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1974 620 New Rear Brake Drums on Perfectly Working Brakes Now Vibrate Terribly


Cardinal Grammeter

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Brakes worked perfectly.

 

When the bottom star wheel brake adjuster housing broke away at the backing plate slot, it put a shallow gouge in the drum - I think I could have had it cut and be within spec - I decided to get a new drum.  Since the other drum OD backing plate seal edge was half missing since rusting away, might as well replace it too.

 

So I put 2 new $17 RockAuto DuraGo drums on it.  

 

Now the rears vibrate terribly like massively out of round.  When brakes are dead cold, it seems like they do not, but when the brakes heat up the smallest amount - like driving a 1/2 mile w/o using them - the vibration is present.  (The brakes are loose and the drums turn w/o any drag.)

 

The vibration is NOT apparent in the brake pedal.  When the truck slowly comes to a stop, I can feel the brakes grabbing, letting go, grabbing, .... etc. 

 

So I'm thinking some kind of brake surface issue.

 

I took them off to inspect.  There was an uneven surface finish:  a band of shiny surface was present on both drums - the rest of the drum area was dull.  Even though there was no significant circumferential variation, I thought maybe it doesn't take much variation for circumferential "patches" of broken in areas (these would grab more), to be realized.  So I roughed them up with a coarse cartridge roll and swapped them side to side.  Minor improvement but still unchanged.

 

Things I will be checking:

*) Replace one side with old drum - If vibration goes away, new drum bad.  If NOT:

*) Replace other side with old drum - If vibration goes away, both new drums bad.  If NOT, well...  that should be impossible. 

 

I am also worried that it may not be a 2-lobed out-of-round elliptical distortion but a 6-lobed distortion due to lug nuts.  

If the new drums are bad, I'll try to observe runout on axle.

 

This is VERY annoying...

 

Edited by Cardinal Grammeter
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16 hours ago, Cardinal Grammeter said:

 

I am also worried that it may not be a 2-lobed out-of-round elliptical distortion but a 6-lobed distortion due to lug nuts

What?????

 

that Star adjuster your talking about. I could move that with my hand. This I guess was my issue with my brakes heating up after installing new shoes. There was always a clunk cluck nearing a stop. When I installed the shoes the brake drums would be smoking hot in about 20miles.  That's with the old and new drums.

If that lower adjuster moves ezely. then that could be the issuel. The tension plate that hold it on was weak so I flipped it over and reinstalled and its not getting hot any more.

 

That's my best guess. I had drums for my Jeep Cherokee were out of round and had to get them turn NEW. What The Fuck!!!!

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6 lugs, 6 lobes.  Same as honing cylinders with a deck plate:  6 head bolts around cylinder, 6 lobes.  Any time you apply a local load to a structure, it is reacted somewhere else.

 

I believe the problem is thermal distortion due to poor casting dimensional tolerances:  

 

Drums had massive balance cuts in them indicating lots of metal in the wrong place.

 

This would produce warping when any thermal gradients from friction area to hub - in this case, thick sections would conduct heat faster than thin sections resulting in different

thermal gradients and hence thermal expansion of the casting.  It doesn't take much to grab the shoe when you consider what a 1/2 turn of the adjuster accomplishes.

 

This was noted when testing:  for the first 60 seconds, there was NO pulsation.  But after a minute or two, the pulsations appeared - and this was with the brakes adjusted loose.

 

I put the OE drums back on and it was pure butter.

 

Returning and upgrading to some Wagners which are supposed to be the finest you can buy.  Definitely better than Bendix and Raybestos.

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On ‎11‎/‎24‎/‎2019 at 7:43 PM, datzenmike said:

Even new ones probably need to be turned by a shop. Crappy manufacturing.

 

Too many folk have no knowledge of drum brakes these days.  Replacement drums have undersized center hole diameter so that they can fit a number of vehicles without stocking an unusual number of stock items.  You look up the required diameter for your vehicle and set up the brake drum lathe.  Turn and you are almost ready to go.  Next take your new brake shoes, adjust the lathe and arc grind to fit the new drums.  Replacement drum brake shoes are over dimensioned [thicker] so that they can be arc ground to fit the state of the drum.  Disc brakes are a snap to install by comparison.

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In the olden days you automatically had your new brake shoes "Arc'd" at he old school auto supply/machine shops. This meant that the shoes were slightly ground to make an exact fit to your drum.....but now because of all the hazardous dust and asbestos that would get in the air and your lungs...that practice has been mostly discontinued. Old Datsun truck drum brakes (especially the single servo front ones) require a proper fit. The new Rockauto drums may have already gotten hot and out of round. Getting these brake adjusted for a safe panic stop pretty much requires finesse and a bit of trial an' error,

 

Vicdat 

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Didn't know that about drum shoes - that they are thicker to allow for grinding.

 

FINAL UPDATE:

 

My plan was to get the Wagner drums from RA but by the time I figured out how to return the bad ones, they were out of them.

 

I found ONE on eBay and all I can say is that it is the closest thing to a "billet" part imaginable - they are 80% machined surface from ANY view angle!

 

...and it was smooth as silk when installed.

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