JG Pasterjak
JG Pasterjak Production/Art Director
2/19/18 9:28 a.m.
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If you hang around a lot of autocrosses—and we do—you may notice a lot of fifth-generation Corvette Z06s running in the SCCA’s A Street class with mismatched wheels. There’s a simple reason for this peculiar condition, and it comes down to a rules allowance and tire availability among the top choices of rubber.

A stock C5 Z06 comes with 17 x 9.5-inch wheels in the front with a 54mm offset and 18 x 10.5-inch wheels in the rear with a 58mm offset. Unfortunately, competitors wishing to run BFGoodrich Rival S tires (which is most assuredly us) find that BFG only makes a Rival S in a 245mm width for a 17-inch wheel. The 315mm-wide BFG Rival S is available for the 18-inch diameter rears, however. Fortunately, SCCA Street Class rules allow for a +/-1 wheel fitment. That means competitors are allowed to up those 17-inch fronts to 18-inch diameter wheels, taking advantage of the 275mm-wide BFGs offered in that 18-inch size.

The tricky part is there’s no such thing as an 18 x 9.5-inch wheel in the Z06's unique split five-spoke design. The rear wheels off of the base Corvette are the proper 18 x 9.5-inch size, and feature a 57mm offset that can easily be moved out to the car's designed front offset by a 3mm spacer. So there’s an easy source for legal wheels, just not one that matches the original design.

We used a pair of reproduction wheels from OE Wheels in a five-spoke design that at least looked somewhat similar to the Z06 wheels. At $125 each, including shipping, we weren’t going to complain.

And while the 3mm spacer still allows the use of the stock lug studs, we played it safe with a set of ARP 3-inch studs which give us all the extra threads we could ever want.

Now our BFGoodrich tires are happy on the proper size wheels, and we’re legal for A Street.

Read the rest of the story

deaconblue
deaconblue
3/3/18 7:04 p.m.

Guys keep in mind that the true factory takeoff C5 corvette base 18x9.5" rear rims, both the 5-spoke thin or the 5-spoke magnesium designs have a 65mm offset.  So you would need 10mm or 12mm hubcentirc wheel spacers to run those on the front of a C5 plus most definitely some of those longer ARP wheel studs.

Also the OE Wheel site lists the offset for the reproduction 18x9.5" rims, like the ones you are using #CV05-D18095-5475-54BM as having a 54mm offset - not 57mm as you noted.  Maybe somethings have changes thought the years?

https://www.oewheelsllc.com/CV05-D18095-deep-dish-black-rims-fit-Corvette-C5-style

JG Pasterjak
JG Pasterjak Production/Art Director
3/3/18 9:10 p.m.
deaconblue said:

Guys keep in mind that the true factory takeoff C5 corvette base 18x9.5" rear rims, both the 5-spoke thin or the 5-spoke magnesium designs have a 65mm offset.  So you would need 10mm or 12mm hubcentirc wheel spacers to run those on the front of a C5 plus most definitely some of those longer ARP wheel studs.

Also the OE Wheel site lists the offset for the reproduction 18x9.5" rims, like the ones you are using #CV05-D18095-5475-54BM as having a 54mm offset - not 57mm as you noted.  Maybe somethings have changes thought the years?

https://www.oewheelsllc.com/CV05-D18095-deep-dish-black-rims-fit-Corvette-C5-style

That's not the first time I've heard of the 65mm offset for the base rears, but I've never been able to find any independent confirmation of it. This is the chart we've been using, and it seems to match up with what we've observed in the wild: https://www.corvettecentral.com/wheel-offset

As for the OE Wheels repros offset being off, well that just an oversight on my part. I guess I assumed that they were repros of rears, so they carried the standard rear offset, but they actually have proper offset from the getgo. Even with the 3mm spacers we're stil legal, though (SCCA rules allow up to 7mm of offset variation from stock). Good catch on that one. Dumb mistake on my part.

 

turtl631
turtl631 HalfDork
3/4/18 8:18 a.m.

Hmm.  I'm planning to run similar sizing on my M3, 18x9.5 front and 18x11 rear.  Apparently the Azenis RT615K in the same 275/315 sizing is a little narrower so it will be more likely to fit in back.  Hope so - that 315 Rival looks pretty pinched on the 18x10.5.  Not many options for really wide 18s in a sticky street tire!

93gsxturbo
93gsxturbo Dork
3/4/18 8:19 a.m.

The magnesium wheels had more offset and stuck in more, approx 1/4" per side as I recall.

The magnesium wheels were used on the export version, blah blah track width narrow roads blah blah. 

 

I had 11" wide rear wheels on my C5 and they fit nice with 305 Nittos.  

deaconblue
deaconblue New Reader
3/4/18 10:06 a.m.
JG Pasterjak said:
deaconblue said:

Guys keep in mind that the true factory takeoff C5 corvette base 18x9.5" rear rims, both the 5-spoke thin or the 5-spoke magnesium designs have a 65mm offset.  So you would need 10mm or 12mm hubcentirc wheel spacers to run those on the front of a C5 plus most definitely some of those longer ARP wheel studs.

Also the OE Wheel site lists the offset for the reproduction 18x9.5" rims, like the ones you are using #CV05-D18095-5475-54BM as having a 54mm offset - not 57mm as you noted.  Maybe somethings have changes thought the years?

https://www.oewheelsllc.com/CV05-D18095-deep-dish-black-rims-fit-Corvette-C5-style

That's not the first time I've heard of the 65mm offset for the base rears, but I've never been able to find any independent confirmation of it. This is the chart we've been using, and it seems to match up with what we've observed in the wild: https://www.corvettecentral.com/wheel-offset

As for the OE Wheels repros offset being off, well that just an oversight on my part. I guess I assumed that they were repros of rears, so they carried the standard rear offset, but they actually have proper offset from the getgo. Even with the 3mm spacers we're stil legal, though (SCCA rules allow up to 7mm of offset variation from stock). Good catch on that one. Dumb mistake on my part.

 

Best way to confirm is to look at the photos on the eBay posting that have the wheels for sale.  Some folks have higher resolution photos of the back of the wheels that can be enlarger and you can clearly see the OE Speedline name, the GM p/n and the size and offset specs cast into or etched into the back side of the spokes. 

deaconblue
deaconblue New Reader
3/4/18 10:10 a.m.
turtl631 said:

Hmm.  I'm planning to run similar sizing on my M3, 18x9.5 front and 18x11 rear.  Apparently the Azenis RT615K in the same 275/315 sizing is a little narrower so it will be more likely to fit in back.  Hope so - that 315 Rival looks pretty pinched on the 18x10.5.  Not many options for really wide 18s in a sticky street tire!

The RT615K is the spec tire for the new Spec Corvette class.  The Rival S tends to be one of the "tires of choice" for the top autocross finishers.

StuntmanMike
StuntmanMike New Reader
4/6/18 11:07 a.m.

I'm pretty sure OEwheels offsets are off too. I don't have any numbers handy but I have run several sets of their wheels and have always been happy with the look and performance of them, I think there is a little more offset than they rate them at for certain ones. Also like it was said, the 18x9.5 rear wagon wheels are dirt cheap. I picked up one set for 100 bucks and another set for a little more than that, then needed a replacement that I got for around 50. I wish they were the 10.5 z06 wheels but for the price and tire availability, can't beat em! Painting them is a must though, they are ugly!

HomeyDClown
HomeyDClown
6/21/18 4:32 p.m.

New to Autocross and the Forum. Thanks for all the info and videos. I bought my wife a 2004 C5 Z06 CE for autocross and ordered the BFG Rival S in 275/35/18 fronts and 315/30/18 rears. Is this the same as you were running in AS? If so were there any issues with the front diameter larger than rear? Any first hand insight would be greatly appreciated. 

JG Pasterjak
JG Pasterjak Production/Art Director
6/21/18 4:56 p.m.
HomeyDClown said:

New to Autocross and the Forum. Thanks for all the info and videos. I bought my wife a 2004 C5 Z06 CE for autocross and ordered the BFG Rival S in 275/35/18 fronts and 315/30/18 rears. Is this the same as you were running in AS? If so were there any issues with the front diameter larger than rear? Any first hand insight would be greatly appreciated. 

That's the preferred setup for AS with BFGs and what we ran in AS. No weirdness to report with the diameter change with regards to ABS or anything like that. 

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