Ok so couple things that I would love some feedback on.

use case for my 1994 m6 base is daily driver and ability to do about 6 road course and autox events per year.
This is getting an ls3 over the winter and the engine dynos at 528 at the crank with 526ft lb so traction in the rear is a consideration.

im doing a bunch of other supporting mods, but for the sake of this thread just looking to double check my logic.

and I definitely have spend prob 5 hours of reading threads on these topics, of course I may have missed some info…

1. Wheels - I’d like to do 12” wide rears and 10.5” fronts. Offset for rear would be +50
fronts +56

Iknow the rears will stick out a bit which is ok because I’m going to install the gs flairs if needed. Although I’m not totally sure how far that will be. 1/2” maybe?

So questions are these

1. will this setup be a poor setup for the track (no drag racing) I’ve seen the square setup cars for track but I dont care about rotating Tires or tire cost. More worried about causing the car to push??

2. how do my offset numbers look?

3. should I do 11” fronts OR drop the rears down to 11”? And why.

thank you all!
love this community.

 

With your post 88 car, I think 50 mm offset in the rear may not be enough for 12” width. 50 mm on an 11” rim is max for for having no tire rub. GS flares will contain the tread width of the tire but will not keep you from rubbing the original body. The issue is tire clearance. These cars will compress enough, especially when racing, to cause rub which will destroy your wheel wells. If you must have 12” wheel width I’d go the max offset you can that still provides clearance with the inside, or stick with 11” width. All the above is especially true if your car is lowered. I think you’d be ok with 50-56 on the front with a 10.5-11” width. I don’t think a 1” or so smaller width tire on the front will cause you to push. However, I have heard 11” width square set up with 315’s on all 4 works well with road racing. I would imagine you are intending to run 315’s and 335’s with those width rims you specified?

 

Another thing I’d suggest is to PM MatthewMiller if he doesn’t reply here. He had plenty of experience road racing a C4 and knows 100% what is a go/ no go based on what you want.

 

Awesome feedback thank you.
I was worried about the 12” clearance in the rear.

im ok with the 11” if thats max before causing major issues.

I had read an article here (was a red corvette) that had 12” rears so that’s what made me think it could work but I’m sure they did a bunch of mods to make it happen.

 

I don’t necessarily think that the 12’s are a deal breaker. I wouldn’t dismiss the idea just yet. Especially if you’re going with wheels that you can choose the offset (or custom spec). We just need to get you dialed in before you take the wallet out.

 

I'm here! C4 Steck said most of what I would have said. I consider 12" wheels on a non-ZR1 C4 with stock body work a no-go. I am well familiar with running 11" wheels with 50mm offset and 315mm tires (25.7" diameter in all cases) on front and rear of an 88+ C4:


These have 7.96" backspacing, and that's about the limit on inside clearance with a 315 tire. So first, you'd need less offset to make 12" wheels clear on the inside: 37mm to get the same backspacing as my 11" et50 wheels. But then the problem is that the entire extra inch of wheel width will poke out of the fenders at the top. Like Steck said, the GS flares just stick on the existing fenders and don't make any extra clearance, so you can run into problems with the tires hitting the fender edges. Not good. That said, it may be possible to make 12s work with 50mm offset and retain enough clearance if you stick with 315mm tires that are only 25.7" tall (315/35/17 or 315/30/18). I just haven't tried it, so you'd probably be on your own. All of this goes for both front and rear, btw. Don't get mislead by people running 12s on ZR1s: they have a lot more fender well width and can easily accommodate 12s. Also, keep in mind that my car in that pic above has about -3 degrees front camber and -2.5 in back. That helps the tops of the tires tuck inside the fenders better. It also improves grip a lot, but you can't get that much camber at either end with stock parts. We can talk more about that if you want.

There is no good reason to go with 10.5" wheels up front. Get 11s with 50mm offset (or match whatever you're running in back). It can only help performance. Don't worry about understeer/oversteer yet. You can easily tune with that different rear sway bars, which are cheap to get in various OE sizes and easy to swap out.

A lot of wheel recommendations are going to depend on what tires you need to run, because most track/autocross-oriented tires only come in a smattering of sizes. I see you live in Wisconsin, so if you really daily this car you are absolutely going to need legit winter tires. All-seasons won't really cut in the snow/ice and they won't survive any real track use. They'll just be slow for autocross use. So I'm ***-uming you plan to have a set of street/winter tires and a set of autocross/track tires? How serious do you want to be about winning autocross events? Are your track days going to be time trial competitions with only a few hard laps, or HPDE sessions with 20- or 30-minute sessions of continuous hard lapping? There are only two top-tier autocross tires these days, and they will work okay for a track time trial if you cool them down between serious attempts, but they will overheat and get greasy for long lapping sessions. Also, unless you are planning to run Hoosiers (i.e. purple crack), then we'll be talking about 18x11 or 18x12 wheels and either 295/30/18 or 315/30/18 tire sizes.

 

And there ya have it

 

Matthewmiller
thank you I actually started to read quite a few of your posts and your a wealth of experience.

Im going to stick with the 11” + 50 on all4 corners as I don’t want to make this more of a headache than it needs to be so thank you for that guidance.

as far a tyres go I won’t be daily driving very often and certainly not during the winter. When I was tracking my gt4 I ran SC3 and they were decent and good on the road so I’m probably be going with them but wouldnt mind trying something different for the first set.

im going to pm you if thats ok as I have some other questions that your likely the authority on. Primarily suspension related stuff

 

My advice on tire choice is to pick whether you want to emphasize autocross competition results or track day consistency and longevity. If one is more important to you than the other, then your tire choice should reflect that. For example, I'm pretty familiar with the SC3 because it came stock on my 1LE Camaro and I have autocrossed and tracked it extensively. It's a step or two off the fastest among UTQG 200+ competition tires, but it handles well and takes heat very well. It didn't actually last super long for autocross use for some reason, but it seems to not need lots of negative camber work well and wear evenly. It's fine as a street tire in the dry, but it's not very good in the wet. OTOH, it's an outstanding track-day tire. It gives up a little single-lap time to the autocross-focused tires; but it never overheats, it gives consistent grip and handling characteristics, and it wears like iron. It's also cheap. They don't make a 315/30/18 in that tire, but their 275/35/18 is recommended for up to an 11" rim width and would be the proper 25.6" diameter. Whereas the 315/30 would be slightly pinched on an 18x11 (like the BFG Rivals in my pic above), these would be slightly stretched. Another great track tire is the Continental ExtremeContact Force, which does come in a 315/30/18. I don't have any first-hand experience with this tire, but everyone who does says they have the same strengths as the SC3. They might be a bit faster, but they're also more expensive.

On the flipside, if you want to emphasize autocross results while still retaining some usability on track, you should strongly consider the Bridgestone RE-71RS. It's faster cold: it's one of the two fastest 200tw tires right now. It will slow a bit on track as it gets hot, but the Grassroots test showed it stabilizing into a still-fast tire for multiple laps. They seem to wear well for an autocross tire, but they won't last as long on track as the SC3 or EC Force. They do like real camber, and they seem to love being stretched. They make a 295/35/18 that you could use and that is 26.1" tall, or their 275/35/18 will be great on 18x11s. I run these on my Camaro for autocross use, and they are fast. The other fast 200tw tire is the Yokohama A052. This tire is generally equal to the RE-71RS, but it is less tolerant of heat and it demands plenty of negative camber to last at all. It gets greasy on track with heat, and even on autocross courses on a warm day they really need to be sprayed with water between runs. They also wear out faster, and they're more expensive. Although they like to be stretched, the 315/30/18 will work fine on 18x11s. Right after I sold my C4 in the picture, the new owner put these on those same wheels. I drove it a bit and they felt good. Since C4s are somewhat light, they worked pretty well. But I still think the Bridgestones are better for your needs since you want to track the car, too.

Right in the middle of the track tires and the autocross tires discussed above is the Falken RT660. These are still a bit faster for autocross use than the SC3 or EC Force, but a half-step behind the RE-71RS and A052. They tolerate heat a bit better than even the Bridgestone, and probably last longer on track. They do tend to "work harden" over lots of heat cycles. They will wake up and get fast again when they get hot, but toward the end of their tread life they are faster on the 5th and 6th autocross runs than the first four runs. OTOH, they'll be happy on track sessions until the end, since you'll have plenty of heat in them after your out lap. Again, my red C4 got a set of these from the current owner (after the Yoks) and it has seen a combo of autocross and track days on them and they work well. These don't seem to need to be stretched as much as the previous two tires, and they make a 315/30/18 that would be great (that's what's on my old car now).

I got it, and I'm happy to kick those topics around. Just due to time constraints tonight, I'll need to try to get to those questions tomorrow. Stay tuned.

 

I would think for 12" wheels add another 13mm of offset. so 63mm. Am I wrong? Mine are 315 on 11s and they stick out past the wheel wells but ive had no problems yet. Its stock diameter though and factory ride height. Thats important.

 

what camber are you running front and back?

was hoping 315 would be right at the edge of the stock well (not the end of the world if they stick out a bit.)

 

Yes, this is wrong. I already went over this in post #6. A 88+ C4 is typically limited about 7.96" of backspacing (maybe fudge it to 8"), which is what you get with an 11" wheel and 50mm offset. A 12" wheel with the same offset is going to have 1/2" more backspacing already, so you're at 8.5" backspacing now at 50mm offset. Adding yet another 13mm (1/2") of offset gives you 9" of backspacing, which is at least an inch too much in most cases. You actually need 13mm less offset with 12" wheels, which means that entire extra inch of wheel width is going to stick out past the fender edges (i.e., add an inch of wheel sticking out past what you see in the pic I posted, keeping in mind that I had way more negative camber in that pic than most street-only cars will ever run). That's way too much poke to look good, but more importantly you risk having the fender edge cut into the tire when the suspension compresses.

As I also wrote in post #6, using a 315 on the 12" wheel might give you a little extra margin on the inside clearance, because it's the corner of the tire that seems to hit things first rather than the wheel. It's possible that a 315/30/18 (or 295/35/18 Bridgestone RE71RS) might clear everything using an 18x12 with 50mm offset. I know that for a max-effort track or autocross build where wheel and tire width is unlimited, in today's world of competition tires I would make the effort to find out; but for a street car I wouldn't bother, and instead I'd just stick with tried and true 11" rim width and 50mm offset.

 

Heres some photos of my dad’s otherwise stock car with 17x11’s and 315’s. Just enough poke to give it the on steroids look without being too much. 50 mm offset

 

Looks great!