poorly designed internal brake cable routing

[Foghat]

Ribble’s Sportive Azzurro frame has a very poorly designed internal rear brake cable routing, causing an irritatingly stiff cable action that even the otherwise slickly-running latest Shimano polymer cabling cannot counter.

The cause of the high friction is the inner cable rubbing the inside of the top tube along the entire length of the internal run. For some reason, the frame designer thought a curved top tube that results in this ridiculous rubbing is a good idea – see the photo below for the curvature (although the photo doesn’t do justice to the extent of the curvature and its impedance of the inner cable).

Now, if Ribble had designed the routing so that there was a full-length run of outer that could be fed through the frame, there wouldn’t be a friction problem – full-length Shimano polymer cables run exceptionally smoothly in my experience, even with tight bends. Unfortunately, the outer cable entry points are fitted with stops that enforce the use of an interrupted outer cable run. The outer sections are all cut properly and are not restricting cable movement. I’ve removed the basic metal ferrules that Ribble fitted and replaced them with long-nosed ones; this has improved things very slightly, but the friction is still unacceptably high – not least because it ruins the modulation capability of the superb Ultegra 6800 brake, and causes an enormous force-required imbalance between the front and rear brake levers. I expect the cable will also wear a groove into the tube that will get deeper over time and eventually compromise the frame.

So I’m minded to execute some rectification on the ill-thought-out design – namely drilling the outer cable stops at the entry and exit points such that a full-length outer can be used. Before doing so, it makes sense to canvas opinion on the feasibility, pitfalls and techniques for doing this.

The entry point has a moulded shape and no covering plate – i.e. it is just a moulded angled hole in the carbon fibre tube with the stopping point fixed in the frame. The exit point is a fair-sized covering plate with a fixing bolt, and this presumably covers a larger non-angled hole and presumably can be removed; I assume the stop is part of the plate.

So I’m considering enlarging the entry and exit holes with an appropriately sized bit and feeding the full-length outer through instead. The fact that the exit point has a removable plate should mean locating the end of the outer and getting it to exit shouldn’t be a major problem.

I would be interested to hear from anyone who’s done something similar on carbon tubing, plus any comments on methods and potential problems.

Thanks.

[mrcharly-YHT]

You can buy inner that has a teflon sleeve. Maybe try this first?

[Foghat]

Yes, I'd thought of that, and it's a potentially viable solution.

Thinking it through though, I suspect there could be problems when it comes to replacing the inner.  The sleeve would need to be shorter than the cable run between the ferrule nose-ends, which means that once it's in place inside the frame, it could be difficult to locate the sleeve and thread the inner into it.  I suppose it all depends on how difficult fishing around for the sleeve at the plate exit point is going to be - and I won't know until I try.....!

[Samuel D]

You could also just route a full-length cable housing externally, where it should be in the first place. Finding a neat way to attach the cable to the top tube would be tricky.

I don’t know how to go about drilling the holes, etc., but you have in your favour the fact that the larger holes will not be subjected to appreciable force, unlike the current arrangement where the holes acts as stops and are subjected to force equal to the cable tension.

Nonetheless, I’d be cautious. The top tube resists considerable bending loads at the head-tube end, roughly where the cable hole is.

[Torslanda]

Talk it through with someone like Carbon Bike Repairs in Derbyshire (Ashby dlZ IIRC) or perhaps engage them for the job...

The outer cable is nominally 5mm. If you can get in straight with a 5mm bit then ream it to 5.5mm to avoid clearance issues. Given there is already a hole there it's unlikely the tube will be overstressed by opening the hole.

If I read it right all you need to drill at the rear is the insert. When you refit the cable drop the inner through with the crossbar vertical and use a pick to get it out then feed a piece of outer up the cable leaving it overlong at both ends  until you're happy with the fit.

[Foghat]

Torslanda - thanks for that and the advice on drilling etc.

It may be that I don't need to resort to such action.  It's come to my attention on the CTC forum that this has happened to other Ribble frame owners, and that Ribble has been known to omit the fitting of a specially designed plastic straw that goes through the top tube, guides the inner and facilitates its smooth action.  So I've requested one from Ribble.

Samuel D - yes, external all the way is a last resort modification option, but it would look awful using zip-ties or whatever to fasten the cable to the frame and would annoy the hell out of me every time I looked at it!  Hopefully it's just a case of a missing straw........

[mzjo]

My Vitus 992 has a totally sleeved rear brake cable and one thing I have noticed is that the entrance and exit holes are as tight as possible, presumably to prevent water getting in. The cable was also a real bitch to fit (due mainly to the totally horizontal entrance and exit points - technique: feed a long inner wire through the frame in which ever way is easiest to catch then feed outer over inner going in the reverse direction and prepare for a battle to get it out through the far end; finally extract first inner wire which is only for fitting and put the real inner through in the proper direction. Make a note never to remove the outer from the frame. This procedure will be complicated by fitting through bar cabling on the same machine - DAHIKT).

If the frame sleeving is missing this might also be a bitch to fit. Presumably this is why there is a plate fitting at one end.

[vorsprung]

Similar problem with my Genesis Datum to the one described by mzjo
The exit hole is tiny tiny
There is some kind of plastic bung on it but I'm dubious about removing it in case this messes up the internal routing by disconnecting the rear
Anyway I'm not sure if it can be removed.

Query sent to mr Genesis, see what happens