Pavel,
For many years I was active on the tandem@hobbes listserv devoted to tandem bikes. It was much like the Thorn Forum in tenor and tone and covered many of the same issues from brand particulars to touring, competition use, camping, various tours 'round the world and of course, all sorts of mechanical issues and questions. I was also involved with some tandem races held locally for a number of years, so I became familiar with what worked and didn't for a number of tandems and their teams. I've owned and extensively ridden my 1989 (non-Thorn, non-Rohloff) tandem since 1992 and worked on tandems owned by a number of others so I can speak to some of the general issues you might face in setting up a new tandem.
You asked...
I imagine that the outside stoker's chainring is sized to give a reasonable final drive, and comfortable range of gears, right?
Yep. That's no different than on a Rohloff single, with one exception: Tandems are notoriously slow climbers and screaming fast going downhill, so you will need to figure which end you will aim for when setting up your drivetrain range. Roughly speaking, tandems have the power of two riders, the wind resistance of one and a little more rolling resistance than a single, so keep that in mind. Going uphill, you've got the weight of two people and that's why they tend to be slower when climbing. It is possible to climb out-of-saddle -- I do with my regular stoker -- but it requires some familiarity with the bike and each other to pull off smoothly. Remember also, Rohloff has some cautions about tandem gearing to avoid a too-low ratio that could result in damage to the internal nylon pins when the torque of two people is applied. I can't endorse varying from their recommendations but I will take this with a grain of salt as some tandem teams cannot generate the same peak torque loads as others. More about this in a moment.
My tandem has a crossover (left-side) timing chain and a 3x6 derailleur drivetrain with a range from 20-92 gear-inches. It is fine for most cruising; for high-speed, we coast. I've had the bike up to 101kmh/63mph downhill with the big Arai drum-drag brake lightly set to limit speed. The bike was good for more but I wasn't, not with two people aboard. Tandem falls are never pretty and going down at those speeds would be unpleasant, so I've never felt undergeared for top speed. Going uphill is a different matter and I've often wished for lower gearing, especially when carrying camping gear for two people in four panniers, a handlebar bag, frame bag, stoker stem bag, a rear rack-top load and towing a trailer full of a week's food and water.
I have a "spare" (used) Rohloff hub with internal shifting and plan to install this on the bike once the current drivetrain is worn out. I brazed on some new/additional fittings and repainted the bike soon after I got it, so have the capability to adapt the vertical rear dropouts from their present 140mm OLN spacing to the Rohloff's 135mm. All cables run inside the down/keel tubes and so the Rohloff with internal shifter is the version I can adapt most readily. The bike's eccentric is in the captain's bottom bracket and used only to tension the timing chain, so I will run the present rear derailleur as my chain tensioner. This will allow me to pair the present 46t and 36t middle and outer chainrings with a 17t Rohloff sprocket to have two drivetrains that are duplicates except at the extremes -- the small chainring will add two lower gears for climbing and the large chainring will add two higher gears for downhill and cruising. The middle 12 gears will be virtually the same so everything will be nearly identical in the mid-range gears no matter whether the "uphill" or "downhill" chainring option is selected. The result will be a range of 15-102 gear-inches, just about perfect for my needs.
I'm not worried about going "too low" on this particular installation and damaging the used/spare hub because my stoker is very limited in the power she can apply. She does very well to ride at all after a horrific back surgery that ended with two 25.4cm rods, a titanium cage over her otherwise exposed spinal cord and a meter's worth of sutures. As it is, I am the primary motive force and climber so I really need the lower gearing and I don't think we will together exceed my output on my loaded Nomad's 36x17 Rohloff gearing.
For a discussion of "illegally low" Rohloff ratios in tandem use, see:
http://thorncyclesforum.co.uk/index.php?topic=11847.0I've attached a copy of my proposed gearing below this post.
How about the inner ring size?
I would strongly suggest avoiding the temptation to "save weight" and go with too-small timing rings. Back in the day when T/A made
the tandem drivetrain, it was common to see bikes equipped with 26t or 28t timing rings. Unfortunately, going too small has a couple drawbacks: a) the small-diameter 'rings accelerate chain wear and b) they tend to exert greater loads on the bottom brackets. Going too large is not ideal either, as really large chainrings (say, 50-52t) can fold under the high torque exerted by a powerful team, say while climbing a steep hill. I've found 40t timing rings to be the happy medium and they have worked well for me with minimal wear over the years.
It is possible your main drive chainring might not be 40t. If it is less, you might want to use the same size for your timing ring, but it would do no harm to size them larger. Unlike a derailleur drivetrain, the Rohloff's chainline is fixed so there is no danger of the single-side drive chain catching on a larger timing chainwheel and causing a mess when shifting.
Are there any special considerations excepting of the fact that both stoker's and Captain's chainrings should be the same size?
Yes, the captain's and stoker's timing rings should be identical in size to ensure your pedaling is synchronized.
As for special considerations, one of the biggest is crank timing. Most people (me included) go with their cranks in-phase because it makes starting so much easier and when cornering deeply, you can make sure both inside pedals are up to avoid a pavement strike. It also allows synchronizes your power strokes, a decided advantage if you are "mashers" and no real disadvantage if you are a spinner.
Other folks prefer to setup and ride their tandems with cranks out-of-phase to some degree, usually 90°. Advantages are a more even power stroke and extended drivetrain life a the cost of more difficult startup and more conservative cornering lean angles. One caution: If you choose go out-of-phase other than 90°, it can cause some problems with bike handling as the power strokes overlap unevenly.
In case it isn't obvious, changing the timing of your cranks is easy: Just make sure the cranks are in- or out-of-phase when you connect your timing chain. Backing off the eccentric and slipping the timing chain to a new position will reset the cranks so you can try it one way and another.
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A couple more points to consider when building up a tandem...
1) An adjustable stoker stem is worth the investment because it allows quick and easy changes in reach and (depending on design) also length (apart from its clamp height on the captain's seatpost). I TiG'd and brazed my own but Thorn offers some very nice ones under their name at SJS Cycles.
2) Stokers generally benefit from a suspension seatpost because a) they cannot see and anticipate oncoming bumps because the captain blocks their view and doesn't always remember to call them out as they approach. Oops. b) Stokers are closer to the rear wheel than the captain, who is nicely suspended between the wheels. As a result, bumps tend to be transmitted more directly to the stoker. A parallelogram sus-post like the Thudbuster or similar designs is nice because it maintains nearly the same distance between saddle and bottom bracket throughout its travel, unlike telescopic posts, which also have more stiction by design but are still more comfortable for the stoker than a rigid seatpost. I made my own adjustable telescopic stoker seatpost and it has worked well for my stoker -- she doesn't want to change.
3) Make sure the stoker's handlebars are wide enough and far enough away from the captain so the captain's hips don't foul the stoker's hands. Nothing is more irritating to both parties. On my tandem where both parties prefer drop handlebars, this meant a pair 46cm wide stoker handlebars were required. As captain, my bottom sits
between the "hooks" of the stoker's drop handlebar with enough room for the stoker's thumbs to clear when grasping the "dummy" brake levers mounted on their 'bar. Here is where it is also good to look at Thorn's stoker stem options. Some allow considerable uplift so the stoker can achieve a more upright position on the bike and this can also resolve some clearance conflicts, even with straight or comfort 'bars at the rear.
4) It is generally easier for both parties if the stoker stays mounted at stop signs and traffic signals; it allows for a quicker getaway as they are already on the pedals where they can apply immediate pressure and only the captain needs to remount.
5) It can be easier for the stoker to mount if they first place a crank in a forward position. While the captain is mounted and bracing the bike with a wide stance and the brakes set, the pedal becomes a handy step-stool for the stoker to mount to their saddle -- no curb needed. The raised pedal reduces the height needed to swing a leg over the saddle and can make mounting the bike much easier if they have back or mobility issues.
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There are social aspects to consider when building up a tandem as well. The stoker (Rear Admiral) always outranks the Captain. If the stoker isn't happy, no one will be happy so do whatever is necessary to accommodate them on the build. The rear of the bike is
theirs, not yours. If their preference for handlebars or saddle are different from yours and the setup looks far from what would make you happy...do what is needed and figure if it is fine for them it will be fine for you. To do otherwise is a direct route to unhappiness for both parties. When riding, you'll soon be able to tell when to stop and resume pedaling and how much pressure to apply based on feedback you obtain from the timing chain and pressure on your pedals. When your stoker needs a rest, by all means rest also. Most teams are mismatched for power output and the stronger rider (whether fitted to the front or back) can get a terrific workout while the weaker rider can exert less effort. Just make sure the weaker rider rider doesn't have to pull more than their fair share and all will be happy.
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One last suggestion: Give a shout to Thorn Forum member Mike Ayling in Australia. He and his wife Mary have a beautiful Tonka Yellow Thorn Rohloff tandem and can answer your questions based on Thorn-specific experience -- particularly with regard to bottom bracket spindle lengths. He posted a link to one of their tours here:
https://www.cycleblaze.com/journals/australia/preamble/Hope this helps.
Best,
Dan.
