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Author Topic: Rear drum brake assumptions  (Read 121 times)

Offline dogshome

  • Posts: 116
Rear drum brake assumptions
« on: April 25, 2021, 08:08:27 AM »
Since the rear brake works, I was assuming a clean up of the pedal was all that was required to tidy up this part of the bike. A load of gunk noticed whilst the battery was out. I assumed incorrectly!

1 Pedal shaft bearing tube seized to bushings. The pedal was somehow moving on the (properly tight) central bolt. This took a drift and favourite hammer (again!) but after some cleanup had little play and caused no issue.
2 Rear adjuster barrel (the part the brass nut touches) did not rotate in the arm coming out of the drum. This leading to the next issue, since I could not rotate it:
3 Arm coming out of drum seized onto spline.

The spline took about a half hour of carful levering, prying open the split and spraying. Eventually coming off with a puff of white oxidisation. The barrel would not shift and I had to get the blowlamp to it. Being made of BMW metal, everything cleaned up and did not get mauled in the various shadetree blacksmithing processes.

That little clip on the pedal arm of the linkage rod is a pain. I did it without removing the exhaust, rear wheel or fingernails, but don't plan on taking it out again soon. So yes, probably worth crawling over absolutely every inch of a 40 year old bike, not just the obviously faulty parts!
肉(r?u)包(bāo)子(zi)打(dǎ)狗(gǒu) (meat+bun(2nd and 3rd)+hit+dog)
* Literally: To hit a dog with a meat-bun.:-O

Offline mrclubike

  • Posts: 1437
  • Jungheinrich Master Tech
Re: Rear drum brake assumptions
« Reply #1 on: April 25, 2021, 09:19:14 AM »
Good info
That is  the kind of problem one sees with low mileage bikes that have been just sitting
Also after we wash our bike or the 10000 K service we should go around and lube any linkage or pivot with some sort of lubricant NOT WD40
That rear arm pivot was sticking on mine because it was  rusting  ,so I replace it with a stainless one from Moto-Bins
If that brake arm bushing doesn't pivot it could fatigue the rod and cause it to break 

You may want to use a quad type Oring on the rare brake shaft
The stock Orings tend to leak into the brake shoe side
I was constantly have a problem until I switched to these
 
When I put a taller rear end in it. I got one of the older ones that does not  have the shaft running through oil

1982 R65 running tubeless Snowflakes
2004 R1150R

Offline dogshome

  • Posts: 116
Re: Rear drum brake assumptions
« Reply #2 on: April 25, 2021, 01:04:06 PM »
The pullrod did have a slight bend in it and yes I think it would have eventually failed. The back brake feels better but the stopping power has not changed. A sudden release of pullrod could ruin your day though :-)

I'll have a look at those seals and get some for next time the back wheel is out. Oil seems to be staying put at the moment. I am still impressed with the materials of the BMW. This kind of shenanigans on a classic British or 80s Jap bike would have resulted in stripped threads, damaged alloys, dead splines and surface rust within milliseconds. The brass adjuster screw seems to be made of the same thing as ships bells, lasts 1,000 years in salt water.
肉(r?u)包(bāo)子(zi)打(dǎ)狗(gǒu) (meat+bun(2nd and 3rd)+hit+dog)
* Literally: To hit a dog with a meat-bun.:-O