Disc brake - Rear cabling

[freeflow]

I'm looking at the cable run for a rear disc brake on a frame I'm having built. Using cable actuated disc brakes on a road bike. Reading around seems to suggest that continuous compressionless housing is the way to go. However I can't understand why this would be the case for a road bike, MTB or cross yes, but not road. By my thinking for a road bike you can use the conventional top tube cable stops, run housing from the rear of the top tube to a cable stop on the rear seat stay and then bare cable all the way to the disc brake provided the path is clear. To my mind you than only have housing from the STI to the top tube and top tube to rear seat stay which reduces significantly the amount of housing that can compress. The key bit is making sure there is a clear cable run from the cable stop on the rear seat stay to the actuator arm.

Is there a flaw in this thinking? I'd appreciate comment.

[Brucey]

I think you are right but I can see a few potential snags

1) the cable ferrules; I think that you would be best advised to use lined ferrules. Jagwire make some nice ones.

2) any bends near the ferrules will be bad news for cable friction, as will any angled exit of the bare cable.

3) if you ride in the wet, expect water to penetrate the cable housing near the seat cluster.

4) you may have a bare cable that lies 1-2" clear of the seat stay. I would expect this to be prone to getting snagged.

5) the direction of the cable pull won't be quite usual, and this may interfere with the brake operation to some extent.

maybe none of these would be a show-stopper; I guess there is only one way to find out.

cheers

[andrewjoseph]

Freeflow, are you sure the caliper is on the seat stay? Many disk road frames have the caliper between chain and seatstays

[mattsccm]

Unless you are careful there is always a curve in the brake outer. This is because the line drawn through the centre the tube bit of the caliper isn't parallel to the seat tube. What would happen would be the cable inner rubbing on the caliper. Possibly you could have the caliper bosses placed so this doesn't happen but I haven't ever seen that so maybe it doesn't work. something to do with the way a caliper sits on a frame or the disk? The same seems to happen with both seat stay and chain stay mounted calipers.
I seem to remember seeing somewhere (here?) the suggestion that 1 long length of decent outer had less give/compression than several shorter bits as each ferrule etc had its own give.
I am not sure the comparison with CX or MTBs has much value. Many of us prefer a exposed cable where possible to a full length outer as it tends to be easier to cleans and lube and as well as remove and full length outers do get mucky inside.
You main thoughts need to be where you place the caliper, seat stay or chainstay.

[Valbrona]

You might want to switch to hydraulic in the future and have the problem of converting cable stops into open-ended ferrules suitable for continuous housing. You would expect all the big three - Campag, Shimano and SRAM - to have hydraulic road disc brakes in the not too distant future.

[freeflow]

Freeflow, are you sure the caliper is on the seat stay? Many disk road frames have the caliper between chain and seatstays

Definitely the seat stay. Its a custom frame and the chain stay mount looked just awful.

You might want to switch to hydraulic in the future and have the problem of converting cable stops into open-ended ferrules suitable for continuous housing.

This is a good point BUT I use a triple chain set and don't have any intention of changing soon. I don't envisage a triple hydaulic STI becoming available any time soon, if ever.

if you ride in the wet, expect water to penetrate the cable housing near the seat cluster.

That'll be exactly the same as my caliper brakes and they seem to keep on working fine.

Many of us prefer a exposed cable where possible to a full length outer as it tends to be easier to cleans and lube and as well as remove and full length outers do get mucky inside.

Funnily enough I saw a tandem at a ride I did over the weekend which had bare cabling, but was routed under the BB's as the calibers were chainstay mounted.

the direction of the cable pull won't be quite usual, and this may interfere with the brake operation to some extent.

I think this is quite likely to be the showstopper as I don't have a disc brake system to play with to check out the angles. Definitely a bit more thinking to do.

I also managed to get into a couple of bike shops on the Saturday to see if I could gain any ideas. The one impression I came away with was that the cable tie cable mounts didn't look too bad.

[reohn2]

The cable run on our disc braked tandem is bare along the toptube with full outer to seatstay mounted caliper no worries :-

[andrewjoseph]

Freeflow, something to consider is if you intend to use a rear rack. Chainstay mounted disk calipers seem to cause more problems.

Our custom road/touring bikes have disk calipers mounted between chain and seatstays with full outers. No problems putting rear racks on.

[reohn2]

Freeflow, something to consider is if you intend to use a rear rack. Chainstay mounted disk calipers seem to cause more problems.

Our custom road/touring bikes have disk calipers mounted between chain and seatstays with full outers. No problems putting rear racks on.

I agree disc calipers mounted between seat and chainstay are more protected too.

[mattsccm]

Plenty of racks out there to cater for seat stay mounted brakes and I can't say that lack of protection on my rough stuff bike has caused problems in the last 5 years. A caliper is a solid thing. On the whole I prefer seat stay mounts as the cable run seems to be smoother and it less cables on the down tube.

[mrjemm]

On my converted Orange P7, it has full housing along the top tube to the seatstay mounted caliper, presumably due to as a mountain bike it would use hydraulics, and thUs hoses. The problem with that for me is that the housing is quite loose in the clips, and along the top tube it veers to one side and lies under an unhoused gear cable, which is a bit ugly, and also causes some scratching of the powder coat.

As for seatstay mounted calipers, I've heard tell that one advantage is not having an upwards pointing entrance for water ingress, as is the case with chainstay mounted, but not sure how important/effective the difference is.

Being an older frame, the mounting of the rack requires a special rack (or spacers), whereas newer frames with seatstay mounted calipers are better designed and work around this, meaning normal racks canbe used. Except my Salsa with it's silly 'Alternator' drop-outs', which required Salsa's proprietary rack until I have new mounts brazed on...

R2, I notice your stoker uses a Brooks, while you're not a fan. Puts paid to my old theory that tandemers like everything matching!

(p.s. FWIW, Not that we ride tandem (yet), but we're the other way around; Mme with Selle Italia/Terry, me the Brooks').

[freeflow]

Thanks for all the input. I'm now minded to go with cable ties and full housing. Not because this is the best solution for a cable operated disc brake but because it makes it simpler to replace cable with full hydraulics in the future should I decide to go that way. I'm planning on using yokazuna (?) housing as it gets very good write ups so compression losses shouldn't be too bad.

The problem with that for me is that the housing is quite loose in the clips, and along the top tube it veers to one side and lies under an unhoused gear cable, which is a bit ugly, and also causes some scratching of the powder coat.

I already spotted this one and have requested three or four cable tie mounts on the top tube to stop the cable housing flapping around.

[Brucey]

it has been my observation that even with a small frame giving plenty of standover clearance, cables mounted on the top of the top tube invariably scuff the paintwork. Probably you are doing this already, but routing the cable/hose on the underside of the top tube is often a better scheme.

It isn't a bad idea to have the guides mounted to the right side at the front (to give a smooth run from a left side lever) and on the left side at the rear (to give a smooth run onto the caliper). The only downside to this is if you shoulder the bike or similar; then the cable is in the exact wrong place.

Although it might look a little odd, it is a good idea to have two guides next to one another (or just one rigid one that won't allow any flexing the other side of it) at the headstock end. This way when you turn the steering the cable/hose won't be flexing and scuffing the paint so badly.

cheers

[freeflow]

Hi Brucey

Some good points there. Paint scuffing won't be an issue as it's a Ti frame. If there is any rubbing then I'll put patches of helicopter tape over the affected parts.

[Brucey]

My Ti road bike hasn't scuffed badly but my Ti MTB has done so, presumably because of the dirt. I now use tape on both as a matter of course.

cheers