11" Backing plates rubbing

[440DART1970]

I just put a real a body 8 in 3/4 rear and the back of my car 1970 Dodge Dart.. My backing plates are 11 inch... I found used 11in drums but not what sure if they're off of honestly but I slid them on and they fit I was told it might have been off of a barracuda.. I was told my back and plates are from a 73 New Yorker.....with rally wheels on....when I tighten up the Lugnuts on the the rims The wheels don't turn. The drums are making contact with backing plates edged. when I put the wheels on without the drums everything spins fine.
.I'm not sure if they're supposed to bleed smooth or if they're finned drums.
Possibly just the wrong drums?

 

[slantsixdan]

Disc brake A-bodies were already overbraked in the rear with the 10" rear drums they came with (real easy to lock up the rears, fixable with 13/16"-bore rear wheel cylinders this way). 11" drums are going to massively aggravate that problem. If you're bound and determined to run the 11" drums, put the even smaller 3/4"-bore wheel cylinders in them. But it's still pointless extra weight.

 

[72bluNblu]

I run 11" backing plates on all of my cars, A-bodies included. I don't run smaller wheel cylinders and I don't have the rear brakes dialed back with an adjustable prop valve, and yet my cars are quite well balanced as they're set up. Yes, from the factory the 10" rear brakes had too much bias, but that was with factory crap-tastic bias ply's. Start running more tire out back and that's no longer the case. Upgrade your front brakes and that's no longer the case. Most folks run much larger/wider tires out back than they do up front, and that extra tire means extra grip and extra weight, so larger brakes may be very useful. The 11" drums work VERY well with an upgrade to 11.75" front rotors. It's not as simple as the 11" brakes are pointless extra weight, with the right combination of front brakes, tires, and suspension they can add some much needed braking improvements.

As for the drums rubbing on your backing plates, you likely have mismatched drums and backing plates. Remember that 11" rear drums came in 11x2, 11x2.5, and 11x3" varieties, and you can't swap drums and backing plates between the different varieties. Since the BBP axle offset was the same for all of them, the axle flange is in the same place for all of those and therefore the offset for the backing plates on each of the 11" drum set ups was different. Meaning, if you have 11x2" backing plates, you can't just buy 11x2.5" drums and shoes and run those. You're going to need to measure the backing plate offset to figure out what brakes you need to have for those backing plates.

To measure the backing plate offset you want to set the backing plate flat on a bench. You'll need to hang the bottom 1/3 of the backing plate off the edge of the bench so that the mounts for the e-brake cables don't keep the backing plate from sitting flat. It isn't much, but they do change the measurement. Then, measure from the bench to the top of the backing plate through the center hole (for the axle) in the backing plate.

If you get ~1.75", you have 11x2.5" plates. If you get ~1.5", you have 11x2" plates. Those I know for sure, as I've measured them. I don't have a set of 11x3" backing plates, but the math would say you'd get ~2" measuring them as I described above. This is because half of the additional width is on the inside of the axle flange and the other half is on the outside to keep the axle flange in the same spot, as there was only one axle offset for BBP 8 3/4 axles.

 

[slantsixdan]

Yes, from the factory the 10" rear brakes had too much bias, but that was with factory crap-tastic bias ply's.

I guess I must be imagining all those A-bodies with front discs, 10" rear drums, and radial tires that I fixed premature rear lockup on by swapping in smaller-bore rear wheel cylinders. Thanks for letting me know.

 

[72bluNblu]

I guess I must be imagining all those A-bodies with front discs, 10" rear drums, and radial tires that I fixed premature rear lockup on by swapping in smaller-bore rear wheel cylinders. Thanks for letting me know.

Like I said, if everything is stock or relatively so you've got a point. But most of these cars aren't stock, and even with radials if you start running different size tires out back than you are up front everything changes. You have to match the brake bias and capability to the car in question, you can't just say 11" drums are pointless. On my Duster the 11x2.5" drums I run aren't enough, I could use more rear brake than I have. But I'm running 13" rotors up front, 275's up front, and 295's out back. If the OP is running 10.95" disks up front and 225/60/15 BFG T/A hockey pucks all the way around the 11" rear drums may be overkill. Regardless, that's not the cause of the problem he's having now.

 

[slantsixdan]

You have to match the brake bias and capability to the car in question,

Yes, fair enough. I'm assuming a broadly stock car, you're assuming a heavily modified one, and only the OP knows for sure...for now, anyway.

 

[Coffee&Cars]

There's more than one way I guess. I put the whole rear drum set up from a Newport on my A- body drag car. I think they were 11"x2". Let me tell you it worked great.

 

[440DART1970]

I run 11" backing plates on all of my cars, A-bodies included. I don't run smaller wheel cylinders and I don't have the rear brakes dialed back with an adjustable prop valve, and yet my cars are quite well balanced as they're set up. Yes, from the factory the 10" rear brakes had too much bias, but that was with factory crap-tastic bias ply's. Start running more tire out back and that's no longer the case. Upgrade your front brakes and that's no longer the case. Most folks run much larger/wider tires out back than they do up front, and that extra tire means extra grip and extra weight, so larger brakes may be very useful. The 11" drums work VERY well with an upgrade to 11.75" front rotors. It's not as simple as the 11" brakes are pointless extra weight, with the right combination of front brakes, tires, and suspension they can add some much needed braking improvements.

As for the drums rubbing on your backing plates, you likely have mismatched drums and backing plates. Remember that 11" rear drums came in 11x2, 11x2.5, and 11x3" varieties, and you can't swap drums and backing plates between the different varieties. Since the BBP axle offset was the same for all of them, the axle flange is in the same place for all of those and therefore the offset for the backing plates on each of the 11" drum set ups was different. Meaning, if you have 11x2" backing plates, you can't just buy 11x2.5" drums and shoes and run those. You're going to need to measure the backing plate offset to figure out what brakes you need to have for those backing plates.

To measure the backing plate offset you want to set the backing plate flat on a bench. You'll need to hang the bottom 1/3 of the backing plate off the edge of the bench so that the mounts for the e-brake cables don't keep the backing plate from sitting flat. It isn't much, but they do change the measurement. Then, measure from the bench to the top of the backing plate through the center hole (for the axle) in the backing plate.

If you get ~1.75", you have 11x2.5" plates. If you get ~1.5", you have 11x2" plates. Those I know for sure, as I've measured them. I don't have a set of 11x3" backing plates, but the math would say you'd get ~2" measuring them as I described above. This is because half of the additional width is on the inside of the axle flange and the other half is on the outside to keep the axle flange in the same spot, as there was only one axle offset for BBP 8 3/4 axles.

View attachment 1715021612

I will check tonight... thank you for all the valuable information

 

[brian6pac]

I have run 11" rears and had no problem till I put 6cyl torsion bars in then the rears locked up so I put in a adj valve and it was sweet.

 

[4mulas]

I run 11" backing plates on all of my cars, A-bodies included. I don't run smaller wheel cylinders and I don't have the rear brakes dialed back with an adjustable prop valve, and yet my cars are quite well balanced as they're set up. Yes, from the factory the 10" rear brakes had too much bias, but that was with factory crap-tastic bias ply's. Start running more tire out back and that's no longer the case. Upgrade your front brakes and that's no longer the case. Most folks run much larger/wider tires out back than they do up front, and that extra tire means extra grip and extra weight, so larger brakes may be very useful. The 11" drums work VERY well with an upgrade to 11.75" front rotors. It's not as simple as the 11" brakes are pointless extra weight, with the right combination of front brakes, tires, and suspension they can add some much needed braking improvements.

As for the drums rubbing on your backing plates, you likely have mismatched drums and backing plates. Remember that 11" rear drums came in 11x2, 11x2.5, and 11x3" varieties, and you can't swap drums and backing plates between the different varieties. Since the BBP axle offset was the same for all of them, the axle flange is in the same place for all of those and therefore the offset for the backing plates on each of the 11" drum set ups was different. Meaning, if you have 11x2" backing plates, you can't just buy 11x2.5" drums and shoes and run those. You're going to need to measure the backing plate offset to figure out what brakes you need to have for those backing plates.

To measure the backing plate offset you want to set the backing plate flat on a bench. You'll need to hang the bottom 1/3 of the backing plate off the edge of the bench so that the mounts for the e-brake cables don't keep the backing plate from sitting flat. It isn't much, but they do change the measurement. Then, measure from the bench to the top of the backing plate through the center hole (for the axle) in the backing plate.

If you get ~1.75", you have 11x2.5" plates. If you get ~1.5", you have 11x2" plates. Those I know for sure, as I've measured them. I don't have a set of 11x3" backing plates, but the math would say you'd get ~2" measuring them as I described above. This is because half of the additional width is on the inside of the axle flange and the other half is on the outside to keep the axle flange in the same spot, as there was only one axle offset for BBP 8 3/4 axles.

View attachment 1715021612

This was an excellent super informative post. Thank you.

In your opinion, do you think the 11.75” disc up front and either 11x2 or 2.5 on the rear would be well balanced on a 69 B body. Yes larger tires out back. Probably 275/60r15. And heavy big block torsion bars up front.

I only referenced torsion bars as another poster referenced 6 cyl torsion bars allowing his car to lock up and become out of balance. I’m still trying to figure out why that would happen to him and was thinking perhaps you may be able to shed some light on that. I normally wouldn’t think that torsion bars would cause changes in braking..?

 

[brian6pac]

When I had the big bars in the car it stopped fine because the front didn't drop in hard braking but with the 6 cyl bars the front would dive taking weight off of the rear so I had to adjust the pressure to the rear till I found the sweet spot then it would haul down hard. Just like the car would spin the tires easy 1st 2nd 3rd gear with no problem with the big bars and HD shocks, then changing to 6cyl bars and 90-10 shocks it would only spin about 1 ft in 2nd gear and smash you back in the seat because the front would lift and put more weight on the rear tires.

 

[72bluNblu]

This was an excellent super informative post. Thank you.

In your opinion, do you think the 11.75” disc up front and either 11x2 or 2.5 on the rear would be well balanced on a 69 B body. Yes larger tires out back. Probably 275/60r15. And heavy big block torsion bars up front.

I only referenced torsion bars as another poster referenced 6 cyl torsion bars allowing his car to lock up and become out of balance. I’m still trying to figure out why that would happen to him and was thinking perhaps you may be able to shed some light on that. I normally wouldn’t think that torsion bars would cause changes in braking..?

That’s a great combination. I run 11.75” disks and 11x2.5” drums on my ‘72 Challenger and have for like 70k miles of street driving with 275/40/17’s on all 4 corners. I was really happy with how balanced the braking was on that car. That was with 1.12” torsion bars up front and XHD springs out back.

The torsion bars absolutely make a difference. As brian6pac said it’s a weight transfer thing, the softer the front suspension is the more weight transfer to the front you’ll get when the nose dives under braking. The B/E bodies do have more anti dive built into the front suspension geometry, but it’s still a factor. Larger torsion bars up front mean less dive. That means less weight transfer forward and more braking from the rear brakes than the same car with smaller torsion bars. Weight balance, suspension, tire sizes and compounds all have an effect on the front/rear brake bias.

 

[4mulas]

I’m getting 9 pairs of 11” plates of various depths with hardware in a small parts deal I did. I’ll use two of the save one and sell the rest I think
Thanks