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Disc brakes : fitting Post Mount callipers to your bike

The Post Mount system was developed by Hayes and is now used widely across the industry. Adaptors are available that will allow you to fit a post mount brake to an IS mount fork or vice versa, so don’t despair if what you have doesn’t seem like it will fit.

Calliper position is easy to adjust with the Post Mount system but it’s important to ensure that the pads are parallel with the rotor before tightening the bolts that hold the calliper in place. It’s remarkably easy to bolt the calliper on at a slight angle, so that one pad touches the rotor at the top and the other at the bottom. Even if this doesn’t make the brake noisy and draggy, it will make it feel mushy rather than sharp and is to be avoided.

Disc brakes - fitting Post Mount callipers to your bike (1)

Step 1 : If you are reusing the calliper bolts, place a spot of thread-locking compound on the thread. New bolts should come with this already applied. Fit a washer to each bolt if one’s not already on there, then push the bolts through the holes in the calliper. Line the bolts up carefully with the threaded holes in the Post Mounts.

Disc brakes - fitting Post Mount callipers to your bike (2)

Step 2 : Screw the bolts in until they’re almost tight; you should be able to wiggle the calliper about from side to side but you shouldn’t be able to rock it backwards and forwards. Pull the brake lever so that the pads both contact the rotor and keep pulling it as you nip up the bolts. If you tighten one bolt fully before tightening the other, you will find that the rotation of the bolt pulls the calliper over to one side and you will never get the pads to hit the rotor straight. Instead, tighten each bolt alternately a quarter-turn at a time until they are tight.

Disc brakes - fitting Post Mount callipers to your bike (3)

Step 3 : You will usually find that this has spaced the calliper perfectly. If it doesn’t work first time, try again, and if the pads are still hitting the rotors unevenly you will need to look at the gap between pads and rotor, work out which pad is too close, loosen the bolts and move the calliper over by hand before tightening the bolts up again. This is a more fiddly way of setting the alignment but it’s not difficult, it just requires patience.

Sizing of disc brake mounts

IS mounts are almost always 51mm apart. An additional standard was briefly used with 21.5mm spacing. The holes in an IS mount are not threaded – the fixing bolts pass through the mount and thread into the calliper. Post Mounts are now more common, though some quirky variations in size appear every now and again: standard Post Mounts are 74mm but 68.8mm and 70mm have been used. All calliper fixing bolts are M6 thread, regardless of mount standard. If you have Post Mount discs and an IS mount frame/ fork or vice versa, don’t despair – you can buy adapters to convert from one standard to the other. They may seem expensive for a small lump of machined aluminium but without them your mismatched brakes simply won’t work – there are no bodges that can be done here. Bolt the adaptor firmly to the IS component, whether that’s the frame/fork or the calliper, then follow the Post Mount adjustment instructions to set up your brake.

Don’t panic if you have an older bike with a non-standard mount, either. Contact the manufacturer who should be able to supply an adapter that allows you to use a more regular standard. If you have a frame without disc mounts but would like to use discs, you may be considering using one of the conversion kits that are available. These place additional stresses on a frame in areas where it was not originally designed or tested to take them, and may be damaging to the tubes. Their success also depends on the precise shape of your frame, so get advice from your bike shop before pursuing this option.