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  1. #1
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    Disc brake expectations?

    I was wondering what your expectations are from disc brakes - be it mechanical or hydraulic?

    I have 2 bikes with disc brakes but have become so frustrated that I'm almost ready to go back to rim brakes.

    Frustration 1:
    3 or 4 years ago I purchased Shimano's XT discs (the first year they featured servo wave). I found installation and set up to be no problem however it was just a short time after initial set up that I began to get pad rubbing. Initially the rotor was 100% true (by naked eye) but after continuing riding like this for some time I could detect the tiniest hint of a wobble. It was so minute that I felt It should still be possible to have the caliper aligned such that it shouldn't rub. But rub it did continue to do despite adjusting countless times and is only getting worse.

    Frustration 2:
    I occasionally ride on mountain roads that are very steep with repeated extremely sharp bends requiring repeated hard braking. After a few applications the rotor seems to get more out of shape and/or pad clearance shrinks causing the brakes to start to rub - a lot. Further more, after a few minutes of (despite crazy rubbing) otherwise smooth strong braking the brakes begin to squeal. Once they start to squeal it doesn't disappear even after the brakes have completely cooled and only goes away after a thorough cleaning with disc brake cleaner. (I have no idea WHY that works but it does so it seems it's not rotor or pad glazing causing the squealing.)

    My wife has a Kona Lisa with Hayes Strokers from the first year of their release. The Rock Shox fork was changed out to a rigid steel fork by my LBS. She has only ridden the bike a handful of times on the road in the dry. As best as I can see with the naked eye the rotor is 100% true and yet there's rubbing that no matter how much I've adjusted them will not stop. And it's an incredibly frustrating loud wail at low speeds.


    I'm disappointed in the infinitesimally small amount of pad clearance that's available with both of these brakes. I'm disappointed in how quickly and how severely the pads rub under a hard road descent (although I'm aware it's probably quite an extreme situation). It's scary as it seems if I went for much longer the brakes might lock up.

    I'm also wondering if it's possible for a rotor to be put out of true by heat from hard braking.

    I expect to be able to set up my brakes to not rub - ever. I this unreasonable?

    As for performance under extreme conditions I'm not sure what to expect from disc brakes. I've heard of rim brakes overheating pads or the rims themselves. Are rotors susceptible to damage from overheating? Can fluid brakes only cope with so much heat before they cannot maintain correct pad clearance anymore? Is it normal for hot rotors to go out of true?

    I'd love any feedback, thoughts or advise, stories of success or frustration or failure. Thanks for listening to my little rant!

  2. #2
    bt
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    most all rotors are a bit untrue. you may need to remove your wheels and then spread your pads apart.
    also maybe you somehow have over filled them with brake fluid?

  3. #3
    dru
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    Shimano calipers can and do get sticky. I've had to retract and extend the pistons numerous times on my old deore and much newer XTR to get the pistons to center over the disc. I've even lubed the outside of the seals on the XTR. A warped rotor is not likely your problem; both my rotors on the deores are slightly warped and they don't rub unless the caliper is being sticky. However, if your shimano calipers use shims you need to be super careful that they are centered over the discs before screwing around with anything else. I can't speak to your hayes issue, don't know anything about them.

    Drew
    occasional cyclist

  4. #4
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    Thanks for the replies. I have spread the pads on the XTs numerous times and they will open a lot more but after 1 or two applications they're back to the same position. I haven't used feeler guages to check the actual pad clearance but it's very very small. I assumed the position that they always return to is where they are meant to be.

    Also, I recently changed the fluid in the XTs and follow the Shimano instructions to the letter. I'm confident that they're no over filled.

    I'm a bit reluctant to go back to my LBS as he charges for everything! Even just to ask him to help fine tune a derailleur adjustment that takes him 2 mins he'll charge for the service. If I go in there asking about brake set-up he'll probably require me to leave the bike there for a few days, charge me for his time and then tell me their working fine and there's nothing that can be done!

  5. #5
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    My 2011 XTR M988 Trail front brake has the same problem with the lever firming up and rotor rub after the brakes have been used constantly on a descent. At least they don't squeal though. It seems to be down to thermal expansion as the brake fluid gets hot. A drawback of mineral oil for brakes apparently, especially if there's any moisture in the system as mineral oil doesn't absorb water. In a mineral oil system the water gathers by the caliper.

    "DOT fluid is hydroscopic (absorbs water) by design, and is the reason it is used as a brake fluid. [If] water gets into any [mineral oil based] hydraulic system that has seals and moving pistons it has a tendency to accumulate at the lowest point (ie: oils float on top of water). This means over time, water accumulates at the caliper, and thus the max temperature that the caliper can rise to before performance issues occur is 100C. With a hydroscopic fluid like DOT, the water gets distributed throughout the system, and although the water absorption lowers the boiling temp of DOT from it's dry state, its still a few hundred Celsius, and thus has a wider performance range [than mineral oil] when your system is "wet".

    In practice, almost all automotive systems use a hydroscopic DOT fluid because it is superior. For bikes, there does not seem to be a practical difference between hydroscopic DOT and mineral oil or silicon based fluids (like DOT 5.0) due to lower operating temps and smaller volumes."
    itsdoable

    Shimano Mineral Oil?

    Love the new 2012 XT brakes

    Does Mineral Oil or DOT Fluid work better?

    DOT brake fluid vs.Mineral oil.

    Here's Shimano's advice on how to get rid of it:

    "Vapor lock may occur if the brakes are applied continuously. To relieve this condition, momentarily release the lever.

    Vapor lock is a phenomenon in which the oil inside the brake system becomes heated, which causes any water or air bubbles inside the air system to expand. This can then result in a sudden increase in the brake lever stroke."
    Shimano brake manual

    http://techdocs.shimano.com/media/te...9830647129.pdf

    Have you tried truing the brake rotors? Because the pad/ rotor clearance is so small it's hard to align the caliper as soon as the rotor begins to warp at all. Straightening the rotor helps make it easier to stop disc brakes rubbing.

    Disk Brake Rotor Precision Truing

    If everything looks ok but the rotors still rub then it could be that something isn't straight. You could try facing the frame/ fork brake mounts and making sure that there is minimal play in the hubs. If the brakes don't rub just spinning the wheels but rub once when you sit on the bike then it may be the hubs.
    Last edited by WR304; 01-29-2012 at 02:51 PM.

  6. #6
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    I haven't tried truing the rotors. I don't have access to a dial indicator and not so confident to try it by eye. It really is a small amount out of true - less than a millimeter - and I don't want to risk making it worse.

    I also will have to double check the wheel bearings but I don't think there's any play there. Everything is well maintained and feels very solid. (rather vague term that, innit!)

    Thanks for the above info and link WR304. I'll re-read it again and check the links when I have a little ore time.

    I really do appreciate the replies - I'm a lone rider without any buddies and so I never get to test anyone else's bike or new gear or talk about products and "how to". MTBR has been a lifesaver for me as without it I may have given up by now!

  7. #7
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    Did your wife's Stroker wail before you swapped out the fork? If not, it may be that the newer rigid fork is giving a 100% interface between the disc tabs and the caliper.
    My brother runs Stroker Trails on his bike, and the rear brake never, and I mean never, runs quietly, and he has done everything humanly possible to stop it. Occasionally discs can be like that, so you either end up learning to live with it, or swap to another brand. Neither is a great option unfortunately.

    Also, what pads are you running in both brakes? Resin/organic or sintered?

  8. #8
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    How do you "adjust" your calipers? Reason I'm asking, the procedure as explained in the manual ( loosen bolts, apply brake, tighten bolts while brake applied) is not always succesful. My shimanos also rub if I do that.
    The best way to adjust them is to simply loosen the bolts and move the caliper by hand until the disc is centered in the slot of the caliper. Push the pads back first if required. Make sure the caliper doesn't shift when you tighten the bolts.
    With this, mine have been 100% rub free ever since, even after hard use.
    wanted: Cannondale Lefty w/ V-brake studs

  9. #9
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    I highly recommend you centre your calipers by eye. As strange as it may seem, it is not unusual to get pad rubbing using Shimano's instructions.

    Just put something white (like a piece of paper) on the opposite side of the caliper so you can easily see how much space is on either side of the rotor. Centre the caliper based on what you see.(Make sure the pads are in their final position before doing this.)

    This is the only way to eliminate pad rubbing on my brakes, and I've read plenty of other people with the same experience.

  10. #10
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    Quote Originally Posted by komekomegaijin View Post
    I haven't tried truing the rotors. I don't have access to a dial indicator and not so confident to try it by eye. It really is a small amount out of true - less than a millimeter - and I don't want to risk making it worse.

    I also will have to double check the wheel bearings but I don't think there's any play there. Everything is well maintained and feels very solid. (rather vague term that, innit!)

    Thanks for the above info and link WR304. I'll re-read it again and check the links when I have a little ore time.

    I really do appreciate the replies - I'm a lone rider without any buddies and so I never get to test anyone else's bike or new gear or talk about products and "how to". MTBR has been a lifesaver for me as without it I may have given up by now!
    Sounds like a sticky piston and/or a weak or worn out spring...

    Wash the brakes with soap and water and rinse...

    Remove wheel

    Remove pads...

    Extend pistons so the move evenly...if not

    Then extend (don't pop them out), clean sides of pistons

    Push back in and extend keep cleaning until they move smoothly and evenly together...

    Add a drop of lube to each piston...

    Reassemble

    Ensure caliper is centered by eye on the rotor (pads will find center).

  11. #11
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    Thanks for the great replies.

    It might be some time until I have a chance to clean the pistons but I will try to do that soon.

    I always sight the caliper by eye in bright light.


    QUESTION - Is there a way to know how far pads/pistons should sit when fully retracted? Should the really FULLY retract on their own - ie. as far as possible? Or, will they usually sit out slightly further than a fully seated position?

    I also recently changed the pads back to the metal ones that had come with the bike to see how they'd go on a bid descent. At that time I stretched the little return springs out a little further but they seemed to have plenty of tension on them already.

  12. #12
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    Couple of other questions:

    Is it normal to have (slight) pad rub when your forks flex - either leaning hard in a corner or turning the fork with weight on it? (Mine is a Niner rigid steel fork and that's what happens with me).

    Also, how much clearance do you usually have between the rotor and pads? Again, when I have a chance I'll measure mine with feeler gauges (metric, mind you!).

  13. #13
    bt
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    while the pads are fully spread using a pad spreader, crack the oil bleeder and let a touch of oil out then quickly close bleeder.

  14. #14
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    If your pads are absolutely brand new, the the pistons should sit flush with the caliper. As the pad wears down a perfectly clean brake will adjust itself automatically to keep the same pad clearance. The problem happens when you mix tight tolerances and dirt.
    Squealing is generally a symptom of a slightly misaligned caliper. This tend to be in the vertical plane as horizontally its easy to adjust. Once the pads start wearing crookedly it only makes it worse as time goes by.
    There is almost no brake in history that has run smoothly and trouble free (no rub). You need to keep on top of the cleaning/maintenance. The benefit is better braking.
    Rimmer - "There's an old human saying - if you talk garbage, expect pain"

  15. #15
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    Quote Originally Posted by komekomegaijin View Post
    Also, how much clearance do you usually have between the rotor and pads? Again, when I have a chance I'll measure mine with feeler gauges (metric, mind you!).
    My SLXs appear to have less than a millimetre of clearance.

    Quote Originally Posted by komekomegaijin
    QUESTION - Is there a way to know how far pads/pistons should sit when fully retracted? Should the really FULLY retract on their own - ie. as far as possible? Or, will they usually sit out slightly further than a fully seated position?
    The pistons on my SLX M666s (2011) stick out a tiny bit when fully pushed in. I also had the M665s (pre-2011) which sat flush with the caliper.

  16. #16
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    Quote Originally Posted by jsilva View Post
    Just put something white (like a piece of paper) on the opposite side of the caliper so you can easily see how much space is on either side of the rotor. Centre the caliper based on what you see.(Make sure the pads are in their final position before doing this.)
    Using a piece of white paper to see the gap is a really good suggestion. I'm going to try that the next time I have to re-align my calipers.

    I usually try and align my brakes outdoors in daylight to get the same effect. If it's sunny the gap between rotor and brake pad is more visible.

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