Strange Rear Brake Problem

[yamahadon]

I have a mystery for you XS650 experts to solve . . .

When the rear suspension of my '73 TX650 is under load, the rear brake begins to actuate by itself. I notice it mostly at the bottoms of steep hills at highway speeds (I live/ride in a hilly area) -- when I reach the bottom of the hill -- when the most weight is on the suspension -- I can feel a drag on the bike. Once I start going up the next hill, it disappears.

A couple more clues: When I hit a bad pothole, the rear wheel has actually locked up momentarily a couple of times! And sometimes I can feel swingarm movement in the brake pedal -- it moves up and down under my foot.

I've been riding both street and dirt bikes, old and new, for almost 50 years, and I've never experienced this phenomenon before. Next time I ride, I'm going to take all the preload off the shocks to see if that helps.
Also, I'll try backing off the adjuster on the rod to see if more play solves the problem. But I don't like much play in my pedal, so that won't be a permanent solution for me even if it works.

Any ideas? I'm not sure it's a model-specific issue -- all drum brakes I've ever seen are essentially the same . . . .

 

[mrtwowheel]

Rear axle not tight and moving back and forth?

Scott

 

[yamahadon]

Nut is tight, and chain tension is just right . . .

 

[Royboy]

Rear brake adjustment too tight. Back it off a bit. Swing arm movement is actuating the brake.

roy

 

[Bob Kelly III]

What Royboy said ! some bikes need more free play in the rear brake than others ...
are the rear swing arm bushings allowing the wheel to move forward and back or side to side ? grab it and give it a big yank and see if you get any slop back and forth ....
.....
Bob.......

 

[TwoManyXS1Bs]

Ensure that the arm of the brake petal shaft (part #31) is vertical, getting its link rod pin as vertically close to the swingarm pivot as possible.

Compare your brake stay and brake actuator rod to this pic.

Ensure that there's no binding, interference, contact with the actuator rod...

 

[Bob Kelly III]

That is a very good point 2M !
many of us should check the actuator angle sense most of us have had the brake peddle off ( and side plate) and it can rotate back quite far .... this will decrease the available pressure on the brake shoes at that steep angle....
so if your thumb screw adjuster on the adjusting rod is a long way in you might better check it !!!!!
.....
2M do you have a link to properly adjusting the rear brake ? I think most of us just take it for granted ! LOL
Bob......

 

[TwoManyXS1Bs]

Bob, recently Weekendrider posted some really good pics of the later frame's brake petal shaft, showing the stop adjuster. This is different from the early frame, with the stop adjuster to the outside, on the petal itself.

http://www.xs650.com/threads/the-ea...day-rear-brake-pedal-shaft.49331/#post-505202

Here's the brake adjustment excerpt from the owner's manual:

 

[TimeMachine]

Because of the clues of this occuring under load and going down hill. The chain tension is suspicious to me. Can you confirm the chain tension is correct or even slacken it up just to help diagnose your brake dragging condition ? Perhaps excessive chain tension is pulling the wheel to one side in effect dragging the brake shoes ?

 

[Bob Kelly III]

2M:Yes ! that is where I saw it ! the bottom Pic shows the brake actuating lever at a very steep angle shouldn't it be at the vertical position and then the brake lever put back on ??? sense it's a splined shaft it can go on in any position I think and that allows it to be put on with the actuating lever way back like that.... cutting your braking force in half ! not to mention the adjustments on the rear wheel being a long way off..... that's what I'm ON about..... I don't remember other motorcycles being like that ...this is new to me !....first time I have seen it !
....lol learn something every day weather you want to or not ! THANKS 2M !!!!
.....
Bob.........

 

[TwoManyXS1Bs]

Bob, the early and late brake setups are different. The main idea is to get the lever and connecting rod's clevis pin as close to the swingarm pivot as possible, to reduce the effects of swingarm movement. A more vertical lever actually reduces the braking leverage, a slanted lever increases braking leverage...

 

[Bob Kelly III]

Ok... your right ...poor choice of words on my part... more distance would be covered by the rod at 90 deg than at say 45 deg
but sense the angle would place the rod lower in its radius the 45 deg angle would indeed have more leverage , at the cost of distance traveled by the peddle...... Correct thinking 2M ...as always ! thank you for catching it and the correction....
so setting the brake lever straight up and down isn't as good an idea as I thought ! LOL I guess they made it like that at the factory for a good reason ! LOL
......
Bob........

 

[TwoManyXS1Bs]

Here's a couple pics of mine. Compare to Weekendrider's later model brake pivot arm pics in that other thread. This is the early 70-73 style.

Notice that the arm is almost vertical, with the clevis pin as close to the swingarm pivot as possible. This minimizes the effects of swingarm travel.

Another pic, clearer shot. When fully assembled, the lever will be more vertical, pointing upward toward the (unused) swingarm pivot hole. The curved clevis isn't Yamaha, I'm using a longer Honda pull rod.

Now, if you can imagine that the lever is tilted back, clevis pin further away from the swingarm pivot, then visualize the swingarm moving upward (when hitting a bump), you should be able to see that this would cause a bit of a "pull" on the pull rod, actuating the rear brake.

During installation, the return spring is trying to pull that lever back. You have to fight that spring to get this thing at the right angle. A bit of a trick, done by partially fitting the brake petal on the shaft splines, angled quite a ways upward, then rotate the brake petal downward, enuff so that the petal's stop bolt will clear the frame's stop ear, then finish sliding the petal onto the splines, without pushing the brake shaft inboard. Kinda like installing the kickstart shaft. This really needs a video to show the assembly motions, and language...

 

[mrtwowheel]

Ah, interesting, and a good reason to hard tail.

Scott

 

[Bob Kelly III]

Hay 2M could you get better braking force by dropping the pivot on the rod lower ? like having power breaks maybe ?
that would of course move it even further away from the swing arm pivot bolt and that is Not what ya want.....
I had a bike ...I do not remember what one it was,.... but both the brakes on it were the best I've ever seen it only took 2 fingers to lock up the front disk and only a light pressure on the back to lock it up as well you could stop that bike really quick ! .... that might have been the Honda 700 shadow , anyway the rear brake kept giving me trouble till I got used to it as it was very sensitive.... I wound up leaving about 2" of travel to the rear brake lever to lock it up and that worked out great ! .... ( i loosened it up a bit )
.... on my 305 honda I had the rear brake shoes off and cut slits at a diagonal in the brake shoes so they could shed water better sense I rode it rain or shine and it did have a tendency to get wet brakes in the rear ..... that worked a treat.... have you ever done that ?
......
Bob..........

 

[TimeMachine]

Hey, where did yamahadon go ? this is a curiuos kinda problem. I still think your clues of deceleration or down hill (chain tightening on under side of sprockets ) are contributing ! Lol -RT

 

[TwoManyXS1Bs]

Bob, no, I've never fooled with grooving brake shoes. But, there are some sophisticated tricks for drum brakes. Getting the shoes close-fitted and arched is one. The brake cam, when rotated, applies different forces. The outer part of the cam moves faster, relative to the shoes' pivot point, compared to the inner. They also have different leverages, distance to the pivot point, which makes the shoes have different expansion pressures. Some brake cams have special contours to mitigate that imbalance. One shoe is leading, the other trailing, and each will apply different braking force, since one is trying to pull away from the drum, the other trying to 'dig in'. Knowing these, you could lay out a plan to optimize braking action, but I can't tell you what that is...

 

[Bob Kelly III]

I dunno.... maybe he's embarrassed as to have asked the question in the first place <grin> but seriously there is NO such thing as a dumb question...... I should Know ! I ask stupid questions all the time ! LOL
......
I've never ran into the problem of hitting a bump and the brakes get put on ... so it's a good question !!!!!
....
Bob.......

 

[Bob Kelly III]

Thanks 2M.... i appreciate the answer !.
....
Bob.....

 

[TimeMachine]

This is all good ! Still wondering if the rear wheel is aligned properly, bearing condition ? His clues are unusual to me .. noticable drag under deceleration , bike is rear braking on its own ? Thats what I understood.. drum brakes of course need good alignment and rigid thru axle too ..