Cranksets for tandem with internal gear hub

Question

I am thinking about assembling a trekking tandem bike. I would like to use an internal gear hub and put both chains on the right side of the bike. I have some questions about the cranksets:

1. What cranksets can I use? Can I replace the sprockets on any crankset with bigger ones (I would need two same-sized sprockets on the middle crankset)? Or do I need some specific type of a crankset?
2. How important is it to line up the chains in this case? What can I use to line the sprockets in the right position?
3. Is it possible to to use different size of sprockets for the front chain to compensate for the difference in strength between two cyclists, or will it make riding too hard because of the shifting phases?

Answer 1

1. You can use whatever cranksets you like and have available. But tandems need either an eccentric bottom bracket on the front, OR a chain tensioner in order to set the tension on the link/timimg chain.

1.1. If you are only pootling around on the flat then perhaps the loss of the front chainring is acceptable. But any hills or a good stiff headwind and you'll be regretting the loss of lower gears. You might be able to offset this with a large rear cassette, a 42 tooth megarange or similar.

2. Chain line is important - the timing chain is often a 1/8th inch chain because it doesn't need any side flex. The rear chain is often a normal 3/32" bicycle chain so it can be used with a derailleur. You could use either (or even a drive belt!) To line up the chainrings, I'd use a long steel ruler, and try to use a matched pair of chainrings and bottom bracket axles.

2.1. Since you have no front derailers, you could possibly flip the chainrings to have the big one close to the frame, and teh timing chain on the outside. Or you could even have both chainrings the same size. Its also possible, (but uncommon) to have just one big long chain doing all the drive.

3. No - the rider's pedalling should be in phase at all times. If one rider is ready for a corner, there's a non-zero chance of pedal strike for the other rider. This is doubly bad on a tandem.

If you want one rider to have an easier time of it, look for different length cranks. They'll still have to do the same cadence as the other rider.

Suggestions:

Get a matched pair of doubles or triple chainrings. Set your timing chain on the big ring, and use the stoker's middle/smaller ring for the normal chain. If its too low geared, swap the timing chain to the middle and put the rear wheel chain on the big chainring.

Once you've settled on a good combination, you could remove the chainrings you're not using.

If you're building this tandem from scratch, separate the IGH hub purchase from the build.

Answer 2

1. It's definitely possible to do #1, although you need to match the bolt pattern and bolt circle diameter (BCD). That's a common change for many cyclists. If you're running both chains on the right hand side standard cranksets are all you need (this is typically why people do that)

2. Chainline is important, and typically you would put both idler chainrings on the same side of the spider and no more is necessary. A different length bottom bracket for the captain or stoker could be used to fine tune that if necessary, but almost all tandems are built with the right hand edge of the BB shell in the same plane.

3. Different size chainrings would work, and while it might be unusual at first I expect you would get used to it. However, cornering will be tricky as you may not be able to get both inside pedals in a high position, meaning either pedal strike or slow cornering. One way to avoid that would be a freewheel between the two riders, normally done via an intermediate drive (three chains!)

One caveat is to be careful about the torque limits for the hub. Rohloff officially support their hubs in tandems, but limit the gear ratio you're allowed to use (without voiding the warranty). Shitmano don't have that restriction, but their hubs tend to fail when used hard and the warranty is limited to a single replacement for each hub purchased... you may end up paying more than the cost of a Rohloff. Sachs also approve one of their hubs for tandems but it's the 5 speed Cargo, so not great unless you're strictly using it in flat areas (Hub-Stripping has more details)

Answer 3

1. Any cranks should do, as long as you can attach two chain rings to the rear spider.
2. My guess is that chainline is unlikely to be an issue if it differs by up to 30mm, as it's a longer run than on, say, a dérailleur system which seems to cope fine with offsets of at least 30mm. What you will need to watch is that the two chains are narrow enough that they don't catch each other on the adjacent rear chainrings. I tried using a super-wide (1/8") KMC single-speed chain and it was so wide that it caught on the adjacent chain. When I switched both (actually 3 chains, two for sync) for KMC X1 they didn't touch.
3. Yes that is possible, but hardly anyone does it, despite most teams being unmatched in power. Firstly it feels odd and would probably make the frame flex/wobble due to the way you're each moving your weight about. Standing up would be very difficult/dangerous. The cadence is a compromise on a tandem. If one person has trouble spinning and has shorter legs it might be worth giving them slightly shorter cranks. Be careful not to give either rider a crank length which would make them uncomfortable.

For further advice on using Rohloff hubs with a tandem I'd suggest the Thorn Forum.