BigBlockC4

I'm looking to run these on 18X13 rear and 18X12.5 fronts. Is anyone running this size tire in the rear or front or both? I'm looking for proper backspacing and offset. Car will be used for road racing, not autocross


Thanks.

B Stead

The setup can be figured out for the static ride height condition:

Assume that the current tires are on the correct spec wheel width. Then just measure the inside clearance from the tire. Next, assume that the wider tires are on their spec wheel widths. So then just figure on one-half the difference in tire section width and the tire section widths can be found in the tire specs. Finally, if the result is such that more inside clearance is needed then that's the amount of additional wheel offset that is needed. In other words, the OEM wheel offset plus the additional required amount.

As for outside clearance there are two choices. The first is fender flares with OEM bodywork cut-away under the fender flares. The second choice is to just stick the tires out and control suspension movement with stiffer springs. Of course, sticking the tires out usually works best when the tire diameter is less than OEM
.

BigBlockC4

I'll be doing flares for sure, but yeah my suspension is already coilovers and will be stiff as hell.

Solofast

C4's like a square setup. Going bigger on the back is a waste because you can't increase the rear roll stiffness without the rear suspension jacking up. If you put smaller tires on the front you will just push.

You still can't stiffen the rear that much without dropping the rear sway bar rate, it's all in the geometry. You would need to lower the rear roll center to get any of this to work.

You're just going to make a mess of things by going gorilla stiff and think that is going to solve the basic problem.

If the car is a road race only car then talk to someone like LG or Doug Rippe and see what they recommend, but you need to address the rear roll center issue if you want to go away from a square setup with equal tire widths all around.

A square setup has advantages too since you can swap tires all around and get more equal wear.

l98tpi

I currently run the square set up 315/17's at all four corners. I am going to run 345/18's on the rear next year for some courses. I don't foresee an issue. I will balance the car each time I switch.

Now, running the 335's on the front you may run into rubbing the fenders too much. You are only going to know if you try it. That's why we have test n tunes, so we can test. The only way to find out is try it.

BrianCunningham

Running with 335's on all 4

With the stock springs it was incredibly balanced

When switched to coilovers it was real tail happy.

I put stiffer spring up front and it's a lot better.

blackozvet

just out of interest brian, what spring rates are you running on the coilovers ?

rfn026

When you run the square setup are you running the same width wheels all the way around?

Richard Newton

theseal

I have 12's with 335's all around the 87. Actually fit better in front than rear

BrianCunningham

Started off with a rate similar to a Z07, bumped it 100lbs/in in the front, a lot better balance.

MatthewMiller

I am relatively new to this site and to C4 ownership. My 96 currently has 17x11s with 315/35s. I'm trying to figure out what class I want to run in next year, and how serious I want to get (i.e., do I want to buy another set of rims and tires, and a trailer to bring them to the event and change them?). Anyway, I read this thread and have questions:
Aren't the 345/35 about 1.2" taller than the 315/35/17s? Won't that cause issues with the staggered tire height?

Is this without flares? No rubbing? Brian, could you also chime in? Are your wheels 12" width also, and are you making them work without flares? Just going by appearance, I'm not sure my 96 can accommodate another inch of wheel width and 20mm of tire width without rubbing.

l98tpi

I don't anticipate any issue. I will balance the car with the 18's on the rear. The larger diameter is the reason I am going to use the 345's on the rear; I will have the ability to alter my final net wheel ratio. Just with a tire change I will be able to change the rear axle ratio.

MatthewMiller

Gotcha. I just hadn't considered that. With the tall transmission gearing in my car, 1st is good for 55mph and 2nd is good for 80mph . I just never think about needing a taller overall ratio for autocrossing! As the roll center will move upward with the taller tires, I would anticipate you'll find your grip shifts toward the front. So you'll probably find that you want to stiffen the front wheel rates or soften the rear to get the balance back to where it has been up until now.

I'm still curious about the need (or not) for flares with 12" wheels and 335 or 345 section widths.

BrianCunningham

12in rim
Need to get the offset right



Still I wouldn't want to parallel park with it

BrianCunningham

BTW if you do run radically different diameter tires, you ABS won't like it.

fair

If you want an easy autocross class to run - like "Drive it to the event on the tires you will race with" - look no further than CAM-S. You can run on the latest batch of 200 treadwear tires, some of which are DAMNED fast (Bridgestone RE71R + BFGoodrich Rival-S), they come in 315 and 335 sizes, too. But only in 18"...



The Rival-S has the big sizes racers full sized car owners want. The RE71R tops out at 275mm in 18" and 285mm in 19".... but there are wider 295 options available in Japan now that might make it to our shores next year.



A friend of mine (and Matt Miller's) ran the 275mm RE71R on his C5 Z06 (below) at an Optima event this year and he crushed it in the autocross. I ran another C5 at the same event on 295F/315R Rival tires (the old compound) and he beat me by a few seconds. The new Rival-S compound is much closer compound to the RE71R, though, and the wider sizes available in the Rival-S make it the obvious choice for SCCA's CAM-S class.



I used to think this was the case, but I ran the 345/35/18 Hoosier on the back of the car below with a 335/30/18 front. Only did a non-square / non-symmetrical height tire setup to be able to use this uniquely wide rear 345 Hoosier.



After 2 seasons with the 345mm on the rear that tire is MAGIC. Yes, it was taller than the front tire by a good bit, but it worked very well on both road courses and on autocross courses. On paper it should have needed changes to spring rates or ABS systems... but it just worked. Top PAX and wins in autocrosses, multiple NASA TT3 track records reset over our old 315mm setup, with no suspension settings changed.



Now I'm always trying to build race cars around a 345 Hoosier rear now, however possible. :P Granted, it only comes from Hoosier in 18" and 19" diameters, so the street use is nill.



This 69 Camaro tube framed car we're building has this 345 rear Hoosier and a 315 front. Will it push more or just have stupid levels of grip? Don't know yet, but I can't wait to find out.




Brian's C4 has some monster tires on it, no doubt about it. But it looks to me like it has two things going for it to that make those tires possible: 1. The ride height is somewhat tall. 2. The tires are poking out at bit at both ends. No offense to Brian, that's just what my engineering eyeball sees. Its still quite amazing that you could fit those!



Note the ride height on my 1992 above. This is a car we initially built for NASA TTC class, where it did very well, but that class build limited the tire width to a 245/40/17 Hoosier R7. This is a "wide" 245, measuring at 9.7" of tread. These are mounted to 17x9.5" wheels.

Well... we're moving classes for 2016 and a set of 18x12" wheels is being ordered today. I looked here on Corvette Forums and found a few instances where a 12" wide wheel was used on a C4 rear, but not on the front - until today. We measured the car carefully and are rolling the dice a little bit.



I do think we're going to have to cut the fenders to clear a 315 or 335 Hoosier at the ride heights we are using. Of course we have a stiff spring setup: 84 Z51 rear with VBP 1170#/in front.



After these wheels arrive I can show how they fit here, if they don't delete my post. Which is very likely to happen.

Note to moderators: I'm not a supporting vendor here, but I'm not selling anything to anybody in this post. Just sharing my racing experiences with various tires and Corvettes here to other racers. Yes, my images have watermarks on them, just like photographers add to all of their pictures shared on the web these days. This is the only forum on the internet that has purged posts I've made because they had watermarked images, and I post on a lot of forums.

CaptainMerricka

What is your offset?