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Rear Brake Disc Too Close to Frame

by andyspaceman
Thu, Nov 03, 2016 17:31
5

I'm putting together a road disc braked bike with a Genesis Equilibrium Ti Disc frameset, DT Swiss Spline R32 wheels and Acor cable/hydraulic post-mount brakes (also sold and better known as Juin Tech R1) that come with 160mm 6-bolt rotors.

The frame has standard 135mm-spaced QR dropouts and an I.S. disc mount, to which I have attached a Shimano I.S. to post mount adapter (SM-MS-R160P/S). The brake caliper bolts nicely on top, no problems.

In order to fit the 6-bolt disc rotors to the centrelock wheels I have a pair of Shimano adapters (SM-RTAD05). All bolts together nicely, no issues.

The problem comes when I put the wheel in the frame, and the rotor is so close that it rubs the washers of the brake mounting bolts and the left hand brake pad, even when it is fully retracted and the caliper is seated as far out to the left as it will go.

The caliper design isn't the problem, because the rotor is also rubbing on the caliper mounting bolt washer which has a position determined by the I.S. to post mount adapter and the frame itself.

I'm disregarding the thickness of the I.S. to post mount and 6-bolt to centerlock adapters as they're both Shimano items and, well, they ARE the standard. I've also tested with a 160mm Shimano centrelock rotor and the problem is exactly the same.

Which means that either the I.S. mounts on the frame are sat a little too far inboard, or the rear hub is a little wider than most (the wheel is nicely centred on the axle). Putting an old Shimano XT 6-bolt hub and rotor in there the spacing is much better, which is pointing to the hub width as the culprit. Rotor face-to-frame distance is about 11mm on my DT Swiss wheels, and about 13.5mm with the old XT 6-bolt.

Has anyone had any similar challenges? The wheels ride beautifully, so I don't really want to change those. Which means either getting the frame's disc mounts faced (tricky with titanium, plus I think there's a maximum of about 1mm that could be taken off) or finding some thinner 6-bolt to centrelock adapters. Which seems the likeliest option.

Any advice?

 

 

 

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5 comments

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BikeJon | 8 years ago
0 likes

I had to get the file out to fit a Center lock rotor (160mm) to some new tapered flat mount forks this week. The lock ring was catching a bulge in the forks but the file sorted it. 

My forks were only £90 from China (free shipping!) but full carbon, tapered, flat mount, internal cable routing, bolt thru and under 400g for CX. Aside from this minor lock ring issue, they are superb out on the road. Very nicely finished too. 

Sorry about the digression and I'm glad you got your issue sorted. 

Avatar
andyspaceman | 8 years ago
1 like

Thanks all, the kindly folks over on the Singletrack forum came up with some smart suggestions.

http://singletrackworld.com/forum/topic/warning-road-bike-content-disc-r...

As identified by therevokid, machining (or filing) down the adapter was indeed the answer. Running smoothly now.

https://flic.kr/p/MVccr1

Avatar
StraelGuy | 8 years ago
0 likes

I don't know about your frame in particular but when I was thinking of replacing my Spyres with Juin Techs, I found a forum post that said they definitely didn't fit my Defy Advanced 2. Seems the Juin Techs are a bit hit and miss fitwise.

Avatar
therevokid | 8 years ago
0 likes

I've had a couple that were "tight" before (all IS mounts) and ended up putting

a thin washer between the frame and the mount to push it over far enough, but this is

the opposite of your problem. Could you get the IS adapter machined down rather than

face the frame ?

 

Not had this on post mount calipers though strangely....

 

Given the relatively small clearances and tolerances involved I'd be surprised if this didn't

happen more often to be honest.

Avatar
Mr. Sheep | 8 years ago
0 likes

Long shot, but I know when I was assembling my bike I remember there being about ten different ways around the IS to post-mount adaptors could have been fitted - are you 100% certain they are the right way around?

In my case, the adaptors came with my Avid brakes, I didn't buy them separately, so they might not be the same as what you have, but I thought I'd mention it incase it helps!

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