Forum

[trophyvase]

‘Self-servo’ is a function of any drum brake with two or more shoes. It is always an aid to braking effect – hence the move to twin leading or four leading shoe brakes. The problem with a drum brake isn’t the result of self-servo but of poor brake set-up or design.

The locking of the shoes occurs when the brake cam is rotated beyond the point of no return so that it is no longer the operating flats that expand the brake shoes but the entire width of the cam is forced between the shoes - a point at which the springs cannot return the brake to its 'off' position. Up to that point the brake ought not to lock.

Or am I wrong again?!

[stinkwheel]

I think back-plate flex has a lot to do with it. They do flex quite a lot in use.

[trophyvase]

Yes SW - I agree.

And that's the main reason why these brakes so often are impossible to set-up correctly. It is a design fault. But not an inherent fault with drum brakes.

[Cranky]

Trophyvase you are not wrong.

[Cranky]

In all my years of relining shoes, land rover and most Rover, all fords and Vauxhall in the 70"s I had never seen what I see here in Thailand. Basically a do as you wait service, they also ask what thickness you want??? This is so that if you have had the drums skimmed they make it up with a thicker lining. 3- 3.5 -4 etc. The flat cam must be as close to zero as you can. Its no good if you have the cam half on.
I have many times made a shim on the end of the shoe to make up for that very important gap. I have also wrapped the cam with metal.

The lever should ALWAYS be set at 90 degrees to the pull.
I have never seen a distorted back plate.

[vince]

Hi, check the backplate is parallel to the swingarm plate. If the distance piece is not the correct thickness the backplate will be carrying the shoes at an angle to the drum causing distortion. Vince

[trophyvase]

Cranky do you mean brake lever always to be at 90 degrees to the pull (at right-angles to the brake rod) on a BULLET?

I ask because that setting angle is by no means universal - in many cases the setting should not be a right-angle but an acute angle between brake rod and brake arm.

[Wheaters]

Ideally, to get the best leverage, at rest the brake operating arm on the backplate should sit somewhat less than 90 degrees to the "pull" rod (or cable for a front brake).

As the brake is applied hard, it should approach 90 degrees, no more.

Worse case is that it's "gone over" 90; in that situation the more force used, the less leverage there is.

I learned this lesson as a 16 year old in the early 1970s when my first little bike (actually a moped), which I'd spent many months rebuilding, failed its MOT test because the front brake arm pulled past the 90 degree point to the inner cable. The brake arm wasn't splined, it sat on a square ended shaft, so it wasn't adjustable. The MOT tester explained it to me and told me to either fit thicker shoes or to shim up the ends of the brake shoes to reduce the clearance from the operating cam. I shimmed the shoes, went back the following day, the bike passed the test and my motorcycling days began.

[stinkwheel]

Interestingly, I just went to video the movement on my brake plate. It's near impossible to capture without mounting the camera ridgidly to the bike because the whole bike moves as you apply the brake and the movement is slight.

However, with close inspection, it's clear that it's not just the brake plate that's moving, the entire hub, complete with chain and sprocket, rocks slightly front to back relative to the wheel and swingarm when the brake is applied firmly. Given the anti-torque stud is firmly attached to the swingarm, there must either be bowing of the brake plate allowing the hub to rock slightly across its axis or the end plate of the swingarm itself is flexing.

What does this indicate though? Maybe there is flex inthe swingarm, although this seems unlikely. Or is there a degree of poor fit between the hollow hub axle, spacers and the wheel spindle and bearings allowing a degree of movement on the cush-drive?

With regard to fitting the wheel, I usually fit everything in finger tight then apply the brake and clamp down the hub nut and anti-torque, then the wheel nut so the brake drum is held in alignment by the shoes as it's tightened down.

What does seem clear is any misalignment of the wheel, either by operator error of design tolerances, would accentuate this effect. If the natural alignment of the hub/brake setup isn't exactly paralell with the alignment of the wheel itself, rotation will tned to occurr.

More research required I think. I'll try to find my camera clamp so I can attach the camera to the bike itself and video this effect properly. Another interesting experiment would be to leave the snail-cams disengaged and sequentially tighten the hub nut then wheel nut with the brake applied so the hub aligns with the brake then the wheel aligns iteslf with the hub then see where that lands-up with regard to wheel position.

[trophyvase]

The 'correct' angle for the brake rod relative to the brake arm is related to and to be determined by the position of the brake arm on the brake cam splines relative to the brake cam itself and hence relative to the brake lever and finally to the brake rod.

Agreed - the thing to avoid is the cam going beyond the point of no return between the shoes.