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Old 06-20-2010, 01:47 PM   #1
LasseNC OP
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Airhead master cylinder diameter

Hello!

I am looking for a replacement mastercylinder for my MZ. They used the same calipers as the old airheads for many years.

What is the diameter and how many discs? :)

Resembles this type.

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Old 06-20-2010, 06:01 PM   #2
jdiaz
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It looks identical to the caliper on my 1986 BMW R80RT, which uses a 13mm master cylinder and dual discs.
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Old 06-21-2010, 12:42 AM   #3
pommie john
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The Brembo master cylinders are ( or were) available in three sizes.
12mm for a single disc. ( Known as the PF12)
13mm for single disc with a more solid feel
15mm for a twin disc.

( There may a 14mm but I'm not sure)


My Norton commando has a single 08 caliper as per your photo and I have a PF12 M/C.

My BMW has twin discs and I use a PF 15 on it.

pommie john screwed with this post 06-21-2010 at 12:48 AM
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Old 06-21-2010, 03:11 AM   #4
Beemerboff
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Some Guzzis had a 11mm which works well with the small caliper, and the big one too , for that matter.

Guzzis used it with twin large calipers, and no one ever complains , but by some strange happening put it on a Beemer and it is useless, according to everybody who hasn't tried it.
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Old 06-21-2010, 09:10 AM   #5
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Quote:
Originally Posted by LasseNC
What is the diameter and how many discs? :)
Ok, this is something that I have spent some time on with my sidecar rig and converting a 1986 R65 to dual-disk.

WARNING: Geeky science content!

First, there were two different sizes of that style (Brembo) caliper used by BMW.
(The early R65 actually used an ATE caliper of that style as well...)

The single-disk models came with 48mm piston calipers,
The dual-disk models came with 38mm piston calipers.

BMW also has FOUR different master cylinder diameters available for the handlebar mounted cylinder units: 13, 14, 15, & 16mm.

The stock single-disk setup was a 13mm m/c with the 48mm caliper.
That resulted in a lever ratio (leverage ratio) of 27.3:1.

The original R100RT came with a 16mm m/c with two 38mm calipers.
That resulted in a lever ratio of only 22.6:1, which turned out to be too stiff at the lever.
The following year and all later Airheads with dual Brembos came with a 15mm m/c and 38mm calipers for a lever ratio of 25.7:1.

Those were the BMW factory configurations with the stock 285mm diameter rotors.

-------------------------

When I went to dual disks on my 1986 R65 I found a second 48mm caliper to match what was already installed rather than sourcing a pair of the 38mm ones.
For the master cylinder, at first I tried going up just one size to a 14mm bore, but found that it just did not have enough volume to pump up the larger calipers properly.
I could easily bring the lever all the way to the grip.
(2x 48mm caliper / 14mm m/c = 47.0 lever ratio)

What I did instead was swap in the 15mm master cylinder from my R100RT and put the 14mm on the RT. That gave a firmer lever for the R65, and more lever feel for the RT.

So what I ended up with is:
R65: 2x 48mm caliper - 15mm m/c = 41.0 lever ratio
R100RT: 2x 38mm caliper - 14mm m/c = 29.5 lever ratio

The R65 lever is definitely softer, but still has plenty of stopping power!
There is just a wide band to be able to modulate the brakes in now.

For the R100RT, the change down one size did ease up the amount of lever effort needed to get an equivelant amount of stopping force, but did not appreciably change the amount of lever travel. I might have even tried a 13mm master cylinder with the pair of 38mm calipers.

--------------------

For comparison purposes, here are the lever ratios for a couple of more modern bikes with excellent brakes.

R100R: Lever ratio - 38.76 (dual disk)
R1100RT: Lever ratio - 34.06
R1150R: Lever ratio - 36.25
R1200RT: Lever ratio - 36.25

So mid-to-upper 30's seems to be a decent target to aim for.

--------------------

Basically, just calculate the total piston area of the caliper(s) and divide by the piston area of your master cylinder to arrive at your lever ratio.

Or PM me and I have a Excel spreadsheet with the areas/ratios already mapped out for the possible combinations for both the early ATE and later Brembo brakes for the BMW Airheads.
I can send you a copy of the spreadsheet via email for you to play with at your leisure...

HTH
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Old 06-21-2010, 09:37 AM   #6
AntonLargiader
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Quote:
Originally Posted by BMWzenrider
BMW also has FOUR different master cylinder diameters available for the handlebar mounted cylinder units: 13, 14, 15, & 16mm.
There's also a 12. I have one on order to try on a G/S with a 320mm rotor.

The lever ratio calculations are fine if the lever geometry (leverage around the pivot point) is comparable. If not, you need to scale the other numbers accordingly. I have most of those levers here so I guess I could measure and post numbers.
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Old 06-21-2010, 11:52 AM   #7
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Quote:
Originally Posted by AntonLargiader
The lever ratio calculations are fine if the lever geometry (leverage around the pivot point) is comparable. If not, you need to scale the other numbers accordingly. I have most of those levers here so I guess I could measure and post numbers.
Yes, that is true, I thought about mentioning the physical lever pivot/contact dimensions as well.
But since I was refering specifically to the interchangable master cylinders available for the BMW handlebar mounted brake units,
I thought that it was safe to neglect that in favor of a somewhat simplified discussion.
For that matter, how far out you grip the lever makes a difference in how much force it takes to squeeze it as well...

I suppose that it could make a difference in making comparative reference to the later style of handgrips on the Oilheads though...

Point taken!
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Old 06-21-2010, 04:31 PM   #8
Beemerboff
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So, to get back to the origonal question , if you are aiming for a m/c ratio in the upper 30s and have have a single 38 mm twin piston caliper , a 9 mm m/c is the way to go, or the the nearest you are going too get to it, whichj is the 11 mm Guzzi item.
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Old 06-21-2010, 05:44 PM   #9
Frank06
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...I have my eye open for a 14mm for my '81 R100RT. It's way too stiff!
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Old 06-22-2010, 05:16 AM   #10
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Quote:
Originally Posted by Frank06
...I have my eye open for a 14mm for my '81 R100RT. It's way too stiff!
My R100RS has a 13mm from the later monolever models. The travel is usually very progressive and feel is excellent. It's much better than the oem 15mm mc it had which lacked feel, was very stiff and likely to lock at lower speeds unless very careful.
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Old 06-22-2010, 08:24 AM   #11
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So, for those of you gathering parts....
I have a 92 R100R with 6 pot Busa Calipers (2) and the original 13mm master.
The Busa Master is a 5/8" (about 16mm) No idea why it isn't metric.....

So, who has a 15mm master for me? I have too much travel!
I could pull the lever to the bars, but, the front would lock before that happened. I would like less travel and harder lever feel.

PM me. Thanks
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Old 06-22-2010, 09:09 AM   #12
BMWzenrider
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Quote:
Originally Posted by C Squared
So, for those of you gathering parts....
I have a 92 R100R with 6 pot Busa Calipers (2) and the original 13mm master.

So, who has a 15mm master for me?
Ok, the single-disk R100R came with the 13mm master.
The DUAL-disk R100R came with the 15mm master that you are looking for.

Now for the bad news...
That is a handlebar unit that is unique to that model bike.
It ONLY came on the R100R/Mystic models...
There does not appear to be any other compatible master cylinder in that size.

I would check with the usual suspects at the various BMW boneyards to see if they have what you are looking for.

Otherwise, the part is listed for $286.58 on the MAX BMW online parts fiche.
32712314027 HANDLE UNIT, RIGHT - D=15MM $286.58

HTH
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Old 06-22-2010, 09:13 AM   #13
BMWzenrider
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Quote:
Originally Posted by boxerboy81
My R100RS has a 13mm from the later monolever models. The travel is usually very progressive and feel is excellent. It's much better than the oem 15mm mc it had which lacked feel, was very stiff and likely to lock at lower speeds unless very careful.
I agree, the stock 15mm m/c was way too "wooden" for my tastes. Especially when adding the extra inertia of a sidecar to the mix.
Would have like to try the 13mm m/c myself, but I already had the 14mm one on hand, and I am a cheap bastard...

For reference to the above lever ratios;
2x 38mm Brembo Calipers with 13mm m/c = 34.2 lever ratio.

Right in the target zone, IMHO...
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Old 06-22-2010, 09:17 AM   #14
BMWzenrider
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Quote:
Originally Posted by Frank06
...I have my eye open for a 14mm for my '81 R100RT. It's way too stiff!
Frank, you have the same dual-38mm calipers as boxerboy81.

I would go the route he did and find a 13mm master cylinder.
There are lots of those available from Airhead GS pilots who upgraded to larger calipers and changed out their m/c...

(No, C_Squared's m/c won't work on your RT.... sorry....)
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Old 06-22-2010, 09:52 AM   #15
BMWzenrider
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Lever ratios for BMW Airhead handlebar mounted Master Cylinders w/Brembo twin-piston brake calipers:
-------------------------------------------
........1-38mm 1-48mm . 2-38mm 2-48mm
m/c - Brembo . Brembo . Brembo . Brembo
11mm - 23.87 _ 38.08 _ 47.74 _ 76.17
12mm - 20.06 _ 32.00 _ 40.11 _ 64.00
13mm - 17.09 _ 27.27 _ 34.18 _ 54.53
14mm - 14.73 _ 23.51 _ 29.47 _ 47.02
15mm - 12.84 _ 20.48 _ 25.67 _ 40.96
16mm - 11.28 _ 18.00 _ 22.56 _ 36.00
-------------------------------------------
NOTE!!! All possible combinations shown, not all combinations recommended or safe to use!
Use extreme care when modifying the braking system on your vehicle.
Your LIFE depends upon it!!!

{11mm m/c is Moto Guzzi part that is stated to fit per other party, not verified by myself.}
Please only quote this posting with all text/disclaimers attached.
Thank you!
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