Airhead master cylinder diameter

Discussion in 'Airheads' started by LasseNC, Jun 20, 2010.

  1. LasseNC

    LasseNC XSessive!

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    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.

    [​IMG]
    #1
  2. jdiaz

    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.
    #2
  3. pommie john

    pommie john Long timer

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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.
    #3
  4. Beemerboff

    Beemerboff Long timer

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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.
    #4
  5. BMWzenrider

    BMWzenrider The Road Scholar

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    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
    #5
  6. AntonLargiader

    AntonLargiader Long timer

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    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.
    #6
  7. BMWzenrider

    BMWzenrider The Road Scholar

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    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... :wink:

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

    Point taken!
    #7
  8. Beemerboff

    Beemerboff Long timer

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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.
    #8
  9. Frank06

    Frank06 Been here awhile

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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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  10. boxerboy81

    boxerboy81 Stay Horizontal

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    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.
    #10
  11. C Squared

    C Squared Now with TURBO!

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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
    #11
  12. BMWzenrider

    BMWzenrider The Road Scholar

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    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 :deal
    #12
  13. BMWzenrider

    BMWzenrider The Road Scholar

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    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... :evil

    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...:clap
    #13
  14. BMWzenrider

    BMWzenrider The Road Scholar

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    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....) :2cry
    #14
  15. BMWzenrider

    BMWzenrider The Road Scholar

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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!
    #15
  16. C Squared

    C Squared Now with TURBO!

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    Yea, I just can't see $250+ for a MC

    I am looking into a rebuild kit and then see if I can have my 13mm machined out to 15mm. And maybe sleeved?

    32712314027 HANDLE UNIT, RIGHT - D=15MM $286.58
    #16
  17. BMWzenrider

    BMWzenrider The Road Scholar

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    Yes, if you look at the chart of ratios that I posted, the best you would do with a single 38mm Brembo style caliper would be a lever ratio of 23.87 with a Guzzi 11mm m/c.

    I would guess that it would feel pretty stiff...

    Better to look around for someone who has a cast-off 48mm caliper from a GS who upgraded to a 4-6 piston caliper... IMHO...
    #17
  18. AntonLargiader

    AntonLargiader Long timer

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    If you're looking on the used market, possibly you could use an Oilhead control housing. These are 20mm, but (and I think this is where dissimilar lever ratios step in) they are used on the exact same calipers where the R100R uses a 15mm MC. So if you're looking for the same effect as the 15mm R100R MC, you should find it in the 20mm R1100/K1100 MC. You can probably change out the clutch side and have matching 1100-style housings.

    Hmm, I actually have all this stuff lying around. Maybe I'll update my K75 with it.
    #18
  19. AntonLargiader

    AntonLargiader Long timer

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    Karl,

    I only found an old K lever and an R1100 lever. OK, also an F650 lever but I can't make sense of it without the other parts. Measured to the center of the fingers:

    R1100 lever: 130/18mm or 7.2 leverage
    K75 lever: 85/18mm, or 4.7 leverage

    That's a huge difference. It confirms my suspicion that the highly revered ratio tables are garbage without figuring in ALL of the information. We have people thinking a 10% difference in the piston ratio is meaningful and the lever can vary by 50%. Go figure.
    #19
  20. Beemerboff

    Beemerboff Long timer

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    A quick check on current superbikes in a showroom is informative, all the brake levers have a long soft, spongy pull, and are adjustable so that it can be minimised.

    Lever advantage looks to be at least 7 to 1 and probably 10 to 1 on most.

    My local brake guy does rebores / resleeves on just about anything you can name, using a proprietry multi head machine so it shouldnt be that hard to find someone stateside who has the same set up
    #20