Hitchcocks Forum

LOCKING REAR BRAKE

I have experienced this problem since buying the (Classic Bullet 500) bike in 2001. The last time I asked (many years ago), no-one else had a snag. Now 12 years later I still have the same problem which develops after about 10 months after "sorting out" the rear brake. It is most likely to occur in wet weather. After a lock up I usually have to straighten the back-plate. I have even modified a back-plate and strengthened it to cope with small lock-ups. To avoid it, I have to test the brake at very low speed, possibly rock it forward and back to "settle" the shoes in a forward bias. That usually sorts it. Prior to each MOT I dismantle and grease up and pass the MOT. I have had a new back-plate, replaced shoes and the working parts of the brake and none of it has changed the liability of locking up. I added metal strips to the back-plate to strengthen it, I have welded extra metal into the brake "holding slot", so that there is no rotational movement of the drum. It appears to be a manufacturing fault. (Hence my surprise that no-one else has had the problem).

Recently I lent my bike to my son, who wasn't used to "settling" the brake before riding off and he managed to lock it up good. I have just ordered multiple brake parts to cover the next few years!

This sounds dangerous! Among the parts you have renewed, have you fitted new return springs? Is the brake drum/sprocket oval?

Sorry Chris, it isn't a common problem and if you have done all those things to the backing plate with no improvement I would be replacing the brake drum, 2001 it probably needs one anyway

Chris, it isn't clear to me what you mean by the brake *holding slot". Like most Bullet owners you have encountered, I find the rear brake on my 1992 500 relatively weak but progressive and at least enough to hold the bike on most gradients.

Can I ask if you are setting up the brake as intended? My understanding is that, with the drum off, the cam bush (7 on the parts diagram) should be initially be set up with the securing screws (17, 18) only just nipped up and the nuts (19) loose. In that state the cam bush should be just about slidable on the backplate. With everything back on the bike, the operating lever (9) should then be operated to apply the shoes so that the brake shoes are centralised. The nuts should then be nipped up without disturbing the screws (the heads being inaccessible inside the drum) so that the cam bush is held fast.

On some older Enfields the front brake had a similar set-up but the securing screws were set so that the cam bush was allowed sliding movement while in use. This would give a more powerful brake but in my view it is not the best set up for the Indian Bullets because the brake rod is offset from the cam bush to such an extent that it is difficult to stop the cam bush from twisting out of square to the backplate.

The difference between the two set-ups is that with the cam bush fixed the force applied to each brake shoe becomes unequal as the linings wear. This is a common arrangement for cam operated brakes and Enfields only differ from BSA, Matchless etc. in that usually the backplate centre hole is oversize and the centre nut has to be loosed and nipped up to centralise the shoes. With the floating cam arrangement the cam bush can move as the linings wear so that the force applied to each brake shoe remains equal.

On another tack, Allan & Norm have questioned whether the drum is oval. I think that would be fairly obvious in normal use, but I wonder whether the cam is turning way beyond its intended operating angle so that not only is the mechanical advantage increased but the friction between the cam and the shoe resists the restoring forces from the springs . Apart from wear of the brake components (shoes & linings, cam and the pivot pin (2)), an oversize drum might give more cam movement than intended.

Finally, is the mousetrap return spring which is wrapped around the cam bush correctly fitted?

My 2002 Bullet, a Hitchcock's GT cafe-racer, will only lock the rear wheel if I brake really hard, it's fully controllable when it does and just releases as soon as I reduce pedal pressure, with no problem whatsoever. It's standard apart from uprated brake linings (and the rearset linkage).

Wet roads will of course increase the tendency for a wheel to skid and therefore lock under braking, but yours is obviously doing more than that.

BobF's reply seems spot-on to me: it's a pretty simple brake design, and if no-one else has ever mentioned such a problem that suggests it may be assembled incorrectly - unless your brake-drum is badly distorted or cracked, or the shoes are the wrong size. Are all 3 rear wheel/hub bearings in good condition? I suggest you compare your entire fitted rear brake assembly with another Bullet: you just might find the solution that way.

Perhaps the cam spindle is moving too far?

Try taking all the load off the spindle actuating arm by slackening the rod right off.

Then see how far the actuating arm will move before the shoes make contact.
If this is even approaching 90 degrees the cams may be locking up.

Agree with what the others have said, but here are my thoughts :- by "locking" brake, do you mean one that once applied, refuses to release , or one that is simply inclined to stop the wheel rotating and causing a skid? If the former condition applies, sounds like you have a cam that is travelling too far - this could be an oversize drum, undersize cam, worn or undersize shoes, etc. If the latter, you should chamfer the ends of the brake linings to prevent grab and reduce the self-servo action of the leading shoe. Also, set the cam lever at a sufficient rearward angle to remove some of the mechanical advantage of the (longer)foot lever and check the firmness of the backplate anchor stud/nut to the swinging arm.

Hi Chris----------- can you post a few photos to give a better idea of what is actually happening ??

I also chamfer my new shoes and take great care re-building the brakes and replaced the Indian made bearings to Jap bearings and have fuss free brakes that can lock the wheel if applied hard but it's great for traffic use , the shoes last for ages .
It sounds like a duff sprocket drum thats oversize but the teeth should be very worn for it to be worn out in the drum .