ExMB

Looks like an update was posted over on planet-9 with pics.

19" BBS CH

EDIT:
Looks like the update and pics were deleted

GrantG

Yes, there are 18" tires in the same width and sidewall as stock, giving 2" smaller overall diameter. Aside from improving the gearing, it could introduce some handling problems with the roll center being moved from the optimum geometry based on the stock 20's (assuming it clears brakes). And of course ride height will be reduced an inch...

ExMB

Not correct.
Ride height also depends with the sidewall height of the tire. Its a combo of wheel diameter and tire sizing.

jphughan

That's true, but choosing tires with fat sidewalls to compensate for the ride height lost due to diameter reduction also makes for poor handling -- plus many performance tires aren't available in high-aspect sizes anyway.

ExMB

True, but if you reread what I wrote you'll see I never advocated matching OD, just that there is more to it than just wheels which in this case was the blanket statement without modifier.

DeerHunter

If you're talking about standard street tires, what you say is generally true. However, IF track tires are available in the correct size (meaning taller sidewalls in order to keep the OEM diameter), they will have ultra-stiff sidewalls that won't make the handling all mushy. As an extreme example, you don't see F1 cars rolling over on their sidewalls, even taking a corner at 3gs or more.

GrantG

In my post you'll see that I was referring to the existence of tires with 18" inside diameters that have the same width and sidewalls as the stock 20" tires which by definition reduces overall outside diameter by 2" and ride height by 1".

ExMB

OOPS. Didn't register at time of posting.

Quikag

I'm going to talk to Finspeed when my GT4 comes in. They make awesome wheels and if they can make a set of 18" wheels that clear my carbon ceramics on my ZR1, they can definitely do it for the GT4. I have two sets of 18" Finspeed wheels for my ZR1 and will likely get a custom set for my GT4 as well.

GrantG

Looks like nice quality forged product, around $800 per wheel (plus options) in 18's (assuming they fit).
http://finspeed.com/performance-wheels/

This option looks particularly interesting:

"Knurled Bead Seat

The Textured Bead Seat option increases friction in the contact areas of the tire and wheel, and limits tire slippage under heavy braking and acceleration. This option is highly recommended for track use and is $40 per wheel."

ExMB

Counterpoint

How will the downforce be affected by going to 18" wheels? We have been told that PCCB equipped vehicle have less of it due to their sizing compared to iron brakes.

GrantG

I think the bigger concern is how the suspension geometry might be mis-matched with the altered roll center of smaller tires (assuming you stay with same widths and sidewalls) - could be a recipe for bump-steer...

vantage

Wouldn't small diameter wheels Lower the car without affecting geometry?

GrantG

I am told by a reliable source that it affects the roll center. I don't claim to understand the details...

85Gold

Like to see the reasoning behind that??

Peter