Confused about disc brakes

[LizardEye]

I'm thinking of switching to disc brakes when I upgrade my wheels, but am slightly puzzled by the different rotor sizes.

So the rotors can be anywhere between 160mm and 200mm in diameter, but the position of the caliper is fixed by the
mounting bolts on the fork. So how can the caliper line up with different sized rotors if it's in a fixed position on the fork?

At least I'm assuming all forks have mounts in the same position.

And do brakes calipers tend to come with rotors or is that a seperate purchase.

cheers for any help

 

[mr Mag00]

the calipers will have spacers on the mounting bolts allowing differing rotors to be fitted. ok?

 

[LizardEye]

Ah right, so it's not the up/down position that changes, its the position in / out from the fork leg.

cheers

 

[mr Mag00]

spot on!

 

[02GF74]

At least I'm assuming all forks have mounts in the same position.

No. The two common fittings now are IS and post mount - in olden days there were other types of mount e.g. Marzochi Z1 that is an unusal one.

The latest calipers is post mount - bolts are radial with disc - this has spacer to move out the caliper to fit differnet disc sizes; these are 140 mm, 160 mm, 180 mm and 203 mm (there may be other sizes).

By default without any spacer, the caliper fits 160 mm disc on front, the 140 mm I've only ever seen on the back.

Front and rear post calipers are identical and will fit IS mount - bolts are perpendicular to the disc - with an adapter.

By default - without any adapters - IS mount on front fits 160 mm and rear also 160 mm. The rear IS caliper is not the same as the front as it has longer legs so the rear, when mounted on the front, will work with a 180 mm disc.

There are many adapters to convert between caliper (IS/post) and fork (Is/Psot) as well as different disc sizes.

Check your bike fittings, front and rear to determine which caliper is best for you, ideally without adapters, but the adapters are not that expensive, in region of £ 5 - £ 10.