Looking at his pic of two blokes about to ride around the world;

made me wonder why they aren’t set so that one is making up for the ‘dead’ part of the pedal stroke for the other? I realise that getting going might be an issue as it would only be one rider providing the power for the first quarter of a stroke but once they’re rolling the power would be more constant and maybe that would be more efficient?

It might look weird to an observer, the legs going round at different phases.

Can’t think of anything else.

Rachel

Ground clearance when cornering.
Thats the first thing that springs to mind.
I’ve seen TT tandems using offset cranks. Have to be careful on the limited corners even then.

Group clearance when coasting and cornering is the main reason. However, some tandems have slightly offset cranks, not 90 degrees different but 10 or 15. Like this in fact.

No personal experience here, but I would guess that having another force kicking your cranks round at what would otherwise be the “off” part of your pedal stroke would be disruptive to smooth pedalling…?

I guess it’s because as you pedal the bike shifts about under you … if you weren’t pedalling in sync the bike would get unbalanced. That’s just a guess.

shifting weight, honking on the pedals, would be horrible if they were trying to shift the bike under them in different directions/ at different times.

the pilot gets an easy time of it on that one lunge 🙂

Changing gear would be a nightmare too. Being able to shift is dependant on a reduction in chain load. Much less variation with offset cranks.

I’d not be using a steel Dawes for a ride round the world. We used to think ours was articulated it was that bad.

DaVinci make a tandem system were each crank is independent of each other. Both are linked to a drive shaft, via free wheels, that in turn drives the rear wheel. It mean one or both can freewheel. It’s a lot easier to learn to ride a tandem than when both cranks are fixed to each other.

DaVinci IDS

thanks all 🙂

mor einfo re: rtw thing;

http://road.cc/content/news/193241-world-first-tandem-circumnavigation-attempt-tandem-men

Slightly offset is not unusual and some people think it’s a good idea. We have done it in the past, but not currently. Large offsets make standing on the pedals interesting, to say the least. When mtbing its good to be able to avoid pedal strikes too.

No personal experience here, but I would guess that having another force kicking your cranks round at what would otherwise be the “off” part of your pedal stroke would be disruptive to smooth pedalling…?

It can already be a problem if you’re the weaker rider the pedals feel like they’re falling away from your feet if they’re in sync.

the pilot gets an easy time of it on that one lunge

Would it? I’d have thought you’d put the weaker rider ahead (on the pedal stroke, the stoker on that bike) so that they were already pushing before the pilot was, otherwise it’d give the ‘falling away from your feet’ feeling?

Think it’s the missing timing chain ^ !

Unless there’s a hidden timing driveshaft assembly to reduce drag, given that it looks pretty aero anyway?

What’s going on with the spokes too? Something to reduce spoke length?
That’s the one thing that used to bother me when I used a tandem – snapping spokes under hard braking.

http://www.bikerumor.com/2009/12/30/specialized-shiv-carbon-fiber-tandem-aka-the-supersonic-divorce-machine/

Made to be as aero as possible, everything that can be integrated is, including the computer, aero bars, brake and shifter cables and even the front drivetrain. The leader pedals a crankset linked to the stokers’ via an internal shaft drive.

aha – thanks. Looks great.

We set our tandem cranks just out of synch, by one tooth on the chainrings, the idea is to make the stronger rider carry the weaker one over top dead centre, it can make quite a difference.

A mate has a home built trandem. When first built he tried off setting the cranks by 1/3 of a turn. It was all kinds of wrong to ride. In sync worked much better.

Believe it or not, this is a religious issue on tandem@hobbes newsgroup. Captain leading by two teeth has been standard. But this varies. I prefer a bigger difference for smaller riders on the kiddyback. I’ve also tried 90 and 180 degrees. All based on feel.

Knowing where the rear pedals are is always helpful when cornering of coasting.

The Da Vinci is a great solution for independent coasting, but “she’s not pedaling” then may have an element of truth.

Tinas – it was the lack of captain chainring that I was referring to so thanks for the link 🙂

My stoker is still waiting for the first “she’s not pedalling” but I’m not buying one of them davinci things just to let her have her way.

shifting weight, honking on the pedals, would be horrible if they were trying to shift the bike under them in different directions/ at different times.

This

It’s normal to set them up about 10 degrees out – so you can start together and get ground clearance, but the peak load on the drivetrain is reduced and there’s no dead spot.

The Pino recumbent tandem I do quite often has a freewheel for the (front) stoker, and I’ve built recumbent trikes where the pedals are 90 degrees out for completely smooth power output – no ground clearance issues there.

I once went dragon boat racing and the difference between all rowing in sync and not, is is amazing. I know it’s a completely different sport but I think the principle is the same.

Try riding a tandem with different sized cogs on the sync chain.

makes for interesting surges

The science is slightly different – I’ve seen an analysis proving that in sync is more efficient for a double kayak, the principle of which would presumably hold for a dragon boat.

The thing is on a bicycle, there isn’t actually any significant loss of efficiency due to the “dead spot” (there isn’t a significant variation in speed through the stroke unlike in a boat). Hence no real advantage to 90 degrees out of sync (notably racers have their cranks in sync). We have ours about 10 degrees out as it seems to help a bit with the feeling of having your feet dragged round for the weaker rider (not that the tandem has been ridden by any stoker other than my kids for 10 years, and I’m not sure they’re so bothered).

BTW the answer to “she’s not pedalling” is “she must be, because I’m not”

There’s some evidence that antiphase rowing is more efficient than in-phase, since it evens out the boat speed (and thus reduces average friction). However this would be a negligible effect for cycling.