Disc mounts an explaination.

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Disc mounts an explaination.

Postby nicklouse » Thu Jun 07, 2007 20:53 pm

there are two main types of mount for your disc brakes.

Post mount as found on the majority of Manitou and Some Rockshox and Marzocchi forks.

Post mount can also be found in 2 sizes 6" and 8". the distance between the mounting holes is 74mm in both cases.
In these cases the post mount caliper will bolt straight on with out any adaptors for the disc size, i.e. a 6" post mount will run a 6" rotor and 8" post mount runs an 8" rotor.
Of cause a post to post 8" adaptor can be added to allow you to run an 8" rotor on a 6" post mount or a post to IS 203mm adaptor can be used to run a 203 rotor on a 6" post mount.

Be aware there was an early Post mount which had mounting hole spacing of 69mm

To run an IS mount caliper on a Post mount fork you need a 20mm bigger disc and adaptor.

IS mount is found on most frames and on the majority of forks but the sizing is different.
A caliper (or caliper with adaptor) when moved from the rear of a bike to the front will require a disc 20mm larger than originally fitted on the rear.
Moving one from front to back needs a disc 20mm less than the original disc size you had fitted on the front.

Some 20mm axled fork have the caliper mounts 5mm further away from the fork center line, this requires a 5mm spacer or a specific 20mm axeled fork adaptor.
Please Note that just because a Fork has a 20mm axle it does not always require a 20mm adaptor or the 5mm spacers.

If the fork does need the spacer they usually come with the 5mm spacers (Marzocchi do this). This Offset IS mount is called a DH INTL STD 8" Rotor Setup by Marzocchi.

Two of the fork producers have also decided to produce mounts for the down hill forks that are unique:

Rockshox with the boxxer mount.
Fox with the mount on the 40 range.

The current IS standard came out in 2000 so it is often referred to as the disc IS 2000.
Before that there was the IS 1999 which is only slightly different to IS 2000, and before that Formula standard which was/is mainly found on pre 2000 marzocchi forks.

Formula Standard (or FS) looks the same but the caliper mounting holes are spaced at 49mm rather than 51mm and the bolts used to mount the caliper are 4mm not the current 6mm, also the Over lock nut to disc mounting face is different.

if you want the tech drawings here they are.
in there true form. Please note the dimensions are for true versions of each. ie. there are very few true 20mm hubs out there most are modified 9mm hubs.
6" post mount
Front IS mount
Front IS 20mm mount
Rear IS mount
Front Qr hub
Front 20mm hub
rear Qr hub

Pictures of Marz 20mm axle fork fittings.


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Re: Disc mounts an explaination.

Postby nicklouse » Thu Sep 13, 2012 07:41 am

Magura has also made some calipers that are disc size specific when fittted to a 6" Postmount.

As always when asking questions about brakes it is always good to ask specifics as there are too many variables.
"Do not follow where the path may lead, Go instead where there is no path, and Leave a Trail."
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