69 Ambo rear drum brake vibration

[tparr]

  When applying moderate pressure on the rear drum brake I get a startling vibration/pulsating and a sound like a horn. Scared the willies out of me the first few times it happened. This just started last year.
  I took the rear wheel off to inspect, thinking perhaps the pads were shot or a spring had broken, but all looked fine. I cleaned everything, filed a bevel on the pads and put it back together. Didn't solve the problem.
  I'm guessing the pads are original. The bike had 23,000 miles on the odometer when I got it and have put on another 20,000 miles.
  Any thoughts on corrective action?

1969 A model Ambo with 43,000 miles

[Antietam Classic Cycle]

Check the drum to make sure it's perfectly round. If not, it will need to be turned. Once the bearings are removed the complete wheel (with tire) will fit onto an older automotive brake drum lathe.

Most of the aftermarket shoes are rubbish, but I've had good results with Ferodo shoes from Vintage Brake.
http://vintagebrake.com/shoes.htm They will need to be arced to match the drum. This can be easily done on a lathe - .020" shims are inserted between the shoes and actuating cams, and the shoes then turned to the diameter of the drum. I mount the shoes to the backing plate and use the axle with a suitable spacer as an arbor.

[guzzisteve]

Have the drum done then you know it's OK. Might just need a hone to get it OK. There was a mod to put a bevel on the shoe's, it's in the 1st MGNOC Tips book. I did it on my 850 T & worked out nice.

Charlie types faster than me

[pehayes]

When  you have the shoes off, remove the rotating ativation cam.  Polish the interior of the bore and the surface of the activation shaft.  Lightly lubricate those during reassembly.

Patrick Hayes
Fremont CA

[n3303j]

First time I've ever heard of arcing brake shoes on a lathe.
I have a lathe.
What an absolutely marvelous idea.
Can't think of a reason it wouldn't work.
Next time I'm doing Ural brakes it will be the direction I take.

Already found a shop with the old style drum lathe.

[73 sport]

    When a lathe wasn't available, I arced many a shoe with a rasp file finishing off with sandpaper.

[n3303j]

    When a lathe wasn't available, I arced many a shoe with a rasp file finishing off with sandpaper.

Me too!
But I had a lathe sitting 10 feet away and never considered it a possibility. 

[Antietam Classic Cycle]

Wasn't my idea, it was suggested here: http://vintagebrake.com/tips.htm

If you want to arc the linings yourself, and have access to a lathe, first mount the relined shoes on the backing plate. Turn on the lathe (300-350rpm) to .020" under drum I.D. in .010" cuts.

I find it works better for me to insert the .020" shims and turn to drum diameter.

[John A]

Make the leading edge of shoes taper or they will shudder.

[tparr]

Thank you all for the input. I went out to the workshop last night, put 4 inches of wood under the centerstand, and removed the rear wheel again to have a look-see.



The rear brake had acted normal for 19,000 miles, then something changed to cause this issue. I noticed a very tiny lip had developed on the brake pad (red line). For some reason the pads are not engaging all the way into the drum leaving a lip on the pad. I sanded a 1/16" bevel on the pad to eliminate this anomaly, reinstalled the wheel and took a frigid test ride. I gave the brake a good workout and didn't encounter any vibration.
  I will need new pads soon so I will be calling on vintagebrake and will be inquiring of any local machine shop with a drum lathe.
  Vintagebrake has a troubleshooting page and this issue is mentioned.

[tparr]

    When a lathe wasn't available, I arced many a shoe with a rasp file finishing off with sandpaper.

Can you explain how this is done?

[tparr]

Make the leading edge of shoes taper or they will shudder.

This was my first thought as well, and I did that last year, didn't make a difference.

[tparr]

This is a quote from vintagebrake.com

"Check to see how far the backplate extends into the drum. Too far in and the sides of the shoes drag on the hub, creating excessive heat. Epoxy a shim to the inside of the backplate. Not far enough, and a ridge forms where there is no contact. Bend backplate or remove material to correct."

[John A]

In damp weather the brakes on my Ambassador would sometimes squeal louder than any horn until they were heated up. It saved me from a car that started to pull out on my left at an intersection. I hit the brakes and the car stopped when they squealed. I was a quarter mile from home and was just getting out for a ride. It’s a feature, not a defect sometimes.

[73 sport]

   To answer tparrs' question: Hold the shoe against the drum, determine where the high spots are and. file them down.

[turboguzzi]

before all that, maybe check the wheel bearings too, if they have play, it can manifest as brake vibration too.

[n3303j]

Can you explain how this is done?

The method I use involves a hard and flat sanding block with 80 grit paper cemented to the flat surface.

First, lightly sand both braking surfaces to give them an even appearance.

Second, assemble shoes to backing plate and install the wheel on the axle with all snugged up.

Third, spin the wheel by hand while applying the brake with as much drag as you can overcome. Maybe you brake and a friend turns the wheel.

Fourth, remove the wheel and the shoes from the backing plate. There will be obvious drag marks in the sanded surface. These are the high spots.

Fifth, sand down the high spots with the block.

Sixth, reassemble and do it again. Keep repeating these steps until the drag marks cover 90% of the friction material. Then you are done. The last 10% will wear in reasonably quickly.

Do not have the drum turned unless it is out of round. Drums are expensive. Save as much as you can. If you are turning a drum you will need a bushing that locates on the drum bearing journals and the spindle of the drum lathe. Otherwise there is no guaranteed concentricity between the drum and the rotational axis.

[Anomaly]

In damp weather the brakes on my Ambassador would sometimes squeal louder than any horn until they were heated up. It saved me from a car that started to pull out on my left aguet an intersection. I hit the brakes and the car stopped when they squealed. I was a quarter mile from home and was just getting out for a ride. It’s a feature, not a defect sometimes.

First, a confession- I have not been following this thread. But... I checked in at the post above and it described a familiar experience-- we have a 1983 Piaggio Ape with drum brakes on all three wheels. Whenever it rains, or is even a bit humid, or has sat unused for more than a couple of days, when you first start out and first apply the brakes, it shudders and grabs and SQUEALS. After a few applications of the brakes, all is back to normal. 2 cents worth just in case your conditions are similar....