight comparison would give you the best idea. Tire weights, wheel weights and then pick the best method to calculate rotational inertia. Then you could figure out how much more energy it's going to take to get the heavier setup moving versus the lighter setup and you can estimate the amount of power left for moving the car. It's not really that cut and dry though.

But get good weights for the wheels and tires. I doubt 45lbs for one setup and 20lbs for the other setup is that accurate.

Also, are the different options going to have tires with different Outside diameters? Because that will affect gearing and is something that should be considered. Also with the 275 be able to put down the power as effectively as the 315?

[enigma]

Do the slicks and skinnies route if you don't drive the car much on the street(1-2 times a week) The lower weight and friction from the tires will see even more gains if you decide to get more serious in the future.