Fox Rear Disc Brake Wheel Fitment

sawdeanz · Nov 27, 2016

Been having a hell of a time finding info on wheel fitment. Almost done with doing a turbo-coupe rear swap on my 90' GT, but am unsure of what I can fit with the longer axles. Right now I have 4 lug 16x8 ponys on the rear but one is ugly and I can't find anyone who sells those. I don't mind the ponys but wouldn't mind something in 17". Was just looking at a set of 17x9 Roh Snypers for sale locally with 275s but he had fender rubbing with the stock rear and I was afraid that with the longer axles rolling the fender wouldn't be enough. I'm kind of a noob to tire fitment, so I assume going to a narrower tire (like 255) wouldn't alleviate the fender issue, correct? Also I can't find info on the offset of those Snypers. So that is why I find myself struggling on what to do.

Anyone with info on the Snypers would be appreciated, as well as suggestions for other wheels to keep an eye out for.

jrichker · Nov 28, 2016

The 93 Cobra used the same rear axle and the axles are the same length. Find the offset and backspacing for the wheels used on it and you will have your answer.

At one time there was a wheel backspacing post that had lots of information in it. A search on backspacing may turn up something.

Mustang5L5 · Nov 28, 2016

Or, run the north cobra race car brackets, and your old 4-lug axles and pull the rear wheels in 0.75"

http://www.northracecars.com/Brakes.html

It's just much easier to fit wider wheels with the stock fox offset vs the added 0.75" with the disk brake rears. In fact, I'm debating buying the Cobra brackets for my Fox to run stock fox length 5-lug axles and run wider rear wheels.

sawdeanz · Nov 28, 2016

If I knew it would be this much trouble I might have, but the one axle is busted which is what prompted the swap. I should be able to have it back on the ground today so I'll see where I'm at with my ponys and go from there. The car is in an unfinished state so body work/fender flares are a possibility too.

Mustang5L5 · Nov 28, 2016

Pony's will prob be flush with the fender or just past it.

I believe pony wheels are 16x7 with a 4.8" backspace. Since you've added 0.75" each side further out, you would need something around 5.5" BS to keep a 16x7 wheel in the same spot.

1993 cobra wheels, for example, are 17x7.5" with a 5.36" backspace.

fess40 · Nov 28, 2016

I ran some knock off cobra R's 17x9 245/45 on my turbo coupe disc rear. I rolled the lips and didn't have any rubbing issues although they did stick out a bit. I eventually went with the North Racecar brackets and like the tucked in look a lot better.

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jrichker · Nov 28, 2016

No one has given any consideration to the fact that 1.5 in. wider track is a handling advantage on a corner carver or street car?

General karthief · Nov 28, 2016

There maybe another factor. I know, I know, there are many, few come up in coversation when it comes to tires. Sidewall flex, a tire that looks like its a tight fit may 'scrub' in a hard turn more so if try'n to accelerate out of a corner, the taller the side wall the more flex occurres, Flex, grip characteristics, road surface all matter, although less on a street car but still matters. Then you have those 'rubber band' tires that are popular today that have no sidewall flex, because they have no sidewall, all scrub, no scat.

Mustang5L5 · Nov 28, 2016

jrichker said:
No one has given any consideration to the fact that 1.5 in. wider track is a handling advantage on a corner carver or street car?

When you negate by installing wheels with deeper backspacing. In the end, did you really increase track?

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sawdeanz · Nov 29, 2016

jrichker said:
No one has given any consideration to the fact that 1.5 in. wider track is a handling advantage on a corner carver or street car?

That's what I assumed too, or at the very least I thought it would look cooler, but it seems like I may not be able to make a significant track width increase due to the outer fender. Counter-intuitively, the 93 cobra apparently had some crazy rims to maintain the stock width. Since I had bent my axle, I was really just taking advantage of the situation to upgrade to disc brakes. I'll see where i'm at with my current wheel set up (255s) when I get the car back. I'm already used to taking bumps very gently so hopefully I can get away with just rolling the fenders.

jrichker · Nov 29, 2016

Mustang5L5 said:
When you negate by installing wheels with deeper backspacing. In the end, did you really increase track?

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It's time to put on your geometry hat and think about levers...
When you make a turn in a car, the weight shifts to the outside wheel. In effect, It is now the fulcrum of a lever since it is now the pivot point of the lever.
The load or weight of the car is some point midway between the fulcrum and the end of the lever opposite the fulcrum, This is the fore and aft center of gravity, which remains constant and does not change except for small amount caused by shifting fuel in an unbaffled fuel tank. This is less than 8% of the car's total weight.
The force that lifts the inside wheel to shift the weight of the car to the outside wheel is the effort or energy applied to the lever.

The car's roll action now has the features of a class 2 lever; so how does the lengthening of the lever affect the amount of effort or force?

General karthief · Nov 29, 2016

Now I really have a headache
I'm no geo-mathtision but the longer the lever the easier to move the load? Hope there ain't no test on friday.

Mustang5L5 · Nov 29, 2016

jrichker said:
It's time to put on your geometry hat and think about levers...
When you make a turn in a car, the weight shifts to the outside wheel. In effect, It is now the fulcrum of a lever since it is now the pivot point of the lever.
The load or weight of the car is some point midway between the fulcrum and the end of the lever opposite the fulcrum, This is the fore and aft center of gravity, which remains constant and does not change except for small amount caused by shifting fuel in an unbaffled fuel tank. This is less than 8% of the car's total weight.
The force that lifts the inside wheel to shift the weight of the car to the outside wheel is the effort or energy applied to the lever.

The car's roll action now has the features of a class 2 lever; so how does the lengthening of the lever affect the amount of effort or force?

I understand that, but my point was if you increase the axles width, and then deepen the wheels backspacing to pull it back into the car, you really haven't increased the levers length if the tire centerline to centerline dimension hasn't changed. You haven't really increased the moment arm if you measure COG to the centerpoint of the contact patch

16x7 wheels with 4.5" BS on stock fox drums
16x7 wheels with 5.25" BS on rear disks (added 3/4" each side)

Your tire contact patch is virtually in the same place, so no change in moment.

Fox Rear Disc Brake Wheel Fitment

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