eacolin

Hi all, I had an idea a while ago for a quick way to create a pseudo-tandem bike by using fork(s) with really wide spacing. Specifically, I thought that if you welded together a fork with spacing wider than a given bike's rear dropouts, you could mount the fork around the rear dropouts. In this way, one bike's rear wheel would be the next bike's front wheel. This obviously wouldn't be limited to two bikes; you could just keep connecting bikes together this way to create a "bicycle built for N". I think a better way to explain this would be with this simple image I just photoshopped together:



Obviously I wouldn't advocate doing it on this particular bike (or frame/fork material), but it was the first big road bike image I found. Some queestions/concerns I have are:

1) Does such a fork already exist? I think the spacing would need to be something like 180mm. Seems unlikely.
2) Do you think that there would be a risk that the fork would push the skewer/wheel out of the dropout? Because it seems like you are sandwiching more than usual with the skewer, and there would be some downwards force from the fork, which I think might shove the wheel out. One obvious fix would be to basically weld the rear dropouts so that they were basically a hole, rather than a slot, or to use a bike with rather horizontal dropouts. Alternatively, instead of mounting the fork around the rear dropout, I imagine you could create a fork which terminated with some kind of clamps that could mount on the seat stays.

Some other notes: I don't think that the rear bike's front brake would necessarily line up appropriately, so it would probably have to be removed. Also, this would only work depending on the rear dropout's location; I would think in some cases the chain stays or seat stays might get in the way.

Any thoughts? Is this a completely foolish idea or should I get welding? I think it could be really fun.

BCRider

I'm pretty sure it would not work out well. The single point connection of this sort with the relatively flexible fork and rear triangles being the connections would mean that the link would be quite flexy through the rear triangle and fork. Secondly the simple friction of the axle clamping the parts together would not be enough to retain the fork in place when faced with the leverage that would be there if the two bikes try to flex to either side. Besides the nature of the three wheels would mean that the fork dropouts are constantly pivoting against the rear dropouts of the forward bike. And that would be very hard on both the fork as well as the rear dropouts not to mention it would work at loosening the axle in short order.

At first glance you'd think you only need to add an upper stiffener. But that won't work because the rear bike has to be free to pivot at the axle so that it can follow the road undulations.

And on top of that front wheels have been pretty standard for axle spacing for decades. So you won't find a fork that is spaced out like you're after. And even if you did the derailleur side of the frame dropouts on a lot of bikes is a pretty crowded area. I doubt it would clamp into place without jamming the derailleur.

So all in all while it may seem like it's a simple and inspired idea it won't work due to the lack of suitable forks and the need for the rear fork to be able to pivot at the axle and the lack of rigidity in the rear triangle of the front bike. In fact in any sort of tumble it's likely that it would fold up the front bike's rear triangle and quite likely the fork of the rear bike. You're putting far too much potential load into far too small a joint that is supported by parts which were never designed for such loading.

A three wheel hinged in the middle tandem is an otherwise interesting idea though. But I think you want to approach it more along the line of a Trail-A-Bike sized for adults.

My first thought was to use the fender and carrier rack brazeons found on some frames to add on a mount which would accept the head tube and headset of the stoker's bike. But that would require relying on just four 5mm bolts into the dropouts and brazeons. And knowing how easily those screws come loose just from the flexing of a typical rack and the rear triangle I can't see that being sturdy enough either. But it's a possible starting point.

eacolin

Hi BCRider, thanks for your thoughts. I think you're right in your points about the flexiness/unreliability of the connection, but now I'm thinking those issues could be solved in part by actually welding the fork dropouts to the rear dropouts, and potentially adding a clamp from the fork to the chainstays. The former idea would not be reversible, while I think the second would. Also, just to be clear, I didn't really expect there to be a fork which would do this (which is why I was saying I'd have to weld it myself), although the Surly pugsley fork comes in 135mm spacing, which might work for older frames. In terms of the derailleur, I think you're right; this might require that a single speed/fixed setup would be needed in order for this to work cleanly. I can't think of any way for the rear fork to be able to pivot in any simple way; that is a really good point. Perhaps this idea is only meant for completely flat rides I do think that using fender/rack mounts would be nice, but I don't think that their location is standardized really, so a new "fork" system would have to be created on a case-by-case basis. Anyways, thanks for the thoughts. Maybe I'll still try to create a suitable fork for fun, or maybe I'll try to figure out how to deal with the "pivot" problem.

BCRider

You can't weld that connection. There needs to be a pivoting joint at this point. Otherwise when riding over a dip the middle wheel won't be on the ground. Or if going over a crest of a long bump the bike will lift the two end wheels up in the air as the middle wheel goes over the crest. None of these is going to make the riders at all happy. There is no such thing as a "flat" ride.

Similarly with three wheels the rear has to be a "trail-a-bike" sort of deal where it can follow like a trailer. Otherwise you'll lean into a turn and the middle wheel will try to lift off the ground. Or even with a shallow lean the three wheels locked in a single line will make one or more scuff sideways.

Then you also bring in the stability of a single track vehicle. Have you ever tried to ride a bike with a bad headset where it wants to "notch" into the center neutral point? If you have you'll remember that it wanted to "hunt" to the side and that corrections to your line of travel were rough and "nervous". A three wheel bike with anything less than fully smooth and non dragging pivot joints for the rear bike would make the combination "hunt" and steer nervously in the same way. All three wheels have to be fully moveable to allow them to caster and track the path of the bike during any upright or leaned riding.

Nope, for a three wheel tandem sort of thing it has to have a two way pivots either at a single point or via two related cross axis separate joints or it's a non starter. The rear bike has to be able to both pivot up and down and side to side freely..... just like a Trail-A-Bike or a single wheel BOB trailer. If it doesn't your combo bike won't even make it out the driveway without crashing.

JiveTurkey

Two words: Bicycle Bungee

fietsbob

More appropriate , perhaps is a system approach, where a tandem has a addition/ subtraction in the design.
so the second rider's portion is added , to a single bike , to accommodate the stoker, Un bolt
the additional section, bars , drive train etc, to use it as a single bike ..
several frame builders come to mind ..R+E in Seattle WA, https://www.rodcycle.com/

and Bike Friday in Eugene OR.https://www.bikefriday.com/

eacolin

OK, well I will put this on hold until I can think of a clean way to get the rear fork to pivot. JiveTurkey, I remember reading about the bungee on bikesnob a while back! Too funny...

Monster Pete

I think you'd need to mount a bearing to each of the rear dropouts, so the axle stub was free to rotate. This would allow the rear bike to pivot up and down as well and left and right. The alternative would be to weld the fork at right angles to the head tube, to be bolted to a hub mounted to the seat post in front. I like the idea of this type of tandem, due to the independent drive for each rider and the ability to quickly convert back to a single bike.

eacolin

This is what I'm thinking to. It will make it a bit more tricky to make.

Atavar

The trail-a-bike idea is the way to go and would be relatively simple,, weld a steerer tube to a sort of reverse trailer hitch to tie to TWO ball hitches on the puller constructed rigid to the seat tube and rear triangle. The ball hitches would permit some flexion while maintaining linear linkage.

If you talk to any multi-trailer trucker though you will find out that the tricky bit is going to be stopping and descents. I would suggest not even having brakes on the front 25% of however many frames you linked. All stokers would need to pay rabid strict attention to what the captain(s) were doing.. if too much deceleration came from the front or too much acceleration came from the back things would get ugly real quick.

Having the handlebars connected to the steerer on each individual frame is another recipe for disaster. Each stoker's handlebars should actually be mounted to the hitches on the rear of the puller's frame.. to allow the stokers to use shoulder muscles to control the amount of stiffness in the linkage between frames.

It is an interesting idea, and I bet an ingenious machinist could make a hitch/linker assembly for a couple hundred bucks worth of parts.

BCRider

Ball hitches would be a crazy heavy method. But you may be onto something. Aircraft "Hiem" ball and socket joints would be far more in keeping with the weight scale and loads on a combination cycle of this sort.

Bezalel

You can use a Pugsley fork but you have to allow it to pivot where it attaches to the front frame.

Jeff Wills

Actually, that's kind of common with recumbent "delta" trikes. With one front wheel and two rear wheels, you simply replace the front wheel of the trailing trike with a hitch on the leading trike. Basically, you take a couple of these:

and make one of these:


There's no limit to how many you can link up. Here's a video from the Monster Cookie ride a couple years back, featuring a 4-trike train:
https://vimeo.com/4382706

And the Hase company's world record tandem "train":
https://hasebikes.com/64-1-world-record-2007.html

fietsbob

A custom project, obviously ... think of a rear substitute fork, with unicycle type end

tipped with a bearing, that is assembled over the extended rear axle , then bolted to the fork tip.

Booger1

Won't work.If the rider in the back is heavy enough,when you go over a bump,the front wheel will come off the ground.That should be fun.

You could make it work,but they won't be regular bikes anymore.Like has been said,it will need to pivot up and down or you will be building Mr.Toads Wild Ride on a bicycle.

I guess a person could add some brazeons to the rear triangle to except rod ends,small pillow blocks or bushings,to bolt the forks to if you wanted.

fietsbob

Im seeing this as a trail-a-bike for big children , grown up in only size.

Jeff Wills

Kind of like the Lightfoot Trailer-Trike: https://www.lightfootcycles.com/trailertrike.php

Deccie

These are real right??? Like these exist in real life? Please ******g tell me these are real I feel insane

please

Deccie

I need to know that these are real PLEASE ******G TELL ME they're real Im actually begging

Deccie

Am I going insane? Please bike men tell me I'm okay

MNBikeCommuter

Editted comment: Good grief. Just realized the age of this post. Some bot got unleashed? Sheeshh....

Aside from the mechanics of linking the two bikes, I'm concerned about the actual control. My daughter and I did an MS150 ride when she was 8 with a Burley Piccolo pushing my Cannondale T600. She was at the upper limit on size at the time but "the ride was for a good cause."

When passing others, she'd kick it into gear (competitive little kid? ) and my bike would "fish tail" to some degree. My "straight line" then fit into a three foot corridor rather than the usual few inches. I can't imagine what control would be like if it'd been an adult back there ramping it up.

The Piccolo mounts on the rack like a fifth-wheel trailer, just like this design. Trail-a-bikes mounting to the seat post are a little more stable, in talking with others. Or maybe their kids didn't have the power of my 8 year old.

Andrew R Stewart 

There was (maybe still is?) an attachment for two bikes to be linked together that's oriented towards an adult bike up front and their kid's bike in the rear. It was on the market a few/bunch of years ago. That no one here knows of it (or has forgotten about it) might suggest the practicality of it in the real world of lawsuits and market forces. Andy (who feels that after 150 years of cycling designs there's very little that is really a new idea)