Best Master size?

[Mr Gazza]

Okay, this might all be slightly academic. I'm trying to get straight in my head which would be the best piston size to go for to operate a single Brembo Goldline four pot calliper.

The benchmark is my twin Goldline setup on my M900Sie with (I think) a 16mm master piston... I just can't conceive of a better brake. I achieved some unbelievable stopping distances on a machine handling course last year.

When I fitted twin discs to my 750SS many years ago, I retained the standard master and observed much more lever travel after the conversion. This was good as it improved the feel and sensitivity of the lever with double the braking power.
I don't know what size a standard single disc master is on a 750SS?

I am assuming that a larger master piston.. Let's say 18-20mm would render very little lever movement feeding a single calliper and would probably be rather unforgiving to too much insensitive use? Probably giving very little feedback?

Presumably larger pistons have come about as the capacity of the callipers has increased?
It seems important to me to balance the master to the calliper(s) to achieve the correct ratio of power to feel.

My thinking is that a smaller than 16mm master would be what I'm looking for?

Thoughts greatly received..

[Dukedesmo]

I've got a 19mm RCS master cylinder on my Monster which has twin 4 pot calipers, also got a 19mm on the 916. These are both radial calipers but I did have the 19mm on the 916 previously with twin goldline calipers and to me these are both spot-on.

Obviously for a single caliper, smaller is required and as a comparison my Guzzi has twin 2 pot Brembo (P8) calipers, originally linked one with the rear and the other individual but I de-linked and setup the front to operate as a 'normal' bike, the original master cylinder (for 1 caliper) was a 12mm and I now have a 15mm Brembo RCS operating both calipers/4 pistons and it is also spot-on.

Now I believe that the P8 pistons maybe bigger than those in a 4 pot goldline? so that may cause some variation and I wasn't 100% certain that it would even suit mine when I did it but the RCS master cylinder has a switchable ratio making it more 'versatile' so I took the chance - I would think a 15mm would be good.


Also I believe that more recent Ducatis (Panigale etc.) have smaller pistons in the 4 pot calipers (for weight saving?) and therefore use a smaller master cylinder?

Not sure how much help the above is, just my ramblings but maybe of interest?

[Luddite]

For what it's worth, the single disc Monsters with a four-pot 30/34mm caliper, used a 13mm master cylinder.

If you wanted to be scientific about it, you could try to replicate the leverage ratios of your 750SS setup, although that might be tricky as you don't know the original master cylinder size.

For reference though, using a 13mm m/c and a P4 30/34 caliper would give a ratio of about 24.33/1

M/c area
13mm = 3.14 x 6.5² = 132.665mm²

Caliper piston area
30mm = 3.14 x 15² = 706.5mm² x 2 = 1413mm²
34mm = 3.14 x 17² = 907.46mm² x 2 = 1815mm²

Total caliper area = 3228mm²

Ratio = 3228mm²/132.665mm² = 24.33/1

To be more accurate, that figure should be multiplied by the lever's interaxis ratio (i.e. the ratio of the pivot to piston distance against the pivot to finger distance) but, as you can't measure the old one and it's likely that both levers would be pretty similar anyway, I don't think it would make much difference in this case.

Hope that helps.

[Pedro]

If memory serves correct, single disc monsters use a 16mm master, twin disc uses a 19mm

[Luddite]

Thought I'd better consult the 'bible' to be sure, and it confirms that the old single disc 600s used a 13mm m/c (part no. 62440091A). It looks like it was also used on the single disc M750 too.



Still doesn't tell us what was used on your 750SS though!

However, if you want to go modern, you could fit a Brembo RCS m/c like Dukedesmo's and that would allow you to fine tune the feel by changing the pivot point. Brembo recommend a 15mm 15RCS for single disc, four piston setups.



The part no. is 110A26330 (they come with long or short levers - that's the long lever number).

If you're thinking of ordering one, I can recommended Simon at WSC Performance. He did me a really good deal on the Serie Oro discs I bought recently. http://www.ukmonster.co.uk/monster/s...ad.php?t=56141

[Darren69]

IIRC the PS13 master has a smaller reservoir than the 16mm one. I can't remember what I had on my 750SS, it was so long ago and had been converted to twin disk by the previous owner anyway.

[Mr Gazza]

Thanks for some fabulous answers guys. You're telling me so much and it's all clicking now.

I have no doubt that the (1992) 750SS and Monster used the same parts, so it's safe to assume that I was using a 13mm piston on that. Yes Darren the dual disc master did have a taller reservoir (and larger piston).

Thank you so much for the formulae Luddite, I can run with that once I have some hard facts about the various piston sizes that I have used and propose to use, to get a better idea of how the different ratios feel and operate.
Thanks also for explaining how the lever ratios can be manipulated. I thought all levers were fixed geometry and also wondered how a ratio could be adjustable without altering the piston size... All is clear now. That is assuming that the pivot pin is eccentric?

Thanks for your observations DukeDesmo, I can relate to all that, as it reflects my own experience.

I am still not sure what pistons I have in my 2000 M900Sie?
There are numbers on the clutch and brake masters respectively, 9k26 12 and 9k13 16.
It does make me think that I have 12mm and 16mm masters?
I had it in my head from test riding an S4 when they first came out, My first try with Goldline brakes. I'm sure the salesman said the brake master was bigger at 16mm than my '98 Monster (coffins)... Maybe he said 19mmm, it was a long time ago!
General consensus seems to be 19mm for Twin Goldline.

But anyhow thanks all for giving me a much better grasp of this...

[Luddite]

Originally Posted by Mr Gazza

Thanks also for explaining how the lever ratios can be manipulated. I thought all levers were fixed geometry and also wondered how a ratio could be adjustable without altering the piston size... All is clear now. That is assuming that the pivot pin is eccentric?

It's a bit like shifting your weight on a seesaw and easier to grasp if you see how the m/c works:



With the pivot further from the piston, you need more effort but less movement whereas with the pivot closer to the piston, you feel less effort but more movement.

Nice design!