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Help in adjusting my disc brake..

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  • October 2, 2019 at 7:08 pm #30165
    Huckfinn

    Hi all,

    I’m trying to work out how to fix the rear disc brake on my new (600km so far) Canyon Grail.  Nothing major but quite annoying and wouldn’t mind to learn to fix it myself!                            Basically :

    1- I noticed the back wheel does not spin as long as the front one when lifting the bike off the ground.

    2- When braking hard downhill it makes this light (but not that light) clunk..clunk..clunk kind of noise.

    3- Tonight I checked the rotor and noticed these regular “straight” kind of dark lines on the interior part of the rotor (picture shows only some of them but they’re regularly repeated in every section in between the fixing bolts).

    4- I also noticed that when braking with wheel off the ground, the wheel tends to move slightly inwards….

    It’s my first disc brakes bike and I would be grateful if someone suggested what to do.

     

    Thanks!

     

     

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Viewing 15 replies - 16 through 30 (of 35 total)
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  • October 4, 2019 at 9:22 am #950673
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    hawkinspeter

    Okay, sounds like your wheel

    Okay, sounds like your wheel is good and solid, so I’m puzzled as to what could be moving to affect the braking. Maybe the rotor has some movement in it, though that’s unlikely if the lockring is anywhere near 40Nm.

    Not bedding in disk brakes can lead to them being noisy and not having the full stopping potential, but it shouldn’t result in a periodic clunking noise.

    You mentioned about the wheel moving inwards under braking with the bike off the ground – I don’t see how that can happen unless something is a bit loose.

    October 4, 2019 at 9:06 am #950671
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    Huckfinn

    Not at all. Wheel is true. I

    Not at all. Wheel is true. I stuck a hard plastic object to bike frame about 1mm from edge of wheel, turned wheel over and over. No contact with hard object (a clear plastic ruler).

    I even thought I didn’t bed in brakes properly (didn’t know I had to… crying). But if that was the case, wouldn’t I have a similar problem with front brake?

    October 4, 2019 at 8:15 am #950669
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    hawkinspeter

    Was there any movement at all

    Was there any movement at all in the rear wheel when you where checking it out yesterday?

    For the problem to resolve and then re-appear sounds to me like something is slipping/moving.

    October 4, 2019 at 7:54 am #950667
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    Huckfinn

    Ok. I’m quite puzzled. Cycled

    Ok. I’m quite puzzled. Cycled to work this morning, it’s all basically downhill. Started well, no noise and no shudder, braking was nice and smooth. But, after a km or so, everything goes back to previous setup: clunk clunk + light shudder when braking hard.. Any suggestion?

    October 3, 2019 at 8:02 pm #950665
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    Huckfinn

    So, well, yeah. Thru-axle

    So, well, yeah. Thru-axle went out and back in. The rotor dutifully cleaned with alcohol. Back in again. Could hear a faint noise with wheel spinning and no brake pulled. Went for the white piece of paper but with a twist: cut a strip of standard white A4 paper an inch wide and let it slip on each side of the rotor, between rotor and inner pad, between rotor and outer pad. Surely enough there was a point where the paper could not slip through because a section of the rotor was just slightly touching.

    Saw today on the net people recommending to very lightly bend rotor with pliers just at the point where it rubs against caliper or pad. Did it (very very carefully and pliers with some cloth around it). An hour at it trying to adjust the fraction of millimeter necessary to avoid rubbing. Eventually stopped, loosened caliper bolts, re-tightened with pulled brake.

    Will see tomorrow but…I have my doubts..

    October 3, 2019 at 6:08 pm #950663
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    hawkinspeter
    Huckfinn wrote:
    Ok, will check tonight!

    PS: after an online search, I discovered that what I described as “clunk..clunk..clunk” is actually  a shudder of sorts (obviously associated with the clunk noise) that happens when braking hard..Do your suggestions still stand?

    The noise could be just a judder, but it sounds like you have some movement of the wheel and/or rotor when braking which isn’t right. Re-aligning your caliper may help with reducing judder – the pads should be parallel with the rotor when making contact.

    October 3, 2019 at 1:34 pm #950661
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    Huckfinn

    Ok, will check tonight!

    Ok, will check tonight!

    PS: after an online search, I discovered that what I described as “clunk..clunk..clunk” is actually  a shudder of sorts (obviously associated with the clunk noise) that happens when braking hard..Do your suggestions still stand?

    October 3, 2019 at 1:21 pm #950659
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    hawkinspeter
    Huckfinn wrote:
    Yeah will do, thanks.

    1-When you talk about adjusting the calipers you refer simply to tightening up the caliper mounting bolts, rights? No other adjustments? Like, for ex, if one brake pad is closer that the other to the rotor you fix the issue by readjusting the mounting bolts?

    2-Rode at luchtime. Wheel spins as well as this morning but clunk clunk is back and when I lift the wheel and squeeze the brake lever the rotor/wheel shifts a touch inward again.

    3-If the mechanic fixed it, why do you think the issue is back after only about 500km?

    1. Yes, just the 2 mounting bolts. Ideally, the pads should self-adjust so a few pumps of the brake lever should centre the pads over the rotor. It is possible for the pistons (i.e. the bits pushing the brake pads) to get stuck so they don’t retract properly. If that happens, I’d remove the brake pads (there’s just one retaining bolt to undo) and use a plastic tyre lever to push the stuck piston back in a bit (plastic is advisable so that you don’t damage the pistons).
    2. I reckon you’ve got a slightly wobbly wheel there
    3. See answer 2
    October 3, 2019 at 1:10 pm #950657
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    Huckfinn

    Yeah will do, thanks.

    Yeah will do, thanks.

    1-When you talk about adjusting the calipers you refer simply to tightening up the caliper mounting bolts, rights? No other adjustments? Like, for ex, if one brake pad is closer that the other to the rotor you fix the issue by readjusting the mounting bolts?

    2-Rode at luchtime. Wheel spins as well as this morning but clunk clunk is back and when I lift the wheel and squeeze the brake lever the rotor/wheel shifts a touch inward again.

    3-If the mechanic fixed it, why do you think the issue is back after only about 500km?

    October 3, 2019 at 10:57 am #950655
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    hawkinspeter
    Huckfinn wrote:
    Ok. Tonight the thru-axle goes out. I’m slightly worried: had a Giant MTB for over 20years. Did 105k km on it and all kinds of fidgeting and adjusting etc. This being a spanking new one I feel like I need to use white gloves on it…don’t know if I’m explaining myself….cheekycheeky

    Once you get used to it, you’ll be fine.

    Removing the thru-axle and popping the rear wheel out takes less than a minute – I do it most times that I wash my bike.

    Adjusting the calipers can be fiddly to get it JUST right, but if you get it wrong you just loosen the bolts and try again.

    Once they’re set up correctly they can be left alone for ages.

    (When fitting your rear wheel, try not to ram the rotor into the frame – speaking from experience, it leaves a nasty gash on brand new carbon fibre frames)

     

    October 3, 2019 at 10:47 am #950653
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    Huckfinn

    Ok. Tonight the thru-axle

    Ok. Tonight the thru-axle goes out. I’m slightly worried: had a Giant MTB for over 20years. Did 105k km on it and all kinds of fidgeting and adjusting etc. This being a spanking new one I feel like I need to use white gloves on it…don’t know if I’m explaining myself….cheekycheeky

    October 3, 2019 at 10:05 am #950651
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    hawkinspeter

    It shouldn’t make any

    It shouldn’t make any difference if the back wheel is on the ground or not. However, the crucial point is that there shouldn’t be any movement in a back wheel (except for the obvious rotation), whereas I suspect that you do have some play in your rear wheel.

    When you get a chance, remove the thru-axle, have a look at it and then re-assemble and see if the rear wheel has any play in it.

    You probably won’t need to bend the rotor – that’s only required if something has made it go out of true or it’s quite old. I’ve never needed to do mine.

    The black lines being on each of the rotor fins sounds like something is causing that, but I can’t think what.

    October 3, 2019 at 9:46 am #950649
    0
    Huckfinn

    Ok guys. Thanks to all.

    Ok guys. Thanks to all.

    So I loosened the hex bolts + squeezed brake lever etc.: this seems to have allowed the back wheel to spin better than before (similar force applied, about 10 seconds spin as opposed to 35 sec. for the front wheel).

    A couple of things:

    -Does the back wheel have to be lifted off the ground when doing this? (I didn’t). I’m asking because now I hear some light clicking noise coming from back brake when cycling downhill and not breaking.

    -The “clunk..clunk” noise is less but still there.

    -If necessary to do it: I’m not sure I understood how to “bend” the rotor…

    -I’ll check tonight also with the white paper.

    -Black lines are exactly identical in every section of the rotor in between the fixing points….

    October 2, 2019 at 9:52 pm #950647
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    Pilot Pete

    Oh and the black lines on the

    Oh and the black lines on the rotor just look like muck to me, possibly it has run down when the bike was stationary.

    PP

    October 2, 2019 at 9:51 pm #950645
    0
    Pilot Pete
    Podc wrote:
    Rear wheels don’t spin as well as front wheels due to the extra drag from the freehub – in my experience anyway.

    That is my first thought too. You can hear and see if a disc brake is binding. Check that the rotor is true – they can easily get a little bend in them. Using the white sheet of paper as a background, spin the wheel and see if the rotor runs true between the pads. If it doesn’t, use the correct tool or a small adjustable spanner tightened over the rotor at the right place and gently bend (I prefer ‘persuade’) the rotor back to true. This may take several attempts before you apply just the right amount of persuasion to get it true again.

    Adjust caliper to get the gap equal each side of the disc rotor. If it is still binding and the wheel bearings and through bolt axle are displaying no play, then I would look at removing the bleed screw in the lever with the wheel and pads out. Gently pry the pistons apart so that they are fully recessed. A little bit of fluid may well out of the bleed port. Mop this up and replace the bleed screw. This makes sure the pads can fully retract (they are not being stopped by too much fluid being in the system. This issue can be caused by bleeding the brakes without fully pushing the pistons home first.

    PP 

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