With the recent exponential multiplication of tall bikes across the country and even around the world, a primer on their construction hardly seems necessary. Still, for the sake of those just starting we provide here the basic steps and notes for the construction of the classic "one bike on top of another" design. By no means that this design is canonical, for there are many approaches by which one may make a bike tall. But purists and the aesthetically inclined may find the classic design more deserving of elaboration.
1. Two bike frames of near-identical length are required. If both head tubes are aligned the bottom bracket of the top bike should rest dead center atop the seat tube of the bottom bike. In a pinch, give or take half an inch.
2.. Remove the seat, seatpost and handlebars from the bottom bike. Remove the handlebars, crankset, and wheels from the top bike.
3. Cut the seat and chain stays (the "rear triangle") off the top bike.
4. Find a piece of steel pipe that will sleeve into the steer tube of the bottom bike's fork. This is usually about half an inch in diameter. It should also be long enough to sleeve an inch or more into the top bike's steer tube.
5. Some steer tubes have an open bottom, so that you could sleeve a pipe through the top and right on through and out the bottom. Find one of these forks or use a grinder open up the bottom of a fork. Either way, this is going to be top fork of your steer-tube extension. Cut the fork blades off the top fork. Put the fork back on the top bike.
6. With the lower bike standing on its kickstand or leaning against a chair, drop your half-inch steel pipe into the bottom steer tube. Then take the top bike and sleeve the top steer tube onto the steel pipe. This should be a relatively snug fit and you should be able to simply rest the top bike's bottom bracket on the top of the lower bike's seat tube, and the whole thing should stand on its own without your support. Make sure you've cleaned and greased all your bearings in the steer tube set because once its welded you can't take them apart again!
7. Examine the point where the top bottom bracket is contacting the lower bike's seat tube. Is there a lot of contact there? You might want to file or grind the seat tube to get more metal-to-metal contact.
8. You are ready to jig! Take two wood boards and place one on each side of your tallbike.
Use old inner tubes, rope, whatever, to lash the boards together snugly, sandwiching the bikes between them. This will align the two frames with eachother.
Eyeball and doublecheck to be sure, but honestly, the best test is to just try turning the front wheel with your hand. If the bikes are lined up sufficiently, the steer-tube of the top fork will turn in a smooth fashion when you move the wheel.
9. Time to weld. Put tack welds at the contact point between the top bottom bracket and lower bike's seat tube. Then tack welds at the bottom and top contact points of steer-tube extension. Let cool. Remove the jig. Complete welds.
10. Add a gusset between the top and bottom bikes in front of the top bottom bracket. Maybe even a second running parallel behind the steer tube. This is too reduce flex stress on the bottom bracket/seat tube welds.
11. Put the crankset back on. Add chain. You can run a chain directly to the back wheel, as shown in Figure 4
Or you can make a triangular chaindrive by looping the chain around a still intact lower bottom bracket, as shown in Figure 5.