Brake cable rubbing seat post

Question

I am building a tandem and I can run the rear brake and shifter cables three ways.

I can run cable housing the whole way. Since it's really far, that's a lot of housing! And I don't think it would look good. But it would protect the cable.

I can put cable stops on the front and back, and run just the inner cables most of the way. The problem is the cables will rub the seat tube if I run them underneath the top tube. Unless I run the cables on the SIDE of the top tube, which would look weird.

Third, I could put little stubs of housing where the cables run past the front seat tube.

I would like to do the first one and just jun bare cable the whole way, and let the cables rub on the front seatpost where they pass it. Is it really a big deal if they touch it? The angle seems really shallow but I don't want it to make the brakes or shifting funny either.

Answer 1

I've ridden a normal bike that had exposed inners on the right-hand side of the top tube - it was unpleasant to ride because my inside-right knee touched the wire a lot.

I'd suggest running the rear rim brake along the top tube, you can choose to run it on the top or underneath. At the front and rear ends of each section of top tube you could either:

1. have a stop brazed on then run a length of exposed inner, then run a short segment of outer around the captain's seatpost to another stop, and a second run of bare inner.
2. Run a single housing all the way along the same path, using cable holders either brazed to the frame, or bolted around the top tube.

Don't simply drag the exposed inner cable on your frame - its a steel wire and will quickly erode your paint, and then start cutting into the steel frame. The wire will suffer too, and you don't want to loose half your braking on a fully loaded tandem !!

Which leads to - If you're thinking of symmetry expect to install two sets of brake stops. It is quite normal for a tandem to have two independent separate rear brakes, a disk or rim, and a separate drum brake in the rear wheel. These require two different activation cables, unless you choose to give the drum brake to the stoker.

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For the front and rear mechs, I'd suggest down the down tube, under the front BB, and along the keel, then under the second BB. From there it would be just like a normal diamond-frame bike.

When I rode mine, one of the missing things was a Visual Gear Display (shimano's fancy jargon was VGD) to show what gear you're in. There's no easy way for the captain to look at their feet to see the chainrings, and the cassette is twice as far away as normal. So, an inline gauge/readout works nicely.

If you're feeling flush, I think that tandems would be a fantastic place for DI2 wireless to be used. The captain and stoker could have their own set of gear-buttons each, and have some shared level of control for the gear setting. But this isn't a cheap way to go, however it does avoid two gear cables, at the cost of carrying around some small batteries on each mech. Food for thought in the future perhaps. And you can both have a Di2 readout displaying the current gear setting.

Answer 2

I think that letting the (inner) cables rub against the seat post is a bad idea. It will cause wear on the cable, scratch your seat post (or seat tube) and probably create noise every time you hit a bump.

Why not go for your third option? Put a cable stop before and after the seat post and run a short length of (almost straight) cable housing around it. Works fine on normal bikes as well, after all.

The first option (continuous cable housing) causes squishy braking, even on normal bikes with shorter cables.

Can’t you run the shifter cable in a straight line along the bottom tube?

I wouldn’t run the cables on the side of the top tube, it causes problems when you lean the bike against something.

Answer 3

There are disc-era tandems that use segments of housing around the seat cluster areas to solve this. The advantage over running full compressionless housing is it saves some weight. One of the sub-decisions is that if the piece of housing that runs into the caliper or RD starts on the seatstay, it's a major point of entry for contamination.

Bare brake cables grinding against anything is bad. It adds friction. The high leverage ratio of mechanical disc brakes means any friction it adds will have an amplified effect on the brake function. There have been a lot of bikes (90s MTBs) that do a version of this with little plastic liner sleeves inside odd little welded-on tubes that wrap around the seat cluster, to avoid having a proper piece of housing there. The liners break down and get frictiony; it's not a good solution.

Full runs of compressionless housing are the standard with mechanical discs for reasons beyond taking the same accommodations as a hydro. Mechanical discs benefit from maximum resistance to contamination more than other kinds of cable brakes.

For the RD, lightest by far and the standard for a road tandem would be routing it down the captain's downtube and around the BB. Most bikes won't need any other housing than the rear loop for this. One of the subtle things about trying to route it along the top tube with minimal housing is that then your rear loop is open to the sky and a magnet for contamination.

Routing both along the dowtube and around (or possibly over) the captain's BB also gives a quality of life advantage in that the riders won't have them in their way during starts, stops, and pauses.