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Old 06-11-2014, 10:24 AM   #11
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Join Date: Apr 2014
Posts: 297
Originally Posted by thebrakeman View Post
OK, here is some drum brakes 101:
All drum brakes (whether on a car, trailer, or other) will have new shoe & linings that are a certain radius. The drum will also be a certain radius:
a. If the shoe radius is larger than the drum, bad things happen. (It's called "brake sprag", or self-energizing, and can result in the brake locking up and not letting go!).
b. If the shoe radius is smaller than the drum, you won't get full contact. It contacts somewhere in the center 1/3 of the lining, resulting in poor output (which is what the OP has).
c. When the lining and drum radius are identical, you get full performance with no self-energizing effect, which is what we want.

Since no lining manufacturer can guarantee to make every lining to exactly match the radius of a drum from another supplier (or even if the same supplier, due to production variation), the only solution is to design so the largest lining radius is always smaller than the smallest drum radius.

This simply means that there must be a break-in or "burnish" period. As the brakes wear, from the center out, they will soon conform to the drum, and you will notice increased output. Turn down the gain on the controller. You will soon find that it settles in and further adjustments will not be necessary.
Thanks thebrakeman! Great info here!

2014 F-150 SCREW Ecoboost 4x4.

2014 Rockwood Roo 233s GVWR #6402
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brake control, controller, brakes

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