Master cylinder selection

yamaman · Posted: September 16, 2009, 10:59 am Post subject: Master cylinder selection

There's plenty of different braking systems around, the following refers to XS brakes, though is applicable in most other situations.

The XS, as most of you know, came out with 2 types of front disc callipers, the early ones (pre '77) had 34mm forks. These callipers were cast iron twin piston opposed 48mm bore.
All the later callipers were aluminium, single piston with a 40mm bore. These all came with 35mm forks.

So what master cylinder should you use in your application?
The Vintage Brake website www.vintagebrake.com/m...linder.htm
has a great chart, with recomended m/c's

icheal Morse (MMM) recomends a hydraulic ratio of around 27:1 (which is to say 27mm of movement at the lever will give you 1mm of movement at the calliper), down to 23:1 when using twin piston opposed callipers.
When using single piston callipers, he suggests a hydraulic ratio of around 14:1 down to 12:1.

Well what the hell does that all mean?????

I'll throw in a couple of examples:
twin disc, old style twin piston callipers.
The closest MMM's chart gets to a match here is a 19mm m/c.
To work this out, we need the area of the calliper pistons....
48mm diameter = 1810mm square, 1810 x 4 (amount of working pistons) = 7240mm square

And the area of the 19mm m/c = 284mm square

Then to get the ratio, 7240 divided by 284 = 25.5:1

Example 2:
twin disc new style single piston 40mm callipers:
MM recommends around a 16mm m/c
40mm piston = 1257mm square x 2 = 2514mm square
16mm m/c = 201mm square
2514 divided by 201 = 12.5:1

I don't offer this as difinitive advice, just trying to get the info out there so you can make your own choices. I'd suggest a chat with MMM before making any mods, he has a lot of knowledge on the subject (a hell of a lot more than me).
I know that some in the past have made the wrong choices, just trying to help some out yet to walk that path!

yamaman · Posted: September 16, 2009, 1:50 pm Post subject: Re: Master cylinder selection

I suppose seeing as I'm almost there, I might as well add in single disc set ups.

Single disc, twin piston, MMM recommends 13mm m/c
48mm diameter piston x 2 = 3620mm square
13mm m/c = 133mm square = 27:1
14mm m/c = 154mm square = 23.5:1

Single disc, single piston MMM says 11mm
40mm piston = 1257mm square
11mm m/c = 95mm square = 13.2:1 ratio

ikes offer a 22mm m/c for the twin piston callipers on a twin disc set up
7240mm square (from above post)
22mm m/c = 380mm square = 19:1

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