Tiresize and wheel dimensions for stance improvement

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

I think something's wrong with your FFR 235 tire size calculations. (I received my kit on July 26th, including FFR's 15" wheels.)

I have my own spreadsheet that I built and my 295/50-15 calcs match yours; 26.61" (676 mm tire height). However, I calculated the 235/60-15 tire height at 26.10" (663 mm).

FFR's 15" front wheels are 8Jx15 with an ET of 13mm. The rear wheels are 9Jx15 with an ET of 12mm.

A few thoughts;
-A lot of tire decisions over the years depend on size availability. I used to run 17s and the first thing I did when thinking about new tires was check for 315s for the rear. That eliminated a lot of tire brands/models. I am not sure where you will be building but I'd spend some time looking at tire availability there.
- If you look here you can see a lot of side profiles w/ different wheels/tires.
https://www.factoryfive.com/galleries/mk4/
It is hard to tell exactly between 17s and 18s but the 15s are obvious. Unfortunately you can't always tell the exact size.
- Be careful looking at the front comparing tire shape to wheel opening size/shape. If the steering wheel is just slightly off center when the pic is taken, the tire will appear to be too far forward or too far backward compared to the body.
- Used to be that 315 was the standard rear width in both 17s and 18s so I am surprised to see references to 295s. Looks like FFR still recommends 315s.
https://www.factoryfiveparts.com/wheels/
- If worst comes to worst you can always modify the body opening. For instance it is easy to mod the front of the rear wheel opening to make it a tighter fit at the front of the tire.
- If there is any way you can swing the $, go IRS. Besides being better in every way, it will pay off when it's time to sell. If I were looking to buy a finished FFR, I wouldn't even consider one w/ a solid axle. Not sure about availability where you live but, if you can buy a salvage yard takeout rear suspension, be sure to get one w/ the brakes included. Free rear brakes (they are good ones w/ built in park brake) goes a long way toward making IRS more affordable.

First of all, thanks for some of the missing info. Very helpful.

With regards to the calculation, I used the link for calcukation: https://tiresize.com/calculator/

To calculate the diameter of a tire with the size 295/60R15 in millimeters, I think you should follow these steps:

1. **Determine the tire’s width in millimeters**:
- The first number (295) represents the tire’s width in millimeters.

2. **Calculate the aspect ratio**:
- The aspect ratio (60) is a percentage of the tire’s width. It represents the sidewall height as a percentage of the width.
- Sidewall height = 295 mm × 60% = 295 × 0.60 = 177 mm

3. **Convert the rim diameter to millimeters**:
- Rim diameter = 15 inches
- 1 inch = 25.4 mm
- Rim diameter in mm = 15 × 25.4 = 381 mm

4. **Calculate the overall diameter**:
- Overall diameter = Rim diameter + 2 × Sidewall height
- Overall diameter = 381 mm + 2 × 177 mm = 381 mm + 354 mm = 735 mm

Therefore, the overall diameter of a 295/60R15 tire is approximately 735 millimeters.

Which is same as the link....

Or am I wrong?

Thanks for you considerations!
Availability
- yes, I have been thinking about that too. And I will do some research on that as well

Centering of wheels
- need to figure out more details on the movement of the wheels, but also needs to be sure the wheel is centered and for that I might modify the body for that.

Modify body
-Yes I have been thinking as wel to modify the body, also to center the wheels. I don't have a big issue in modifying the body. I think it's necessary anyway on some parts, as the reflection on bodyside between the arches has some issues anyway.

IRS or 3-link
I was always thinking to go IRS, but in reality, I won't use it on track so I have some doubts... :o

You're losing something in the translation or conversion.

I've built several Mk3 and Mk4 FFR roadsters with 15" Cooper Cobra Radial GT tires and the specs are as follows:
235/60-15; 26.1" diameter, 9.1" section width and 7.2" tread width
295/50-15; 26.8" diameter, 12.2" section width and 9.7 tread width

You have errors in your math &/or info. For example:

Quote:

---

Tires Front: 235/60R15 (=Diameter: 734mm, Width: 295mm, Sidewall: 178mm)
Tires Rear: 295/50R15 (=Diameter: 676mm, Width: 295mm, Sidewall: 147mm)

---

The actual diameter of the front is 26.1; when you multiply this by 25.4 it is 662mm, not 734. Also you're showing the width of both the front and rears as 295mm. It isn't; front is 235mm and rear is 295mm.

Not sure where you're getting your info on tire sizes but as Craig mentioned 245/40-18 and 295/40-18 are not the usual recommended sizes for an 18" rim our cars. Those would be 255/35-18 and 315/30-18. The specs for those are:
255/35-18; 25.0" diameter, 10.2" section width and 9.3" tread width
315/30-18; 25.5" diameter, 12.6" section width and 11.4" tread width.

You might find that your results will be more accurate if you actually refer to the tire manufacturers' specs rather than trying to do the math or go through a conversion.

Jeff

In this video I've got 245/45-17's (25.67") on the front with 285/40-17's (25.98") out back.

https://youtu.be/9WEe6-wdNtA


In this video I'm running 255/40-17's (25.08") up front with 295/35-18's (26.18") on the rear.

https://youtu.be/iwslgKJUaKc


Hope This Helps & Good Luck!

The Tire Rack is a good technical reference source for many brands, dimensions and price. https://www.tirerack.com/
They have a tool to image-test wheels, sizes and colors on a variety of cars, too bad they do not include a Cobra. Maybe FFR should reach-out.
jim

The only way to get true diameter is from the manufacturer of the specific tire. Tires with the same numbers on the sidewall vary.

Comparing wheel wells. Every original Cobra is different. There were several different rear fender styles that were very different. Narrow, wide, lip, no lip, etc. FFRs are like Dick Smiths #198. Wheel centering is also all over the place on originals, as is badge placement, vents, exhaust, hoods scoops, etc. These cars aren't even symmetrical.

I will second Tire Rack as a source. I use them for everything tire related. Beyond size specs look at their tire tests. They do a lot of them. Two tires you might be thinking about may have been tested at different times so sometimes you have to read through both to try to figure a way interpolate the results.

I 100% trust u on it, I just wanted to understand where my mistake in the calculation is.

With regards to standard 18" sizes. I took the measurements from FFR homepage, when you configure the kit. Here I copy/pasted it from the homepage:
16693 - Vintage Halibrand Replica Wheels, 18” X 9” Front and 18” X 11” Rear & Mickey Thompson Tires, 245/40R18 Front and 295/35R18 Rear+$3,699.00

I guess there is a mistake there, so I will use the 315, instead, thanks for noticing it to me.
I will go with:
255/35-18; 25.0" diameter, 10.2" section width and 9.3" tread width
315/30-18; 25.5" diameter, 12.6" section width and 11.4" tread width.

And yes, i made a typo on the width for the 15" tire. Thanks for pointing it out.

Quote:

---

Originally Posted by GoDadGo

In this video I've got 245/45-17's (25.67") on the front with 285/40-17's (25.98") out back.

https://youtu.be/9WEe6-wdNtA


In this video I'm running 255/40-17's (25.08") up front with 295/35-18's (26.18") on the rear.

https://youtu.be/iwslgKJUaKc


Hope This Helps & Good Luck!

---

Thanks. They look great!
Helps a lot. Do you think there is space for a bigger tirediameter? Or these are to the limit?

And what ET and backspace did you use on the the 2 cars?

Thanks! I guess the page is down atm, as I got an error in opening it.
I will try again tomorrow!.

Quote:

---

Originally Posted by rginah1974

Thanks. They look great!
Helps a lot. Do you think there is space for a bigger tirediameter? Or these are to the limit?

And what ET and backspace did you use on the the 2 cars?

---

It's the same car with different rims.

What works best on my MK-4 Roadster is the 17 X 9 and 18 X 10 combination.
That set up tucks the front tires in so I have no rubbing plus the diameter stagger yields a 25.08" front tire with a 26.18" rear tire.
It cheats the eye since the rear fender-wells are so much larger than the front fender-wells so both tires have the same tire sidewall height.
People can't believe that I've got two different rim diameters, but it works very well plus I've been told that they look really good.
For me finding the right offsets and tire combination was a paramount concern because form and function must go hand in hand.

Gray Rims
The 17 X 9 have a back spacing of 5.94 or 24 MM offset.
The 17 X 10 have a back spacing of 6.30 or 20 MM offset.

Black-Machined Rims
The 17 X 9 have a back spacing of 6.05" or 30 MM offset.
The 18 X 10 have a back spacing of 6.37" or 22 MM offset.

Hope This Helps!

Quote:

---

Originally Posted by rginah1974

I 100% trust u on it, I just wanted to understand where my mistake in the calculation is.

With regards to standard 18" sizes. I took the measurements from FFR homepage, when you configure the kit. Here I copy/pasted it from the homepage:
16693 - Vintage Halibrand Replica Wheels, 18” X 9” Front and 18” X 11” Rear & Mickey Thompson Tires, 245/40R18 Front and 295/35R18 Rear+$3,699.00

I guess there is a mistake there, so I will use the 315, instead, thanks for noticing it to me.
I will go with:
255/35-18; 25.0" diameter, 10.2" section width and 9.3" tread width
315/30-18; 25.5" diameter, 12.6" section width and 11.4" tread width.

And yes, i made a typo on the width for the 15" tire. Thanks for pointing it out.

---

That is funny. Just before typing my post above I went to the FFR website and looked in the roadster parts section for wheels. There they list 315s for both 17 and 18. I haven't messed w/ tires on a MkIV but on my old MkII the diagonal tube that runs behind your shoulder out to the door latch area was a problem for larger diameter tires. So if thinking about larger OD than the standard 315s, I'd ask if others have tried it. Also note that for a given tire size, tires of different brands can vary more than you might think. Way back when people were trying to fit 295-15s on MkIs and MkIIs it was common to try to choose the narrowest 295. Tread width could vary by a full inch. We were usually worried about thread and section width but I don't remember much discussion about OD.

Quote:

---

Originally Posted by GoDadGo

It's the same car with different rims.

What works best on my MK-4 Roadster is the 17 X 9 and 18 X 10 combination.
That set up tucks the front tires in so I have no rubbing plus the diameter stagger yields a 25.08" front tire with a 26.18" rear tire.
It cheats the eye since the rear fender-wells are so much larger than the front fender-wells so both tires have the same tire sidewall height.
People can't believe that I've got two different rim diameters, but it works very well plus I've been told that they look really good.
For me finding the right offsets and tire combination was a paramount concern because form and function must go hand in hand.

Gray Rims
The 17 X 9 have a back spacing of 5.94 or 24 MM offset.
The 17 X 10 have a back spacing of 6.30 or 20 MM offset.

Black-Machined Rims
The 17 X 9 have a back spacing of 6.05" or 30 MM offset.
The 18 X 10 have a back spacing of 6.37" or 22 MM offset.

Hope This Helps!

---

Thanks a lot! Really helpful.

And yes, it's a good solution to have 2 different rim sizes as it will help give more feeling of power on the rear wheels. Some cars use this as well. Like the Chevrolet Corvette C4 (1984-1996) if I remember well.

Are your offset values positive or negative values (30mm or - 30mm, and 22mm or - 22mm)?

You think there is some more space in ET on the rear to widen a bit more?

Quote:

---

Originally Posted by CraigS

That is funny. Just before typing my post above I went to the FFR website and looked in the roadster parts section for wheels. There they list 315s for both 17 and 18. I haven't messed w/ tires on a MkIV but on my old MkII the diagonal tube that runs behind your shoulder out to the door latch area was a problem for larger diameter tires. So if thinking about larger OD than the standard 315s, I'd ask if others have tried it. Also note that for a given tire size, tires of different brands can vary more than you might think. Way back when people were trying to fit 295-15s on MkIs and MkIIs it was common to try to choose the narrowest 295. Tread width could vary by a full inch. We were usually worried about thread and section width but I don't remember much discussion about OD.

---

Yeah, I just checked in the parts catalog:
Recommended Tire Sizes:
Front: 255/35 ZR18
Rear: 315/30 ZR18

������:confused:
Weird....

Quote:

---

Originally Posted by rginah1974

Yeah, I just checked in the parts catalog:
Recommended Tire Sizes:
Front: 255/35 ZR18
Rear: 315/30 ZR18

������:confused:
Weird....

---

Why is that weird? That's the combination a whole bunch of us have successfully used for 18's. It's possible to go up to a 275 for the front. Tight on a Roadster. Easy on a Coupe. I have Goodrich Rival S tires on my Coupe. There's .1" difference between the front and rear diameter. I challenge anyone to notice that. Exposure in the wheel opens is also a function of ride height. Note just diameter or body outline. I applaud you for doing your homework. But the formula for these is proven by a lot of builds. You're not the first.

Been running 275X40X17 & 315X35X17 on FFR wheels for years. No fit issues. 275 are a bit heavy for some steering inputs without power steering. Nothing wierd

https://thefactoryfiveforum.com/atta...7&d=1581372038

BTW best to use Tire Racks specs which are actual measurements. There are no standards in the listed sizes. Have seen over an inch in tire tread on the ground fron two tires with the same mm listing. Goodyear & Yokahama in that case.

Quote:

---

Originally Posted by BEAR-AvHistory

Been running 275X40X17 & 315X35X17 on FFR wheels for years. No fit issues. 275 are a bit heavy for some steering inputs without power steering. Nothing wierd

https://thefactoryfiveforum.com/atta...7&d=1581372038

BTW best to use Tire Racks specs which are actual measurements. There are no standards in the listed sizes. Have seen over an inch in tire tread on the ground fron two tires with the same mm listing. Goodyear & Yokahama in that case.

---

Very nice build Kevin!
And thanks for the info.

Do you happen to know the ET/offset and backspacing of the wheels?

Quote:

---

Originally Posted by edwardb

Why is that weird? That's the combination a whole bunch of us have successfully used for 18's. It's possible to go up to a 275 for the front. Tight on a Roadster. Easy on a Coupe. I have Goodrich Rival S tires on my Coupe. There's .1" difference between the front and rear diameter. I challenge anyone to notice that. Exposure in the wheel opens is also a function of ride height. Note just diameter or body outline. I applaud you for doing your homework. But the formula for these is proven by a lot of builds. You're not the first.

---

For me the "weird thing is that FFR gives a different tiresize in the complete package, respect to the recommended tiresizes in the parts catalog.

And yes, i know many have been built and look amazing, but it's difficult to find the information of tiresizes and brand/model, wheels details, like Offset and backspacing etc.
And I guess this is the "easiest way" to find the difficulties increasing the tire diameter.

Thanks to the replies I notice that the limitation on tiresize is also because of the 2 different dimensions on the car and to find a brand that has a tiremodel with the 2 different sizes

Quote:

---

Originally Posted by rginah1974

Very nice build Kevin!
And thanks for the info.

Do you happen to know the ET/offset and backspacing of the wheels?

---

These are the FFR wheels that I have.

Technical Specifications:
Wheel Set Part Number: 14865
Front Wheel Size (Diameter x width): 17"x 9"
Rear Wheel Size (Diameter x width): 17"x 10.5"
Front Wheel Backspace: 6.00" (24mm Offset)
Rear Wheel Backspace: 6.7" (27mm Offset)
Wheel Lug Pattern: Ford style 5 lug x 4.5" bolt circle
Material: Cast Aluminum
Color: Gun Metal