Rear disk for 8.75 with SBP?

[Projectile Dart]

I don't think I am inventing anything new, but has anyone made a junkyard swap (or any swap) for a 8.75 rear with sbp to upgrade it to disks?

I just picked up a set of rear disks from a Liberty and will be following the Mopar Action Magazine articles step by step to add disks to my rear end but in the process of studying everything and measuring all the parts out, I think it is completely possible during the milling process to adapt the backing place to correctly position the SBP axles within the brake calipers as intended. After that, the only custom part you would ever need is standard off the shelf Jeep Liberty rotors that need to get the SBP drilled into them.
I work in a machine shop and can take the time to figure all this out but the real question here is, Is there any interest for it or no? If not then I wont waste my time and will just continue with my BBP conversion with no more distractions
If I end up doing this and if there is enough interest I would strongly consider selling modified backing plates and possibly even the re-drilled rotors on a pre-order basis to save you a trip and hassle to find a different machine shop to figure it out.

Please let me know your thoughts/concerns

 

[Projectile Dart]

P.S. Is there any possible interest in anyone wanting to purchase only the modified backing plate? Pretty much exactly what is done in the MA article for the standard BBP axles
I ask because if so, I might invest more time into making actual jigs and fixtures so they can be reproduced accurately and cost effectively.

For those who aren't sure what I am referring to
Part 1
Part 2

 

[ch1ll]

I voted no but now I’m having second thoughts. Add a maybe to the poll.

 

[33IMP]

I think the people that want to keep the small bolt pattern probably have rare or expensive small pattern wheels that they want to keep.
I think the kind of enthusiast that wants discs on the back also wants the big bolt conversion, for a much,much wider wheel selection.
I have some small pattern wheels i would like to keep, so a front disc conversion that still keeps the small pattern interests, but to do a rear.... i would go big pattern on both ends
But absolutely do one for your car! Love to see procedures and results.

 

[Professor Fate]

The Mopar Action articles are about adapting Liberty/Grand Cherokee rear discs onto factory LBP rear ends, or SBP rears (A body 8 3/4) with aftermarket LBP axles.
I see a couple of potential problems you'll need to overcome:
1.) The small bolt axles have a different offset than the large pattern axles- the large pattern axles protrude approx. 1/2" (IIRC) farther out of the axle tube than the small pattern axles. This is why you can't run stock SBP drums and backing plates on aftermarket LBP axles. For this same reason, the Liberty/G.Cherokee discs are designed with the greater offset of the LBP axles. They bolt on to the legacy LBP rears; but the "hat" of the rotor is too deep and will set the disc too far to the inside, interfering with the caliper brackets on SBP axles. If you found a rotor with a smaller offset, it could work- but then you'd lose the parking brake.
2.) The center register is considerably smaller on the SBP axles than on the LBP axles, which could allow the rotor to "drift" on the axle flange, especially under aggressive braking. You don't want an off center rotor. This could be remedied with a spacer ring, but then you're back to issue #1.
Honestly, the only way I can see this working is by redrilling the rotor to SBP; and using a (1/2"?) wheel spacer between the axle flange and the rotor, which would need a minimal large center register to locate the rotor, and an additional SBP register to locate /center your SBP wheels. That would require longer wheel studs but should allow the parking brake to remain functional, too.
Good luck!

 

[Projectile Dart]

The Mopar Action articles are about adapting Liberty/Grand Cherokee rear discs onto factory LBP rear ends, or SBP rears (A body 8 3/4) with aftermarket LBP axles.
I see a couple of potential problems you'll need to overcome:
1.) The small bolt axles have a different offset than the large pattern axles- the large pattern axles protrude approx. 1/2" (IIRC) farther out of the axle tube than the small pattern axles. This is why you can't run stock SBP drums and backing plates on aftermarket LBP axles. For this same reason, the Liberty/G.Cherokee discs are designed with the greater offset of the LBP axles. They bolt on to the legacy LBP rears; but the "hat" of the rotor is too deep and will set the disc too far to the inside, interfering with the caliper brackets on SBP axles. If you found a rotor with a smaller offset, it could work- but then you'd lose the parking brake.
2.) The center register is considerably smaller on the SBP axles than on the LBP axles, which could allow the rotor to "drift" on the axle flange, especially under aggressive braking. You don't want an off center rotor. This could be remedied with a spacer ring, but then you're back to issue #1.
Honestly, the only way I can see this working is by redrilling the rotor to SBP; and using a (1/2"?) wheel spacer between the axle flange and the rotor, which would need a minimal large center register to locate the rotor, and an additional SBP register to locate /center your SBP wheels. That would require longer wheel studs but should allow the parking brake to remain functional, too.
Good luck!

You are correct, in the MA articles they adapt the brakes for LBP and to make it all work they have to machine the Jeep backing plates down to roughly .100" to match the same thickness of the original drum backing plate and correctly locate the splines in the center section. To to this they machined the face closest to the rotor to "sink" the axle shaft into the backing plate.
I have a set of both aftermarket LBP axles and stock SBP axles and my measured difference was about 0.187" between the mounting flanges. Since the backing plates are just shy of .300" (I measured them at about an average .289" but they aren't perfectly flat) in theory I have about .200" to cut off and about .200" of a difference between the 2 types of axles. I would be able to cut the backing plate from opposite sides, and "sink" the backing plate into the axle housing, I would just BARELY have enough meat to get it down to the needed .100' and the flanges might track about .010"ish wider on each side... I'm sure these cars had wider tolerances than that off the factory line which is why nearly every part of the car has some level of adjustment, but considering how close all the measurements are, its almost too much of a coincidence to ignore..
The center register size difference is another issue. I had planned on precisely drilling the bolt holes so that they sit with a precision fit on the studs so that there would be nearly 0 play in the rotor for it to shift around and test it. If that wasnt enough I could make a minimal spacer (about .050"-.080" between the axle flange and the disk with the inside hollowed out to register the SBP hub, and the outside cut to the LBP hub) Again, we are getting to the point of if no one is interested in this sort of thing then why invest my time in it.
Seems like not too many people are looking for something like this, and since I already converted my front end to LBP I am going to continue with the rear. Maybe if I get blessed with a slant 6 4 speed early A-body wagon someday I will revisit all of this and do the SBP rear disks for myself along with a full write up

Thank you to everyone who voted!

 

[Professor Fate]

Pic of offsets that I stole from @72bluNblu :

If you do go ahead with this, keep us posted- it would be interesting!

 

[gold66cuda]

Doctor Diff makes a small bolt rear disc set up.

 

[trapster]

Another thing to consider is you may not be able to use the stock smaller wheels with the larger disks anyway. Plus I believe the Disk set up make the fender make clearance an issue as it adds to your overall width.