bar-mounted mc upgrade question

Hi all,

Bike is a 1980 R100RS but with '84 Brembo forks / brakes. It came with under-tank master cylinder and I am upgrading to handlebar-mounted.

I just have one question (well, so far anyway) - what is the difference in braking efficiency between the 14mm MC and the 15mm one?

I just bought the 14mm one, it has the dual holes for my existing throttle cables, and I want to keep that setup. But looking closer at the somewhat confusing varoious fiches, it looks like the 15mm one might be more correct for my bike.

The 15mm one does not have the provision for dual throttle cables. I really don't want that little cable splitter and having to buy three more cables if I can get away without it.

If the performance differnce between 14mm and 15mm is not that noticeable, I will keep what I have.

Thanks in advance,

Eric

Do you know the size of the under tank master cylinder? For an '80 R100RS, the fiche suggests it was 17mm. Something on Snowbum's site suggests that making a conversion, decreasing 1 or 2mm in bore size would be fine. I always have trouble understanding the fluid dynamics here, but if you go to a smaller bore, you will have to pull the hand lever farther to push the same amount of fluid as the larger bore. If that's the situation, then I guess the concern would be is that going to be too much of a lever pull...will you run out of movement on the lever.

The Vintage Brake website is gone which had a sizing chart. But I found something from a Ducati website that might be illustrative. I'm not sure I understand it but check it out:

http://ducatimeccanica.com/brake_ratios.html

Thanks Kurt. That chart is confusing to me, I'm not sure what the numbers mean. In other words, my wheel cylinders have 38mm single piston. Looking under 15mm and 14mm is a number a lot ower than his 27:1 ratio in the paragraph above. This probably has more to do with my lack of understanding than in lack of data...

I'm with you! You might try google a phrase like "how to size motorcycle master cylinder" and check out some websites. Beware, lots of math!!

20774 said:

Beware, lots of math!!

Click to expand...

Oh great LOL!

If you decrease the size of the master cylinder, it will increase the lever travel as previously stated, but it also increases the hydraulic advantage and makes the brakes require less effort. If the bike has dual Brembo brakes, you may want to go with a 13MM Magura master cylinder as that was shown as standard in the fiche for the 85-87 R80s with dual Brembos. I just spent a few minutes standing on my head and trying to read the numbers on the master cylinder of my 86 which has factory dual Brembos and it does indeed appear to have "13" cast into the bottom of the MC.

The set-up on my bike has a single hose going from the master cylinder to the left caliper and a rigid metal pipe going from the left caliper to the right one. It originally had about 9 feet of hose and a very complicated looking junction block under the tank. I replaced it with a single braided line direct to the left caliper. That improved feel a lot. I think I sourced the line from Spiegler.

My bike has EBC floating disks and grippy "Competition" pads, but they still take more effort than current bikes.

One of the search results was a spreadsheet calculator. I measured as closely as possible the pivot-to-finger distance, and the pivot-to-piston distance, and plugged them in with 27:1 ratio as goal. Using 2 pistons at 38mm diameter each, the spreadsheet yields "14.3626mm" mc piston diameter. But very slight adjustments in the pivot lengths yield from 13 to 15mm mc piston. So it seems like a matter of feel.

I would think the pivot lengths would be specified somewhere so you don't have to make rough measurements. It's very confusing still. I can't attach the spreadsheet (invalid file), so here's a copy/paste:

Master Cylinder Piston Diamater Calculation
All dimensions in millimeters
Desired ratio 28 :1
Caliper large piston diameter 38
Caliper mid piston diameter 0
Caliper small piston diameter 0
Number of large pistons 2
Master cylinder lever pivot to finger 50
Master cylinder lever pivot to piston 25
Master cylinder piston diameter 14.3626

Eric -

Thanks for that! You could save the spreadsheet off to a PDF and upload that. Or you could take a snapshot of the work area of the spreadsheet and upload that image. Just some future ideas.

BTW...do you have a link to the website you used?

It's at the bottom of the first post on this page:

https://www.wristtwisters.com/threa...the-proper-size-radial-master-cylinder.48345/

He has the extension as "xlr" instead of "xls". I just downloaded it and changed the extension.

I just wish I knew the spec for this Magura 14mm master cylinder inetad of having to roughly measure the pivot-to-finger and pivot-to-piston.

But since my rough measurements give me roughly a 14mm piston for a 27:1 "feel" ratio, I think I am in good shape with the 14mm one I have now (just came yesterday from EME).

Thanks...requires a dropbox login which I'm less than inclined to check out.

I already have one for work. I can email the spreadsheet to you if you're interested.

I think you'll be fine with the 14.

If it helps, my Street Triple has an adjustable MC -- 19mm-21mm. At 21mm the travel is shorter and requires more effort; I find 19mm suits me better. That said the difference in feel is underwhelming.

ccolwell said:

If it helps, my Street Triple has an adjustable MC -- 19mm-21mm. At 21mm the travel is shorter and requires more effort; I find 19mm suits me better. That said the difference in feel is underwhelming.

Click to expand...

They have been discussing bore diameter. How is your bore diameter adjustable? Or what is??

Well, for other reasons I am going with the 15mm. I was swayed by Tom Cutter's observations from a post on the airheads list. I'm not sure if it violates the spirit of any guidelines, but I can post them in this thread if it would be OK.

But the actual decision-maker was the fact that the 14mm one I bought came with an incorrect throttle cam cover and is apparently one of a defective lot from Der Vaterland and needs to be sent back. So I am returning that one, going with the 15mm one, and getting of course the single - splitter - dual throttle cable setup.

13mm Master Cylinder For Dual 38mm Dual Brembo Calipers Is IDEAL

I loathed the rock hard lever and poor braking power of my 1984 R100RS brakes with the 15mm master cylinder ever since I bought it 4 years ago. I read everything I could find about master cylinder to piston ratios and ended up going with a 13mm unit. It is night and day better - much more progressive lever feel (but absolutely not spongy), and noticeably better stopping power. Hard to believe that BMW specified 16mm (to late 1981) and 15mm (through 1984) master cylinders for the 38mm Brembos. For the 1988 model year, also running dual 38mm-pistoned Brembos, BMW got it right with a 13mm unit, but for whatever reason, in 1989-1991 went to a 14mm
unit. You wouldn't think that a 1 or two millimeter difference in size could be felt or make a difference in braking performance, but it sure does. I imagine a 16mm bore would be horrendous on both fronts.

I also recommend going with sintered pads, NOT organic.

Jim

If I were you and could stand the expense, I would convert to the one into two throttle cable setup. Yes, with splitter.

Compared to my '78 S, my '84 RS with the splitter setup hardly ever required throttle synchronization adjustment once correctly set. Like never for years.

The split set up removes almost all possibility of cable stretch affecting anything.

It's a great upgrade.