After about two years of head scratching and spannering I’m now stumped and asking if anyone can shed some light on this one.
The rear disc brake on my outfit is very poor – I cannot lock it if I try, it passes the MOT on the rolling road but I find it inadequate.
My outfit is a 1978 BMW R80 with a palma sidecar, the wheels are 15” EML, the rear brake is hydraulic disc. The rear master cylinder is genuine BMW, the disc is a cast one (trimph/BSA) and the calliper is from a mini cooper car. Everything moves OK (no stuck pistons) theoretically I believe it should pull the bike up, but when moving very little breaking effect comes from the rear end even when pressed hard.
My conclusion is the master cylinder is restricted by the little lever mechanism on it employed to reverse the direction of effort supplied by the pedal (turns a pull into a push). The standard BMW rear disc set up is noted for it’s poor operation and stopping power so perhaps this points to the master cylinder as the rest is not BMW.
I’m thinking of converting it to drum brake but this will mean loosing my 15” EML wheel and sourcing lots of bits.
What are your thoughts, I love this little outfit but the rear brake is not up to touring fully loaded with the family aboard.
George
Perhaps the volume of the stock Beemer master cylinder is less than required to efficiently operate the Cooper caliper.
This problem sometimes occurs with sidecar disc brakes linked to a single caliper front brake system.
A larger capacity (bore) master cylinder or a smaller capacity slave cylinder may be in order to solve your dilemma.
Lonnie
Northwest Sidecars
This is something I've mused over. The caliper is much larger bored than the ordinary BMW one it replaces - the diameters of pistons are larger. Because of this the mecanical advantage should work in my favour - but the pedal travel will be larger - a change in velocity ratio. There is a simple calculation - MA/VR = ME (mecanical efficency) We it's not very effective. perhaps my swept area is too large and the pressure not high enough per square MM etc. etc. Thanks for the input.
George
Appearantly You simply displace much less volume than the gap of pad retraction needs.
As I may remember my Minicooper calipers had 2 pistons of 35or 40mm diameter, while my first sidecar break had a piston of just 18mm diameter.
check dimentions and keep the relationship of the hole system somewhat in a resonable range.
(My former Ford Festiva never breaked well, while the equivillant Kia Pride has nice breaks => Ford had Kia install way smaller break reciever pistons than the original (about 27mm against 35))
Another thing that causes often trouble are worped discs or egg shaped and cristalysed (overheated) drums. As well as worn cams and bores gave me once trouble on my NSU Supermax. The same effect can give week (soft) hydraulic hoses and uneven worn pads.
Regards
Sven
Thanks for the post. I have two mini cooper calipers on the front end and the normal BMW handlebar master cylinder pumps or moves enough fluid to work very well. I have just ordered all service parts for caliper and a new braded hose and will inspect the master cylinder when I take it all to down to give it a real good going over.I've also found some red rubber grease (brake cylinder grease).
Best cheers
george
I will let you know how I gert on
