Re: Re-installing rear brake drums

Posted by DavidPackard On 2023/7/29 14:14:51

Retuning back to the first few postings from ScottG:

Post 1: As context to the above questions, my car is 1955 Clipper Constellation that is driven regularly and is in reliable mechanical condition. I pulled the drums to locate a slight grinding noise that would arise under hard braking conditions. I found that the secondary shoe was just barely contacting the inner face of the drum which I believe can be corrected with a brake shoe adjustment.

Post 6: I only removed the rear brake drums to inspect the shoes and springs after the grinding noise started. After seeing that the shoes were not visibly worn, the retaining hardware was intact and the drum braking surface was not scored, I put everything back together and adjusted the brakes per the manual. I hoped that the shoes simply needed adjusted and that a bit too much travel was allowing the shoe to move outward and contact the inner face of the drum. Unfortunately, the noise never changed although braking performance improved a bit.

Post 8: I've put about 2000 miles on the car with no problems and the previous owner was driving it as well. Unless I missed something, the car was braking fine and then starting grinding out of the blue.

Post 16 suggests new bearings, axle seals, return springs, hold down hardware, hoses, and rebuilt cylinders. Bearing clearance and brake adjustment were conducted per service manual guidance, yet the noise persists.

Post 23: Photos posted to document the locations where the drum web is contacting the shoe(s). This information is indicating one shoe is contacting near the hydraulic cylinder, while the other shoe is contacting near the star-wheel adjuster, essentially diagonal across.

Post 27: Basically the brakes were fine and then they weren't. I tried adjusting them, then replacing all the hold-down hardware and return springs, then checking axle bearing clearance (with new bearings and seals), adjusting everything again, and, most recently, disconnecting the hand brake just to make sure it wasn't pulling things out of place. So far nothing has changed the nature of the grinding one bit. At least if I had put something together wrong I could blame myself and then fix the problem.

Post 32 documents that the affected drum was ‘cleaned-up’, and there was no objectionable run-out.

My take on the symptoms would be: Since for many miles this car did not have the symptom, then the shoes are likely compatible with the drums. There was no mention of the car striking a curb, which would reduce the probability of something is bent, and that accident damage is the root cause of the symptoms.

I do like the idea that the shoes are riding too far outward. A few months ago there was a discussion of the numerous lengths of hold down pins, and the precaution to check the length before use. I have first-hand experience on a modern car where the ‘hardware kit’ contained pins that were quite a bit longer than the originals. For all of the symptoms to be explained the original springs would need to have lost a bit of free length, AND the replacement parts are such that the spring force pushing the shoes in contact with the backing plate nibs insufficient.

The design intent of that rear axle would be the outer cup of the bearing is in intimate contact with the backing plate, and that sets the spatial relationship between the axle taper and the braking components. The amount of shimming does not change this relationship unless the bearing outer cup has hit a shim instead of the backing plate. That condition would effectively hold the axle taper inboard, and lead to a loss of clearance. That clearance loss would equal the thickness of the interfering shim(s).

I would look at the old hold-down hardware versus the newly installed hardware, and I would triple check to make sure the shims are not inhibiting the intended contact between the bearing cup and backing plate.

I’ll admit we need a few different failure modes to explain the timeline of all of the symptoms. Unless an axle nut was loose, I’m thinking the initial difficulty could be a relaxed hold down spring, while later in the saga modern hardware may be the culprit, but there remains the shim interference issue.

dp