Hey everyone,
I'm looking to be purchasing some new wheels and tires for AX to improve on my Cooper R/T for grip, but I am nervous about dropping $2000+ on wheels/tires to have them not fit correctly. I have not moved my parking brake and trailing arms are currently stock.
I would prefer to have a set of 4 wheels that are all the same size so I can rotate fronts and backs, but can live with multiple sizes if I must. Looking at a few different wheel sizes, bigger will fit better on the car, but I've also read that smaller wheel size will give me a bit more acceleration, and since they are the same width, no loss of traction. I don't think I can get wider on all four spots, read somewhere that 285s might hit in the front fender.
rankly the backspacing is what's throwing me the most. Seems like I would need the same backspacing for them to not rub on either the fiberglass spring, or the parking brake, which would make me sit about 2in wider overall... I am leaning toward the 17s but hesitant to pull the trigger, I am planning to only run these for AX, so if I get a bit of rub at full lock on the fronts, that fine, If I am turned that far while racing, I have bigger problems.
Wheels I'm considering:
275/40R17 on a 17"x9" w/ 4" or 4.5"BS
275/40R18 on a 18"x9" w/ 4" or 4.5"BS
help/advice is appreciated. Thanks!

 

275 40R 18 with 4.75" Backspacing. Look for 18" options much better Tire selection.
No suspension mod's. Suspension is VB&P street slalom kit in the pictures. Car is 1978. Front may have issues @ full turn, but my car has R&P steering which reduced full turn from factory.

 

Agreed with 18" wheels having much better tyre options / availability. They also fit better in larger back spaces - handbrake bracket and leaf spring end are likely to fit inside the wheel in the rear, and the tie rod end will fit inside the wheel in the front. That is not the case with 17" wheels. Overall diameter of the wheel and tyre combination (as separate from wheel diameter - my street tyres on 17" wheels are a larger overall diameter than my 18" track tyres) does make a difference in terms of acceleration/gearing, but the smaller diameter also decreases contact patch and grip, so it's all a tradeoff.

 

Thanks guys, I'll start looking into some 18's.
Metalhead, I was just reading through that suspension thread, your car looks like what I basically want mine to turn into some day (still a ways to go), drive it to work, AX on the weekends.

 

are you running same setup for both front and back?

 

forgot to ask, what brand wheels are those and are they 9 inch all around?

 

What are the actual rules these days for a C-3 in auto-X if you change wheel diameter. Are you moved up to a higher class?

But anyway in past posts I stated that it is the front tire diameter that causes lock to lock frame rubbing. So the rule is less tire diameter you can run more back spacing. I run 4.5 with 26.2 front tires and they barely rub at full lock. I ran some 27's and they rubbed so you just never turned to the max like in a slow parking lot.

My racing 295 road racing slicks were something like 25.4 dia. and I used 5 inch BS on 17X 10 wheels

 

I'm running in CAMS, which has pretty lose requirements. Literally only has to be a 200TW tire of any size/wheel. Makes sense a bigger tire would rub first. Even the 18s will be a bit smaller than my 255/60s.

 

Lowering the car with shorter tires is very good for autocross. It shortens the sidewall giving you very good response, and lowers the car's CG at the same time. Suggest picking your tire first, as fat as possible, and then size the wheel to fit. You want the wheel as wide as the tread, or at the maxi width suggested for that tire. That also improves response a lot. Cargotzman has a great 18*9.5 inch wide setup. They need 4.75" BSP. That makes them offset 0.5" out just like original Ralleyes. Probably why they fit so well. I checked and I think they are the minimum width necessary for the 275 width tire, which has 10.5" of tread. Ideally I would suggest an even wider wheel for the same tire, if you can get it to fit, I do not know if it will. He has got the offset nailed down. But I wonder could you add .25" or even a 0.5" on both sides? The offset must stay the same. It would have to be an even addition on both sides to keep the tire still in/out. It would stretch out the sidewalls better and not move the 10.5" wide tread at all. It would be really tight!. Measure closely. I do not know if there is even room, but I believe he did not even have to remove the parking brake brackets, so there may be a little extra wheel clearance if you need it. The rear frame rail gets close, and the rear sway bar too, but those are tire issues. I will be doing the same measurements when I get to that stage in my build.

When we put the same tire on a wider wheel at the track, it had multiple benefits. The sidewalls were straight up & down and stiffer. Steering response improved by a couple inches at the steering wheel. Tire temps were more even so better grip. I was able to lower tire pressures by a lot say 4-6 psi. Tire temps stayed even and more sq. in. of rubber were on the ground for more grip. G-Force readings went up. Transitions got quicker. Less rubbery feel to the steering. I was able to run less static camber. Camber in the turns was still better. It was all good, there was no downside. I went from an 8" to a 9.75" on a 245-45-16 tire. And "everyone" said it was too wide because of the recommended rim width was 7.5" to 9.0" with 9.5" of tread. Not! I ran 24psi at races at 1.25G and everyone said that was too low too. Not! Then why was I beating them? And had unbelieveably even tire temps and wear? And only 0.5 degrees neg camber to boot?

 

to add to what leigh1322 said. When you look online tobuy a certain tire look at the recommended rim width. You might see something like 8 to 10 inch rim width. The 10 is the best option for keeping the widest contact patch of rubber on the road.

Someone ask me once about why are you driving around on the street with 650# pound QA1 semi coil overs and a 520# rear mono spring and you say that the ride is nice. The key is additional front and rear track width is leverage. Like a long breaker bar for extra TQ. My rear street wheels are 13 inch width. with 6.5 BS and 2 inch wheel adapters. So my rear track width is 2 inch adapters plus the outside 6.5 inches. So your stock C-3 with lets just say 4 inch BS and an 8 inch wheel has a track width of ..... I have an additional 9 inches of rear track width comparatively requiring the rear fender flairs. I've been driving around for 30 + year with 335 or 345 rear width tires and 500+ rear pound springs and put 174,000 miles on my Vette since I bought it brand new.

wide rear track does wonders for stability and cornering "G" forces.

I did address bump steer and camber gains to get even tire temps and ware from lots of testing. Tires, tires, and tires are everything to max traction. Buy the best Speed rated AA - A sticky tires with the lowest tread ware rating. It sad that tires cost so much now!

When you are out racing this is what you want people to see!

The Rear from being behind you. Big road racing slicks and 4 inch exhaust tips to keep back pressure down!

 

100%
Racing is all about putting the widest tire you can on the ground, with the lowest sidewall height, and then keeping them as flat as possible... all the time.
We are fortunate that our C3s take a much wider tire than most cars, and that the fenders can be rather easily flared to fit some monstrous ones.
Once you have all that monstrous traction, our engine bay is designed to rather easily accommodate engines with monstrous power.
A very Powerful Combination!
My '70 Z28 was much more limited on tire width. 9.5" wide tread was the max that would fit in the front. Our C3s seem to fit 10.5" wide tread even with stock fenders.

Seeing around the carb / hood scoop seems to be the biggest issue we have here! LOL!

 

I'm also in the market for similar new tires and wheels for spirited driving but not auto-cross or track. Hopefully, the OP won't mind a few similar questions too.

Michelin Pilot Super Sport 275/40R18 tires are what I'd like to run on my 72 C3 with VBP Performance Plus suspension.

I'd like to use an off the shelf wheel from American Racing.

Is the 18 × 9 the correct wheel or should it be the 18 x 10 or something else entirely?





Keeping it simple, I could probably settle for 245/40R18's on 18x8 as that would allow better tire selection than the current 15x8 steel rally wheels allow. Not many decent 245/60R15 tires left and none are up to the Pilots etc.

Struggling with the Back Space and wheels sizing. The more I've read the less I know.

Thank you

 

cagotsman, I love your wheels, this is the first of 'em I've seen!

 

Just for reference point and to reiterate that 18 inch wheels make it easier. 18x10 with 5.5 inch back spacing. 285/35r18 200 TW stickies.They rub slightly on the steering box bolt up front. No clearance issue in the rear.

 

Yes, big motor intakes require taller hoods. Which does block the view.

 

I feel you there man...

 

Love the look Jim!
Any other details? Trailing arm mods. spacers, ride height, etc.
I would like to do a similar size.

 

These are the wheels

ET Wheels by Team III Wheels (etmags.com)

These are 18x9.5" with 4.75" BS for the factory rear suspension anything more than 4.75" Backspacing varies from car to car. Easiest way to determine what your car will tolerant from the factory 15x8 4" backspacing is, use a 1" width of material (simulate 5" BS)
and see if anything on the wheel will hit anything on your rear suspension.

 

You have a PM with lots of detail.

quick version for anyone else. The car is currently sitting on complete Van Steel coil overs, tubular front arms and offset rears. These wheels did fit with the stock front suspension and the VBP Dual mount rear that was on the car previously. The rears will NOT clear the factory sway bar though. The front steering has been changes to a Borgeson Box with the frame brace. The left front tire rubs the front bolt for that brace if I really crank it in a parking lot. I've just learned not to do that. It sits at 25 7/8 Front and 26 1/2 rear ride height at the center of the fender openings. There are no spacers currently. I've considered a 3-4mm spacer for the front, but haven't got that far.

 

Alright, so I'm getting there, but still have some gaps. My understanding was that I needed to keep my backspacing at around 4"-4.5" to avoid hitting my parking brake or trailing arms. Can I fit over those with a 18" wheel, so I just get to the trailing arm issue, and apparently sway bar clearances too... I appreciate everyone's help/knowledge so far. Thanks for chiming in!

For tires, I'm planning to get the 275/40R18 Yokohama ADVAN A052, which has a recommended rim of 9-11, but says measured 9.5".

If stock rims are offset 0.5" then I would have 4.5" backspacing... instead of the 4" I thought I had. Have not had time to measure mine. I will do this before I purchase new rims.

Quote:

These are 18x9.5" with 4.75" BS for the factory rear suspension anything more than 4.75" Backspacing varies from car to car.

so that would be a offset of 0mm, (9.5" / 2 = 4.75"), and 4.75 would be close enough to the factory spec of 4.5" that I could probably get them in there.

For wheels, the internet has been confusing me because I've been referencing this chart from discount tire, but their math doesn't seem to work... If my current 8" rim has a 4.5"BS, it would have an offset of +12mm. but they are saying 0mm to be there... (also 8" with 4"BS would give 0mm offset, not -12mm) so I'm doing my own math. I'd love to be able to just buy standard rims like Black Bird wants to do, but unless I skimp on wheel size, I don't think I can.

9" wide rim gives a 4.5"BS with 0mm offset, or a 4"BS with -12mm. This one seems like it would fit, but be on the smaller side for the rim.
9.5" gives 4.75"BS with 0mm, or 4.5"BS with -6mm, or 4.25"BS with -12mm. This one would match the measured size of the rim, so probably my choice. It also lines up with what cagotzmann said.
10" gives 5"BS with 0mm, 4.75"BS with -6mm offset, or 4.5"BS with -12mm. I would like to be able to get a 10" rim, but clearance concerns, especially after reading Jim's post... I think I need more
Does this mean that if I got the offset from a 5.5 up to a 4.75 (meaning -18mm offset (or -24mm) that I could use these? Seems like the fronts would stick out extra far with that BS being out, and potentially rub the front fenders.

I could theoretically get two different rims, front and back, but I'd rather not do that because I want to be able to rotate through the AX season. Rather not have to bust tires to rotate, just swap locations.

Anyone see any glaring errors here? My last AX on my 255/60R15 Cooper R/Ts is this weekend, then it's shopping time. They held up well to the abuse, but they didn't hold ON to the ground very well...

Again, thank you all for the help!