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  1. #1
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    New question here. Disc Brake Danger (-?)

    Setup: Juicy 7's, Minute 2.00 fork, SC Blur.
    Problem: Under sudden heavy braking the front wheel is moving in the dropouts. I know this because three times now my first ten rides on this bike I have found that after heavy braking or a sudden hard stop, the front brake begins rubbing. When I release the QR I hear and feel an slight movement of the front wheel and then upon re-tightening the QR, the brake is perfectly aligned again. Yes, I know how to operate my QR's and yes, I do crank them down with a good bit of force. I did a quick web search on this topic and opened this lovely can of worms:
    http://www.ne.jp/asahi/julesandjames...quick_release/
    This website describes in detail the forces that are generated by disc brakes (the front in particular) and how those forces essentially try to pull the wheel down in the direction of the dropout openings. The author then goes on to describe the mechanism by which a quick release would unscrew with this movement eventually causing the wheel to come off, although personally, I think this is getting a bit extreme. Regardless, the force analysis still points out a fundamental flaw in the current design of bike disc brakes and serves to explain the movement of my front wheel causing misalignment of the disc in the caliper.
    Anyone else have this problem or anything similar ???

  2. #2
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    despite what some experts tried to make you believe, there are millions of disc brakes out there , where this is not a problem. There are some where this is an issue however.

    Ok you need a good q/r to start out with. Forget titianium skewers or any frilly lightweight stuff. Best are Shimano Quick Releases. Than tighten them down with the max allowable torque.
    Problem solved

    Oh by the way, the whole discussion startet in England, because somebody on a TANDEM lost the front wheel .. there were more intriqing points to this story as the fork was selfmade and so forth. Some people suspect a major connection to a drum brake manufacturer as well. No matter what, its not as black and white as it seems to look.

    Thorsten

  3. #3
    Baliw
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    Quote Originally Posted by micstew
    I did a quick web search on this topic and opened this lovely can of worms:
    http://www.ne.jp/asahi/julesandjames...quick_release/
    This website describes in detail the forces that are generated by disc brakes (the front in particular) and how those forces essentially try to pull the wheel down in the direction of the dropout openings. The author then goes on to describe the mechanism by which a quick release would unscrew with this movement eventually causing the wheel to come off, although personally, I think this is getting a bit extreme. Regardless, the force analysis still points out a fundamental flaw in the current design of bike disc brakes and serves to explain the movement of my front wheel causing misalignment of the disc in the caliper.
    Anyone else have this problem or anything similar ???
    There are two aspects to this issue. One is the direction of the force that a disc brake exerts on the axle of a front wheel, and two is the mechanism by which a screw clamp is unscrewed by repetitive movement along the clamping surface.

    Issue one: there's no doubt that a disc brake system on a front fork will try to eject a wheel out of the dropout. A simple force analysis will show this, but most convincing of all is a simple test. Release the QR on your front wheel, roll the bike with you walking beside it, and apply the front brake. The wheel will try to pop out of the dropout. This tells you the tendency of the disc brake to eject wheels.

    Therefore, if your front QR is insufficiently tight, the front wheel can certainly pop out when the disc brake is applied, but lawyer lips on your dropout may most likely prevent a catastrophic ejection of the wheel. Still, it's not a good situation. How likely is this to happen with a "sufficiently" tight QR? Jury's still out, but I think the majority of users encounter no problem. This isn't the same as saying YOU won't either, because one of the unanswered questions here is, how tight is "sufficiently" tight? You now have a perfect opportunity to find out just how tight you need to set your QR to prevent your front axle from moving when you disc brake. Keep increasing the tightness settings and see if you can arrive at one which prevents your axle from moving.

    The second issue is the proposed mechanism by which a QR can loosen because of movement along the dropout. The hypothesis is that an insufficiently tight QR can move along the front dropout when disc braking, and further riding (and forces from the ground) can push the QR back to its original position; then another disc braking pushes it out, and so forth, and all the time it's sliding in and out along the dropout, the QR is slowly loosening by virtue of the force on the QR threads having a tendency to unscrew itself. While this unscrewing mechanism can be observed in many screw systems subject to vibration (eg aircraft engine bolts, which require lock-wires), it remains to be determined how likely this happens in bicycle forks and QR systems. In the event this happens, lawyer tabs will most likely also prevent catastrophic wheel ejection.

    If there's anything to take out of all this, it's that you need to make sure your front QR is "sufficiently" tight before each ride (marking the QR ends against the dropouts saves a lot of time). For the ultimate peace of mind, however, a captured through axle is the answer, until fork manufacturers shift their disc brake mounts to the front of the forks.

  4. #4
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    The obvious:

    Are you sure the bearings in your hub don't need a lil' tweak?

    What kinda hubs are you running? I'm willing to bet a fictional sum of money that they're your problem.

    Just for fun, I'm guessing you have a King Hub. I'm not putting any fake money on that though.


    One more reason to run 20mm.
    Last edited by singletrack; 03-08-2004 at 10:16 AM.

  5. #5
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    Quote Originally Posted by micstew
    Setup: Juicy 7's, Minute 2.00 fork, SC Blur.
    Problem: Under sudden heavy braking the front wheel is moving in the dropouts. I know this because three times now my first ten rides on this bike I have found that after heavy braking or a sudden hard stop, the front brake begins rubbing. When I release the QR I hear and feel an slight movement of the front wheel and then upon re-tightening the QR, the brake is perfectly aligned again. Yes, I know how to operate my QR's and yes, I do crank them down with a good bit of force. I did a quick web search on this topic and opened this lovely can of worms:
    http://www.ne.jp/asahi/julesandjames...quick_release/
    This website describes in detail the forces that are generated by disc brakes (the front in particular) and how those forces essentially try to pull the wheel down in the direction of the dropout openings. The author then goes on to describe the mechanism by which a quick release would unscrew with this movement eventually causing the wheel to come off, although personally, I think this is getting a bit extreme. Regardless, the force analysis still points out a fundamental flaw in the current design of bike disc brakes and serves to explain the movement of my front wheel causing misalignment of the disc in the caliper.
    Anyone else have this problem or anything similar ???
    The danger is real, but usually only under extreme conditions. I know of one case of a wheel coming out on a single (not tandem) mtb. If you use a Shimano QR skewer (and check it often), use a fork with the lawyer tabs intact and a hub with knurlling on the end caps there is little danger. If you use a fork with forward facing dropouts (rare) there is no problem at all.

    The case on the web site you found was a rigid fork without lawyer tabs on a tandem. Tandems can brake much harder than a single because it is nearly impossible to endo a tandem.
    mtbtires.com
    The trouble with common sense is it is no longer common

  6. #6
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    Replies...

    Meister - I'm already running Shimano QR's and cranking them down (there's a subjective measurement if I ever heard one). Agree, this is not black and white, but the specifcs of the discussion origin (James Annan losing the front wheel on his tandem with custom (not self-built) fork and disc brakes) do not change the reality that the disc forces are trying to pull the disc side of my wheel toward the opening of the drop outs. I don't suspect my wheel is likely to be coming off or anything, but the movement and the resulting misalignment of the disc is quite concerning.
    Juramentado - Your comments are right on the money (although they don't make me feel any better about the situation). I'll be marking my QR to find that "sufficiently tight" setting. If I can't manage that, I'll be headed back to v-brakes.
    SingleTrack - Hub bearings?? Hubs are brand new, Chris King ISO Disc (good guess). Are you suggesting that they are not adequate for disc use? I'm just glad we were only betting fake money...
    Shiggy - Yes, the example on the Annan website is a tandem, but there are other examples of problems with solo MTB's. Again, the catastrophic failure (loss of front wheel) is not my main concern. Rather, the unlikely to get reported condition of hub movement in the dropouts causing misalignment of the disc. If all that's securing this system from unwanted movement is the knurling on the hub end caps, I'd say we have a significant design error.

  7. #7
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    Good guess based on experience.

    All King Hubs get loose especially when they're new. You just need a 5mm key and kings funky tool to tighten um up (I've done it with a spanner fine)

    No big deal, not knocking kings, but this happens all the time.

    Tightening the skewer would mask this problem in the short term.

  8. #8
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    Hubs Are Tight

    ST - Hub is tight. It did loosen up as you suggested but I just tightened it last week. The problem is that the brake forces are pulling the disc side of the wheel toward the dropout opening. The wheel moves a bit (ie: the hub cap slides down the dropout) and then stops. When I say a bit, I mean barely, but still enough to make the brake rub.
    Can't believe I'm the only one who has seen this - I do brake hard sometimes (put myself OTB last week avoiding a dog) but I'm all of a 146 lbs so we're not talking about unusual forces here.

  9. #9
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    Quote Originally Posted by micstew
    Can't believe I'm the only one who has seen this - I do brake hard sometimes (put myself OTB last week avoiding a dog) but I'm all of a 146 lbs so we're not talking about unusual forces here.
    You're not the only one, and it's obvious if you look at the testimonials at that webpage you quoted. I suspect that those who experienced it haven't made a big deal of it, and just attributed it to loose QRs, so you don't see too many complaints anywhere. It's an issue though, one that requires some vigilance in QR setup.

  10. #10
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    Quote Originally Posted by micstew
    ST - Hub is tight. It did loosen up as you suggested...
    I say borrow a wheel and see what happens. It'a gotta be the hub/dropout interface. Shimano skewers get REAL tight.... You could have a shop put the alignment tools on the dropouts to see if they're, uh, aligned.

    Either that or just send the fork back to Manitou. Answer, while they make a good product (Now) still do tend to send out some funky messed up stuff. It's a serious liablity issue, they'll probably replace your lowers just to be safe.

    ALSO: sometimes the axle of a hub will extened past the droput and make contact with the QR. The skewer will feel tight, but the axel will still be a lose. The is pretty rare and usually only happens when 12 year old kids rebuild hubs (It happened to me when i was 12. ouch) Still, It's worth checking.

  11. #11
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    Quote Originally Posted by singletrack
    I say borrow a wheel and see what happens. It'a gotta be the hub/dropout interface. Shimano skewers get REAL tight.... You could have a shop put the alignment tools on the dropouts to see if they're, uh, aligned.
    Why has it got to be the hub/dropout interface, and what anomaly on that will give the symtpom described in the OP? Are you trying to avoid facing the fact that disc brakes on forks with QR dropouts can produce the described phenomenon?

    Quote Originally Posted by singletrack
    Either that or just send the fork back to Manitou. Answer, while they make a good product (Now) still do tend to send out some funky messed up stuff. It's a serious liablity issue, they'll probably replace your lowers just to be safe.
    Or just blame him for improper setup.

  12. #12
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    I've worked on a lot of Minutes w/ Discs and never seen this.

    Quote Originally Posted by Juramentado
    Why has it got to be the hub/dropout interface, and what anomaly on that will give the symtpom described in the OP? Are you trying to avoid facing the fact that disc brakes on forks with QR dropouts can produce the described phenomenon?


    Or just blame him for improper setup.

    If it only slides so far then stops, it could well be the the dropouts aren't in-plane with each other, and the hub is finding the most level surface. It could also just be hitting the lawyer tab and stopping there.

    Certainly disc torque could move the hub. But since wheels aren't flying off disc equipped QR bikes left and right, I tend to think there's a simpler explanation.

    I'm inclined to think that there a problem with this one fork, rather than saying whole concept of QR forks with disc is flawed and all those forks should just be thrown away.

    Of course, I wouldn't run a QR on my bikes, there's only one way to attach a wheel for me:
    Attached Images Attached Images
    Last edited by singletrack; 03-08-2004 at 07:02 PM.

  13. #13
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    Quote Originally Posted by singletrack
    If it only slides so far then stops, it could well be the the dropouts aren't in-plane with each other, and the hub is finding the most level surface.
    You'll find that a QR will tend to bite into the metal, and the dropouts will have to be grossly misaligned indeed for these to cause any movement of the QR.

    Quote Originally Posted by singletrack
    It could also just be hitting the lawyer tab and stopping there.
    That's an event, not a cause.

    Quote Originally Posted by singletrack
    I'm inclined to think that there a problem with this one fork, rather than saying whole concept of QR forks with disc is flawed and all those forks should just be thrown away.
    Interesting comment because you then go on to say that:

    Quote Originally Posted by singletrack
    Of course, I wouldn't run a QR on my bikes, there's only one way to attach a wheel for me:
    Do as you say and not as you do?

    While I feel a lot better using through axles myself, I also use forks with traditional dropouts and disc brakes. I think a combination of judicious QR tightening and vigilance can make the dropout/disc brake system workable.

  14. #14
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    off the deep end

    What are you trying to say? That the bike industry is conspiring against us? That they're intentionally trying to sway riders towards a system that's been proven unsafe at any speed? Yeah, thanks Ralph....

    Jeez man, micstew should try a different wheel and see what happens. Are you somehow suggesting that this is bad advice?

  15. #15
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    The Bike Industry...

    Quote Originally Posted by singletrack
    ...That the bike industry is conspiring against us? That they're intentionally trying to sway riders towards a system that's been proven unsafe at any speed?
    ST - Appreciate your input here, but as for the bike industry, I don't think they are together conspiring on anything and I think that's a big part of the problem. It's not that the system's been proven unsafe, its that no one to my knowledge has done any testing to conclude that the current hub, dropout, and QR designs comprise an adequate system for securing the wheel under disc braking forces. It works, usually, for most people, under most conditions, and when it slips a bit I suspect the typical action is to reset the wheel and retighten the QR - no blood, no foul. Clearly, the wheels are not all going to come flying off at any minute. But it does seem to me that we really don't know how close we are to movement of the front axle under braking, given reasonable ranges for braking forces and QR grip. My extremely limited anecdotal evidence at least suggests to me that we are lots closer to that line than I would like.
    Clearly the current design is less than optimal. But "a combination of judicious QR tightening and vigilance can make the dropout/disc brake system workable." Unfortunately, I think this is the bottom line: "workable" is as good as it gets unless you go to through hole (obviously preferrable as it is inherently more stable and has been proven through motorcycling for years - trouble is, I have yet to find a through hole axle fork that doesn't exceed the Blur's maximum allowable travel) or go back to rim brakes. If there is a failure on the part of the "bike industry" at this point, it is that they have, apparently without testing, taken us into the middle ground (disc brakes acting within a rim brake proven system), driven of course by customer demand.
    I don't want to start any flaming posting wars here, but I would like to hear additional thoughts on all of this before I spend another $1500 for a new fork, wheels, and rim brakes.

  16. #16
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    Well since you asked

    I know you *probably* don't want to hear it, but I think the King hub is the problem.

    I pulled my punches before on this, but....

    I'm not gonna tell you what to do, but if you try another wheel, I think you'll eliminate the problem. King makes the best headset, but their hubs are lacking. I'm sure that this comment will evoke the wrath of numerous MTBRers, but it's true. Throw your rotten apples now.

    /wipes splatter from face

    Anywho. Try a cheap wheel. An XT or Deore. I'm willing to bet VAST sums of fictional money that this won't happpen anymore.

    Then, sell yer King wheels to someone.... on MTBR... and buy some DT/ Hugi's.

    But please, first try a non-King hub before you sink any money into it.

    Happy Trails

  17. #17
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    20 mm...

    Quote Originally Posted by micstew
    ST - Appreciate your input here, but as for the bike industry, I don't think they are together conspiring on anything and I think that's a big part of the problem. It's not that the system's been proven unsafe, its that no one to my knowledge has done any testing to conclude that the current hub, dropout, and QR designs comprise an adequate system for securing the wheel under disc braking forces. It works, usually, for most people, under most conditions, and when it slips a bit I suspect the typical action is to reset the wheel and retighten the QR - no blood, no foul. Clearly, the wheels are not all going to come flying off at any minute. But it does seem to me that we really don't know how close we are to movement of the front axle under braking, given reasonable ranges for braking forces and QR grip. My extremely limited anecdotal evidence at least suggests to me that we are lots closer to that line than I would like.
    Clearly the current design is less than optimal. But "a combination of judicious QR tightening and vigilance can make the dropout/disc brake system workable." Unfortunately, I think this is the bottom line: "workable" is as good as it gets unless you go to through hole (obviously preferrable as it is inherently more stable and has been proven through motorcycling for years - trouble is, I have yet to find a through hole axle fork that doesn't exceed the Blur's maximum allowable travel) or go back to rim brakes. If there is a failure on the part of the "bike industry" at this point, it is that they have, apparently without testing, taken us into the middle ground (disc brakes acting within a rim brake proven system), driven of course by customer demand.
    I don't want to start any flaming posting wars here, but I would like to hear additional thoughts on all of this before I spend another $1500 for a new fork, wheels, and rim brakes.
    I was in the market for a new fork which came in a 20mm or QR version. I asked the locals and posted for info on 20mm set-ups. The most convincing info. to run a 20mm came from my LBS "QR are for road bikes" and there were boatloads of QR loose/ loosening stories. I currently run a 20mm Zoke and a QR Vanilla 125R. If I had a problem such as your I wouldn't discount any advice - as suggested I'd borrow another wheel, try a different skewer etc. There are many disc QR users w/o problems. Are they running on the edge w/ a QR? Well in my opinion QR's look so damn wimpy. When I looked into a 20mm set-up this salesman said " It's going to be hard to change your tire & it's going to add weight, no worth it in my opinion" Hmm I'd rather have a solid front end than some missing front teeth. I'd say for those that are worried - Go 20mm, If not you will have a light weight set-up, easy on the tire changes but questionable strength.

  18. #18
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    I second your criticism of King hubs

    My man, I could not agree more that King hubs are overrated. Their inevitable loosening frustrates me greatly.

    I agree with you, as well, that Hugi hubs are better.

  19. #19
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    Since I have the 20mm axle setup I am not worried about the QR issue, but I was under the impression that QR's can be over tightened and this causes excessive bearing wear in the hub. I know it seems like the lock nuts should hold back against the QR tightening, but I have had wheels where the bearings were slightly loose in their races that I have simply cranked the QR harder and taken the little bit of play out.

    If this is true and you have disc brakes it seems like the issues are multipling for the QR disc setup.

    Quote Originally Posted by Juramentado

    If there's anything to take out of all this, it's that you need to make sure your front QR is "sufficiently" tight before each ride (marking the QR ends against the dropouts saves a lot of time). For the ultimate peace of mind, however, a captured through axle is the answer, until fork manufacturers shift their disc brake mounts to the front of the forks.

  20. #20
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    Quote Originally Posted by DrGlen51
    I was under the impression that QR's can be over tightened and this causes excessive bearing wear in the hub.
    This is true if the hub has been adjusted to be tight before being mounted on the frame. What many people do is adjust the hub so that there is a little play on the bearings before mounting on the fork/rear dropout, then tighten the QR until the play disappears.

    The amount of play to allow in the hub before mounting is pretty much trial and error, keeping two things in mind: adequate tightness in the QR and enough initial play in the hub to allow for this tightness.

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