mityaz

Hey guys,

I was talking to a few manufacturers and ended up ordering Signature wheels in 18x11.5 and 19x13. My plan was to run something like r888r in 315/30/18 and 345/30/19 and maybe Hoosier R7s in the same size (if i ever wanted to get adventurous). What do you guys this about those wheel sizes? The gentleman from the company assured me that 19x13 rear is not too wide and will be ideal for the 345 rear. 12'' width is a bit too small and causes the "bulge" that some say doesn't give the tire a precision feel.

My main concern is that i don't want to limit myself with the tire choices with these wheels.

The "track" refers to advanced HPDE on North East tracks like Watkins Glen, Poconos, NJMP, Limerock...

What do you guys think? @Poor-sha ? lol

RichieRichZ06

I think you will be OK with the Hoosiers, but you might run into Ice mode issues with the 888r sizes. The front is only 25.5" diameter, but the rear is 27.2". That's more than a 1/2" over the stock front to rear difference in diameter. The ZR1 seems to be much more sensitive to tire size changes that result in ice mode type of issues. We've been through this on another car that was only trying to run a 1/4" more difference than stock. It was fine on road course, but had all kinds of braking issues on auto cross.

mityaz

So according to my friends and the guy who I order thru, the C7 corvettes should run fine with that discrepancy of the wheel size.

Stock ratio of rear-to-front diameter is 1.031 (3.1% difference in diameter or circumference). The ratio with the 315/30/18 and 345/30/19 r888r tires would be 1.067 (6.7%). The question is what is that percentage that triggers the traction control issues. And how about adjusting it? (in my jeep i can adjust for the tire size thru superchip programmer so that the speedo is reading correctly).

RichieRichZ06

The guy selling you the wheels and tires will always tell you it "should" work.

It might be fine or it might have the same issues that we had on a 2019 ZR1 running auto cross with tires that have less of a difference in diameter. The car nearly hit the K rail twice due to ice mode when hitting the brakes at the end of a straight section of the course.

I don't know if there is a specific difference that starts to **** the car off, but the stickier the tire and the larger the difference in diameter, the more likely it will happen. The earlier C7s don't seem to mind as much, but we've had it happen on a couple different 2018 and 2019's. I assume there is a difference in the EBCM and programming because the part numbers for 2018+ are different than the earlier cars.

mityaz

The price of the wheel is the same, so the guy is not basing his suggestion on that. He is genuinely trying to help.

Is the EBCM unit the same for all 2018+ C7s including ZR1s? Maybe people with other 2018+ C7s can chime in too

RichieRichZ06

I was mostly kidding about the wheel sales thing.

Yes, all 2018+ models have the same EBCM (including the ZR1), but the programming for each suspension/brake package is likely different.

Poor-sha

It sounds like you've already ordered so is there even an opportunity to change them? I run 18x11 front and 18x12 rears because it gives the most flexibility for proper diameter track tires without the issue of the wheel contacting the UCA - mainly that means Pirelli or Hoosiers. 18s are also what you will find available for most track scrubs since that is what most real race cars run. I don't understand the comment about the rear tire bulge causing a lack of precision feel.

mityaz

can you give me the specific tires that you run on those 18'' wheels? I want to look up the diameter ratios. I wouldn't mind running an 18'' rear but the r888r toyos don't come in the 345 size.

I could probably still change the order as it was only placed yesterday.

The comment about the bulge is that if the sidewall is not stretched then the contact patch of the tire can have lateral movement in relation to the wheel (kind of like the rubber around a subwoofer that allows the diaphragm to move in and out).

Poor-sha

In Pirellis it's a 305/645-18 or 305/660-18 front and a 315/680-18 rear. I'd have to go lookup the Hoosier sizes on tire rack but I mostly run Pirelli on the 18s.

mityaz

So with those choices you were running a 1.051 ratio and a 1.02 ratio respectively. Still not quite the stock 1.03 but a lot closed than my 1.067 target with r888r

Poor-sha

I moved to the 645 front because I could see in the wheelspeed data that the fronts were slightly off with the 660 front. With 645/680 the wheelspeed data all lines up in a straight line.

mityaz

well, i can't say that it makes sense, as the 1.05 ratio should be farther from the factory spec than the 1.02 ratio.

I wonder if the car learn how the wheels spin in the "average" driving conditions and the ratio sort of "updates" based on overall driving. I'm sure the car can realize that it's not constantly slipping all over the place.

Poor-sha

More likely the fact that the printed tire sizes aren't exactly the same from brand to brand.

Painrace

I ran CCW 18 X 11 on the front and 18 X 13 on the rear of my 2016 Z06 with A7 Hoosier 315s up front and 345s in the rear. Very neutral set up.

Jim