Tandem crank synchronisation

[srw]

We've always had the tandem set up the way it came out of the box - with the stoker's and pilot's cranks in synch. By which I mean that when my cranks were at 12 o'clock, so were hers. It's a Rohloff, and we've had a couple of problems with hub flange failures. Reading around the subject one of the causes of this appears to be a strong (!) tandem team over-doing the torque.

In my very simplistic world in which I know nothing about engineering, if both riders are pushing through the transmission at the same time this will double the torque transmitted, and there will be a peak twice every revolution. So I've experimented with putting the cranks out of synch. Now when my cranks are at 12 o'clock, the stoker's are at 9 o'clock.

Does this actually smooth out the torque transmitted and reduce the peak, as I think?

Has anyone else experimented like this? My initial reactions are favourable - it gives a smoother ride, and a much better transmission of power for hill-climbing. On the downside, starting off is trickier, and it's more difficult to find a "dead zone" in which to change the hub gear. And it makes honking out of the saddle impossible rather than merely impractical!

 

[PpPete]

I got mine a few degrees out of synch by mistake once. Uphill I really did not like the sensation at all, but that may be because the stoker on that occasion, although remaining seated, tends to mash up and down on the hills rather than pedal smooth circles.

 

[byegad]

Tandem recumbent trikes fitted with SRAM Dual Drives do this, for similar reasons. The SRAM is torque sensitive, although maybe not as touchy as the Rohloff, which has a plastic tang inside designed to fail in order to save the expensive bits. I investigated a Rohloff set up for a recumbent trike (Solo) and their dire warnings about minimum chainring size put me off as I would need to accept a minimum gear inch value that didn't make sense here in County Durham, where we have a few hills.

As to timing your cranks at 90 degrees to each other , the difficulty I foresee with doing this on an upright tandem would be in starting off, where you need, as I recall from some tandem rides nearly 50 years ago, to 'launch' from a standstill in order to get balanced. This is not an issue with a recumbent trike.

Another point is that from what I've read on forums about the kind of failure you describe is that it can be caused by the angle of bend of the spokes at the hub end. IIRC some spokes were 'better' than others at causing the hub flange to fail. Frankly the people to speak to would be Rohloff, given their boasts a few years back about the reliability of the hub they may be willing to guide you.

 

[Tim Hall]

Interesting that you've had flange failure problems. Did it actually split? Wowbagger, over on yacf, is on his third I think, having had two replaced under warranty by Thorn. I'm guessing yours is a Thorn too.

As to being out of sync, I can't comment, as we've got a Pino, where the stoker's pedals have a freewheel. She's almost never in sync with me.

 

[srw]

We're now on our third hub casing, as of a couple of weeks ago - both replaced by Thorn and Rohloff under warranty. After extensive internet searching I know of two other cases of split hub casings, one of which is Wowbagger's (his second happened while we were both on ACF, I think).

At one level I don't care, as long as the two companies exist, as they do both explicitly give lifetime guarantees. At another level I'd really rather avoid a return if possible. Byegad's comment about SRAM is interesting. Starting off is less of a problem than you might think. The stoker sits on the saddle ready to go, I have the initial push-off and she is able to join soon after, while I'm hoicking myself up onto the saddle.

In response to pppete, for us the hills are beautifully smooth. I think it's a combination of two riders who consciously pedal in circles, plus a tandem with a lot of lateral stiffness.

For what it's worth I love the simplicity of the Rohloff, especially on the tandem.

 

[mickle]

A ninety degree crank offset will help to even out the torque over the whole rotation as you describe. Tandems devouring their own rear hubs is not a new phenomenon and this is an age old 'fix'. The main problem with such a set up, and the reason it isn't more widely used, is the increased risk of catching a pedal in a corner. I'd be inclined (!) to try a 45 degree offset if you don't mind passersby thinking that you haven't set it up properly...

I'm surprised that you are breaking flanges. Could there be other factors? Crappy spokes, poor build, excess spoke tension, flexible rim, too much weight on the back...? Massive thighs might be a contributory factor but can't be the whole story. What kind of spokes are you using and what lacing?

 

[byegad]

I've been mulling this over all day and was about to suggest between 60 and 45 degrees as a compromise. I'd say 45 degrees would give easier positioning of pedals for cornering.

 

[srw]

I've been mulling this over all day and was about to suggest between 60 and 45 degrees as a compromise. I'd say 45 degrees would give easier positioning of pedals for cornering.

We did some experimentation yesterday. The bike would have to reach something like 30 degrees from the vertical for one of the pedals to catch on the road. That is about three times the typical maximum lean we make, not being a speedy crew. In any case if we do lean into corners either we're pedalling, or one of my cranks (sometimes the inside one!) is vertical.

It's a fascinating experiment. Effectively what we've done is convert a 2-stroke engine into a 4-stroke engine, and the result is a much smoother and, it feels, more powerful machine. This can only be anecdote, of course, but several little rises were much easier and faster than they usually are. There's a short downhill into a 30mph speed limit we ritually try to break. Normally we reach about 28. Yesterday we topped out at 32.