Hi guys,
First of all I love this forum and it has been a lifetime dream to build a Cobra kitcar. I have always wanted to do it, since I was 18 years old (32 years ago!). It's never too late, so now I'm getting prepared on building my car. If anybody has experience in homologation the FFR in italy (or Netherlands), I would really appreciate it to contact me and let me know about their experience).

As I am a car designer(doing this job professionally for 25 years now for OEMs), the beauty of the car is really a must, and that we have with the timeless design of the cobra (and Daytona and gt40(not FFR though)).
The second thing that is of importance is the stance on the wheels. And the third is the tire position and filling of the space within the wheel arch (flush and distances from tire to wheel arch.
I am sure most (maybe all of you) know this already.

I have been doing my background checking and Superformance uses the following 15":
Wheels Front: 15" x 8J
Wheels Rear: 15" x 10J
Tires Front: 255/60 R15 (=Diameter: 686mm, Width: 255mm, Sidewall: 152mm)
Tires Rear 275/60 R15 (=Diameter: 711mm, Width: 275mm, Sidewall: 165mm)
FFR uses the following 15":
Wheels Front: 15” X 8” (probably J as well)
Wheels Rear: 15” X 9”
Tires Front: 235/60R15 (=Diameter: 734mm, Width: 295mm, Sidewall: 178mm)
Tires Rear: 295/50R15 (=Diameter: 676mm, Width: 295mm, Sidewall: 147mm)
To calculate the dimensions I use this very useful site:
https://tiresize.com/calculator/
Can set it up in mm or inches.

There are some things to be noticed on FFR dimensions:
1. Front tire diameter is bigger than rear.
2. Rear tire diameter is quite small for example respect to the SPF

The front tire dimension is quite big and fills very well the wheelarch. For a good stance the rear tire diameter should be bigger than the Front, or at least the same diameter. This is NOT OK on 15" FFR.

Less issues comes with the FFR 17" wheels and tires:
Wheels Front: 17” X 9” (probably J as well)
Wheels Rear: 17” X 10.5”
Tires Front:245/45R17 (=Diameter: 653mm, Width: 245mm, Sidewall: 109mm)
Tires Rear: 315/35R17(=Diameter: 653mm, Width: 315mm, Sidewall: 109mm)

Rear tire diameter is same as front (which is OK),
but diameter is quite small and leaves a bigger gap. You can reduce the ride height, but in the front and rear of the tire the gap won't reduce and stay bigger. As well the comfort will reduce of course.

As well less issues comes with the FFR 18" wheels and tires:
Wheels Front: 18” X 9” (probably J as well)
Wheels Rear: 18” X 11”
Tires Front: 245/40R18 (=Diameter: 653mm, Width: 245mm, Sidewall: 99mm)
Tires Rear: 295/35R18 (=Diameter: 663mm, Width: 295mm, Sidewall: 104mm)
SPF:
Wheels Front: 18”x ?
Wheels Rear: 18” x ?
Tires Front: 275/35R18 (=Diameter: 650mm, Width: 275mm, Sidewall: 97mm)
Tires Rear: ?

FFR Rear tire is slightly bigger than front (which is OK),
but diameter is quite small (like the 17").

What I don't know(FFR):
- ET of each size wheels (ET= offset) front and rear.

This is quite important to understand more about how to improve the stance, and as well what I should put in the order list.


My assumptions:
- The tire diameters are biggest on the 15" wheels. And if the smallest diameter is the rear: 295/50R15 - - > Diameter rear is 674mm, width 295mm

I have the following thoughts:

- If the 18" front is 245/40R18 (diameter is 653mm) and 15" is 235/60R15 (diameter is 674mm), if we keep the width same as 15",couldn't we increase the 18" tire to: 235/45R18 (=668mm)? (This would have to be with same 15"ET size).
- if we also change the width to the current 18" front width (=245), so: 245/45R18, the tire diameter would become 676mm (nearly same as 15"). This would be much better in ride comfort and stance. But what I don't know, is that if it will hit suspension. It could be solved probably by changing ET, but then we need to understand if the suspension can handle the forces(probably can), and what impact it has on the drivability) as it changes the distance from kingpin angle section on floor to center of wheel, etc.

And same thoughts for the rear of course.

If we stick to the max tire diameter on the 15"rear, with 18" we could stick to same size as front,or maybe go to: 255/45R18 (but maybe still too narrow)

There must be a reason that FFR stuck to a much smaller tire diameter on the rear, respect to the front, does anybody know the reason?

I think my ideal setup would be:
Front 235/45R18, (668,235, 107)
Rear: 295/40R18 (693, 295, 117)
But what mods would this bring?

Please let me know all your thoughts and experiences. And I also hope you can give the missing details about rear wheel and tire of SPF.

The next step would be width, but maybe we go step by step😁


Thanks guys!
Ramon

PS. Still haven't decided if I go with 3-link or IRS