RedHotBolt

I run 18 x 10.5's all the way around on my C6 at the track. I bought cheap (heavy) wheels to mount scuffs on. Last weekend Bobby Archer was mounting some tires for me and he commented how heavy my wheels were and how unsprung weight is bad. I used to read that same statement as a kid when I was racing Bultaco's (motocross) but did not know what it meant. I understand it now but since I'm not racing is it really that big a deal for HPDE? If so, how exactly does it manifest itself? Also where do I find four 18 x 10.5's that are not heavy?


Jerry

wamara

[QUOTE=RedHotBolt]I run 18 x 10.5's all the way around on my C6 at the track. I bought cheap (heavy) wheels to mount scuffs on. Last weekend Bobby Archer was mounting some tires for me and he commented how heavy my wheels were and how unsprung weight is bad. I used to read that same statement as a kid when I was racing Bultaco's (motocross) but did not know what it meant. I understand it now but since I'm not racing is it really that big a deal for HPDE?

Well, how important is it to you to perform/drive at your best? In a competition situation, obviously you won't be as competitive. By how much? Tough to say - so many variables. For simple fun high speed driving, probably not too important except you'll never get a feel for what your car can ultimately do, so it's a negative from a learning standpoint. Only you can decide how big a deal this is. I also assume there is an argument that the heavier wheels put more stress on components, etc., but doubt this is a concern for most casual events.

If so, how exactly does it manifest itself?

Your car will be slower - it saps more power to run those heavy wheels (not just the weight itself but the inertial effect of getting them rotating, slowing them, accelerating, etc.).

Also where do I find four 18 x 10.5's that are not heavy?

OEM Z06, CCW classics, Forgeline - how much do you want to spend?

Good luck with your decision.

AU N EGL

For doing HPDEs does it really matter?? No and Yes.

For competition one big way to gain HP is to reduce the cars weight. Some ppl say for every 100 lbs removed that is equivilent of 10 Extra HP . Each competition class has a minimal weight for car and driver which is measured after the race is completed. If the car is under that weight the car is disqualifed.

Wheel weight or unsprong weight is an other area. 1 lb of unsprung weight is like 7 lbs of normal. Reducing wheel weight. rotational weight, helps - Is this true ?? Who knows but I would not be supreised if there is some measure here. I know my back feels better with the lighter track wheels vs the normal street wheels when I change them.

The other challange is wheels that are too light, or cool looking thin spoke wheels. Are they strong enough to hold the latteral force of cornering?? I bet not.

On a related note there has been several shops that did dyno tests on cars with differnt size wheels and differnt weight wheels too. Bigger and heavry wheels produced less power then small and lighter wheels.

Going from a 18 to a 19" wheel even though the OD was about the same, the added wieght of the 19" reduces the outcome 5-7 HP.

So for racing and weight, yes it makes a difference. For HPDEs and a little track time and fun, no not really.

and I agree here "OEM Z06, CCW classics, Forgeline - how much do you want to spend?"

How much do you want to spend?? if you do decided to go this route stay away from Repoduction wheels as then tend to be heavy.

Falcon

I weighted some wheels I have in the garage. I have the AFS (aftermarket) wheels and some original Grand Sport wheels. The original Grand Sport wheel (9-1/2 x17) is 5 lbs lighter than an AFS (aftermarket Grand Sport aftermarket) 9-1/2 x 17. I'm sure the 11" are even heavier.

I'm sure some engineer on here can compute the rotational differences of this 5 additional #'s. I'd bet it is significant.

AU N EGL

Jody, with tires??

My CCW classic street rears with MICH PS were 51 lbs and the CCW classic tracks with Good Year, slicks was 39 lbs.

Falcon

Yea, with tires. The same tire (710's) was on each wheel, so the only weight difference would be the wheel.

I remember how much lighter the 710 was than the VictorRacer.

tcmc5

I had AFS 17x11's and they were 7lbs heavier than the OEM GS 11's.

MitchAlsup

The vanValkenburg book indicates that every pound you remove from rotating mass is equivalent to removing 4 pounds of nonrotating mass. I believe that the Carroll Smith books give a similar nmber.

AU N EGL

Thanks Mitch.

AN example of my street wheels at 51 lbs each and my track wheels at 39 lbs each is a 12 lbs savings or 12 x 4 = 48 lbs per wheel.

yellow01

Rotational energy is linearly related to Inertia.

It depends on the distribution of the mass, but rotational inertia (which you'll have to fight to speed up or slow down) increases linearly with mass and squared with radius (distance from centerline).

Simplifying the wheel to a hoop with no spokes (as in only the rim portion) increasing from 17" to 18" assuming no change in weight (mass) and assuming rolling diameter remains constant (so angular velocity, rpm, remains the same) there is a 12% increase in rotational intertia. Athough in reality the tire may be less massive since it would have to be lower profile to maintain OD so this would counteract this a bit.

Going from 40 to 45 lbs, keeping the same radius and ignoring the mass distribution for simplicity would increase a maximum of 12.5%.

These are sweeping oversimplifications.

Here's a good site (good reference table at the bottom).
http://library.thinkquest.org/16600/...linertia.shtml

Falcon

Thanks there, Yellow01!

See, I told y'all!

JiminVirginia

I run three different wheel/tire combos on my MINI S: OEM 17" wheels with runflats (winter), 17" Team Dynamics Pros with Kumho MXs (summer), and 16" SSR Competitions with Falken Azenis (for autocross/track). The OEM combo weighs 55 lbs, the SSR/Falken combo about 43 lbs, the TD/Kumho combo in the middle.

The difference is readily apparent: the car is so much nimbler with the lightweight setup it's amazing, and it's easier to stop, too. But the SSRs were over $300 apiece, too.

For my ZO6 I have CCWs and OEMs, and I'm not sure the weight is much different. Can't tell the difference on road or track really. But the OEMs are really pretty light--for OEMs.

RedHotBolt

Thanks for the feedback. Since I only do HPDE's it's not important to be the fastest but I think I may look into a set of OEM Z06 wheels and see how much difference it makes. It's not like I would have any trouble selling them if I did not want to keep them. Every time I see a set of rears they sell almost as fast as the add is placed.


Jerry

TexasTechZ06

I would lean twords whatever you can get that is forged. I am running heavy aftermarket street wheels that are cast and I am shopping for forged for HPDE weekends. I am doing it for the sake of stregnth and not so much for weight. I saw a car drop a cast wheel off the track right behind me (he was playing follow the leader with me in an eclipse instead of driving his own line). When he corrected he overcorrected went from one side of the track into the curb of the other side and his cast wheel exploded where I believe a forged wheel would have just bent. Chunks of the wheel caused additional damage to his car. I would rather replace a $500 wheel plus tire than a $200 wheel plus tire and a fender!!!

ghoffman

That is probably a fair assumption. As you combine that savings with a lighter weight 2 piece rotor you will improve it more. This is a factor in your tire choice as well, and since the tire is mostly at the outer radius. As far as unsprung weight goes, on a perfectly smooth track surface it would not matter that much but all tracks I have been on are not billiard table smooth so the unsprung weight is very important. The natural frequency of a simple mass-spring is defined by the square root of the spring rate over mass. So in the example above if you went from 45 pounds to 40 on the wheel it would reduce the total unsprung weight from around 80 pounds (wheel, tire, rotor, uprights, bearings) to 75 pounds. (Normally you would now divide by gravity to get mass but on a percentage basis it is the same) But anyway, 80/75**.5 = 1.03 so you are looking at about a 3% improvment in response over bumps. Repeat the above with -7 pounds for a Hoosier tire, -9.5 for 2 piece rotors/hats (on a C6Z this is very possible) -5 for a CCW Corsair wheel and it now looks like 80/58.5**.5 for a 17% improvement.

John Shiels

well that sums it up in detail Gary

Allthrottleandsomebottle

For HPDEs and learn how to drive schools........doesn't matter in my experience.
For Competition........big time, every oz. adds up
My dyno and time tests have show every pound you remove from rotating mass is equivalent to more like 2.5 lbs of nonrotating mass.
The book is always a good starting point
Go out and have fun with them

froggy47

Just to add a couple of data points, I own both the afs and oem Grand Sport rims.

Fronts about 4.5 lb difference, rear about 5.5 on a digital bathroom scale.

YMMV