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