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Thread: 78 R100RS rear brake disc cracks

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  1. #1
    Registered User Air Race Addict's Avatar
    Join Date
    Aug 2010
    Cheney, WA

    78 R100RS rear brake disc cracks

    So, has anyone ever seen the rear disc/rotor crack between the cooling holes? My 78 R100RS has cracks between almost every set of holes going center outwards. I had a rear bearing fail on my way home from the Stanley Stomp. The bearing failure seems to have been caused by a failed seal that let water into the wheel when I had to ride through a creek to get to the camp. I had no indication that I had a problem until I was slowing down to pull into a roadside attraction and heard a sound like I was dragging something. Of course the trip ended at that point. Fast forward, got the wheel into the shop for new bearings and seals and the mechanic pointed out the cracks. Freaked me out. What could cause the cracks? That's my question. Of course I checked my front discs very carefully and thankfully no issues. Any thoughts? 125000 miles by the way. Same with the bearings. Just lubed every tire change.
    I'm in the market for a new/gently used brake rotor. (Same part # as the front disc by the way).

  2. #2
    If that rotor was hot (as they do get) and got dumped into cool water, that would cause dramatic temp change which often causes cracking metals.

    Not sure if that is it, but it sounds like it could be.

  3. #3
    Registered User
    Join Date
    Aug 2013
    Camas, WA
    Yep, that's odd. The rear sees much lighter duty than the front. I've got an '81 RS down near Portland, I'll look at mine, but don't expect to find anything.
    Hmmm, lots of small radial cracks, the disk has overheated, and those are essentially stress reliefs, rears shouldn't get that hot, it's the fronts that do the work, so... The brake is dragging? I can think of two reasons: the seals could be gummed up / piston is binding, (corrosion), or could it be possible that your toes are brushing the brake pedal, keeping it lightly engaged? Just a thought.
    BTW, I plan to convert my RS and my CS rear brakes back to drums at some point. I think that's plenty for the job and they'll be easier to modulate. Especially in our nine months of rain.

  4. #4
    rsbeemer 22600's Avatar
    Join Date
    Jan 2011
    New Braunfels,Texas/Taichung, Taiwan

    overheated disc

    I have an old race car buddy that told me he has seen a lot of disc crazing on cars. Says it from overheating from cheap brake pads or track racing and using them a lot and happens over time. But , also said lots of times it's not serious, it's just hairline cracks that look worse than they are because of the shading from the brake dust. Anyway, he said buy the best quality pads you can and the disc will run cooler. I don't know why, but it has something to do with the material they are made of. I would replace them if it were mine just to be safe; I see them on IBMWR Market Place sometimes for sale.

    1978 R100rs MOA#22600 125cc Kymco , 180cc Kymco Racing King
    The pessimist complains about the wind; the optimist expects it to change; the realist adjusts the sails.

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