Instructables
Picture of Strategy for Joining Two Bicycles
This Instructable grew out of a response to a question by janmcevoy, who is contemplating joining two bicycles side-by-side for more riding stability when bringing home groceries by bicycle. It is really for her, but others may view and comment.

Shown is my bicycle on the left and my wife's bicycle on the right. If I were pairing two bicycles, I would pick two that are more similar, and would probably not use one with drop handlebars.
 
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Step 1: Keep the bikes next to one another

Picture of Keep the bikes next to one another
Struts or braces can be used to pair two bikes in a side-by-side configuration. The green lines represent the location of bracing to keep the two bikes abreast of one another so one does not lead or lag behind the other. The bracing together with the chain stay forms a triangle, which is very rigid and a basic component of structural engineering.

If two bikes are to be paired like this, the bracing cannot interfere with the normal movement of the pedals, the feet, and the legs. This diagram assumes the rider will ride from the bike on the left. The bracing could be flipped if the rider were to be on the right side bike.

I would remove the crank and the chain on the bike not ridden. Not only does this lighten the bike, but it also offers more places to connect things.

Step 2: Another view

Picture of Another view
This provides another view of the lateral bracing to keep the bikes abreast.

Step 3: Tie the front together

Picture of Tie the front together
The front of both bikes will need to be tied together. The yellow line shows approximately where the fronts of the bikes could be joined to keep the front separation the same as that at the rear.

Step 4: Vertical stability

Picture of Vertical stability
The bikes need to be stable vertically as well as laterally. The green lateral brace from earlier steps is not shown in this view so it is possible to concentrate on the structure and the location of the vertical bracing. It is in an "X" pattern. It attaches below the seats and at the rear of the chain stays. Where the members of the "X" cross one another, they are fastened together to make two rigid triangles.
zymurgic1 year ago
Have you actually tried this? I would say that given that steering a bicycle is more about weight shifting than actually turning the steering column that linked parallelograms (as seen on some three-wheel motor scooters) would be better than trying to make a rigid rectangle.
jtx86 zymurgic3 months ago

It's a faith-based instructible. You just have to take the author's word for it.

Phil B (author)  zymurgic1 year ago
No, I have not tried it. I did this as a suggestion for the lady who wanted something like this.
great idea.
i intend to join two identical dumpster BMX bikes with three transverse steel tubing rods secured by 12 "u" bolts. steering : i thought to simply drill each handle bars top at near centre and join then with a light rod. the entire arrangement will be a bit unwieldy but will provide a means for a pal to cycle with his spina bifida son
I love popular mechanics!!!!
Phil B (author)  jackjackboom2 years ago
Take a look at this Instructable. It will take you to all back issues of Popular Mechanics and Popular Science on-line. You can read those magazines to your heart's content.
Thanks for your time Phil B, very nice instructable!
I will start to build it right now.
Another alternative for joining two bicycles into a Tandem Equivalent is available at: http://TinyUrl.com/TandemEquivalent 

This solution would not work for the expressed purpose of carrying groceries, but it is good for two people who want to cycle together.  One big advantage is that it's easy to carry the two bikes and the tandem equivalent on a normal car bike rack.
clutz4 years ago

I have been thinking along this line also but for two riders.  I know a few people with disabillities who ride trikes in their neighborhood but aren't able to access the beautiful trails we have around here because their endurance limits their range and their trikes aren't transportable except with a truck or very expensive rack.  This is also a problem for people with traditional tandems.  By combining two normal size bikes and making them able to be seperated you can have the stability of four wheels and also allow  a stronger rider to assist a weaker friend while enjoying a change of scenery.  In my area (Seattle) the buses also have bike racks and routes connect to many trails.  It also gives you the opportunity to pick up a friend who's willing to ride but doesn't own a bike or lives/works in high theft area.  Sorry I'm rambling but anyhow I found two commercial options.  They are a little pricey but not everyone can figure this out on their own.  You can also just take a look for ideas.

http://www.thegrannybike.com/  or

http://gandydirect.com/products/pedal-partner-kit/

Lokisgodhi4 years ago
It seems to me that removing the fork off the second bicycle and using it like a sidecar would solve steering geometry problems. 
Phil B (author)  Lokisgodhi4 years ago
That would be fine, as long as very little weight was placed toward the front of the second bicycle.  I think steering geometry problems under general use will be less than some anticipate.
panhead555 years ago
In reference to jtobako's concern as to the cornering... this problem can be overcome by using 'Ackerman' steering. Basically, the tie-rod arms are attached at an angle so they both point directly towards the rear axle. This will allow the inner wheel to turn at a slightly tighter radius than the outer, thus eliminating the 'scuffing' problem. To get a better understanding, just take a look at any go-kart or riding mower.
parisbabe5 years ago
another cool instructable would be an attachment you could use for riding with a dog running along side
the one instructable that i want to see is for a bike side car
YES A SIDECAR WHAT A CLASSIC THAT WOULD BE ANYONE HAVE IDEAS
Phil B (author)  pineapplenewton5 years ago
That would be interesting. It would be complicated by the need for a circle kept free of bracing and structural components for the feet and the pedals to work. It might be easier on a stretch frame for a cargo bike where there is extra space between the seat post and the back wheel.
janmcevoy5 years ago
Thanks a million Phil B - or, if you want it in real Irish Go raibh mile maith agat! . I didn't expect such prompt or comprehensive help, I'm very grateful. Will definitely try this as soon as I can find a second, cheap or free, bike! And re your reply to jtobako, yeah, believe me - on these hills, I won't be doing any speed or any sharp cornering. I'm lucky to get up them without having to get off and walk...
Phil B (author)  janmcevoy5 years ago
I had another idea for materials you could use to make the bracing between the two bikes. It is 1/2 inch electrical conduit, sometimes known as electrical metallic tubing (EMT). It is not very expensive and quite rigid. It is also thin enough that a vise or a big hammer can be used to flatten the ends of a piece, which can then be drilled for "U" bolts. Done right, it can also be bent without putting a crimp in the tubing. it is easy if you can get access to a conduit bender. You can also cut a wooden disc about 8 inches in diameter and use it as a form around which to bend conduit. It is another option with some advantages over some other materials.
Phil B (author)  janmcevoy5 years ago
Jan, Thanks for your note posted here. I am glad you saw my suggestions for you. This is probably one of the very, very few Instructables done for the benefit of just one person; but others seem to be interested in it, too. On bicycle forums people frequently ask how they can get a bike on a low budget. Frequent answers include 2nd hand stores, rummage sales, and scrap yards. I do not know about Ireland, but in the USA the local police usually have an auction of unclaimed property about once a year. Often there are lost or stolen bicycles in the lot. At a scrap yard you might be able to find a frame from one bike and a wheel or two from another. I once bought a decent set of derailleurs at a scrap yard for only $5, just for comparison.
jtobako5 years ago
I think a plywood board across the upper part of the rear wheel frame and an as-wide-as-possible board connecting the fork tubes in the front would cover most stability issues. Make the steering bar as adjustable as possible, because the turning radius is going to be messed up. If the rest of the bracing and load will handle it, remove the front wheel entirely.
Phil B (author)  jtobako5 years ago
Plywood has good stability. It does also add wind resistance.
jtobako Phil B5 years ago
If you are worried about the wind, cut the center out. I'd be more worried about the cornering (swerving) ability at any speed, where the front tires aren't lined up to turn at different radii (the outside and inside wheels want to turn on the same circle rather than circles with a common center, if that makes sense).
Phil B (author)  jtobako5 years ago
The lady who wants this bike setup will use it to travel four miles to buy her groceries and bring them home again in somewhat hilly country. I expect her cornering will be pretty tame and mild. Even if the geometry is not perfect for swerving, it will likely do for her just fine. And, when you start cutting large holes in a plywood piece, it is no longer nearly as strong as it was before.