Large or small diameter master cylinder for a single rotor brake setup?

[Rudemouthsky]

   I had a minor fiasco with the custom lines I ordered for my build being unsuitable (my fault, and now stuck wit em') so I've decided to switch gears and use a single caliper/rotor setup for my 944 project bike. I have 65mm spacing on my rebuilt forks, which limits me a bit. My sales guy is pushing hard for me to keep the 19RCS master for this project but I have my doubts...Brembo themselves do not recommend the 19 for anything other than dual caliper applications. His theory is: bigger piston = more power to the single rotor. I'm not sure if I buy that...espec considering the number of retailers that only recommend the 15RCS when the bike is primarily ridden on the street. Which it would seem to me, if the power comes on too aggressively with 2 rotors, this would only be exacerbated when using one.

I'm trying to avoid a setup that has little to no feel and unfriendly onset of power for a commuter bike.

An example of a setup that sounds nice to me for a mildly hopped up 900 is;

A single 6 pot ISR billet caliper mated with a Braketech Iron Rotor
A 4 pot 4 pad Brembo with the same rotor, both using the 15RCS

In summary, I'm seeking opinions on which size master to use; larger or smaller.  Thanks!

[ducpainter]

There's a reason Ducati uses a smaller diameter master on the single rotor bikes.

The sales guy is wrong.

[koko64]

+1 ^
Be good to get some advice from a brake specialist.
The big bore M/C might work with the 6 piston caliper depending on the piston diameters/stroke, but you better check. Brembo's spec on the 4 piston caliper is clear enough.
Some of the feel Vs power issues are personal preference, but you can spend a lot of money sorting that out without a brake guy pointing you in the right direction.

Hopefully some specialist folk in these matters chime in.

[Rudemouthsky]

Thanks Tony and Nate.

I do feel pretty confident in my riding experience to the point that I don't want to leave any braking power on the table if I don't *have* to. I don't wanna be doing endos every time I need to slow down in a hurry but I also wouldn't mind having a 1-2 finger brake at any speed.

[thorn14]

I'm running a 16mm master with single rotor 2pad 4 piston with a stainless disc and random organic pads, but am going to the braketech iron and 4pad 4 piston once I order brake lines.

Here's some more info I've found when poking around initially.

http://www.vintagebrake.com/mastercylinder.htm
http://www.ducati-upnorth.com/tech/radialbrakes.php

Definitely downgrade the master cylinder size for single rotor, as you don't need as much force.

[ducpainter]

Like Tony says...

a large bore master will greatly reduce the feel. Your brakes might be like a light switch.

With a radial master you should be able to play with lever ratios to get both the power and feel.

A good friend used to run a single disc on a 750 Monster track bike...the thing would get so hot he couldn't keep rotors on it...of any brand.

[koko64]

Good articles thorn. They say it well. Cheers

[Rudemouthsky]

Thanks Thorn.

If you're interested in my brand new Fren Tubo braided lines, lemme know. They are exactly 2" longer than the stock M750 lines, not sure if that'll work for you or not.

Can any of you guys tell me if these brackets:

http://motowheels.com/i-8518113-corse-dynamics-axial-caliper-adapters-65mm-to-40mm.html

Combined with this caliper:

http://www.ebay.com/itm/Brembo-Front-40mm-Axial-Billet-2-Piece-P4-30-34-Caliper-Left-/261452977835

Will work with my S2R800 fork bottoms? afaik, all the Duc 65mm mounts are the same, whether they be Ohlins, Showa, or Marzocchi.

Bracket adapters kinda make me nervous for things like brakes, but oh well.

That's good to know about your friend with the single rotor, but I don't plan on flogging mine too bad. Some aggressive riding in the metroparks on the weekends here and there.

[koko64]

From the links it seems that's the adapter kit brackets to go from 65mm calipers to the 40mm Billet ones. You going twin or single in the end?

[Rudemouthsky]

From the links it seems that's the adapter kit brackets to go from 65mm calipers to the 40mm Billet ones. You going twin or single in the end?

Leaning heavily toward a single caliper now.

[DarkMonster620]

why don't you try this one,

http://www.cyclebrakes.com/html/accossato.html
http://www.cyclebrakes.com/AMAG027.jpg

And, BTW, don't you have a twin rotor set up? with rotors, calipers and 65>40 adapters?

[thorn14]

Why are you going down to a 40mm mount? I don't get the advantage of the caliper you posted so maybe I am missing something.

[Speeddog]

On master cylinders:
Bigger piston = less "leverage", firmer lever. Less braking power for a particular force at the lever.
Smaller piston = more leverage, softer lever. More braking power for a particular force at the lever.

Oddly enough, I've tried some of the combinations you're thinking about.

I've got a single 4-pad Brembo caliper on my M750, CP211 pads.
Originally running on a Brembo snowflake rotor, as that's what was on the bike.

Originally set up with a 16mm goldline master, as that's what I had in my spares box.
The lever felt like it was bolted to the bar.
Not much 'feel', and lever effort was higher than I wanted.
If you want it to feel like an 80's motorcycle, it'd be about right, at least as far as the lever effort to decel ratio is concerned.

Switched to a Brembo big-pivot 13mm master.
Lever feel is normal, but a bit softer than I like.
Lever effort is fine.

I'd like a firmer lever, if there were a 14mm Brembo big-pivot, I'd get one.

After I wore out the snowflake, I got an iron BrakeTech.
Feels like a bit more grip.

I've had no braking power problems with the single disc setup.
It *is* a 750, and I don't ride in the Swiss Alps, nor ride like a maniac.

Dunno what master would be appropriate for the 6-piston caliper, would need the piston diameter specs.

As far as an adapter and that billet Brembo 30/34.....
For that money you could get a pair of brand new Brembo 4-pads, a new line, and still have a Benjamin left over.

Another thing to keep in mind, the lever geometry is different between radial, small-pivot, and big-pivot.
That is, the leverage between your hand and the piston is different.

[Speeddog]

Regarding the lever geometry.

I took some measurements of the 3 styles of levers.
Not super-accurate, as not everyone has their fingers in the same spot, and there are differences between OEM and aftermarket levers.

I measured from the outboard side of my first finger to the lever pivot, dividing that by the distance from the lever pivot to pushrod pivot/socket center.
That would give an approximate ratio for the leverage a two-finger squeeze has over the piston.

Radial - - - - 5.67:1
Big-pivot - - 4.16:1
Small-pivot - 4.02:1

This explains my experience with the small-pivot master feeling a bit squishy compared to a same size piston small-pivot master, it's got more leverage.

Using the above leverage ratios, a 16mm piston small-pivot master will have about the same lever feel as a Radial with a 19mm piston.

[stopintime]

ISR recommend a 17.5mm master for a pair of their 6 x 30mm piston'ed calipers and a 14mm for single set up.

http://www.isrbrakes.se/products/calipers/22-052.shtml

...............

I've had no braking power problems with the single disc setup.
It *is* a 750, and I don't ride in the Swiss Alps, nor ride like a maniac.

, but I do   and there's no way I'd go for a single disc - fast road riding or track