I'm trying to match up the front master cylinders I have to the front calipers/rotors of the project bikes languishing in the barn, and I've been referring to the magic Vintage Brake ratio chart. http://vintagebrake.com/mastercylinder.htm (http://vintagebrake.com/mastercylinder.htm)
I started out looking for a suitable master for my RD350 with 6-piston TL1000R caliper and 320mm disc (I know): four 27mm pistons and two 24mm pistons is 3193.38 square mm. I have on hand a 1/2" master (CBR600F2) that the chart says will yield a 25.2:1 ratio, which is right in the sweet spot. But...that F2's four 25.335mm pistons are 2015.45 square mm, which the chart puts at 15.9:1, well into the "wooden" category. I can't imagine Honda getting it wrong, so I checked the Ol Faithful RZ for comparison. It has two stock 38.18 pistons (2288.6 square mm) acted upon by a 5/8" master, which the chart says is an 11.46:1 ratio; oughtta be awful. It isn't, and hasn't been for 20 years. I must be doing something wrong, and it's surely my math but I can't find my mistake(s). Anybody feel like crunching numbers?
I think your numbers are right but the lever feel may have a lot to do with the master cylinder brake lever pull to piston movement ratio. If 2 master cylinders have the same bore but one pushes, say, 2 square cm of fluid with a full stroke of the lever, pulling it back to the handlebars, and the other only pushes 1.5 square cm, the one that's 1.5 is going to produce more pressure than the 2 cm one, with the same amount of force your hand is exerting on the brake lever. BTW, something must not be right with my RZ front brake. It doesn't have a quick bite and requires a harder pull than my RD400 that has a 1/2" master. :pop:
Edit: oops, make that cubic cm not square
^ hes right. IMHO there's a bit too much focus on bore vs lever geometry and fluid displacement in these discussions, typically. both need to be considered. start with bore, then its more of a trial and error with 'feel' in regards to lever feedback. Ive never seen 'x lever travel = y amount of fluid displaced' upon master cyl specs, and it does matter.
MY opinion/observation is that newer masters typically will have more master piston travel (than older ones) per x amount of lever travel, therefore allowing for a smaller bore master, due to lever fulcrum geometry. .02
It's never simple, is it? I hadn't gotten around to considering master cylinder stroke, but I was pretty sure disc and/or wheel diameter were going to come into play. It seems trial and error are (is?) going to be the path forward.
And btw 1976RD400C, my RZ doesn't have the stock master any longer. The 5/8" unit it's using is a remote reservoir Honda or maybe Suzuki item that a younger me thought looked cooler, and by luck it improved brake operation. I'm still a sucker for the remote reservoir look, but I'd like to think I'm considering the physics of the system these days.
Thank you gentlemen!
I have been using radial master cylinders on the last 4-5 projects I have done.
But I hate lever travel and prefer one finger braking.
I received some advice over on the RD/RZ site, similar to what has been said here, and found this gem as well:
https://www.wristtwisters.com/threads/calculator-for-finding-the-proper-size-radial-master-cylinder.48345/ (https://www.wristtwisters.com/threads/calculator-for-finding-the-proper-size-radial-master-cylinder.48345/)
Ordered a CBR1000RR 16mm radial master cylinder tonight; wish me luck.
And today it's here:
(https://i.postimg.cc/PrsQLJyK/31-D9-FC5-C-AC68-4369-A7-AC-EAEBFED0-BC16.jpg)
aaand if you look at the geometry of the pivot to the piston, this is what makes radials work so well. nothing magic overall.
I bought a 17mm Brembo RCS for my track bike and decided I needed a 19mm. So I kept the 17mm RCS and mounted it to the RD. I'm running a single "Gold Dot" Yamaha caliper and FZR disc. You can adjust the piston stroke to give more power/feel. It might be overkill but it sure stops well now. Previously had a Mike's XS "Slimline" MC and honestly, it was crap. Radial is the way to go imo.