Fox 36 Shimano XT 203mm rotor installation with post to post adapter

PB Forum :: Mechanics' Lounge
Fox 36 Shimano XT 203mm rotor installation with post to post adapter
  • Previous Page
Author Message
flag Spirocheter
Posted: Nov 6, 2015 at 12:54 Quote
I'd be grateful for any tips here. I'm in the process of building a Tallboy LTc, Fox 36 forks which has 180mm rotor standard.

I've bought a 203mm disc and an adapter. I had initially bought the Hope adapter, but then had a catastrophic failure once brakes were bled and whilst in testing. The brake lever went all the way the brake grip and fluid everwhere on the floor. Turns out the piston had got stuck squewed so the fluid could leak out. No way of getting it back.

Looking at the calliper and rotor, I thought perhaps the rotor wasn't aligned over the disc / rotor properly. I assumed the Hope adapter was incorrectly proportioned so I ordered the Cheaper Shimano product... And a new rotor. Again though, not enough of the disc sits within in the grip of the Pistons, so I expect there'll be an unequal pressure and the piston will likely fail again (I've not applied any pressure yet).

So I'm perplexed, the adapter seems to be correct spec, but looks out?

Any help would be much appreciated, thanks in anticipation (pics below, side on view and oblique view).
(My guess is, I've got the wrong adapter again, but I'm not sure what to order for the third time)!

Fox 36 Float 150 Rc2 29" Fork 2015 (apparently 180mm post mount)
Shimano XT RT-86 6 Bolt Ice-Tech Rotor 6 Bolt Style, 203mm
Shimano M8000 caliper

Apparently I have the 180 - 203 post to post mount adapter?!

Sorry duplicate trying to delete

Close up of M8000 XT oblique view with adapter on 203mm rotor
flag rileyager
O+
Posted: Nov 6, 2015 at 14:38 Quote
My guess is that the forks mounting posts are designed to take 180mm rotors without adaptors, so you probably need a 180mm adaptor to put your caliper in the right place for the 203mm rotor.
flag Spirocheter
Posted: Nov 7, 2015 at 1:39 Quote
Thanks bowbikebuilder, the forks do state they do not need adapters to run 180mm rotors, but I thought in order to run 203mm rotors I would need 180 to 203mm adapter. I can find different adapters which say 180mm rotor which I suspect is the correct item (fixed with two bolts straight through as opposed to four bolts), but I can't understand why this item should be correct.

I have not been able to find any guides which explain how to find the right choice of adapter. Clearly I'm out of touch with bike building, it has been a few years! I find it odd the adapters say 180mm when I could run it with 203mm? Any light on this to aid my understanding would be great.

I thought this item looked potentially right (but I stand to be corrected):

http://www.ebay.co.uk/itm/Shimano-Front-Disc-Brake-Post-to-Post-Mount-Adaptor-180mm-rotor-SM-MA-F180P-P2-/262092482547?hash=item3d05ede7f3:g:MGoAAOSwo0JWHWqC
flag Spirocheter
Posted: Nov 7, 2015 at 3:33 Quote
I should like to add some more detail here, in case anyone else is searching for answers to this topic in future.

In the process of trying to obtain the correct adapter, I should it is not an uncommon thing to install Shimano XT M8000 203mm rotors. I should have thought Fox would have a guide to instruct how to install and which would be the appropriate part. Instead, it’s like I’m having to learn some sort of black art (perhaps I should have consulted the dark web).

http://www.ridefox.com/help.php?m=bike&id=517#mountingdiscbrakes

(nothing here about how to fit larger rotors with brake adapters)

It is not easy from any of the bike e-retailers (wiggle / chainreaction) to work out which adapter to use (I should have thought also in their interests to facilitate selection of correct adapter). I live in a rural community so it is a long round trip with parking charges to get to a decent bike shop for advice (plus I’ve not always found advice regarding mechanical issues reliable). I didn’t incidentally have any problem finding the correct post mount adapter for the rear (160 to 180mm rotor fitted at the rear).

The most useful guide online I have found is here:

http://blog.artscyclery.com/keeping-the-rubber-side-down/rubber-side-down-disc-brake-adapters/

My search turned to using google (dangerous I know for the uninitiated). I am a medical doctor and cringe when people have been using Google!

One of the few threads I found included:

http://forums.mtbr.com/brake-time/post-post-adaptor-180-post-203-rotor-921446.html

(Someone has had this problem before). As usual per some forum threads though, once the user obtained the answer, it was not clear for any other users to follow (not for me at least)!

So from this thread, I determined I required a +23mm adapter, why though it should be a 160mm to 183mm doesn’t make sense (to me at least). There is no link either to the correct part.

I’ve emailed JensonUSA since I bought the forks from themselves. Also emailed Fox Forks USA. I’ll update this thread for any other users. Please let me know if there’s anything I need to clarify and I’ll edit the post.
I’ve wasted hours on this so I’m keen to give a road map to help prevent others wasting so much time also!
flag nigeJ
Posted: Nov 7, 2015 at 3:33 Quote
Hi, how old are your 36's. I bought a pair of 2012's and they were for a 200 rotor, without any adaptors. I ha a 180 front so had to get a new 200 disc and adaptor to take the caliper up to size. All this was Hope stuff and didn't have any problems.
flag Spirocheter
Posted: Nov 7, 2015 at 3:45 Quote
Thanks Nige.

They are 2015. I just phoned the bike shop (about a 30mile round trip) and they said I need a 180 adapter, then to fit shims to take it up to 203mm. They'll do the adapter and shims for £10, so for the bother of getting it right, I'm going to take a trip!

I'll be sure to upload an image to the thread to clarify things!
flag Spirocheter
Posted: Nov 8, 2015 at 10:57 Quote
Sorted,as promised pics attached for anyone searching the issue. For reasons still not clear (see below), the required adapter is 180mm (this adds +20mm) and it also requires shims to take it up 3mm further for the required 23mm.

What doesn't make any sense, is which bikes do my other adapters fit! . The Shimano product being clearly marked as for 203mm rotor.

Apparently most guys are running the larger rotors, be it 200 (SRAM) or 203 Shimano.

The part I bought was: Shimano SM-MA-F180P/P2, but note you will require shims (installed in the correct location - see pictures). Otherwise the slot will jam on the disc and your wheel won't rotate.

Fox 36 with 180 20mm Shimano adapter plus shims now perfect fit

photo
flag rileyager
O+
Posted: Nov 8, 2015 at 11:03 Quote
Those conical washers that you have mounted on the top of the calliper can also be mounted under the calliper which would negate the use of those shims. Specs for fork post mounts also differ between manufacturers so no mount will fit everything perfectly without modification.
flag Spirocheter
Posted: Nov 8, 2015 at 12:46 Quote
Thanks again bowbike, that does make sense. I've learnt a little more which is no bad thing. Cheers
flag nakos
Posted: Nov 9, 2015 at 23:38 Quote
I have noticed one huge flaw while looking over this thread. You need less a 23mm adapter to convert from 180mm post mount brackets to 203mm brackets. The issue is that rotors are measured by their diameter, but caliper mount adapters only add distance to the radius.

A 180mm rotor has a 90mm radius
A 200mm rotor has a 100mm radius
A 203mm rotor has a 101.5mm radius

To go from a 180mm rotor mount to a 203mm mount, you should only have to add 11.5mm to the original mounts on the fork, since the difference in the radii is 101.5mm - 90mm = 11.5mm.

I'd hope that an adapter that is made for a 180/203 conversion is the right length, but if you bought a +23mm adapter thinking that it would work, then that could be your issue. Adding 23mm would be enough to convert a 160mm mount to 206mm.
flag Gaci
Posted: Jan 23, 2016 at 5:50 Quote
Guys would this two options worked without adding shims, i really dont like the idea of shims:

http://www.bike24.com/1.php?content=8;product=97900;menu=1000,2,15,117
http://www.bike24.com/1.php?content=8;product=5624;menu=1000,2,15,117

With magura i guess is the +23mm adapter so the QM26, but is fuggly as hell, so my next bet is shimano SM-M90-F203, am i right?

Need for new fox 36.

Tnx
flag sandwich
FL
Posted: Jan 31, 2017 at 16:33 Quote
cobba, sorry to resurrect, but I'm facing a similar question.

I have a fox 36 and shimano 203mm rotor.

There are two available post mount adapters that are very similar,

http://www.jensonusa.com/Shimano-F203PPM-Disc-Brake-Adaptor

and

http://www.jensonusa.com/Shimano-F203PP-Disc-Brake-Adapter

It looks like OP had the first one, then switched to the second one to make it work. What's the difference? What's the P/P for, and why is it different than the p/PM?
flag sandwich
FL
Posted: Feb 5, 2017 at 15:14 Quote
cobba wrote:
sandwich wrote:
What's the P/P for, and why is it different than the p/PM?

P/P is for 160mm post mounts
P/PM is for 180mm post mounts

Fox 36 has 180mm post mounts

thank you for the response, I ended up finding it in another of your posts. Can't believe it's so difficult to find that info!
flag bikingjk
FL
Posted: Feb 20, 2017 at 13:21 Quote
Anyone know where to get 1.5 mm spacers from? Can't find any anywhere!
flag Schummy
Posted: Feb 21, 2017 at 1:49 Quote
The adaptor you seem to need is the Shimano Disc Brake Adapters SM-MA-F203 (180mm to 203mm), Post/Post Front/Rear.
  • Previous Page

 


Copyright © 2000 - 2024. Pinkbike.com. All rights reserved.
dv42 0.013274
Mobile Version of Website