Changing the Transmission of our utility tandem
Our oldest tandem a Dawes Duet has sentimental value so we have kept it to go shopping, plug along tracks and such like.
The transmission was always a bit iffy - the rear derallieur in particular has always been slow and inconsistent. I have tried various things with cables etc but have now decided that it is time to give the bike a bit of a makeover.
At the Eroica last weekend we saw a tandem running an SRAM 1x system. I have a bike that runs this system and love the linearity without having to worry about crossover point like with a double/triple (and we all know how temperamental a flont changer can be).
So this thread is about cheaply upgrading my Dawes to just have a rear changer with a big freewheel. I have discussed the viability of this with several regulars on here (John and Martyn). Martyn was concerned about gear range at the top end but given the use of this tandem I can forego that (like many of you we have another tandem we can go fast on).
The bike currently has sychronisation chain on one side and triple chainset to seven speed on the other (although I have changed the wheels and used a spacer so the freehub body will support 10 speed). I am planning to move to double chainset with synchronisation chain on the same side as transmission (easier to transport given we lie it down in the car).
My questions are:
1. How do I work out the bottom bracket length I need? - Is it just the length for a double chainset recommended for the cranks I am going to buy?
2. Are there any gotchas with changing the side of the synchronisation chain?
3. Will tensioning the synchronisation chain still work the same?
4. I am proposing to use an ordinary chainset on the rear drive not SRAM's specially toothed one. But given I am just going to run a normal 10 speed set up are there any problems with this?
5. Any questions I should have asked?
Timing chain tension is unchanged.
In-line drive puts the front chainring on the right. If it is currently on the left you will need a new front chainset or have the cranks e-threaded.
Measure the length of the existing rear BB as a starting point. The final drive ring should be roughly in line with the middle cog. If it is way off centre the chain might fall off (with a single ring set-up) Try the reccommended length or try a spare bb to test.
Hope this helps.
Many tandems use rear spacing of 145mm or more and this may be reflected in the chain line. For example our Cannodale RT2 hollow spindle had an extra spacer on the right as against the MTB standard for 135mm spacing and a special front mech clamp. Personally I like the straight chain to be one cog in from middle so I took the spacer out and fitted a normal clamp.
I don't think you will have much fun if you use a chain ring intended for front changer use unless you use a guide of some sort especially with same side drive. Chains hoping one onto another are the bane of same side drive. On our 2001 Thorn Adventure the synch kept on dropping onto the drive granny ring so in the end I swapped it to cross over. Our 2002 Thorn Adventure has their special 30t synch chain wheels and has been trouble free. If you don't want to use a wide narrow you could use a high tooth type intended for single speed use for all three chain rings.