Maximum Meat - Fitting Wide Rubber on a Superformance

[RedBarchetta]

Disclaimer: The following may apply to other Cobra manufacturers. Perform your own due diligence, accordingly.

This thread will illustrate how to maximize the amount of tire (on a 15” diameter rear rim) you can tuck under a Superformance, especially on an earlier numbered car. Bear in mind that newer SPFs had some reconfiguration to the rear suspension/axles/wheel housings that allow for really monster tires with 17” rims (335s) without much of a hitch. For me, the key is to wedge as much tire under the outer wheel lips to achieve the right stance and look, without sacrificing both straight line and turning performance.

INTRO
My present driving opportunities with my SPF are mostly limited to bombing around town, stoplight to stoplight, and the occasional run through a local canyon or a cruise on the highway to the beach and back. I presently do not track or auto-cross the car, so maximum straight line traction is my highest priority without sacrificing hard street cornering abilities (i.e. canyon runs and freeway on/off ramps at twice the posted speed limit). So I set out to shoe-horn sticky tires with maximum size. Personally, I have never liked the idea of constantly losing traction during the 1-2 and 2-3 shifts, especially on the street with curbs, light posts, trees and other objects to potentially contend with. Some here will disagree, but that's okay.


WHEEL MOD #1
A not-too-common modification to the standard Trigo/A-Mold factory rims on a SPF is to eliminate the spacer ring on the rear hub adapters. By removing this ring and shortening the wheel studs by a corresponding amount, you essentially move the wheels inward about 1/2”. However, this mod alone will cause clearance problems with the brake calipers (in my case, Wilwoods). So you have to have the inner hubs machined 1/4" on the circumference to allow for this clearance. The wheels have plenty of “meat” to perform this without weakening their structural integrity. Here’s a pic of the rear rim…you can clearly see where the machining was done.



That mod alone allows you to comfortably run tires like a 295/50-15 or a wide Goodyear Billboard/Blue Streak AND to drop the ride height to 26.5-27” (measured from top of wheel lip to the ground). Without this mod, you cannot comfortably drop the ride height without running into potential clearance problems on the outer wheel lip.

WHEEL MOD #2
When I made the decision to move to a Mickey Thompson 325/50-15 drag radial, I determined that to maximize the tire’s performance would require widening the factory rim from 9.5” to 10.5”. This would reduce the inward pinching on the beads and actually spread the crown of the tire flatter across the pavement. I made several measurements to determine how much inward clearance I still had to the inner wheel well, coil overs, etc. Crawl under your car and you will be amazed as to how much inboard clearance is available. Then, working with Lynn Park (Trigo), the rims were sent to a local fabricator that specializes in wheel widening and narrowing. The fabricator uses new rim halves to widen the wheel, while machining the edges and using automated welding equipment (welds on both sides) and sealing with polyurethane. Here are a few pics of the finished rim with the clean machine welds.




They actually balanced pretty good, requiring about 2 to 3 ounces of weights per wheel.


THE FINAL PRODUCT
I will let the pics speak for themselves (though the lighting was mid-day and not great). The tire dimensions are 28” height (but they are 27” on the car, even at a robust 32psi), 11” tread width and 13” section width. Fronts (for now) are still the Dunlop 255/60s that I’ve had before. They are 27” tall (around 26.5” on the car) and fill up the front wheel wells nicely. I am in the process of swapping these for MT 255/60 drag radials...they have the same vertical dimensions but offer an additional 1/2" of tread width.




I’ve had a chance to put a few miles on the car (with just the MTs in the rear) and the traction with these things is exceptional. No, it's @&#%$* crazy sticky! Even with 32 psi (to keep the sidewalls stiff during cornering), they grip hard on straights and in corners. Adding the same type tire in the front will only improve the handling. This is a fair weather car for me...the car does not see wet weather.

Ride height is presently set at around 26.75" in the back and around 26.5" in the front. The oil cooler shroud sits 4.75" above the ground, with the same clearance at the oil pan. So far, aggressive cornering has not resulted in tire rub. If/when I ever track the car, I will swap this combination for something with more clearance in the rear, as well as raise the car by 1/2" all the way around.

I hope this was helpful. I get a lot of questions on the first rim mod, which I performed a couple years ago.

Last but not least, I would also like to give acknowledgements and thanks to CC members "D-Cel" (Jason) and "Greg Schroeder." They were, in my mind, the pioneers here on CC that proved that you can run a MT 325/50 on our cars and on the street without any worries. Jason runs MTs on all four corners and has managed some very serious canyon carving along Mulholland without any issues in roughly two years of driving with this tire set-up.

Thanks, guys!

-Dean #747

[dlang14]

Hey Dean,

Great write up. Did you add any sort of liner to your wheel wells to prevent those sticky meats from tossing rocks up? Thanks,


Daryl

[Powershift1038]

Wow, Dean. I agree with the above "great write up" comment. Your car looks great with those big fattie tires back there too. I run a 305/50/15 on the rear of my car and thought they looked fat. But, after seeing whet you have just done to your car.. Just.. WOW. Nice job.

[RedBarchetta]

Originally Posted by dlang14

Did you add any sort of liner to your wheel wells to prevent those sticky meats from tossing rocks up?

Thank you, Daryl. And YES, I actually did (and should have mentioned that). I purchased 1/8" black foam rubber from a local arts & crafts store (1'x2' sheets). Then I carefully cut them to fit only the inner wheelwell surfaces that have the exterior as the opposite face. It took about 6-7 individual panels per wheel well to accomplish this, and I trimmed them within 1/2" of the outer wheel lip. After masking off everything, I used spray contact adhesive and essentially put the puzzle together. After it set, I then used black rubberized truck bed liner and laid down 2 good coats over the foam rubber, carefully spraying just far enough over all the edges so that I don't get any peel-up later on. They turned out looking pro and best of all I don't hear any pebbles or debris hitting the underside.

I will need to repeat the process when I get the front MTs, and those front wheelwells have more curvature to them (i.e. will be more of a PITA to do, but worth it).

-Dean

[RedBarchetta]

FWIW, I'm also on the hunt for a spare set of rear 5-pin Trigos, 15"x9.5", with the standard 3.5" offset. If anyone has a decent pair lying around, I'm game.

-Dean

[greg schroeder]

Looks awesome Dean! The wide wheel really gives the tire a great look on top of being in the correct location under the wheel well.

I think you'll find they don't throw rocks under normal driving. Not until you do a burn out or extended speed driving will they heat enough to throw stones.

[RNT]

Dean...So what happens now with 1-2 shift and 3-4 shift? Can you do wheelies?

[RedBarchetta]

Originally Posted by RNT

Dean...So what happens now with 1-2 shift and 3-4 shift? Can you do wheelies?

If the half-shafts don't explode, I'm sure it's possible. Good thing I don't have a straight axle on this car...

[aldersonjames20]

So, what is the widest tire that you can get on an early spf like yours without having to do mods except custom backspacing? Is there a difference with 15 and 17 inch wheels fitting wider tires?

James

[RedBarchetta]

James, you can fit a 295/50 (~10" tread width and ~12" section width) on the standard 9.5" rim and with the factory Trigo adapter spacer in place. However, the edge of the tires will come right up to the fender lip and this limits your ability to drop the ride height. Mod #1 is worth the hassle of having the inner hubs machined and shortening the pin studs for this reason alone.

15" vs 17" wheels really don't change the maximum width you can handle, but altering the backspacing is the key. Also, the tires that are sized for 17" wheels all tend to have a lower aspect ratio (i.e. 40 or 35 series, vs the typical 60 and 50 series you see for 15" tires). The lower aspect ratios produce a superior handling tire (due to the shorter sidewall), but some Cobra afficianados will argue that the larger rim/smaller tire combo don't (a) look right and/or (b) fill up the wheel well properly. I've looked at enough Cobras to appreciate both, so it comes down to personal choice.

If I ever track my car, you can bet the farm I will be running 17" wheels and tires. Most of my driving today is just bombing around town, stoplight action and the occasional canyon run. That's why I went with this set-up...to maximize my traction in those instances and to not get unnerved when I lose traction on the 1-2 and 2-3 shifts. Some here like that feeling...I just don't trust my driving instincts with a high-powered, short wheelbase car to correct the slippin' and slidin', especially on the street with curbs, light poles, pedestrians, etc.

-Dean

[greg schroeder]

I prefer the look of your 10.5 inch wide wheels Dean, but I thought I'd display how the same tire looks on the 9.5 inch wide wheel with 4 inch back space. The deeper backspace moves the wheel in a bit. This is the only wheel placement change I made.


On my car the driver side had a big lump of fiberglass inside the inner wheel well. I had to correct that so the tire wouldn't rub.

[RedBarchetta]

Thanks for posting the pics, Greg. You are correct in that the offset is really the key. The expense and hassle of having the rims widened another 1" will improve the footprint and how the tire sits on the rim, but for most folks the visual and street/track performance would arguably be negligible (heck, you're running high 10's...enough said).

Another factor is how straight the body is set on the frame. My right rear tire actually comes up closer to the edge (by about 1/4") on my car than the left rear. I have also heard this to be true for several of the older cars, so I'm not sure if it has something to do with the 8.8 IRS and axle lengths or what.

Seeing your ride height adjusted darn near spot on where mine is gives me comfort that I won't experience a whole lot of rubbing.
-Dean

[Remod]

As usual, nice work Red. Good pics always help too.

But those pipes....they look LOUD!

You should sell them to me!

[RedBarchetta]

Originally Posted by Remod

But those pipes....they look LOUD!

Huh?!? What did you say?!?! Speak up!

[Remod]

I Said, In Exchange For A Portion Of Your Hearing Back, I Will Gladly Take In Trade Your Custom Fabricated Sidepipes!! I DON'T NEED MY HEARING ANYMORE, I'M MARRIED! :lol:

[Barry Hall]

I took one of those canyon rides with Jason (D-Cel) and can attest to how much those tires REALLY DO stick. It's amazing. I'm thinking of putting them on my Backdraft but worry that the BMW running gear in the back wouldn't handle the 1-2 and maybe the 2-3 shift.

[Lowell W]

Great write-up, Dean! Thanks.

Of course you know what's next...new rear axles...

Lowell

[RedBarchetta]

Yeah, I'm checking between the seat cushions for spare change as I write this.

[D-CEL]

Dean,

You-da-man! As always, great post. Youll love them on the front too!

Lets get together and do a 31 spline diff and half shaft mod. I have a spare 8.8 center section in the garage.

Jason

[Semper Fi]

My car is newer...a 2002. It has some serious rubber in the rear! They are 335/35/ZR17.