B body rear brakes onto A body axle

[SGBARRACUDA]

ENOUGH SAID! Well done.

Wow. Ok,

1. Small bolt pattern axles can only be used with small bolt pattern brakes. That's it. Even if you re-drill the stock axles to BBP, you still have to use SBP backing plates and SBP drums and re-drill them as well. The axle flange offset is unique to the A-body SBP axles, nothing else has that offset.

2. Big bolt pattern brakes can only be used with big bolt pattern axles. That means you can not use big bolt pattern rear drums and backing plates (10x2.5, 11x2, 11x2.5, or 11x3) with ANY stock A-body 8 3/4 axle, as they were all SBP from the factory.

So, if you have 10x2.5" drum brakes and want to use them, you need to buy aftermarket axles and advise the company you plan on using BBP brakes so the axle flange offset is correct for the brakes you intend to use. Call Cass at Dr. Diff, he'll set you straight.

Otherwise, you need to use A-body 10x1.75" brakes (10x2.25" drums are for the front). If you re-drilled the stock axles to BBP you still have to use the SBP brake hardware, you'll just have to re-drill the SBP drums to BBP too. There are no drums you can order that have the 5x4.5" bolt pattern and use SBP axle flange offset.

A-body SBP axles use a different axle flange offset than BBP axles. They can not be interchanged. The BBP axles all used the same flange offset, so you can use BBP brakes off of anything. And the backing plates and drums are NOT interchangeable between the SBP brakes and the BBP brakes. The difference isn't just in the drums, it's also in the backing plates. So, 10x1.75" backing plates can not be used with 10x2.5" drums and shoes or vice versa, the depth of the mounting flange on the backing plate is different. Either re-drill your SBP hardware to match your re-drilled SBP axles, or buy BBP axles to fit your A-body rear axle housing and use the BBP brakes.

 

[BigBlockMopar]

Remember that when you increase the brakeshoe width on the rear axle, there's more brake contact to the drums and the brakebias will also shift to the rear.
Meaning the stock proportioning valve will not meter the correct amount of fluidpressure anymore and overbraking on the rear wheels will occur.
If not delt with properly, one will find out the hard way in a panic stop or worse on wet roads.

An adjustable proportioning valve will be necessary.

I telling this because I've installed 2.5" wide, 10" diameter drumbrakes on my Dart myself, and even with the 11.75" C-body discrotors and large 2.75" calipers up front, the rear brakes still want to lock up during panic stops and more easily on wet roads.

 

[72bluNblu]

72bluNblu:
You have the gospel on the A axle. Can this be made a sticky? I have found a source nearby that has the 10" A axle backing plates and drums. The drums have been re-drilled to the BBP. This setup will get the wheels/tires centered in the well.
Thanks everyone for chiming in.

Uh, not necessarily on a Duster. It'll depend on what backspace you use.

Even the A-body 8 3/4 puts the wheels closer to the springs than the quarters, this will be especially true with the SBP axles/brakes (BBP adds about a 1/4" to 5/16" to the track width). Although it is wider than the BBP 7.25/8.25 rear axles by about an inch, it still isn't enough to center things up with factory wheels.

Even with 15" rims you'd want zero offset in the back to center things up with the widest tire you can run (275), while in the front you'd want +6 to +12 for the offset to maximize your tire sizes. And if you're not worried about maximizing your tire size, but you don't want the rear to look narrower than the front, you actually would need a negative offset in the back to push the wheels out toward the quarters. When I was running cop rims on my Duster (15x7, 4.25" backspace, 225/60/15) the rear looked absolutely goofy until I swapped in a B rear and kept the same wheels.

I run a 68-70 B body rear in my Duster with the perches relocated. Probably the exact same rear you had in your car (55" flange to flange). Using 17" or 18" rims it's very close to making the offset the same front and rear. A 65-67 B body rear is actually about perfect to match the front offset with the rear on a Duster/Demon/Dart Sport if you're trying to maximize tire width front and rear.

This is what I'm talking about. This is with the cop rims I mentioned, and 225/60/15 tires. This is a BBP 7.25, so it is narrower than an A-body 8 3/4, but an 8 3/4 would only move the wheels out about a 1/2" per side. You can clearly see that there's several inches to the quarter, while the wheels are tight on the springs. That's with a +6 offset on a 15x7 (4.25" backspace)

Now, I don't know what rims/tires you're planning on running. If you're staying with 15" rims the A-body 8 3/4 isn't as hard to deal with, there are still plenty of 15" rims available in 0 or negative offsets. If you're moving up to 17's/18's, having the wider rear is a much better match to the available offsets.

I will say that I wouldn't mess with the SBP brakes. Replacement parts are not always easy to find, and selection is limited. The drums themselves have fallen out of production several times, although they are currently being reproduced. And of course, any time you need new drums you'll have to have them re-drilled for the BBP if that's what you've done with the axles. Makes more sense to buy the aftermarket BBP axles and use BBP brakes, because they came on darn near everything from the mid seventies all the way up to the late eighties/early 90's.

Remember that when you increase the brakeshoe width on the rear axle, there's more brake contact to the drums and the brakebias will also shift to the rear.
Meaning the stock proportioning valve will not meter the correct amount of fluidpressure anymore and overbraking on the rear wheels will occur.
If not delt with properly, one will find out the hard way in a panic stop or worse on wet roads.

An adjustable proportioning valve will be necessary.

I telling this because I've installed 2.5" wide, 10" diameter drumbrakes on my Dart myself, and even with the 11.75" C-body discrotors and large 2.75" calipers up front, the rear brakes still want to lock up during panic stops and more easily on wet roads.

Uh, are you really running C-body brakes?

Meaning, you changed out your spindles to C-body spindles and re-drilled them because the lower ball joint spacing is different between A and C bodies? And you're running 1973 C-body spindles, because those are the only ones that use the same upper ball joint diameter? And of course they're 5/8" taller too.

Or are you running the later B/R body 11.75" brakes which bolt right on to 73+ A, and F/M/J spindles?

 

[340fourspeedDuster]

With the 55" wide, B body rear axle, the factory 14" ralley wheels and 225 wide tires would rub the body when going up a roll curb at an angle. This was one reason I went to an A axle.

 

[72bluNblu]

With the 55" wide, B body rear axle, the factory 14" ralley wheels and 225 wide tires would rub the body when going up a roll curb at an angle. This was one reason I went to an A axle.

I take it those were the factory 14x5.5" rims then? Which are too narrow for 225's anyway, btw. It's all about having the proper backspace for the axle you have.

 

[BigBlockMopar]

Uh, are you really running C-body brakes?

Meaning, you changed out your spindles to C-body spindles and re-drilled them because the lower ball joint spacing is different between A and C bodies? And you're running 1973 C-body spindles, because those are the only ones that use the same upper ball joint diameter? And of course they're 5/8" taller too.

Or are you running the later B/R body 11.75" brakes which bolt right on to 73+ A, and F/M/J spindles?

Don't want to hijack the topic but I'm using the stock '73 A-body disc spindles with a homemade outer bearing collar to make up for the bearing difference in the '73 C-body rotors (11.75"/1.25").
With 2 simple washers behind the bracket the calipers lined up perfectly.
Been daily driving this setup for over 3 years already.

In my case it was a case of making use of what I had on the shelf and see if I could make it work.
It's not a 'lightweight' upgrade and setup ofcourse, and I would def. like to go to aluminium calipers in the future, but that would offset the brakebias unfavorable in my case, so for the time being I'll keep using this.

Nice thing about this setup is the trackwidth remained the same, or perhaps even a tad narrower than the 'stock' A-body discbrake system.

 

[d5667]

Another option is 8.25 backing plates and drums off of dodge trucks

 

[Kernel Sanders]

I knew I had this around somewhere and it took me a bit to find it...

This may help explain why you cant use the backing plates from a Large Bolt Pattern rear with stock Small bolt pattern axles

 

[72bluNblu]

Don't want to hijack the topic but I'm using the stock '73 A-body disc spindles with a homemade outer bearing collar to make up for the bearing difference in the '73 C-body rotors (11.75"/1.25").
With 2 simple washers behind the bracket the calipers lined up perfectly.
Been daily driving this setup for over 3 years already.

In my case it was a case of making use of what I had on the shelf and see if I could make it work.
It's not a 'lightweight' upgrade and setup ofcourse, and I would def. like to go to aluminium calipers in the future, but that would offset the brakebias unfavorable in my case, so for the time being I'll keep using this.

Nice thing about this setup is the trackwidth remained the same, or perhaps even a tad narrower than the 'stock' A-body discbrake system.

Ah, bearing spacers. That would do it too. Are you using the C-body caliper brackets? Or B/R body brackets?

I knew I had this around somewhere and it took me a bit to find it...

This may help explain why you cant use the backing plates from a Large Bolt Pattern rear with stock Small bolt pattern axles

That is EXACTLY what the problem is, and why you can't run BBP brakes on SBP axles. Great illustration. I will be using this again I'm sure! :thumbup:

 

[340fourspeedDuster]

Thank You Kernel Sanders!! Great pics showing back space.

 

[Marcohotrod]

Another option is 8.25 backing plates and drums off of dodge trucks

comments on this

 

[d5667]

Why should I comment? Your mind is made up. They won't work right?( although I run them on a 67 dart and 68 Cuda)-carry on