Bike Repair – Fit Post Mount Disc Brakes

Bicycle with disc brakes.

Here’s how to get your PM disc brakes correctly set up for safe braking and smooth running, mile after mile, and without any rubbing and squeaking.

Disc brakes come in two main varieties: those that use the International Standard (IS) mounting system developed in 2000, and those with the older Post Mount (PM) system, as used by Hayes. IS brakes are generally heavier and more fiddly to set up, and although IS has been the main standard for some time, the old PM system is fast becoming recognized for its benefits and is being offered by traditionally IS brands, including Shimano and Hope.

Fit is to PM adaptor

PM callipers usually incorporate a two-part mount using an IS adaptor to ensure they will fit IS frame and fork tabs. Assuming your bike has IS tabs, you need to fit the IS adaptor to the frame using a 5mm Allen key.

Slacken calliper bolts

Ensure that the frame mounting bolts are tight and then, while holding the calliper on to the adaptor, loosen the calliper bolts a little so that the caliper can freely float from side to side; half a turn of the Allen key will do. If the hydraulic hose is in its chainstay guide, take it out so the calliper can move freely.

If you have PM frame tabs, ignore the IS adaptor; remove it, bolt the calliper to the frame and slacken the bolts so the calliper is free-floating.

Lock the lever on

Use a zip tie or elastic band secured round the brake lever and bar to pull the lever inwards and push the pads on to the rotor. The pistons in the caliper should be moving an equal distance so that the pads sit evenly and firmly against the rotor.

Check piston gaps

If the pistons aren’t moving an equal distance, the pads might need to be reset. Drop the wheel out of the frame, squeeze the brake lever once so that the pads move together and then check the distance between them. If the rotor is unable to fit between the pads, insert a clean, flat screwdriver head or a proprietary pad tool and gently prise them apart until the gap is sufficient for the rotor to fit without rubbing. If this doesn’t cure the problem, it might be because the pistons are sticking, in which case the answer will be to get the caliper serviced by a reputable mechanic.

Tighten calliper bolts

Centre the calliper over the rotor and then, while holding the calliper on the adaptor or frame, tighten the Allen key bolts bit by bit, alternating between them both. Keep a close eye on the calliper itself to ensure that it doesn’t ‘walk’ off-center because of friction on the bolts. If this happens you’ll need to slacken the bolts off, re-center the calliper and carefully begin tightening them again.

Check and true rotor

When the calliper is tight, remove the zip tie from the brake lever and return the hydraulic hose to its guide. Check that the rotor is centered; look for equal amounts of daylight between the rotor and both pads. To check the rotor isn’t warped, which could make it rub, spin the wheel.

If it is, gently true it using either disc truing tools or an adjustable spanner. Remember that a little force goes a long way with an adjustable spanner.

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