Help with Post mount bodge & Avid BB7

[Fatal Swan]

My first foray into disc brakes - bought some Avid BB7s with 160mm HS1 rotors. The rear is on fine with the IS adapter. In the front they're going on a set of Manitou Mars Carbon forks, post mount - good lightweight forks working nicely so I'd like to keep them if possible. I was expecting them just to bolt on to the post mount with no adapter but the mount itself doesn't seem to extend out of the fork leg far enough, see first photo (sorry for the quality). Comparing against another set of Manitous I have I can only think that the original owner of the forks cut or filed them down for some reason. (However even on the magnums the caliper still fouls the fork between the mount bolts so it wouldn't fit on that fork, either.)

My plan is to use spacers of some kind (brief bodge attempt with the bits that were lying in the second picture). I guess I'll have to use trial and error to get it exactly the right distance for the rotor. Would really appreciate any advice/warnings/better solutions as I'm completely new to disc brakes!

 

[jimo746]

Re:

Odd, I would expect the post mount caliper to just bolt straight on and sit right for a 160mm disc on the front.

The Avid brakes I've had in the past though all came with those spherical style washers to adjust the caliper position, so your method of mounting the caliper looks fine to me.

 

[02gf74]

what you have looks to be correct; Avids use the spherical washers but you need to have them both sides of the calliper.

If you cannot find them, I got some spare so can post to you.

 

[dyna-ti]

160mm rotor ,thats your problem there. Post mount on a manitou uses an adaptor
My manitou R7 is the same. Same post mount with the top mounting further out than the bottom. Sorry, i cant say if all the post mounts are the same though i think they are.

I think its the case that you always use an adaptor. Mine is 185mm and the adaptor is shaped to take up this difference in lengths and leave a parallel surface for the caliper to bolt on to.
I dont know if youre ever supposed to run a 160mm :? its possible and it would look similar obviously only not sticking out as far but given the top section is thin and the bottom thick,there isnt really enough metal to thin it further for a 160.
Im meaning as bought new,not grinding it down or anything so i would assume that if youre fork is post mount then the rotor you need to run starts at 180mm :?

Do you follow ?

Buy a Hope adaptor H, and a bigger rotor.

 

[legrandefromage]

I have the same forks and the brakes went straight on - but they were Clarks cable. The same spherical washers are found on v-brake pads btw.

 

[EarlofBarnet]

PM to PM adapter maybe help with clearance if the washers don't?

 

[Fatal Swan]

Right, thanks for these replies, a few different ideas so let me try and make sense of them.

Re: jimo746's point, just bolting the caliper on isn't possible as the bulbous part in the middle of the caliper just bashes into the fork leg between the bolts. The conical washers I have on in the photo were originally to attach the caliper to the (IS) mount that came with it but I'd assumed that when you remove the mount you don't use the washers.

If it's supposed to be used with a post-to-post adapter as in dyna-ti's comment, but for the rotor size, I was under the impression that the Mars fork was only compatible with 160mm max rotor - I found reference to this on a couple of different threads as I'd looked this one up before buying them (I don't have any need for bigger, either).

I didn't know post mount to post mount adapter's existed until now, though if the mount 'shape' is the same as the one on the fork (as per the left one in the photo above) then I'm likely to end up with the same issue of the caliper fouling the mount. Looks like what I need is something the shape of the one on the right in the photo though my guess is that one will actually space out the caliper too much. I'll start hunting for something like this to try then.

That photo is helpful even if I end up sticking with the 'washer' solution for now as it reminds me that due to the angles I'll need more spacers on the bottom than the top...

Cheers

 

[Fatal Swan]

So, thinking more on the ideal solution, a post-to-post adapter (assuming I found one the right size), I don't think it can work. My makeshift first attempt in the photo in my earlier post is actually pretty close to the spacing required and as you can see in the photo, even then the caliper only just clears the fork leg by a few mm. If I installed an adapter instead to make a similar thickness at the bolts, then the middle part of the adapter is definitely going to be fouled by the caliper, so it's the more or less the same problem.

I've done a bit more searching this evening and found a reference to some post mount 'spacers' like these, which seem to be just the kind of thing I had in mind.

I guess the ones in the photo are probably a bigger spacing than I need though as my makeshift first attempt is actually pretty close. No idea where to get these kinds of spacers from though...

On the same thread (on mtbr.com) I found what seems to be a helpful diagram to determine the spacer sizing to fit the right rotor:

Armed with the measurements in the diagram, I think my next plan will be to use more makeshift spacers to get exactly the right distance for the 160mm rotor, then once I've got it right through educated trial and error, measure the total length and see if I can find some large spacers like in the first photo with the correct length as a more permanent arrangement. Anyone have any better ideas??

Cheers

 

[Fatal Swan]

A bit more progress... these big spacers are found in some Avid post mount kits:

http://www.merlincycles.com/avid-post-s ... 69933.html

Will do some measurements to check if the spacer sizes included will suit.

 

[02gf74]

I'm using 160 rotor on a pair of manitou forks and avid calliper.

Will post a photo of it, seem to recall noticing the lack of parallelelismum.

Not come across post forks not able to take post mount calliper for 160 rotor, only issue i know about is fitting IS calliper to post for 160 rotor, there are clearance issues with fork/adapter/calliper.