Brake caliper mount interference

[stormdot5]

I'm currently trying to put together an electric bike using a cyclocross frame and a q100 cst motor. I chose it partly because it's fitted with dual cable disc brakes. I'm having an issue with the disc brake mount on the motor.

Where the disc would mount, it would protrude into the mounting lugs for the brake caliper. The simple solution would be to offset the motor with spacers, however I'm already pushing it as the motor is 139mm wide and the dropouts are 135mm wide in an aluminium frame. I could use a smaller locknut on the axle on the cassette side, but I don't yet know whether that will be possible with the chain on the smallest rear sprocket.

Is this a common issue or just because of the frame I've picked? I assume the mount and caliper might be spaced better on a mountain bike.

I think I need to move the disc 3.5mm inbound to compensate. Is it possible to machine the motor disc mount surface to accomodate? The disc mount bolts would end up with less thread to sit in and I'm not sure of internal clearnace with the ends of the bolts on the internal motor components.

Some pics to show the issue

If anybody has encountered similar issues, and managed to overcome them, any help or info would be appreciated. Thanks.

p.s. anyone know how to get embedded images to be not-humungous?

 

[Drunkskunk]

Welcome to the forum. That post mount brake boss is clearly in the way, and there isn't enough material in the side of the motor to allow you to successfully machine down the side to get the clearance you need.

Really, you have 3 choices, and 1 of them sucks.

The first is add spacers till it fits, but being an aluminum frame, that's not a good choice. You're chance of having a frame crack is high, and potentially disastrous.

The second is to cut that brake boss off, then use a "Shark fin" style disk brake mount. Shark fin mounts are designed for adding disk brakes to frames that don't have brake mounts, which your bike will be. It would just be a matter of picking one that gave you the clearance you need, or even making your own. This isn't a bad idea, but it does involve a lot of labor, time, cost, and chopping up your frame.

The third is to get a frame with a more conventional brake mount.

 

[mybike]

Personally I would go for the 3rd option getting a new frame just looking at those pictures.
I say you picked the wrong frame.

 

[aroundqube]

I would go with a suitable frame too. Not sure if these the work - http://porkchopbmx.com/evolution-v-brake-aluminum-mounts-clamps-adapters-pair-silver/ , but converting to a v-brake for the back may be an option.

 

[stormdot5]

Just an update for anyone who find this in search. I tried a third option, which worked for me, may not work for you.

I bought a brake disc off of ebay for £8 to play with. I talked with a guy at work and we thought it was doable to get it to fit, with some adjustment.

The disc had relatively thin extensions from the mount to the mounting area to the working disc. Using some supersize 5mm thick M24 washers and bolt we clamped the centre of the disc squarely, protecting the mounting area, with the washers stopping about where the motor started to curve away. Then we clamped the outer disc in between two large bearings, large enough the hold the edge of the disc but keeping the extensions free. Then I struck the centre bolt with a hammer. The extensions bent but the disc mounting area stayed flat. This moved it to about 5mm of offset in one hit. This left the brake disc working area at the same angle as the extensions. To sort this out we just placed the bearings back on the working area and gave it a good solid hit a few times, checking each time to see how far we had to go. Perfect alignment was impossible with this method, as the disc couldn't bend past flat, and so would always spring back a little. Still, it turned out rather well.

The disc is now offset about 4mm (exactly as intended) with .15mm misalignment in the mounting area, which will correct when torqued up. The disc surface I havn't DTI'd yet.

To fit, the axle required a washer in the brake disc side of 2mm, and the dropouts end up being pressed about 2mm apart, basically a tiny bit of positive pressure on the motor axle nuts. The disc doesn't quite clear the caliper mount, and required a little light filing to allow the extensions to run freely past it. It's so slight that just letting the brake disc rub the paint off may have done the trick. Maybe a fraction of a millimeter more.

After a test run and some pad/caliper adjustment, the brake is working perfectly. I'm now able to keep the back wheel next to locking all the way through an emergency stop. There is no drag from the rotor and no vibration under load.

http://imgur.com/a/ZdOIX

Just waiting for the 180mm front rotor to turn up and this bike will have all the braking it can take.

p.s. embedding imgur albums and preview size pics?

 

[ecycler]

Cool solution. That is thinking outside the box for sure. Keep us posted on how your brake pad wear works out over time with this setup.