Backing plate vs shoe size, how to tell?

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rumblefish360

rumblefish360

I have escaped the EVIL Empire State!
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I'm putting together a 8-3/4 rear for the wife's '67 Cuda. It has the large bolt patten axles all ready, so that's a plus. It has 10 X 2-1/2 drums on it now. I have these mystery 11 inch backing plates. How wide of a shoe can I fit on it? Is it a matter of drum depth or is there a backing plate issue with the 11'inch drums?
 
It is NOT just a matter of drum depth. There was only one axle offset for BBP axles. 11" rear drums came in 3 varieties, 11x2", 11x2.5", and 11x3". Since the axle flange is in the exact same place for all of those, the backing plates are unique to each shoe width. So, you can't mix and match drums and shoes with 11" backing plates, you must use the drums and shoes that are correct for the backing plates.

Now, to measure the backing plates 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! Make sense?
 
72bluNblu said:
It is NOT just a matter of drum depth. There was only one axle offset for BBP axles. 11" rear drums came in 3 varieties, 11x2", 11x2.5", and 11x3". Since the axle flange is in the exact same place for all of those, the backing plates are unique to each shoe width. So, you can't mix and match drums and shoes with 11" backing plates, you must use the drums and shoes that are correct for the backing plates.

Now, to measure the backing plates 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! Make sense?
Click to expand...
Dang, I follow you but I usually just measure the drum diameter and the drum depth. I must have been lucky.
 
toolmanmike said:
Dang, I follow you but I usually just measure the drum diameter and the drum depth. I must have been lucky.
Click to expand...

If you have the drum that goes with the backing plate to measure the depth it's not a problem. :D

The problem comes in when you have a random set of 11" backing plates like rumble does, ie, you don't have the drums that came with the backing plates. If all you have is the backing plate you have to measure it because they're actually different for each of the 11" drum options, you can't just buy the drums and shoes you want because they may not fit that backing plate.

Here's a picture of the measurement I'm talking about. You can see the e brake cable mounts are hanging off the bench. The angle of the picture makes it difficult to tell, but the measurement is 1.75" for the 11x2.5" plate pictured. And yes, I know there's an oil spot on that shoe. :banghead:

IMG_2848.jpg
 
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72BnB, thanks! Makes sense, easy to understand. Just the information I needed.
The cream and cherry topping is the pictures so I know what goes where hardware wise.

LOL, would you have an 8-3/4 picture to hardware set up too?
 
rumblefish360 said:
I'm putting together a 8-3/4 rear for the wife's '67 Cuda. It has the large bolt patten axles all ready, so that's a plus. It has 10 X 2-1/2 drums on it now. I have these mystery 11 inch backing plates. How wide of a shoe can I fit on it? Is it a matter of drum depth or is there a backing plate issue with the 11'inch drums?
Click to expand...

lay backing plate on flat surface. make sure parking brake indent is not holding it up. might have to put parking brake over edge of table. plate must lay flat. now measure thru center to table. if it measures 1.75" =11x2.5" pad. 1.5" =11x2". 2.0" =11x3". these are for 11" backing plates
 
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