73+ disc brake backspacing

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Slantsix64

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hey guys so im getting new rims made for my brother cuda hes got the 73 and up bg bolt disc brakes, my question is how much more does the wheel get pushed out on each side when running this set up. i have read half inch on both sides if thats true im going with 4.75 back spacing thanks!
 
What size rims and what size tires do you want to run? If you’re running 14” rims, 4.75” of backspace might be too much and put the rims into the brakes. If you’re running 18x8’s and want to run a 245, it won’t be enough.

Details are important!
 
What size rims and what size tires do you want to run? If you’re running 14” rims, 4.75” of backspace might be too much and put the rims into the brakes. If you’re running 18x8’s and want to run a 245, it won’t be enough.

Details are important!
hey im running 15x6 225 60 my question is how much does the 73 disc brake set up make your tires stick out? i read someone saying half an inch?
 
It does widen the track, how much depends on what brakes you had before but it can be almost a 1/2” per side.

I ran 225/60/15’s on 15x7’s with 4.25” of backspace on my ‘74 Duster with the 73+ disk brakes, that worked fine. The equivalent backspace on a 15x6 would only be 3.75”, that would put the centerline of the wheel in the same place as a 15x7 with 4.25” of backspace. But I assume you’re talking about a ‘64?

You can run more backspace if you want the wheels more tucked in or need the clearance on your ‘64, but with 15” wheels you generally don’t want to run more than 4.5” of backspace because you run into brake clearance issues. Some wheels have better brake clearance than others, but that’s the general rule.
 
It does widen the track, how much depends on what brakes you had before but it can be almost a 1/2” per side.

I ran 225/60/15’s on 15x7’s with 4.25” of backspace on my ‘74 Duster with the 73+ disk brakes, that worked fine. The equivalent backspace on a 15x6 would only be 3.75”, that would put the centerline of the wheel in the same place as a 15x7 with 4.25” of backspace. But I assume you’re talking about a ‘64?

You can run more backspace if you want the wheels more tucked in or need the clearance on your ‘64, but with 15” wheels you generally don’t want to run more than 4.5” of backspace because you run into brake clearance issues. Some wheels have better brake clearance than others, but that’s the general rule.
Coool thanks 4.5 BS i think it is, its going on a 1968 barracuda
 
Does this help you?

Screenshot_2015-12-24-20-57-06.png


Screenshot_2015-12-24-20-58-35.png
 
Coool thanks 4.5 BS i think it is, its going on a 1968 barracuda

Oh, well on a ‘68 Barracuda I’d keep it at 4.25”. On a 15x6” that would still move the wheel a 1/2” inboard compared to the 15x7’s with 4.25” backspace. And the fender clearance on a ‘74 Duster vs. a ‘68 Barracuda is pretty similar. If anything the Barracuda will have more clearance everywhere except maybe the lower front corner.

If you look at the wheel offset it makes that difference more obvious, the narrower rim with the same backspace actually moves the centerline of the wheel. A 15x6” with a 4.25” backspace has an offset of 20mm, while a 15x7” rim with a 4.25” backspace has an offset of 6mm. So the centerline of the wheel actually moves a 1/2” in board on the 15x6”.

The issue with running more is brake and ball joint clearance. You should be ok at 4.5”, but the design of the rim does play a part in the caliper clearance, so it’s not the same for every wheel.



I’ve posted that graphic a few times myself, but I’m still pretty sure those are just generic numbers. At the very least I know that on my ‘71 with 10” SBP drums the track width didn’t widen by that much by going to ‘73+ disks. I’m pretty sure it wasn’t that narrow to start.

But that diagram doesn’t say what brake set up gave those numbers for the earlier cars. After all there were 9” and 10” drums, and the KH disks which all used different spindles and all could have had slightly different tracks. It lists the rear track as a single number too, even though the 7.25 and 8 3/4’s are different widths. The rear track width going from a 7.25 to an 8 3/4 is a good 1” different. Screwed me up the first time I bought rims for my Duster, I figured the 7.25” was the same width as an 8 3/4, turns out it’s not even close and I ended up with a set of rims that didn’t fit.
 
Oh, well on a ‘68 Barracuda I’d keep it at 4.25”. On a 15x6” that would still move the wheel a 1/2” inboard compared to the 15x7’s with 4.25” backspace. And the fender clearance on a ‘74 Duster vs. a ‘68 Barracuda is pretty similar. If anything the Barracuda will have more clearance everywhere except maybe the lower front corner.

If you look at the wheel offset it makes that difference more obvious, the narrower rim with the same backspace actually moves the centerline of the wheel. A 15x6” with a 4.25” backspace has an offset of 20mm, while a 15x7” rim with a 4.25” backspace has an offset of 6mm. So the centerline of the wheel actually moves a 1/2” in board on the 15x6”.

The issue with running more is brake and ball joint clearance. You should be ok at 4.5”, but the design of the rim does play a part in the caliper clearance, so it’s not the same for every wheel.




I’ve posted that graphic a few times myself, but I’m still pretty sure those are just generic numbers. At the very least I know that on my ‘71 with 10” SBP drums the track width didn’t widen by that much by going to ‘73+ disks. I’m pretty sure it wasn’t that narrow to start.

But that diagram doesn’t say what brake set up gave those numbers for the earlier cars. After all there were 9” and 10” drums, and the KH disks which all used different spindles and all could have had slightly different tracks. It lists the rear track as a single number too, even though the 7.25 and 8 3/4’s are different widths. The rear track width going from a 7.25 to an 8 3/4 is a good 1” different. Screwed me up the first time I bought rims for my Duster, I figured the 7.25” was the same width as an 8 3/4, turns out it’s not even close and I ended up with a set of rims that didn’t fit.

Not an expert by any means and not debating, but the dana 60 I had built w/ disc brakes changed the rear track 1" according to the builder , I didn't even check it , it dropped in so perfectly, just had to make my brake lines .
Was cut down from a pick up rear -----------
 
Not an expert by any means and not debating, but the dana 60 I had built w/ disc brakes changed the rear track 1" according to the builder , I didn't even check it , it dropped in so perfectly, just had to make my brake lines .
Was cut down from a pick up rear -----------

Rear disks pretty much always add width to the track. And with a shortened rear axle it could be anything. The wheel info I gave was for the front anyway, which is what the OP wanted.
 
IMG_0137 (1).JPG
Rear disks pretty much always add width to the track. And with a shortened rear axle it could be anything. The wheel info I gave was for the front anyway, which is what the OP wanted.

Missed the ''front'' wheel part !!

Yeah , mine is kind a wiered on wheel offset, going to go from an 8" wheel to a 10" , looks like I`ll have to have a 4" backspace. Need to pull a wheel and re measure for sure, have 15x8 prostars w. 3 1/2'' b.s. on it now , w/ about 9/16" spacers, and the 295 60 d15's clear everywhere but are close , the 295 65 15 drag radials were closer but cleared too.
 
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Thanks for all the replies I went with 15x6 4.5 back spacing I read that the 73 big bolt pattern disc brake set will add almost 3/4 of an inch on both sides so I went a half inch more to 4.5 back spacing came out nice!

Resized_20191012_113732.jpg
 
Thanks for all the replies I went with 15x6 4.5 back spacing I read that the 73 big bolt pattern disc brake set will add almost 3/4 of an inch on both sides so I went a half inch more to 4.5 back spacing came out nice!

View attachment 1715408038

Well done...and damn, those cop wheels/hub caps look great on there. Any additional pictures would be welcome.
 
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