Inbred rear disc mounting question

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Inbred rear disc mounting question

29 posts & 11 voices | Started 9 months ago by tron | Latest reply from oneoneoneone

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tron - Member

Afternoon all.

I've just put a new set of brakes on my Inbred and the back brake is puzzling me. It's the M665 SLX brake CRC currently have on offer, with a 160mm RT76 rotor. The frame is a 2005 or 2006 DN6 Geared frame, bought in one of the rigid Deore groupset bikes deals.

The brake is mounted with the post mount to IS adaptor that came with the brake.

The brake doesn't seem to be sitting where it should be - there is a fair bit of pad that isn't sweeping the disk.

Here you can see how everything is mounted up, and the unswept bit of disc

Here you can see how a fair bit of pad is stuck up above the top edge of the disc:

Any ideas on what's going on and how to fix it? Or is this common on back brakes (I seem to remember similar issues with my old Maguras)?

Posted 9 months ago #
vdubber67 - Member

It's because you've left the sticker on.

Sorry, I don't know.

Posted 9 months ago #
Underhill - Member

Adaptor on the wrong way round? That's what it looks like to me

Posted 9 months ago #
tron - Member

Just been and tried that. Swap the adaptor round and the back of the caliper fouls the disc.

No arrows on it like there are on most Shimano adaptors, but it came assembled with the caliper that way round...

Posted 9 months ago #
lump - Member

180 adapter 160 rotor ?

Posted 9 months ago #
tron - Member

I don't think it's that either - I've grabbed a spare 180 rotor, and there's no way it would fit together with that adaptor. The caliper needs to be about 5mm lower down than it is.

TBH as I had a similar problem with my old Maguras, I'm beginning to wonder if the mounting tab's in the right place...

Off to the shed with a set of digital calipers and this:
http://www.hayesdiscbrake.com/wp-content/uploads/2010/04/98-17886_6inch.pdf

Posted 9 months ago #
tron - Member

Assuming that spec from Hayes is correct (does anyone know otherwise?), my IS tab is welded on in the wrong place...

Posted 9 months ago #
brant - Member

Crucial dims are the distance of the IS mount holes from hub centre, and the spacing between them.
We can see the spacing is correct, so it's the 78.1/39.9 dimension.

Posted 9 months ago #
tron - Member

I had a crack at taking a photo of the callipers set to the correct dims, but you couldn't make much out of them at all due to parallax error.

Measuring up, I'd say the two mounting holes come out at ~82/42mm from the centre of the drop out. I think the only way to be 100% would be comparing with a jig or facing tool. The IS bolt holes are 51mm apart, otherwise I'd have noticed a long time ago!

Posted 9 months ago #
andyl - Member

the rearmost tab hole should be a bit behind the centre point of the axle looking at my scandal and Merlin frames. Different seat stay angle and position I know but definitely looks like the tab is wrong on the frame.

Posted 9 months ago #
jimw - Member

I have just looked at the spare hope F mount that I have which is for a 160mm rear rotor and the rear mounting post looks shorter than the adaptor in the photo. It came off my orange 5 when I changed from 160 - 183mm disk

This one:

http://www.chainreactioncycles.com/Models.aspx?ModelID=13738

edit:

Could yours be this one for a front post 180mm?

http://www.chainreactioncycles.com/Models.aspx?ModelID=17390

Posted 9 months ago #
shedfull - Member

It looks like a Shimano 160 rear adapter:

Posted 9 months ago #
tron - Member

I can see where you're coming from Jim, but I'm pretty sure it's the right adaptor. It came already attached to the rear brake, which was brand new, and if you hunt about a bit the blurb says the SLX rear brakes come with an IS to PM 160mm mount.

Unlike the other Shimano adaptors I've seen, it isn't all labelled up. I'll see if I can compare it with a mate's one later...

Posted 9 months ago #
tron - Member

Just checked it against a mate's adaptor, and it is the right one.

Posted 9 months ago #
andyl - Member

it is definitely the right adapter and the right way up.

A word of warning about shimano adapters - the front 180mm is NOT the same as the rear 160mm. They have the same dimensions for caliper offset but the holes are about 1mm off centre on the rear one which moves the caliper too far in towards the wheel if used on the front (which means if you use a front 160 to mount a 140mm disc on the rear you need a 1mm spacer between the frame and the adapter). The front 180mm is also a bit chunkier than the rear 160mm to cope with the higher loads.

Posted 9 months ago #
tron - Member

Would a front 180 adaptor used on the back move the caliper back or forwards?

Posted 9 months ago #
andyl - Member

180 front on the back would be the same as a 160mm back but would need 1mm shims between the IS tabs and the adapter to move the caliper towards the cassette side.

160 front can be used to fit a 140 disc on the rear (again needs shimming)

Other makes may not need shimming but shimano definitely have their rear adapters slightly off centre.

Posted 9 months ago #
tron - Member

Hmm. There doesn't seem to be an off the peg fix then.

Thanks for the info Andy!

Posted 9 months ago #
robdeanhove - Member

It looks like a well used frame, judging by the state of the end of the chainstay. Did the old brakes have an adapter or were they a direct IS mount?

If you still have the original brakes, pop the caliper on to confirm it lines up with the new disc, then you know it's definitely the wrong adaptor that you have, not an issue with the frame or disc size. At this stage it'd be good to nail it down to that part.

I suspect that, assuming the above confirms the caliper adapter is wrong, a new adaptor will do the trick. I had something similar with an adaptor that was too small (wrong in the opposite direction) that I shimmed out with a washer for a few days while I waited for a new one from CRC, which only cost a couple of quid and I was riding happily in <48hrs.

Posted 9 months ago #
andyl - Member

Rob - nope. not the adapter as already stated many times.

Tron - only way to fix this without anything drastic is:

A) machine the current adapter down to bring the caliper in towards the frame. Needs to be done on a milling machine to ensure the caliper sits perfectly inline with the disc. Both upper and lower need machining to keep the bolts perpendicular to the face.

b) get a front 160mm adapter and first attach it to the frame with some shims to space it in about 1mm (get some nice quality stainless shimming washers) and then use some more shims when fitting the caliper to increase the gap - a bit like some of the brake manufacturers supply instead of offering different adapters.

Number 2 will be the easiest and most people have front 160mm adapters lying around.

I suspect you used to have a make of brakes that had spacers between the caliper and adapter already which would explain why they fitted? You could also probably just use this adapter but be away it may also need some sideways spacing due to the shimano caliper.

Posted 9 months ago #
andyl - Member

ahh no it's avids that have shims between the caliper and adapter:

That is what you need to do with a shimano F160 adapter and some shims between the adapter and frame tabs or you will be stuck with the inboard pad rubbing constantly.

Posted 9 months ago #
blahblahblah - Member

I don't think you have anything to worry about. I had this exact issue with a rear SLX brake when I built my new bike up two months ago.

After seeing uneven contact on the disc I double checked the calliper alignment and all was good. I just went for a 3 hour ride and by the end the pads were bedded in and making proper contact with the disc braking surface.

From the photo you have it looks like you the disc is running through the calliper correctly. Give it a little time to bed in.

Posted 9 months ago #
andyl - Member

It is the wear path on the disc that concerns me. It should be pretty much exactly level with the inner edge of the braking surface when it cuts across the top of the spokes. You have about 4mm of breaking surface untouched.

Maybe that is due to a step on the pads? But if not you need to get that caliper moved so the whole braking surface is being used.

My carbon 456 with swapouts is set up absolutely perfect with XT calipers so that the whole brake surface is used and the spoke isnt touched at all.

Posted 9 months ago #
tron - Member

A bit of clarification:

1) It's the right adaptor. I checked it side by side against a mate's known good Shimano 160R last night.

2) The old brakes had the same issue, I just never noticed as they were second hand, so the wear pattern on the disk didn't show. You also can't see in through the back of those calipers. I pulled the pads out once to bleed the brakes, and they had a fair old step in them.

3) All the current braking kit is brand spanking - Rotors, Calipers, adaptors, bolts and pads. There are no steps in the pads etc.

Anyway, I've been out and had a mess with some of the old Magura mounts (160F/140R type, as my old Magura calipers seem to have +20mm built in), and some long M6 bolts. Put it together with 9mm (!) of washers like Avid used, and everything runs as it should, with the pad acting on the correct part of the disk. By that point, you're adding leverage to the threads on the mount etc. No idea how much impact that makes, cos I'm not an engineer.

Posted 9 months ago #
andyl - Member

the washers and long bolts are not ideal but as long as you use nice flat ones and as thick as you can find then you will be fine. it is mainly compressive (unless rolling backwards!) force on the fixing.

Ps is the sideways alignment okay? Check for the disc being pulled over when the brakes are applied.

Posted 9 months ago #
tron - Member

It's pretty scrapheap challenge style at the moment! It's hitting the right part of the brake, but there's no point tightening up a load of hex head bolts onto a V-brake washers!

I'll get a proper set of M6 allen bolts and see if there's enough slack in the caliper holes to align everything.

Posted 9 months ago #
andyl - Member

boltmeup on ebay are pretty cheap for bolts and come quick.

Posted 9 months ago #
tron - Member

Gave on-one a buzz today - it is an old frame, but that mount hasn't moved since I bought the bike.

Looks like I'll be DIYing a mount

I'm not keen on having a bunch of washers waggling about and falling off every time I need to adjust the caliper alignment.

Could I Araldite some aluminium blocks to the top of a F160 mount, drill and tap it and expect that to last (using long bolts to go down into the mount proper)? Or would I be better off drilling the blocks slightly oversize so all the thread contact is in the proper mount?

To be honest, I'm not sure there's much stopping me tapping two holes in a bit of aluminium for the IS mount, then hacking and filing away until I've got a half decent mount. Except for the fact that it'd look ugly and probably still weigh more than a factory 160 mount with some alloy spacer blocks glued to the top...

Posted 9 months ago #
oneoneoneone - Member

If your worried about washer etc then just file/cut the old one down. Then run the tap through it.

Will look like it's the proper thng any way.

Posted 9 months ago #