master cylinder ?

[guyom]

hi Guys, i want to change the master cylinder of my BMW R65 LS (dual front brake). Simple reason : i brake the lever fixation and due to that, i just changed the gas handle.. et relocalize the start / kill switch in an other place. So i just need to simple master cylinder but i am a bit lost...

I know that i have ATE brakes (R65 1981) dual brake line so 2 discs of 260mm and piston diameter ... hum 36mm ?  (which sound a bit bigger....) floating callipers ? not sure also...

so any one could help me... which type of master cylinder i need ??

As an example, Beringer brake company propose différent type :

http://www.beringer-brakes.com/en/moto/technologies/aerotec-master-cylinder-8.htm

Brake
 - BR10 (Ø 14,5mm piston) for only one floating or opposite 2 piston caliper (small size).      
 - BR12 (Ø 17,5mm piston) for only one opposite 4 or 6 piston  caliper. Integral braking for Quads.
 - BR14 (Ø 20,6mm piston) for 2 opposite 4 or 6 piston calipers. Integral braking for Quads

i know there is Nissin brakes that could fit also...

Anyway, any help is welcome.

[Matt Chapter]

The size of your master cylinder is on the bottom of the cylinder.  You would want one of about the same size. I've heard of some as small as 13mm and as large as 16mm.

You could also try and find a replacement perch, from a scrap yard.  www.beemerboneyard.com is one, there are others.

[Barry]

Same size or a little smaller. 

I suspect you will have a 16mm and could perhaps get a better brake feel with a 15mm.

[guyom]

thanks guys, Beringer proposed me

BAR14/BAO14 (Ø piston 20.6mm) for 2 caliper of 2/4-or-6-pistons.

[Matt Chapter]

(Ø piston 20.6mm)

A 20 mm piston seems well away from the norm!  You have dual rotors, but only a single piston per caliper.  I think 20 mm is probably too big.

[wilcom]

As an example, Beringer brake company propose différent type :

I went to the site and they are suggesting master cylinders for single cylinder bike and multi cylinder bikes along with ATV's. I find this confusing. When choosing the size of the master cylinder it should be the calipers that dictate the size, not how big the motor is.

If you still have the old master cylinder it will be stamped with the bore size. 13-16 mm......... get the same size as the one you took off.

As Barry suggested, 1 mm smaller might give better feel

[Matt Chapter]

single cylinder bike and multi cylinder bikes

Replace "bikes" with "calipers" and you're in the zone.  Also remember pistons are cylinders. 

[Barry]

Choosing the correct master cylinder is about the hydraulic ratio between master and caliper pistons and here is an  illustration of why I suggested a smaller master cylinder.

You will also need to do the calculations if none standard calipers are to be used.

Example for my 79 model:

Master cylinder 13mm = piston area of 132.7 mm2

Caliper piston 36mm = Piston area of 1018  mm2 
Caliper has 2 pistons therefore total area 2036 mm2

Hydraulic ratio = 2036/132.7    = 15.3

Hydraulic ratio for  twin disc model with 16mm master cylinder =  20.25

It's generally accepted that BMW were very conservative with hydraulic ratios and the early single disc model is particularly low which provides a very firm brake lever with little travel  but a relatively high effort is needed to get the brakes to work. 

For target numbers this web set recommends hydraulic ratios  in the range 23 to 27 in order to get decent feel at the lever.

http:// https://www.vintagebrake.com/mastercylinder.htm