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Thread: Brake Rotor Thickness

  1. #1
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    Brake Rotor Thickness

    Clymers and Haynes both show .14" as the minimum service limit for both front and rear totors. The text in the Haynes Manual also indicates that the minimum thickness is marked on each disc.

    On my 93 K75S with ABS, the front discs are stamped "min .14" and the rear "min.18".

    I purchased a used rear rotor from a guy who had it replaced because his independent garage said it was "Under thickness" at .17. I measured it at .1764.

    Does anyone know if this is under thickness or not? If not, what are the stampings on the rotors about?

    Also, my front rotors appear to have been thinner to begin with, do not appear badly worn at 27K miles, yet are still measuring near .1412.
    John Peck
    Maple City, Michigan
    R1150R
    F650GS

  2. #2
    bmwmick
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    John,
    The fronts on my K1100LT show 4.5mm minimum thickness, rear shows 4.6mm minimum. These values are stamped on the rotors AND in the manual.

    Mick

  3. #3
    A bozo on the bus deilenberger's Avatar
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    Originally posted by bmwmick
    John,
    The fronts on my K1100LT show 4.5mm minimum thickness, rear shows 4.6mm minimum. These values are stamped on the rotors AND in the manual.

    Mick
    Mick - considerably different rotors.

    Why is the rear rotor listed with a different min thickness? Well.. the very early K bikes ('84 to late '84) used the same rotor (different carrier) as the front one - ie - one with slots in it.

    For some reason - these rotors tended to crack in line with the slots. This isn't a good thing, although I know of no crashes due to it.

    BMW's fix was to use a solid rear rotor, and set the minimum thickness a tad higher.

    I have used a drilled rear rotor on a K100 (Braking rotor) with no adverse effects.

    To the original question - well - the rotor you got used is under spec, so if the seller will refund your $$$ I'd try to get the money from him and look for another one.

    Best,
    Don Eilenberger http://www.eilenberger.net
    Spring Lk Heights NJ NJ Shore BMW Riders New Sweden BMW Riders
    '07 R1200R (current ride) and some bimmers.. and a Porsche

  4. #4
    bmwmick
    Guest
    Originally posted by deilenberger
    Mick - considerably different rotors.
    snip
    To the original question - well - the rotor you got used is under spec, so if the seller will refund your $$$ I'd try to get the money from him and look for another one.

    Best,
    Don,
    The fronts ARE different, my fronts are the same as the K1/K100RS 4V bikes.
    The rear rotor (ABS) should be the same as his '93K75 (ABS). The rear solid rotor was changed in '94 to a drilled rotor.
    I agree, his are slightly under spec.

    Mick

  5. #5
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    Thanks for the help, guys.

    My seller told me it was under spec, but I took a chance because my manual appeared to tell me it wasn't. The good news is that it was only $25 so I'll call it an education expense and move on.

    I'm inclined at this point to fit a new part and get on with my riding season.

    Any thoughts on aftermarket sources v. the original BMW part?
    John Peck
    Maple City, Michigan
    R1150R
    F650GS

  6. #6
    A bozo on the bus deilenberger's Avatar
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    Originally posted by jepeck
    Thanks for the help, guys.

    My seller told me it was under spec, but I took a chance because my manual appeared to tell me it wasn't. The good news is that it was only $25 so I'll call it an education expense and move on.

    I'm inclined at this point to fit a new part and get on with my riding season.

    Any thoughts on aftermarket sources v. the original BMW part?
    Unless the aftermarket source is cheaper - the original part is more than adequate for the job. If you're ambitious you can put a "Braking" semi-floating rotor on the existing carrier - but it requires some machine work. Other options are available, but they all cost more than the original rotor.
    Don Eilenberger http://www.eilenberger.net
    Spring Lk Heights NJ NJ Shore BMW Riders New Sweden BMW Riders
    '07 R1200R (current ride) and some bimmers.. and a Porsche

  7. #7
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    I'm back on the road with a new stock rear disc and new stock pads front and rear.

    The reason this came up is that the right rear pad failed and ruined the rear disc.

    I'd checked the pad thickness recently and all were still within spec in terms of thickness. However, post mortem, the right rear pad was down to the bare metal of the backing plate, as if the remainder of the friction material let loose suddenly.

    All of the remaining pads are still well within spec (2-3 mm) at 28K miles, but I replaced them all because I was concerned about additional failures on the remaining 11 year old original pads.
    John Peck
    Maple City, Michigan
    R1150R
    F650GS

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