You are absolutely correct! I feel a tad embarrassed as I've always thought it was just 11/16 for dual brakes & 9/16 for single disk. Everyone I know has always referred to them as such. After further inspection of my master it does have a small 15 stamped on it. All the ones I've seen before have a 11/16 or 9/16 stamped on them. Do you know which bikes the 11/16 came on? They are the same shape as the one I have which is clearly a 15mm after close inspection. Thank you for the correct information as well. 🍻

 

So the Brembo master is smaller than the 11/16 FXDLS master I'm running now?

 

I'm really curious now and a tad concerned. Obviously the bagger brembo is designed to be used with dual disk but I'm wondering if the 11/16 would be the better option.

 

I agree and am confused. There must be a reason. I talked to guys that never swapped their 9/16 master when they went to duals because they claimed it gives better lever feel. I don't know if that's true or not, but I wanted to do the conversion the "right" way, so I got the bigger master.

 

My thoughts exactly. I have also seen guys run the 9/16 master with duals but I was under the impression that the 11/16 master flowed more fluid making it superior.

 

Think of it this way. A small MC you need to make the lever travel more, but is normally easy to apply the brakes. Too big and the lever has little travel, also normally had to apply the brakes.

Brake ratio chart, There may be an easier to read one out there:
http://www.vintagebrake.com/mastercylinder.htm

Tom

 

That's a great read! Thank you. Now I guess I have to figure out my caliper diameter. Are they referring to the diameter of the caliper piston on that chart?

 

Yes, the piston size. And when the note shows 2 or 4, that would be the pistons in each caliper. The only thing the chart does not list is 4 piston calipers with 2 different size pistons. My Brembo's are that way.

If your adding a second caliper, "normally" you would go a little bigger on the MC. For instance a 15mm with single and a 18mm with dual.

Tom

 

Do you happen to know why the bagger brembo mc is 15mm then since they are installed on bikes with dual rotors? And thanks again. I really appreciate the information here!

 

If you're adding a second caliper of the same size you need to double the square of the radius (half the diameter) to get the same "feel". For example, if you have a 15mm master, the fluid output is proportional to 7.5^^2 or 56.25, so, double that (112.5) and take the square root (10.6) so a 20 or 21mm master would move 2 calipers the same distance as the 15 moves one.

Maybe the Brembo calipers have smaller or fewer pistons, or maybe the lever is designed to somehow moves the piston through a longer stroke.