LT1*C4

I'm hoping this is the right forum to post in, but I'm assuming since the question involves 2 areas in which Autocross/Road racing fans are always trying to improve upon, this would be the best place to ask.

I'm currently running ZR1 style (GS offset) wheels on my '92. The wheel and tire size are as follows:

Wheel/Tire dimensions:

17x9.5 front / Kumho Ecsta XS 275/40R17
17x11 rear / Kumho Ecsa XS 315/35R17

I've always loved the look of ZR1 wheels on these cars but my only complaint is that this wheel/tire combination is really heavy. I didn't know just how heavy it was until I actually weighed everything while recently doing some suspension upgrades to my car.



The weights are as follows:

17x9.5 (front) - 28lbs (x2)
17x11 (rear) - 30lbs (x2)

275/40R17 - 27.4lbs (x2)
315/35R17 - 32.4lbs (x2)

The grand total of this combination is aprox. 236 lbs !!

116 lbs worth of wheels and 120 lbs of tire!!

That's a heck of a lot weight to be pushing around when you think about it. On the bright side though, there's a heck of a lot of rubber contacting the ground, which goes a long way towards enhancing the car's maximum grip.....


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Here's my dilemma which I'm hoping some of you who road race/autocross your cars can give me opinion on:

I've recently run into a pair of C5 rims for sale. Not quite as flashy as the chrome ZR1's I'm running now, but still a nice looking rim that works well with the lines of the c4. They should bolt right up without the use of spacers, seeing as how my Vette is a later model (88-96) which as far as I know, can run C5 offsets without issue.

I'm interested in these wheels because of their weight savings. These aren't just run-of-the-mill C5 aluminium wheels, but the low production Magnesium (Speedline) rims that were a fairly expensive option on the C5's (I'm assuming their price is what made them so unpopular).



The following are the wheel measurements/weights of the Speedline:

17x8.5 front - 17 lbs (x2)
18x9.5 rear - 18.9 lbs (x2)

Grand Total: 71.8 lbs

That's 44.2 lbs LESS rotating mass than the ZR1's I'm running.

Unfortunately (and here's the dilemma) - since the wheels are narrower front and back, the largest tire I can safely run is a 255 in the front and a 285 in the rear. That's a pretty big reduction in contact patch and I'm not sure how much that will affect the amount of grip the car will have and whether or not it's worth the weight savings.

The tires I've been looking into are Pirelli PZero's, but even though they're narrower than what I'm running now, believe it or not, they actually weigh about the same as the Kumho's I'm running. Don't ask me how, but that's what Pirelli's website states so there's isn't going to be any weight savings as far as tires are concerned.


So here's the question. Given the choice between, narrower wheels/tires that offer a substantial weight savings, or a much wider wheel/tire combo, that put more rubber to the ground but weigh substantially more, which route would you recommend?

Should I just stick with what I'm running now or are the lighter Magnesium's worth the expense?

Any info would be appreciated. Thanks in advance.

drivinhard

What is the total weight of your car?

have you already bought the mag wheels?

If not, look at the C5Z OEM wheels as something in between as far as width and weight. They will likely be cheaper than the mag wheels anyway.

They'll be 1" wider than the mag wheels, and only slightly heavier. 295/18 Hoosiers on 18x10.5" OEM are ~44 lb.

44 lbs is a huge amount of weight for the suspension (unsprung) and driveline (rotational).

CHJ In Virginia

It really would depend on what your planned usage of the car is going to be. If it is a weekend or daily driver and you might occasionally run a twisty back road, I would stay with your current combination. The GS wheel and tire combo will provide all the traction / performance you can legally use. The weight difference will not be noticable in a street driving situation. On the other hand, if you frequently track the car and are running time trials or AutoX, weight will make a big difference. The C5 mag wheels are light, but just too narrow for max tire sizing and that is not a good trade off. I would invest in a true light weight set of wheels and sticky tires as a SECOND set and swap for track events. Best of both worlds.

LT1*C4

Last time the car was on a scale, it weighed roughly 3240 with a 100% full tank of gas and a bunch of crap in the back hatch (probably about 15-20 lbs worth). Since then, I've shed some weight here and there on the car, but I'd imagine no more than about 20-25lbs reduction at the most since then.

The car is primarily a street car, which will be Autocrossed/road raced whenever I get the chance, but it's far from a full race car, just for the record. It still has a full interior with all the creature comforts it came with and will most likely remain that way.

I looked into the C5z wheels, but think they look much better on an actual C5 than on a C4.

The spokes look a bit too "thin" on a C4 IMO. I like the look of the mag's much better as their spokes have a lot more "meat" on them and flow better with the lines of the C4.

I haven't bought the mag's yet, no. I first wanted to here from those who are much more knowledgeable then I am, if they considered the weight savings of the magnesium wheels would "outweigh" the extra grip of offered by the wider (and much heavier) Zr1 wheel/tire combo.

AU N EGL

For every pound or rotting mass remove, is ~ 6-6.5 lbs of static mass removed.

Saving wheel weight is very important.

RX-Ben

moments of inertia matter more than lbs re rotating mass. Heavy tires are a killer.

Jason

For a street car, I'd go with what looked best.

lawyer

I run original GS 17 x 11 rims that weighs 24 and 27 respectively. Therefore all GS rims do not weigh the same so look around for a lite weigh set. The more mass you take off the wheel tire the faster you can run

lawyer

Meant to say I run Original GS 17 x 11 rims that weigh 22lbs but also have aftermarket 17 x 11 rims that weigh 24 and 27 lbs respectibely

LT1*C4

Thanks to all who've responded so far.

Well, I've come to the conclusion that while the mag's are definitely a nice weight savings, the extra grip offered by the wider wheels/tires I'm currently running probably offsets their added weight.

The mag's just aren't wide enough and putting much narrow tires feels like a downgrade rather than an upgrade. - So I've decided to pass on them.

I'm actually looking into a set of Fikse FM5's in an 18x10 / 18x11 combo to replace my ZR1's now. Amazing how I just went from looking at a set of $700 used wheels, to a a set of wheels costing 5x as much within the span of a few hours.

At least I'll be able to still shave off 30+ pounds of rotating mass and still have wheels wide enough to stick some decent sized tires. Hey, you gotta pay if you want to play right?

Any recommendations as to which tire size's to go with, would be appreciated.

drivinhard

IMHO unless you've tested it and gotten some data, I'm not sure you can say that. For a 32xx car, you can get "enough" tire on a 8.5 and 9.5" wheel.

RacePro Engineering

First of all, you asking a "racing" type question about a car that sees mainly street duty. And while it is a very good question, and you have some knowledge of the rotating mass aspects, your "correct" answer will be either right for the track, or right for the street, but not both. Therefore the suggestion to maintain 2 sets of wheels and tires is a good one.

The mechanical grip of a tire has to do with its contact patch, sidewall stiffness, and the softness of its "compound". In general, a narrow slick offers much more grip than a wide DOT tire. (The slick should also weigh a lot less.) So, we wouldn't get too hung up over tire width until you are using racing rubber.

There is much to be gained from lower unsprung weight and less rotational mass ON THE TRACK. For spirited street driving, how will you measure the difference? Good racing wheels are light weight, expensive, and relatively fragile. They do not like things like pot holes, railroad tracks, and curbs that were closer than you thought. They are not fond of bad weather.

If we were forced to pick only one, between lower unsprung weight, less rotational mass, and real grip, we would have to favor grip, because better grip helps in all three aspects of track driving - acceleration, decelleration, and cornering.

Ed LoPresti

froggy47

Shameless plug.



http://forums.corvetteforum.com/c4-p...port-rims.html

Kubs

To be honest, for a street car and any sort of autocross or track days a beginner is going to do, you will not notice a difference in wheel weight, or a difference in grip from a 285 or 315. In my opinion just run what is on the car now and enjoy it.

Kubs

Tire COMPOUND will be more noticeable than the difference between a 285 street tire and a 315 street tire.

LT1*C4

I ordered some new wheels yesterday afternoon. Basically they're the same width's as what I'm running now, but taller (went with 18's all around) and of coarse, much lighter than either the ZR1 replica's I'm currently running, OR the original A-molds.

Now all I have to do is figure out what size tires to go with. I'm pretty sure I'll be going with 275/35/18's in the front, but I'm not sure I can still run 315's out back. I'm afraid they'd probably rub so a 295/35/18 may work better for my application.

Any C4's owners out there running a 315 on an 18x11 rim? Do they rub at all, or should I stick with a 295 size?

Thanks again.

Kubs

If you go with a 315/30/18 it will be the same height and width as the 17" tire.

ScaryFast

Depending on your backspacing they will rub. If you purchased the same backspacing as a ZR1 or GS 11' wheel they will rub the inner fender and the trailing arms under load.

That said, I've been running that set up for years. I simply call it "self machining".

varkwso

It takes a pretty good wheel to be lighter then an A-Mold GS wheel...

froggy47