Front Master Cylinder

rbt11548 · Mar 2, 2009

cash said:
In my early days of messing about with m/c sizes I found the 11mm could be an absolute pig to bleed unless there was enough stroke to give plenty of flow, and for me 11mm gave too much lever movement. I've used 13mm set-up for near 14 years and find it more than adequate, with two fingers I can stand the Commando on its front wheel. (not intentionally I add)
I reckon anything between 13mm and 11mm will be OK but that 0.1 less might be the last straw.

Cash

Thanks Cash, just trying to get one without breaking the bank, have fitted RGM's grimeca kit with disc caliper, longer lever etc., so listening to other guys on the site the master cyl addition is the best way forward.

JimC · Mar 2, 2009

The smaller master will take longer to push fluid to the caliper bleeder. Design and/or bleeding technique will make a difference. I know you can use conventional bleeding on my ABS BMW till you are dizzy and never get the brakes bled. Power bleeding turns it into a five minute job. I use a small pump up sprayer with a hose connected to a barb fitted to the reservoir cap . Works like magic.

As to the 27:1 ratio, I would not vary much. I think a 12mm master is probably adequate with a 41mm two piston caliper. This is based upon personal experience. As well, Michael Morris of Vintage Brake knows brakes.

Ron L · Mar 2, 2009

As well, Michael Morris of Vintage Brake knows brakes

And if you read Mercury Morse' comments, he is aware of other factors.

Quote
"This critical ratio is of paramount importance in determining "feel". It has been my experience that there is a "sweet spot" in the range. I like ratios in the 27:1 range-2 finger power brakes, feeling some line and/or caliper flex. 23:1 is at the other end of the spectrum-firm. Ratios lower than 20:1 can result a feel so "wooden" as to have a toggle switch effect: nothing happens until the wheel locks. Disc and wheel diameters must be taken into consideration. A 10 inch disc working against an 19" wheel just doesn't have the leverage ratio that a 13 inch disc working a 17" wheel does. The hand lever ratio counts too: witness the adjustable master cylinders from Lockheed and Brembo."

JimC · Mar 2, 2009

Why does Brembo not make radial mc 's with a dia. smaller than 16 mm ?

I don't think Brembo is aiming at the single two piston caliper market. I'm not real familiar with all Brembo motorcycle brakes, but I think most modern motorcycles are twin caliper four or six piston models. When you calculate the total caliper piston area of these, a 16mm master may very well end up near a 27:1 ratio. Maybe a radial master cylinder will work well with a closer ratio. I think the Vintage Brake chart is for nonradial master cylinders.

I have a single 12" floating rotor with a Grimeca two piston caliper and a Grimeca 11mm master and I have a very good brake on my Combat. In fact, it is probably equal to the Brembo twin disk setup of my 91 K75 BMW. Someday I'll check out the ratio of the BMW setup.

Cheesehead Commando · Mar 3, 2009

Ok, fellas, now I am getting dizzy. The original question referred to the stock MkIII's master cylinder. What is the stock diameter, 11 mm? Assuming a stock 19 inch front wheel, a similar or stock caliper, and a 13 inch (non-stock) rotor, what m/s size would get me into the proper range?

Corona850 · Mar 4, 2009

Cheesehead Commando said:
The original question referred to the stock MkIII's master cylinder ... and a 13 inch rotor ... what m/s size would get me into the proper range?

The stock master cylinder is 5/8" bore.
The diameter of the rotor hasn't got anything directly to do with the hydraulic ratio per se.
It depends what the caliper piston diameters are. The stock Lockheed caliper has 44mm diameter pistons. The Grimeca and AP calipers that are often used are 41mm diameter.
For the 44mm diameter caliper piston, a 12mm master cylinder gives you 27:1 (damn close anyway).
For a 41mm diameter caliper piston, a 11mm master cylinder gives the magic 27:1 ratio.

Whether that ratio (i.e. 27:1) is the ultimate solution or not is subject to a lot of opinions, as this thread demonstrates.

T.C. · Mar 4, 2009

I have to ask....

I was really hoping someone else would ask this, in case the answer is really obvious....

But, if the conventional wisdom is that 11mm is the ideal M/C size with the stock caliper, why do all the conversions for the factory M/C use 13mm?

If you're going to all the trouble and expense, it seems like that would be the thing to do. Is there some technical/structural problem with the factory unit that won't allow an 11mm sleeve?

slupdawg · Mar 4, 2009

I would second that question.........

Keith1069 · Mar 5, 2009

I have the RGM conversion and while it's a big improvement it's not the ultimate in feel or leverage. I guess the answer is it has to be a compromise with the bendy/twisty Norton forks and spongy feel with the 11mm conversion. 16mm (5/8") down to 11mm more than doubles hydraulic leverage. Might be something else though. Maybe they don't want it too effective so they can sell their large twin disc setups or is that cynical of me?

rbt11548 · Mar 5, 2009

Re: I have to ask....

T.C. said:
But, if the conventional wisdom is that 11mm is the ideal M/C size with the stock caliper, why do all the conversions for the factory M/C use 13mm?

It might be a case of a good compromise, as Keith says ' spongy feel with the 11mm conversion' also Cash saying 'the 11mm is hard to bleed' and with overwhelming opinion that the original 5/8" is too big, the 13mm does seem like hitting the middle ground. I think sometimes we over analyse things, yes we all want the a good efficient brake but I think it's all maybe down to the individual, what the person ideally wants, what is available and what they can afford.
When talking to a mechanic at the local bike shop re the fork settings for modern bike with preload, damping, rebound etc., he said almost everyone knows the positions for the basic set up for heavier loads, faster riding etc., adjusting to suit. He said only a factory racing rider, mechanic or suspension specialist could tell the difference between the infinite number of minute adjustments available, and the ordinary rider, although able to tell a change from the basic adjustments can get thrown by over compexity. It could be the same with the brakes?, of all the brake components available what combination works for one person may not suit another and at the end of the day I think it must be an individual choice on what you think is best, but the good thing about this forum is the amount of people with experience out there willing to give you help to decide which is best for you.

JimC · Mar 5, 2009

I see some posts mentioning a 11mm master cylinder without referencing the caliper diameter and number of caliper pistons. If I were to choose a disc brake system I would first decide twin or single. Then the choice of rotor, moving on to caliper choice. Once those choices were made I would opt for a decent caliper area to master cylinder area ratio, using the Vintage brake chart as a guide.

I would not consider bleeding an issue. After all, how many times do you bleed a brake system as opposed to how many times do you use the brake? Any time you increase the caliper to master ratio the feel becomes more spongy. The reason a stock Commando front disc feels solid is because of the lack of force advantage the master cylinder has.

Cheesehead Commando · Mar 6, 2009

Thanks, guys. Now its making sense. Start with the caliper, then work towards the m/c. I agree that 13mm is middle of the road, gets ya in the ballpark. I will probably use the stock caliper, as I can't see a reason to dump it since its rebuilt, etc. So even the 13mm conversion that's being sold gets me close to the 27:1.

JimC · Mar 8, 2009

BTW, don't forget the brake hose. Very important to have a non-expanding type, such as a braided stainless steel one. One that expands will contribute to that spongy feel.

Johnnymac · Oct 31, 2011

Sorry to dig up the old thread...

I'm looking at upgrading my MC. I'm looking at the Brembo MC from Colorado Norton Works or the Andover Norton Upgrade. Does anyone have a picture of the Brembo setup on a MKII? It would be nice if I could keep the exiting switchgear. Matt at CNW said that you can keep the RH Switchgear if you can rig up a mounting back. Had anyone done this?

Thanks!

AussieCombat · Nov 3, 2011

This is what was on mine when I bought it, It has the original Lockhead master cylinder with the modification.
This Grameca works exceptionally well, one finger, under most normal riding conditions.
I take my wife on the back quite often and have never had a fade or 'MOMENT" where I've
thought that they're are not good enough.
Actually I worry more about the added pressure on the Head stem bolt more than anything.
I think they look a little bulky, but they work.
A new braided line is on it's way.
AC.

Front Master Cylinder

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