BRAKE ARMS: INSIGHTS & EXPERIENCE
I liked the letter by Dave Bushell in RC154 about brake arm positioning. I agree with his conclusions, but suggest the reasoning is slightly different. If we assume that the cam is symmetrical, and the faces on the shoes flat, then as the cam rotates the high points of the cam move the same linear distance from the neutral axis. However, the inner end of the cam (ie. the end towards the wheel spindle) has LESS leverage on the shoe than the outer end, not more as Dave suggests. What does happen is that the inner end moves the shoe further – the usual trade-off between leverage and movement. So the shoe moved by the inner end of the cam will contact the drum earlier, and as the lever is moved will apply more force to the drum than will the other shoe (if of course we are considering unworn linings).
If the operating lever opposes wheel rotation this will be the leading shoe (and vice versa for a lever following wheel rotation). If we take the cam dimension across its long side as say 1”, and the drum diameter of 8”, the mechanical‘ disadvantage’ would be very approximately 7:8 and the movement at the ends of the lining and hence the pressure applied would have a similar ratio.
Of course once the linings have worn to a significant extent, this effect will disappear. I believe this may be why some brakes improve more than others as they bed in. Interesting that the Triumph in the Lewis Leathers ad opposite Dave’s letter has the lever the ‘wrong’ way round!
I also followed Reuben Fowles’ Guzzi rebuild with interest – what a lot of work, but worth it for a stunning result. I particularly liked the very neat wiring layout although I personally would have used a single changeover relay for main / dip headlight which would simplify the wiring a bit. I’d also use a solid state flasher unit rather than the thermal type fitted. And I hope he locked down the locking nut on the rear brake rod in the same picture before riding away! Ian Soady, member 3405
I was prompted to respond to Dave Bushell’s question in relation to the orientation of the actuating lever at the front brake end of the cable and the likely effect on brake efficiency. The simple version is that in a generic sls set-up the actuating cam operates on adjacent ends of each shoe. In many set-ups, the lever actuating the brake cam rotates in the same direction as the wheel (eg. faces backwards on the right side of the forks, forwards on the left side). So when the cable pulls the lever and rotates the cam, the cam moves the trailing shoe a slightly greater distance than the leading shoe. Drawing a simple diagram will show how this is so.
The result is that the trailing shoe contacts the drum before the leading shoe, preventing further movement of the lever and, therefore, useful contact between the leading edge of the leading shoe and the drum (reducing the leading shoe effectiveness). This results in a less effective brake, due to limited opportunity for the leading shoe to do its thing, than would be the case if the lever on the end of the cam rotated in the opposite direction to the wheel, ie. if the lever pointed forward on the right side. This seems to be the Scott case discussed.
Proof of this is simple: have you ever noticed how the front brake is seemingly more effective when the bike is being wheeled backwards? It’s not just due to reduced weight on the wheel. The same is true of many rear brake set-ups, where the lever is often pulled forward, opposing the wheel direction of rotation, the result being a seemingly sensitive / more powerful brake. It’s not just that you have a Size 9 pressing on the lever.
The manufacturers knew this: there are many period photos of factory and private BSA Gold Stars with the front brake back plate repositioned to allow the lever on the drum to be reversed (a simple, reversible modification). This resulted in the operation of the lever in the opposite direction to the wheel direction with the bike in motion, and a more effective brake. Keep up the good work! Kerry Wilton
I can confirm Dave Bushell’s experience with drum brakes. A forward facing brake arm forces the leading shoe onto the brake drum against the direction of rotation, whereas a rearward facing arm gives maximum leverage to the trailing shoe. Twenty-odd years ago I did his mod to the full width brake of my Model 120 Panther, which transformed an already effective brake into an absolute stunner. It would readily lock the front wheel of the heavy outfit on dry concrete.
Expecting a similar result, I did the same with the twin drum set-up on my Enfield 500 twin, only to find that it made no perceptible difference, in spite of skimmed drums and special linings. There must have been a reason for this, but my experience with the Panther convinced me that the principle is correct. BobTorrens