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I want to buy a new hydraulic disc brake for my old mountain bike, which already has a disc brake. As I am very far away from any bicycle shop I want to buy the parts on the internet. The shipping costs will be high, so I don’t want to send anything back.

There are different standards to mount the brakes to the bike. Post mount and IS2000, and I have heard that post mount has different sizes/types, e.g. post mount 6 and 7.

Now what I want to know:

  • What do I have to check or measure to be sure, that the new brake fits my bicycle?
  • Which adapters do I need (depending on the size of the discs and the type of fixation of the calipers)? Especially, when the new brake has a different disc size than the former one?
  • Which screws? Are they standardized and fit on all bikes and brakes independent of bike and brake manufacturer?

I want to know this in general, because I also want to replace the brakes of a friend’s bike with hydraulic disc brakes too.

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(within reason) Any disc brake will fit on any bike with the right adaptors.

The adapter you need will be based on the size of the rotor (disc) you want to run, the mounts you have on the bike (IS/post) and the brake you're trying to fit. Most callipers will have post mount fittings so require a adapter to fit to any IS mounts. Most frames still have IS mounts on the rear, while most newer forks have post mounts. They can be identified through how they are orientated, post mounts go into the fork or frame, IS mounts are orientated the same as the bikes axles.

When purchasing brakes Shimano callipers come separate of rotor and mounting adapter, Avid comes pre-packaged with everything so you simply buy the size you require. Smaller brands vary.

You need to be aware that the size of rotor makes a difference and some forks/frames are only rated up to a certain rotor size; any larger could cause the parts to fail.

There are two different standards for fixing the rotor to the hub - standard 6-bolt and shimano centrelock.

The bolts are standard thread (for all examples I've seen) but the length will vary for post mount brakes. You need to ensure that you use the correct length screws for post mounts. Short bolts can (some say will) lead to catastrophic failure.

The other point to bear in mind is that the hydraulic hose length for the rear brake may have significant extra length (c.300mm) and requires a moderate level of technical skill to shorten. The rest of the install can be done by hex wrench and T25 driver/wrench. A torque wrench is recommended but not mandatory. Other considerations will be cable routing, securing the hose and whether the brake mounts need facing.

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  • Do all brakes come pre-bleed and with the caliper/hose/lever pre-assembled?
    – Fred the Magic Wonder Dog
    Commented Jul 21, 2014 at 22:50
  • You didn’t say anything about the different types of post mount, which are PM6 (post mount 6), PM7 and PM8 (and maybe more, I am not sure). This is very important to know, otherwise the adaptors wont fit.
    – erik
    Commented Jul 23, 2014 at 10:07

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