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Thorn Tandem bb size & crank rings?

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Pavel:
I can't seem to find it in any of the literature, but does anyone know what the bottom bracket size is on the previous generation (Rohloff model) Tandems?  Is it 68 or 73?  It would make sense to me that it be 73, as the Nomad's is, but I want to make sure.  I happen to have two unused Shimano UB 55's, one with a 113 and the other with a 110 spindle dimension. 

Also, as I know nothing of tandems, especially in regards to how to set up the pedals and the single and double ring, any advice would be appreciated.  I imagine that the outside stoker's chainring is sized to give a reasonable final drive, and comfortable range of gears, right?
How about the inner ring size?  Are there any special considerations excepting of the fact that both stoker's and Captain's chainrings should be the same size?

Any info ... much appreciated, thanks.  :)

Danneaux:
I have spotty comm links at the moment but will be back with some recommendations.

Best,

Dan.

Danneaux:
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...

--- Quote ---I imagine that the outside stoker's chainring is sized to give a reasonable final drive, and comfortable range of gears, right?
--- End quote ---
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=11847.0

I've attached a copy of my proposed gearing below this post.

--- Quote ---How about the inner ring size?
--- End quote ---
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.

--- Quote ---Are there any special considerations excepting of the fact that both stoker's and Captain's chainrings should be the same size?
--- End quote ---
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.
=====
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.
=====
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.
=====
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.

Pavel:
Thanks very much for your precise and encyclopedic reply Dan.  I believe the sum of your posts here on this forum has gone a long way to many purchases of Thorn bicycles, over the last decade or so.

Your observations make my way going forward a less daunting (though not any less expensive :D ) and has given me an idea to start with. I shall likely set the bike up with the cranks set to a ninety degree offset.  This will lessen a burden for us in that I can normally clip in with no thought as the pedals rotate, but Vicki will take time to get accustomed to it, and having it when we start out, and after stops so that I have my foot down with one pedal clipped in, and hers then a level platform should make it easy for her to clip in at any pace she desires, and then she can stay clipped in on most stops.  It should also help set us off as she will be on the power end of the stroke as I raise my drive leg off to a 45% raised start.  It should help too without as much verbal communication, to my mind.  I don't think, I'd have thought to do that without your mention of it.  Thanks!

I plan to put on my Nomad's Rohloff with it's stout Andra 30 rims, and probably the Schwalbe 1.6" supremes, for a start.  The Andra 30 rims are the heaviest, but rated for a lower max weight, while the Andra 40 rims are lighter, which would make them less robust to my way of thinking, but the only Ryde rims rated for 180 KG. But it's my belief that Thorn specs tandems with the Andra 30s, right, so that is a bit confusing.  Do you think taking the trouble and expense to rebuilt the wheels for the tandem to Andra 40s is worth the bother, and is there any real advantage in that higher rating? When they rate a wheel for 120 kg, I take it that is not for the set, but rather per wheel?

At a later date I will buy a new Rohloff and lace it up with 650B rims, disks (not that I like them, but what other choice is there if one wants larger wheels?) and a rim lighter than the Andra.  I knew it was overkill back in 2011 when I placed the order, but what a marvelous overkill it was, and fuel many an enjoyable days of daydreaming of future epic rides, especially while I was flat on my back in the hospital.  For the Nomad, It's time for lighter wheels and lighter ambitions - Slightly, ever so slightly.  :)

Danneaux:

--- Quote ---Do you think taking the trouble and expense to rebuilt the wheels for the tandem to Andra 40s is worth the bother, and is there any real advantage in that higher rating?
--- End quote ---
Pavel,

Last year I freshened my tandem and rebuilt the wheels -- the original SunMetal CR18 rims supplied to the now defunct builder proved to be faulty and despite being a registered owner I was not notified of the recall. Happily, the rims still worked well for me until recently.

After a lot of thought and looking at the Andra 30s on my Nomad, I decided to go with Andra 40s for my rebuilt tandem wheels.

They were unusually true out of the box and laced up and tensioned nicely. They are wider,\ but the extra width between beads doesn't make as much difference in the profile and inflated width of my 26 x 2.0 Schwalbe Duremes as I would have thought. As I recall it is only a couple of millimeters difference at the same pressure, but if you would like I can measure the actual difference for you.

The Andra 40s are not simply "scaled" 30s...the interior rim profile of the 40 is different and the rim sidewall is not as tall/wide as the 30 and doesn't extend as low toward the rim.

Do beware, after a LOT of research and many dead-ends, I "discovered" a couple of things about Andra 40s I had not expected:

1) They are apparently drilled in batches, some with spoke holes offset to the left of the valve hole, others to the right. The best hope is to order your pair at the same time to ensure both rims match. This difference sounds trivial and largely is -- so long as you are lacing the new rims to new hubs. However, if you are planning to reuse an older hub, it can mean a different lacing pattern and this places stresses in a different direction and causes the spoke elbows and heads to emboss the hub flanges in the opposite direction. Our own Julian (membername Julio) discovered this when switching from Andra 30s to 40s on his Nomad some time ago.

2) It was very difficult for me to find the exact ERD (Equivalent Rim Diameter) of the Andra 30 and much of the listed information showed the ERD varying from the Andra 30. I asked a number of sources including the maker and all got it wrong in one way or another. I finally found in this dimension the 40 was closest to the 30, as I found by measuring an actual rim on my own -- unfortunately after I had already pre-ordered a wheel's worth of spokes based on "expert testimony" that didn't prove out. If you need the actual ERD I found, I can look in my files but I'm not at my computer at the moment (I'm roadside atop a bike and using my phone instead).

My Andra 30s have proven extremely reliable on my Nomad even with expeditionary loads (bike and cargo weighing in at 56.7kg/125lb plus my own 78kg/172lb atop it. However, the tandem is a similar weight (Nomad 20kg/44lb dry weight vs. tandem 20.9kg/46lb dry weight) but carries not only two people but two people's stuff when camping...plus hauling a trailer full of food and water for extended time "away" so I tried to choose a rim based on those demands and the extra tire pressure needed to support it all. Until they failed due to the factory flaw (the rim ferrules were clinched improperly and fractured), the SunMetal CR18s happily supported me and my Dutch pal when he came for a "glamping" "luxury" tour up gravel roads in the mountains. My trailer weighed 56.7kg/125lb and it, the bike, and both of us with water and food spotted in right at 272kg/600lb.

The key difference there was the CR18s were shod with 1.5in tires, so they ran higher pressures than a wider tire, but were narrower. This is important. I wanted to run 2.0 tires after my wheel rebuild and wider tires result in greater forces and an outward "jacking" effect on the rim beads and sidewalls, especially when inflated near the upper end of their inflation range -- as would be necessary to support the greater weight of two people and a touring load. This was the main difference in use between my Nomad on its Andra 30s and the tandem, so I went with the 40s for the tandem as the wider rim better tolerates the jacking forces of wider tires run at higher pressures. I haven't regretted going with the 40s on the tandem, but the difference is effectively transparent in ordinary use. The 40s are not much heavier than 30s, but they're still heavy rims -- a factor that is not as noticeable on the tandem as on a single. Remember, most weight on a tandem (rotating weight excepted to a lesser degree) is effectively halved compared to a single because it is divided between two people.

A further note: I laced my Andra 40s to a rear derailleur hub, not a Rohloff. I found my 40s had spherical spoke nipple seats (and I used Sapin Polyax nipples to match the seats and better aim the spokes toward the hub flanges), but they didn't appear to have the deliberate directional drilling intended for Rohloff hubs as some 30s tout, so this might be a factor for you going forward and for me when I convert my tandem to Rohloff. I didn't see any 40s available with Rohloff-specific drilling when I made my purchase a little over a year ago, but you might be able to find them now. Check with SJS Cycles first as they can best advise on this point.

Best,

Dan.

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