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Help in adjusting my disc brake..

by Huckfinn
Wed, Oct 02, 2019 20:08
35

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!

 

 

If you're new please join in and if you have questions pop them below and the forum regulars will answer as best we can.

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35 comments

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Pilot Pete replied to Podc | 4 years ago
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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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hawkinspeter | 4 years ago
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Re-adjusting the position of the caliper is an easy enough task and is worth knowing how to do.

Loosen the two hex bolts that hold the caliper to the frame so that there's some lateral movement of the caliper. Then there's two methods that can be used:

  • Squeeze the brake lever so that the caliper will centre itself over the rotor and tighten up the caliper mounting bolts to keep it in place. (This is what the mechanic did)
  • Eyeball the gap between the rotor and the brake pads. A light background makes this easier, so hold some white paper the other side if it helps. Move the caliper so there's the same gap either side of the rotor (usually it's only about 1mm or so) and then tighten the caliper mounting bolts.

I've found it can be tricky if when tightening the caliper bolts, it shifts the position of the caliper, so watch out for that happening. Once you're happy with the position make sure you tighten the caliper bolts up to the recommended torque (from memory it's probably around 5Nm).

I prefer eyeballing the rotor as it also highlights any problems if the rotor is bent when you spin the wheel.

Once the caliper is in the right place, it shouldn't need re-adjusting often - maybe after replacing pads or a rotor but otherwise the thru-axle should keep everything firmly in place.

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Huckfinn | 4 years ago
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Hi, and thanks!

I'll check tomorrow morning on the potential side to side movement, but the reason why I think it's only a matter of adjustment of the brake is because a few weeks ago I asked a veteran cyclist/mechanic why it would be that the back wheel wasn't spinning much. He had a look and said: it's the brake! So he unscrewed it from the frame, asked me to pull the brake lever, fidgeted with the caliper, retightened it to the frame and...voilà, everything was fine. 

Unfortunately he doesn't live nearby....

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hawkinspeter | 4 years ago
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Maybe dodgy wheel bearings or possibly the thru-axle isn't correctly fitted. Can you feel any side to side wheel movement with your hands when the bike's off the ground?

The marks on the fins are curious as they shouldn't be touching anything. Can you eyeball it when spinnng the wheel and see if there's anything it could be hitting? Also, it'd be worth double checking that the rotor is good and tight.

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Huckfinn | 4 years ago
1 like

Here's the photo.

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