Rim Choice with Dr. Diff's 13" Brakes

-
SeattleQQ1Fish

SeattleQQ1Fish

FABO Gold Member
FABO Gold Member
Joined
Jun 6, 2019
Messages
203
Reaction score
174
Location
Texas
I need some help determining what rims and what backspace to run on my 67 barracuda with Doctor Diff’s 13” brake kit in the front and his 11.7” rears. Rear end is a Strange S-60.

The car will be 1 - 1.5” lower than stock and has QA1 UCAs.

I want to run 275/35/18s in the front and 275 or 295/35/18s in the rear.

I know some have had to machine their rims to fit the 2.7” hub bore, but I’m not really able to do that. Is there a rim you all have had success with?
 
SeattleQQ1Fish said:
I need some help determining what rims and what backspace to run on my 67 barracuda with Doctor Diff’s 13” brake kit in the front and his 11.7” rears. Rear end is a Strange S-60.

The car will be 1 - 1.5” lower than stock and has QA1 UCAs.

I want to run 275/35/18s in the front and 275 or 295/35/18s in the rear.

I know some have had to machine their rims to fit the 2.7” hub bore, but I’m not really able to do that. Is there a rim you all have had success with?
Click to expand...

I've learned that 72bluNblu is your guy.
 
Since the kit is based on Cobra brakes, I think you'd be safe with Mustang Bullitt rims. A quick search turned up a variety of 17 & 18" sizes. Probably good with any aftermarket rim that uses the Ford center bore. Or you could leave a step on the hub to center the rotor, and have the remainder of the hub turned down to the Mopar diameter if you really wanted to.
 
@autoxcuda Might be able to shed some light on sizes to fit your Barracuda.

Why do you want to run such a large tire on the front, it will need a 10-11" wide rim. Are you racing?

I have those brakes, and use American Racing VN425 Torq Thrust SL.
 
Professor Fate said:
Since the kit is based on Cobra brakes, I think you'd be safe with Mustang Bullitt rims. A quick search turned up a variety of 17 & 18" sizes. Probably good with any aftermarket rim that uses the Ford center bore. Or you could leave a step on the hub to center the rotor, and have the remainder of the hub turned down to the Mopar diameter if you really wanted to.
Click to expand...
The calipers are Mustang, but the rotors are c4 Corvette, and I don’t know about the hubs. I don’t think they’re Ford. I’ve read that a lot of people have had to machine Mustang rims.
 
SeattleQQ1Fish said:
The calipers are Mustang, but the rotors are c4 Corvette, and I don’t know about the hubs. I don’t think they’re Ford. I’ve read that a lot of people have had to machine Mustang rims.
Click to expand...

Hubs are custom. Mustang hubs of that era are non tapered and sealed. The Dr Diff hub uses the stock dual, tapered roller bearings.
 
SeattleQQ1Fish said:
I need some help determining what rims and what backspace to run on my 67 barracuda with Doctor Diff’s 13” brake kit in the front and his 11.7” rears. Rear end is a Strange S-60.

The car will be 1 - 1.5” lower than stock and has QA1 UCAs.

I want to run 275/35/18s in the front and 275 or 295/35/18s in the rear.

I know some have had to machine their rims to fit the 2.7” hub bore, but I’m not really able to do that. Is there a rim you all have had success with?
Click to expand...

I run the same brakes up front on my Duster with 275/35/18’s and lowered maybe a smidge more than that. My front rims are 18x9 +35 which is about where you need to end up. 18x9.5” would work too, offset should be in the +27 to +35 ish range unless you want to start modifying things. For a Barracuda the additional concern would be the lower front corner of the fender, it’s more restrictive than Dusters or Darts. The height of the 275/35/18’s is ok but the added width will reduce the clearance on turning.

As for the hubs and modifications to the wheel, it really depends. My Enkei RPF1’s had to be machined. The DoctorDiff hubs are actually a little smaller than the stockers, but are still pretty large compared to other cars. The issue is usually the wheel cap. For example, the hub bore on my RPF1’s is listed at 73mm, which clears the hub. The problem is that the wheel cap is only 63mm, and the hub is so tall that it has to pass all the way through the wheel. So I had mine bored to 73mm all the way through and made custom wheel caps. A lot of aftermarket wheels use a 73mm hub bore, but you also have to look at the wheel cap diameter and how thick the rim is at the mounting surface. Mustang wheels usually have to be machined. Some of the torque thrust style wheels have larger bores, the vision legend 5’s I have on my Challenger are advertised at 83mm and they clear easily. But those are 17’s not 18’s.

In the back, what spec did you use for the width of the S-60? I can get backspace info but I need to know the width of the rear axle. The other thing is if your springs are in the stock location or not. 275’s will fit with the springs in the stock location, 295’s will not on a lowered car without other modifications.
 
72bluNblu said:
In the back, what spec did you use for the width of the S-60? I can get backspace info but I need to know the width of the rear axle.
Click to expand...
I'm getting ahead of myself with the rear wheels. I don't have the S-60 in my possession yet, so I can't provide measurements. Luckily, the old 7 1/4 is still under the car, so I can still roll it around the garage. It'll be a good look with 14" Cragars in the back and 18's in the front.

72bluNblu said:
The other thing is if your springs are in the stock location or not. 275’s will fit with the springs in the stock location, 295’s will not on a lowered car without other modifications.
Click to expand...
My springs are in the stock location, and I plan to leave them for now. I wasn't sure if 295's would squeeze in there or not. I was leaning toward 275's anyway because I like the idea of having the same size tire front and rear. Am I correct in assuming that the offsets will probably differ from the front?

72bluNblu said:
For a Barracuda the additional concern would be the lower front corner of the fender
Click to expand...
Do you think an 18x9 as opposed to a 9.5" will keep me out of the front fender? My car came with 245/60/14's + 4.5" of backspace and they rub so bad. I swear it makes me cringe every time I take a turn.
 
SeattleQQ1Fish said:
My springs are in the stock location, and I plan to leave them for now. I wasn't sure if 295's would squeeze in there or not. I was leaning toward 275's anyway because I like the idea of having the same size tire front and rear. Am I correct in assuming that the offsets will probably differ from the front?
Click to expand...

If you use a stock width rear you’ll need different offsets, yes. But if you haven’t ordered your S60 yet you can specify whatever width you want. If you want to run the stock width springs you could use a 68-70 B body width with 18x9 +35’s and 275’s and you should be within a few mm’s of where you need to be.

SeattleQQ1Fish said:
Do you think an 18x9 as opposed to a 9.5" will keep me out of the front fender? My car came with 245/60/14's + 4.5" of backspace and they rub so bad. I swear it makes me cringe every time I take a turn.
Click to expand...

Are those 14x8 or 14x7? If they’re 8’s the 275’s will be over a 1/2” further inboard on 18x9 +35’s. Heck even if they’re 14x7’s the 275’s will be about 1/8” further inboard.
 
72bluNblu said:
Are those 14x8 or 14x7? If they’re 8’s the 275’s will be over a 1/2” further inboard on 18x9 +35’s. Heck even if they’re 14x7’s the 275’s will be about 1/8” further inboard.
Click to expand...
They are 14x7's. I remembered the backspace wrong though, it's 4" not 4.5.
 
SeattleQQ1Fish said:
They are 14x7's. I remembered the backspace wrong though, it's 4" not 4.5.
Click to expand...

Ok, well, that would put them over a 1/2” further outboard than the 275’s with an 18x9” +35 with the 13” DoctorDiff brakes. So your tire to fender clearance would actually improve compared to your current wheels.
 
It would not shock me, if you had to cut/bend the bottom front fender lip.

Has someone already done something to the fenders?

Are you fenders/car all painted done??
 
autoxcuda said:
Has someone already done something to the fenders?
Click to expand...
No, the fenders are unmodified. The PO drove it like that for at least five years.

autoxcuda said:
Are you fenders/car all painted done??
Click to expand...
The paint looks good from 15 feet and that’s good enough for me. I’m keeping the car forever, so I don’t have too much of a problem modifying the fender if necessary.
 
For reference if you have to:

Looking back I went an inch more than I had to.

upload_2021-6-6_10-50-46.jpeg


upload_2021-6-6_10-51-7.jpeg


upload_2021-6-6_10-51-43.jpeg


upload_2021-6-6_10-52-2.jpeg
 
72bluNblu said:
I run the same brakes up front on my Duster with 275/35/18’s and lowered maybe a smidge more than that. My front rims are 18x9 +35 which is about where you need to end up. 18x9.5” would work too, offset should be in the +27 to +35 ish range unless you want to start modifying things. For a Barracuda the additional concern would be the lower front corner of the fender, it’s more restrictive than Dusters or Darts. The height of the 275/35/18’s is ok but the added width will reduce the clearance on turning.
Click to expand...
I couldn't find 18x9 +35 rims that I liked anywhere (so many things are backordered,) but I found a pair of 18x8.5 +35 Enkei PF01s that I went with. 275s seem big for that rim, but am I safe with a 265/35/18, or maybe a 255?
 
SeattleQQ1Fish said:
I couldn't find 18x9 +35 rims that I liked anywhere (so many things are backordered,) but I found a pair of 18x8.5 +35 Enkei PF01s that I went with. 275s seem big for that rim, but am I safe with a 265/35/18, or maybe a 255?
Click to expand...

275/35/18's are too big for an 18x8.5, so are 265/35/18's for that matter. A 255/40/18 will fit though.
 
72BluNblu makes a really good point that a lot of people are tempted to ignore. Even if you could pinch a 265 on there, it’s pretty likely that you wouldn’t see any performance benefit since it requires distorting the sidewall and compromising the contact patch. You will also get extra rotating mass and probably some extra cost too.

Personally, I’d suggest erring on the smaller side of the acceptable tire sizes for your wheel width. So with an 8.5, a 255 is about the max useful tire (they really want a 9” wide wheel IMO) and 245 section is probably closer to optimal. As a bonus, it will give you a better shot of clearing everything without rubbing and pretty much anything modern that you put on there will look huge compared to stock and provide a massive improvement in grip too.
 
Last edited:
p7lDKAggBOhcP3iqMXc_NhLvSiVA6eamFkYua8l_TV04sxqQxkBm5ONkhDskTKp43Nvr0W2=w1250-h937-no?authuser=1.jpg

I have the American Racing Torque Thrust M in 18x9 +35 with 275-35-18s. I had had the wheels machined before I did the Dr Diff brakes anyway, but it wasn't a big deal to get them cut given the wheels were less expensive to start with. The center caps are rocket racing wheels and they machined it for them at the same time. But they clear the brakes perfectly.
 
72bluNblu said:
275/35/18's are too big for an 18x8.5, so are 265/35/18's for that matter. A 255/40/18 will fit though.
Click to expand...

I went with the 255/40/18 on the Enkei PF01s +35 and everything appears to fit. The machine shop had no issue removing enough material to fit over the hub. They even machined the hubcentric ring that Tire Rack sent with the rims. I can't believe how light those rims are; a night and day difference compared to the Cragars.

IMG_0216.JPG


autoxcuda said:
It would not shock me, if you had to cut/bend the bottom front fender lip.
Click to expand...

The tire just barely clears the fender. so close

IMG_0215.JPG
 
72bluNblu said:
In the back, what spec did you use for the width of the S-60? I can get backspace info but I need to know the width of the rear axle. The other thing is if your springs are in the stock location or not. 275’s will fit with the springs in the stock location, 295’s will not on a lowered car without other modifications.
Click to expand...

I ordered a B-body S60 from Cass, and I'll be using the 11.7 discs. Will 18x9s +35 with 275/35/18 be a proper fit?
 
SeattleQQ1Fish said:
I went with the 255/40/18 on the Enkei PF01s +35 and everything appears to fit. The machine shop had no issue removing enough material to fit over the hub. They even machined the hubcentric ring that Tire Rack sent with the rims. I can't believe how light those rims are; a night and day difference compared to the Cragars.

View attachment 1715873551



The tire just barely clears the fender. so close

View attachment 1715873552
Click to expand...

That’s close!!!

Like, too close. I’d want a bit more than than for street use.

SeattleQQ1Fish said:
I ordered a B-body S60 from Cass, and I'll be using the 11.7 discs. Will 18x9s +35 with 275/35/18 be a proper fit?
Click to expand...

What year B-body? There’s different widths for different years.
 
SeattleQQ1Fish said:
'68-'70
Click to expand...

Ok, with 275’s, 18x9’s with a +35 offset and rear disks will be pretty close to the springs with the stock spring location. You might need a small spacer.

I run a 68-70 B 8 3/4 with the 11.7” rear disks and 18x10’s with a +38 offset and 295/40/18’s. But I also run a 1/2” spring offset. The math says the 275’s and 9” rims should clear without the 1/2” offset, but, it will be close enough that I’d say you’ll have to mount it up and see for sure. Especially considering it’s an aftermarket S60 rear being compared to a factory 8 3/4. Fortunately, if it’s a little too tight a small spacer should solve the issue.
 
You must log in or register to reply here.
-

Back
Top