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Discussion Starter · #1 ·
Was curious if I also will need to remove brake ducts??
Thanks
 

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The brake ducts are gone for sure.......
not 100% if you'll still get rubbing but I'm about to do a similiar setup and will follow this to find out myself.
 

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1. With normal offset they will stick out of the wheel well. You have to raise the rear ride height to prevent the upper fender lip from contacting the tire. Otherwise you will split and crack the fender.

2. You will need to remove the rear brake duct.

3. You may have inner fender liner contact without tubbing.

4. You will also have clearance problems with the inner rim shoulder and the end of the spring perch bracket. This may look OK in static condition but will make contact under hard acceleration and cornering. The steel bracket will shave the wheel rim shoulder....so if you see Aluminum shavings or "curly-q's" on the track your in trouble. This is more of a problem on 17" wheels but should be checked on 18" wheels as well. You may need more positive offset or a spacer but this compounds problem #1.
 

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Discussion Starter · #4 ·
Heard enough to go 305/30/18's all around.

Just doing an event that I can get some real cheap VRL used slicks for so wanted to try them out.

I was going to run 305/30's in the front anyway.

It's more of a comparison to see the difference between them and the Pilot Cups and my Toyo's.

Slightly different sizes but should pin down what I want to use in the long run.

Intersting though because I see so many guys running 335's in rear and I don't think they do anything besides take out ducts. Guess they discover these other issues the hard way.
 

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You can also buy offset spacers (use with the longer wheel studs) to kick the wheel out from the fender some more too. It's a pain installing the longer wheel studs, but worth it if you want to run that big of a tire.
 

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Silver Bullet said:
Heard enough to go 305/30/18's all around.
You will need to run with TC and AH turned off or these tire sizes will cause problems. For a 305/30-18 rear, you want a front tire closer to 275/30-18. You can put 315/30-18s on the back and 285/30-18s on the front, which is what I did.
 

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RocketSled said:
You will need to run with TC and AH turned off or these tire sizes will cause problems. For a 305/30-18 rear, you want a front tire closer to 275/30-18. You can put 315/30-18s on the back and 285/30-18s on the front, which is what I did.
Is what you mean that without the stagger, there will be problems with the AH system??

Well, I have a 285/30-18 and a 335/30-18 setup in the BFG Comp T/A. Will that affect the AH? There is a definate stagger to the diameter of the tires, I haven't measured the 2 tire heights but guessing I'd say about an inch.
 

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If you click my Sig, and go to the bottom of the page that opens up, you'll see a link to the tire computation spreadsheed. Download it, enter your tire sizes, and it'll tell you.

If I remember, 335s will work with 285s. But 335s will rub.
 

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Thanks for your help!! :thumb: :cheers:
 

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What type of AH issues do you expect from the size differences. I have run the same size 25.5 all around on the 18's at 3 different tracks and have not noticed any odd stuff in comp mode. Once in awhile you get the AH message on the IDC saying it's working. But nothing spooky or odd with the way it reacts so far. Just wondering what to be on the look out for. The rake is set to compensate for the same size tires.
 

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With 315/30-18s all around, the rear tire is proportionally smaller by 97.36% and the front is larger by 0.37%. the delta between the original-tire and new-tire front/rear ratio is about 3%.

The rear tire actually turns faster by 3%, compared to the front tires, vs. how fast it turns with the OEM tire sizes. The AH and TC computers constantly monitor the speed of each wheel. Loss of traction and/or the conditions that could produce a skid, is indicated when the back tires spin faster than the fronts. The both the AH and TC have some amount of "margin" built in. This is the amount of slip they're willing to allow before they interfere.

Say the computers allow 5% of slip before activating either TC, AH, or both (I don't know how much the computers actually do allow, but based on experience this seems like a decent guess). Your off-ratio tire sizes consume 3% of the margin. So compared to the original OEM tire ratio, where the TC wouldn't activate until it thought the rear tires were going 5% faster than the fronts, with your ratio the TC cuts in when the rear tires are only going 2% faster.
 
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