I've been trying for a couple of years now to figure out a way to make a tandem bike out of two existing bikes such that each rider has their own independent pedaling and gearing...
...and it just occurred to me: take the front wheel off the rear bike and somehow attach its steering fork to the front bike's rear wheel axle.
That seems like it ought to work... though it might be necessary to replace the axle (?) to make it wide enough.
@blankideogram The rear bike's steering would be fixed, not steerable. (We have an actual tandem bike, and the rear rider's handlebars are much the same; they're for holding, not steering.)
I don't think what you predict about differential speed would be a problem, though; the two geared wheels (wheels 2 & 3) would just turn according to how fast the bike was moving along the road.
The lifting issue doesn't seem to be a problem for kiddie-trailers. https://www.walmart.com/ip/WeeRide-Co-Pilot-Child-Bike-Trailer/24772106 Want that for adult.
@woozle It must be possible to invent a hub gear, so each rider could choose their own preferred cadence, or something close to it. Then maybe mechanically, or probably electronically, make both chain rings rotate at the same speed, by re-gearing one of them...
@blankideogram All that's really needed is this, but for an adult: https://www.walmart.com/ip/WeeRide-Co-Pilot-Child-Bike-Trailer/24772106
I hadn't realized they were so inexpensive...
@woozle Maybe seek these folks out?
https://www.durhambikecoop.org
@woozle So you're looking for an adult tag-along trailer (trailer bike)? Totally doable if you build the lead frame+wheels beefy enough and gave it slow enough steering.
You wouldn't attach it rigidly to the lead bike, though, or your steering and handling would be (probably dangerously) scary.
It'd be a really long bike but it'd be all kinds of fun!
@jond I don't have the shop space or equipment to build anything custom; even extending an axle is kind of a stretch, but it seemed potentially more doable.
I was thinking that my idea wouldn't work because of the pivoting issue... but if it's attached to the front wheel prongs, which *are* able to turn...
...I'd think at least in theory it should work.
I could certainly do a mockup and photograph it, just to be clear on what I'm suggesting.
@woozle Sounds cool. You generally want the pivot axis for the trailer to pass (as close as possible) directly through the lead bike's rear wheel ground contact point. (Imagine turning a bike with a verrrry long beam sticking off the back and a weighted wheel back there -- makes it obvious that it'd impede the front bike's ability to pivot on it's rear wheel.)
Light trailers will often just put the pivot directly above the wheel and not worry about moving it down.
@woozle You'll definitely want to find some way of making the fork "mount" a vertical pivot (letting it rotate on some bushings?). A lack of a vertical pivot would put a lot of stress on the lead and trail bikes: there will be moments where each frame will carry the *combined* weight and on a long moment arm!
@jond By "vertical pivot", do you mean the axis is vertical or the movement is vertical (horizontal axis)?
@woozle Sorry, unclear: allow pivoting in the vertical plane, e.g., by letting the horizontal axis of the end of the fork pivot.
@woozle (I mean, you need a pivot on the other plane, too, but your headtube pivot of the fork sounds like you've got that covered.)
@jond Then as you suggest, some sort of bushing (preferably with ball bearings) seems like it would work.
The only remaining problem would be whether the head fork could take the additional strain... and how to keep it from popping off the axle. Tricky. The fork would probably have to have closed holes for the axle, rather than open U-slots, too.
Someone just needs to make an adult-size tag-along backend. :-|
@woozle The more I think about this, the more I think this might be a deathtrap without some careful work. Imagine the steering forces under braking, for instance: if the mass of the trail bike is an appreciable fraction of the lead bike then you're likely in for a wild ride. But I'm not sure?
If you sit down and sketch it out I'd love if you could share the sketches. I bet they'd be illuminating!
@jond I'll have to ponder it some more... I don't think braking would be the issue; more like going through turns.
The steering fork is designed to deal with vertical forces, not twisting -- which you would get if, say, the rear rider is leaning one way and the front rider is leaning the other. ...which normally wouldn't happen, because everyone wants to stay balanced, but it could happen momentarily.
Also, if the middle wheel goes over a bump while in a turn... something could break.
@woozle Seen this done; I believe the stoker bike had a threadless headset which was simply overtightened to prevent steering, and the captain bike had a bunch of spacers removed from its rear axle so that the stoker's fork dropouts could go straight on. Suspect they'd probs. also replaced the cap's 135mm rear hub with a 120mm hub (maybe from a fixie or Brompton or something) so there was room on its axle for the stoker's front dropouts.
Still: a proper tandem would be far safer..
@ej We actually have a proper tandem -- but the front and rear pedals are locked in synch, which has proven to be an issue.
@woozle If it helps, "frewheel cranks" do exist, which enable a rider to freewheel independently of the other. Fitment is only possible for a limited range of BB shell types and/or synch-chain setups though (I think!)
https://www.sjscycles.co.uk/cranks/shun-alloy-104-pcd-4arm-freewheeling-cranks-170-mm-silver/
@ej Now that's something I figured had to be possible, somehow -- but the place we used to take the tandem for service hadn't heard of any such thing.
That place is out of business now (too bad; we liked them), but once we start using the tandem again and figure out where to take it for service, I may have to show them that page.
@woozle Yeah, it's a little esoteric. I used to be the bicycle mechanic for a charity that maintained a fleet of customised machines for disabled folk who couldn't otherwise ride - we had some stuff that defied like 200 years of conventional wisdom on how bicycles should operate!
If you can't source them elsewhere (and if I'm around - I go to sea for long periods, y'see) I'd be happy to post you a set of the SJS/Shun ones, if that helps you out.
@ej I've made a note of your offer, and will ping you if we can't find the necessary parts/info when we're ready to actually do something about this. (Probably a couple of months at least.) Thank you! ^.^
@woozle If my mental picture is right, you're planning a three wheeled bicycle, with all wheels in a row, and only the front wheel turning to steer?
It wouldn't work because the middle wheel would skid and drag sideways when you tried to turn, maybe even when you try and go in a straight line.
You'd also get skidding whenever either rider spun their wheel faster than the other.
Also one of the wheels would lift off the ground whenever the gradient was uneven.