This is my first instructable, so I hope it is clear, enjoyable, and inspirational! (give me your feedback on anything and I will revise) I re-use the same photo a lot because I don't have the bike in my possession right now to take others. Hopefully it is all clear anyway.
Goals for this tallbike:
-to be sufficiently tall, but can be easily dis-assembled, sort of like a folding bike. for fitting in a trunk or on a trunk mounted rack, or even in a rear passenger compartment if necessary.
-to ride as much like a "regular" bike as possible, specifically steering and pedalling.
-to be Relatively quick and easy to build with a minimum of welding, cutting, and custom fabrication.
-to be as strong and sturdy as any other tallbike building method
-to be disassemble-able with a minimum of tools and time
-a couple of similar-sized bikes from the trash or wherever. If you're reading this, I assume you probably have a bike pile. I used department store BMX frames because they have nice straight forks (more on that later), thick steel that is easy to weld, and they are pretty to easy to find in the trash.
-a stem with a bolt on plate that holds the handlebars, not the kind of stem with a pinch-bolt.
-a steel seatpost for the bottom frame. This gets welded to the top frame.
-a front hub. find a wheel that is taco'ed beyond repair or has a bunch of broken spokes, or even a broken spoke flange. Make sure it is not too small or big in diameter to clamp into the stem!
-standard bike tools like allen wrenches, adjustable crescents, chain breaker, pedal wrench, channel-lock pliers, etc
-a couple 4-foot long 2x4 pieces of lumber or other nice flat, straight pieces of lumber
-at least two 4" or 6" c-clamps, speedclamps, or something you can use to clamp to boards together.
-Welder. Whatever you have on hand that wont burn too hot. I used a $89 stick welder from Harbor Freight, some 1/16" rod, running at about 50Amps, plugged into regular AC outlet.
-Your other welding tools: helmet/goggles, gloves, chipping hammer
-Angle grinder, if you need/want to clean up welds.
Step 1: Headtubes
This building method eliminates welding in a longer head tube or chopping up forks and welding in a fork steerer extension. It also allows adjustments, dis-assembly, and helps during the rest of the build.
Step 2: Picking a Fork and Stem!
the bottom frame has a stem with a bolt on handlebar clamp.
instead of a handlebar there, you use a spare front hub complete with axle.
the fork of the top bike bolts onto that axle.
The key of X:
Pick up the stem you want to use and measure the distance from the center of where the handlebar would be to the center of the expander bolt (the bolt that goes vertically through the stem to the little wedge shaped thing that goes into the forks). Call this length X.
On the forks, measure the distance from the center of the dropout (where the wheel axle goes) to the center of the steering axis. This is easiest on straight-bladed forks like I used. With curved forks this is trickier to measure. If this measurement is within a few millimeters of X, you're good to go. If not, find a clever way to bend your fork legs fore-/back-ward simultaneously to get that distance to X.
Alternately, you can put the hub in the stem, slide the axle into the fork ends and eyeball it to see if everything will be aligned.
Step 3: Disassembly and Frame Attachment.
Now it is time to begin assembling and aligning the frames.
On the bottom bike, if you haven't installed the front hub into the stem clamp, do so. Make sure it is centered left to right and clamp it down as hard as you like. Tighten the stem expander bolt so it is secure to the fork.
With the bottom frame standing up, resting on the fork ends and the bottom bracket (the fat tube that the cranks went through), lift the frame that will go on top up above it. Bring the upper frame down with the works and slide them onto the axle of the stem hub. Tighten down the axle nuts. Now your bikes are connected on the front end! Hold them up one on top of the other. I figure this is at least as strong as a decent weld job up here. The axle is probably the weekest link. This clamping system is also a little more forgiving of misalignment than a weld.
Step 4: Vertical Alignment
I tightened the clamps enough to hold the upper frame in place, but so I could still pivot it around the front axle attachment point. Rotate the upper frame until it looks like the two headtubes are on one steering axis. Try turning the handlebars on the top bike. If they rotate freely, everything is aligned and that is where you will want the top frame to be. (NOTE: if you could make a universal joint out of two front hubs 90 degrees from each other, the headtubes wouldn't even have to be perfectly aligned this way. I think the offset for the fork and stem would have to be the same).
On the bottom frame, raise the seatpost until it hits the upper frame. On mine, it hit right behind the bottom bracket. If the upper bike has a kickstand plate, that would be a perfect thing to weld to. Tighten the seatpost at that height.
Step 5: Weld!
Rotate the top frame back down so the seatube is touching it. Tighten your sandwich board clamps really well once everything is in position.
Weld it up! I found it was a lot easier to do this weld if I turned the entire assembly over so the handlebars and seat were on the ground, just like some people do when fixing a flat bike tire.
The result is a real sturdy attachment from top bike to bottom bike, and only required welding in 1 place!
Another idea I'm going to try is to use another fork instead of the seatpost on the bottom bike. Then welding an axle to the top bike where the fork would hit it. Then they can bolt together there too.
Step 6: Final Assembly!
Figure out a way to install brakes. Easiest to hook up only front brakes, otherwise it may be difficult to find long enough brake cables.
If you opt out of brakes, you can always just stick your left foot down onto the rear tire to stop. This works surprisingly well to stop, but it's not so great for your shoes.
Go riding! Figure out what you can and can't do on the bike!
Warnings and such:
DON'T RIDE IN FLIP FLOPS (see Brakes section above). Start Slowly! Do not be surprised if you end up doing a massive wheely and end up standing in the road with the bike flipped out in front of you. Adjust handlebars and seat as necessary to avoid this.
The bike pictured, though welded in only one place is really strong and steers just as easily as a normal bmx. It was sturdy enough I was able to bunny hop it a couple of times. It didn't collapse, and the bottom frame seatpost didn't telescope downward or anything. I weigh 210lbs. I was not at all worried.
Step 7: Dis-assembly for Transport!
STEP 1. Loosen either the clamp that holds the hub in the stem... OR Loosen the axle nuts so the fork can slide up out.
2: Loosen up the seatpost clamp. If the bike has a separate clamp (not welded to the frame), make sure it doesn't fall off or that you don't lose it! Some of the separate ones have a little set-screw that hold it onto the frame.
3. Undo the chain around the upper crank. Now the top bike should be totally separated.
For the pictured bike, I simply stuck the two frames next to each other so they took up a minimum of height. I wrapped a couple bungee cords around them. In this manner you can make your tallbike fit into the back seat of a 1995 Honda Accord.
Step 8: Re-assembly After Transport!
Have fun! Let me know what you think.