This is a message which I posted to rec.bicycles.tech on the subject of hooking up multiple bike computers to the same pickup on a tandem.
On Sat, 07 Aug 1999 18:04:42 GMT, firstname.lastname@example.org
> Anybody put 2 computers on 1 pickup for their tandem...If so How?
It has been done. One of the regulars on the bikecurrent list (see cyclery.com) has spliced the wires on cyclometers before.
That might be me (I've done this using two Vetta computers).
The problem is that water and contamination of the joints result in high impedance shorts that the the cyclometers see as dead shorts. Also, it is sometimes difficult to properly splice the wires, since they are all individually coated with insulating lacquer. Unfortunately, I haven't saved the relevant message in the bikecurrent Reviews (http://www.fan.nb.ca/reviews/). I'll search my archives for it though, and post it there if I find it.
I haven't had any such troubles. My connections are well insulated using heat shrink tubing. The only area where I could get an impedance short is at the RCA jack that I used to make the stoker's computer quick disconnect. This hasn't been a problem however, even when riding in rain.
I know one frequent contributor to this forum (Joshua Putman) who has his bike computer wires twisted together, and hanging free in the rain. He rides much more then I do, and in plenty of bad weather, and doesn't have impedance shorting problems. I think the worries of this on the bikecurrent list are overblown in practice.
As to the original question, here is what I did:
- Start with two identical computers. I used Vetta C-15s because they were cheap at the time and had very easy to solder wires. You might consider buying a 3rd if they are cheap, just so that you have a backup if one breaks down the road (when it breaks in two years the model might not be available anymore).
- Hook up the captain's computer normally.
- Cut the wire from the captains computer around the headtube. Strip off a bit of insulation from the wires.
- Attach the stoker's computer to their handlebars, and route the cable under the top tube, back towards the headtube.
- Cut off the pickup (keep a little bit of wire on it so that you can splice it back later if you ever want to). Strip the insulation.
- Twist together one of the stoker's wires with one of the captains wires. Use a multimeter to see if you matched two of them. You want to end up with the right contact on one computer mount in parallel with the right contact on the other computer mount, and visa-versa. If they aren't matched then just swap with captains wire you used.
- Place small heat-shrink tubing over each of the wires to the pickup. This will insulate the wires from each other. Place a larger piece of heatshrink tubing over both sets of wires. This will cover the smaller stuff and provide extra protection.
- Solder the cables up. You'll have two solder connections of three wires -- one to the stoker computer, one to the captain's computer, and one to the pickup.
- Put the heatshrink tubing in place and heat it up.
- Cabletie everything into place.
On our bike I also put an RCA jack inline with the stokers computer. It hides underneath the captains saddle normally. This was because we used to put the tandem into the back of my girlfriend's car (a Honda Civic coupe), and doing so required removing the stoker's handlebars. On a bike where you rarely remove the stoker's handlebars this is not necessary.