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I recently bought a new Polygon Strada 5.0 mountain bike with Shimano BR-M355 hydraulic disc brakes and have had issues with the brakes rubbing. I fixed the rub but now it is happening again.

The fixing process takes time and I'm asking if there are any suggestions/ideas on how to prevent this from happening again.

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  • Can you tell us the brand and model of bike along with the type of brakes? – mikes Sep 15 '15 at 1:17
  • Brand : polygon, model:xtrada 5.0 mountain bike, brakes type : Shimano br m355, i think it use shimano alivio group set. – Padang Sep 15 '15 at 2:12
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Likely the brake caliper assembly isn't mounted quite parallel to the rotor and needs to be slightly adjusted. This is a pretty easy process that takes less then a minute to complete:

  1. Using a 5mm Allen Key, slightly loosen but do not remove the two caliper fixing bolts so you can move the caliper assembly side to side.
  2. Depress the brake lever so the rotor is being clamped by the pads and then re-tighten the caliper fixing bolts to 6.0 - 8.0 N.m (53.0 - 69 in. lbs.)

If you ever need to remove the caliper assembly from the mounts (for instance to change your brake pads) you'll use a similar process to re-align the assembly to the rotor. In that case you would re-install the caliper assembly but only screw the caliper fixing bolts in part of the way and then follow step #2 above.

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  • Thanks for the point. I repair my brakes rubbing same as the suggested, only difference is i press the brakes while re-tighten the caliper fixing bolt. But sometimes after using a brakes or remove and reinstall the front wheel the problem happen again. Are this problem common for a new bicycle? – Padang Sep 15 '15 at 6:55
  • It is not just common to a new bicycle. – paparazzo Sep 15 '15 at 10:54
  • Great advice here,thanks. Brand new bike,purchased online, but rear disk brake rubbing. No matter what I did it still rubbed. I then Decided to go back to basics. I loosened the rear axle and felt the wheel drop into position. I'm guessing it wasn't tight enough and moved in transit. Anyway, problem solved and I adjusted perfectly using the method recommended above. – user28807 Aug 20 '16 at 14:33
  • If you assembled it yourself, might be a good idea to get someone else to give your new bike a one-over for safety and assembly. Never hurts to be confident; may save your teeth or skin or bones if something else wasn't done right. Most of us have observed bikes being ridden with front forks facing the wrong way! – Criggie♦ Aug 20 '16 at 21:22
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Generally, brake pads rub mean one of the two things:

  1. Caliper is not set up properly. Misaligned with the brake disc plane, not centered on the disc, brake pads worn out asymmetrically, etc.
  2. Too much hydraulic pressure in the system. It's either too much fluid in the system or brake lever idle travel set up too low (on brakes which do have this).

These issues may be addressed by following the steps (respectively for each issue).

  1. a. Loosen the caliper bolts as Glenn suggested.

    b. You may move the caliper with your hand slightly to ensure it is free on the bolts and to break it if it jammed (metall diffusion, dirt and such).

    c. Rotate the wheel. This will help the caliper to settle in centered or may show if your brake disc is warped.

    d. Gently apply the brake so the wheel stops in 1-2 revolutions.

    e. Clamp the brake lever to secure the caliper in its position and tighten the bolts. Do not release the brake while tightening the bolts! If you do so, caliper may become again misaligned due to gravity or inner tensions.

  2. a. Consult with the manual to find out where bleed port on the brake lever is located.

    b. Get your Torx (at least for SRAM brakes, not sure if applicable to Shimano) key ready as well as some rag.

    c. Slowly unscrew the bleed port. You may see a drop of brake fluid running away. This may indicate that your brake system had the above atmospheric pressure which prevented the brake pistons to retract properly.

    d. Do not open the bleed port fully! Do not touch the brake lever! Doing so will break your brakes and you will have re-bleed them.

As a side note, it is recommended to perform a "break-in" procedure if you happen to change the brake pads, rotor or after cleaning mentioned parts. This is required to set the pads correctly in the caliper and spread a thin layer of material on the disc -- this increases the brake ratio significantly. To perform a procedure, just speed up on your bike to a moderate velocity and then perform a nice hard braking to a near-halt speed. Repeat 10-20 times to warm the brakes properly. It's a cool warm-up for the rider as well :)

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  • Removing brake fluid is not a fix that should be suggested unless you are explaining more of the process. There is a large risk of introducing air into the system in many brakes or taking out too much fluid and should only be attempted by someone who understands what they're doing and is comfortable doing it. removing the pads and resetting the pistons should also be included as a solution. – DWGKNZ Sep 22 '15 at 20:19
  • Sorry for a long reply. There is some risk of depressurization involved indeed. The process is very simple, but still has to be performed with caution. You just unscrew the bleed port, watch the drip form (if any), then close it back again. I can't remember where I got this advice, but it helped me a few times I bled my brakes myself. Trying to do it good I put too much liquid sometimes, heh. But it should be considered as a last option after resetting the pistons, as you suggest. There's a short video explaining some of the solutions. – Alexander Goncharenko Sep 24 '15 at 23:15

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