Rear drum brake pulsing

[Matt Chapter]

I've noticed recently with some (but not full) pressure on the rear brake I can feel the brake pedal pulsing (moving up and down).  This seems to be a "recent" development, and when I had a new rear tire mounted I thought I saw the brake linings had worn unevenly. I ordered some new ones, but after closer inspection the old friction material hardly seems worn at all!  Last night I measured the hub and it seems to be worn but within spec (201 mm, spec is 200 mm to 201.5 mm).  What could be causing the rear brake to feel like this?

[Barry]

Are these the new shoes that are causing the pulsing or the original ones ?   

Normally pulsing indicates an oval drum but if the brake shoes have been off,  I wonder if they are truly centred in the drum so that on light application only one shoe is making contact.

Airhead rear brake shoes seem to last near forever. Did you notice the ventilation slots on the back of the drum casting where you can inspect the lining thickness without removing the wheel.

[Tony Smith]

Matt

Find a piece of road where you can get up to 50`60mph safely and then do a number of stops with the rear brake only and held just to the point of almost locking.

I reckon you will find that after 3 to 5 of these stops your rear brake will be perfect.


Oh. Two points.

I do this regularly on my wife's bikes as she is a light, even "timid" user of brakes, her brakes on her bikes and her car frequently pulsate  that cause by the way is minute particles of friction material that have been "lifted" from one part of the brake lining or disc pad and then redeposited as the brake pressure was not enough slough the particles completely off the surface.  There is actually some science behind this which you used to be able to read about on the Vesrah website (about 10 years since I looked), but basically light application means that only a small part of the shoe or pad is in contact and that bit get very hot at a microscopic level, vaporises and then redeposits in a cooler area, do it often enough and you have areas of vastly different friction co-efficient.

Second point, after you have done 3 to 5 "crash" stops with the rear brake alone, don't be tempted to put your bare hand on the wheel near the hub. I can verify that it will be "spit-sizzle" hot and that it is not good for finger tips.

[Barry]

Be careful you don't lock the rear wheel and drift sideways. 

If you release the brake in that situation it will suddenly grip which can result in a high side.  I locked the rear once at relatively low speed and when it snapped back it frightened the life out of me.

[Matt Chapter]

Are these the new shoes that are causing the pulsing or the original ones ?   

Normally pulsing indicates an oval drum but if the brake shoes have been off,  I wonder if they are truly centred in the drum so that on light application only one shoe is making contact.

These are old shoes, I suspected the cause was that they were worn.  I installed these within a few months of moving to Austin, i.e. sometime between November 2007 and April 2008.  They're hardly worn at all!

Tony I will try your suggestion, I wouldn't characterize my usage of the rear brake as "timid" so much as "sparing", but I can imagine the results being the same.

[wilcom]

Can you duplicate the condition on the center stand and in gear? Run it up to 30 or so and have someone eyeball the rear hub as you feel those pulsations. Hopefully you see no issue and you are back to looking at the shoes and drum circularity. 

[Semper Gumby]

Hi Matt,

As the owner of a bunch of drum brake BSAs I can tell you that nobody remembers is that when you go to adjust the drum brake first -

1) Loosen up the rear axle.
2) Hold down the rear brake pedal.
3) While holding pressure on the rear brake pedal, re-tighten the rear axle nut.

This should center the rear drum and prevent pulsing but assumes there is not something else bad wrong or out of round.  If you still have nojoy, send all the bits to Vintagebrake.com.  When you get it back you will have the best drum brake money can buy.  8-)

[Matt Chapter]

1) Loosen up the rear axle.
2) Hold down the rear brake pedal.
3) While holding pressure on the rear brake pedal, re-tighten the rear axle nut.

I don't believe this is a problem, since I've got the single sided swingarm / monoshock.  Unless all 4 of the lug nuts are off center, which is not impossible, I guess.