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Thread: rotor question

  1. #1
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    rotor question

    I have a newish bike with Hayes MX2 disc brakes. First mechanical discs for me, and overall I'm quite happy with them (woohoo, they work in the rain!) Front brake is fine, the lever is at a good height and the amount of cable travel feels good. No issues there.


    Rear brake had a tendency to drag a bit, plus there is a lot of cable travel and lever movement, I'd prefer to engage a bit higher. I was able to resolve my dragging issues on the rear brakes by backing the fixed (inner) pad out until the pad is just peaking out. I do notice that the rear rotor is not flat, so there is a lot of runout. Have tried cold setting / tweaking the rotor with a 12" crescent wrench, but no luck.


    Questions:

    should I expect the rotor to be flat? I've no idea how it could have gotten distorted, certainly not from overheating. At what point is a rotor considered defective? Granted, these are low end brakes. Not sure what realistic expectations should be. But if the rotor were flat(er) the pads could be closer together and the brakes more responsive (subjectively, of course).

    would the rear brake have a different feel simply because there is more cable to move?

    thanks!

  2. #2
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    Rotors should be flat, but the larger they are, there will be some warping to them, so if just slighly off flat, not too much to worry about; but it does sound as though yours may be quite off.

    Fot the dragging, as this is a cable system, this could be caused by the cable routing, if the cable goes through some sharp bends this could lead to some drag, see if this can be improved for better response, if the braks is dragging, it indicates that the pads are not releasing, or are always contacting the braking surface, and need to be further away.

    Is one of the pad on the rear always in contact with the rotor, and the other a long way away?

  3. #3
    Senior Member BCRider's Avatar
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    You got the right idea for tweaking the rotor but the wrong tool. Stick to just flexing it with your hands. The crescent wrench should only be used for repairing crash damage so you can get another ride or two from a disc. It's far too easy to flex the rotor way too far when using it for final rotor truing.

    For the mechanical brakes to work well you MUST have the fixed inside pad all but touching the rotor. Otherwise you have to use too much of the lever pressure to flex the rotor until full contact is made.

    If your rear rotor is not flat on the wheel I'd suggest you start by removing it from the hub. Check the seating shoulders and rotor spider for burrs or grit that may not be allowing the rotor to sit flat. Then check the rotor laid or held to a piece of glass and see if it lays flat or can rock around. If it rocks a bit when laid up to the glass on either side then your first job is to ease it flat again. Go easy, a little at a time goes a longer way than you think. Once flat then re-bolt it to the hub. Put the wheel back in the bike and see if the rotor is wobbly as it passes through the caliper. Gently but firmly flex the rotor with only your hands either out or in at the high and low spots until it runs nice and even within the caliper's slot. At this point adjust the inside pad towards the rotor until you get the lightest possible scuff at a point or two in the wheel's circle. A slight flexxing at the touching spots should reduce the scuffing. Then move the pad towards the rotor one more click. If it scuffs very lightly at two or three spots I'd say it's time to leave it. Don't worry about teh scuffing. If it's actually as light as it should be it won't slow you down at all. Besides, the pad will wear enough to make the scuffing go away after a few stops.

    To adjust the point of engagement at the lever you need to adjust where on the cable the arm grabs. You can do this by loosening the pinch bolt and moving the arm on the caliper and then tightening it on the new spot of the cable. Smaller fine tuning adjustments are then made using the barrel adjuster at the lever.
    Model airplanes are cool too!.....

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    coprolite fietsbob's Avatar
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    + If you can get a low compression housing that will help.
    Nokon and the like are solid metal beads and tubes,
    with the liner keeping them together like beads on a necklace.

    jagwire has Kevlar braiding over their housing offering.
    Last edited by fietsbob; 04-25-11 at 04:06 PM.

  5. #5
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    thanks BCRider, really good feedback. I'll have a go at removing the rotor, I have a large thick piece of glass that I use for checking the flatness of woodworking tools.


    I should have said "scuffing" rather than "dragging" in my original post.

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