Make a Portable Tallbike!

Step 4 Vertical Alignment

Take a couple 2x4 or 1x4 or some other nice flat straight lumber. Place one on either side of the frames, oriented vertically. The frames will be sandwiched between them. Put clamps on and tighten them down somewhat. The goal is to ensure that the top and bottom frames are both in the same vertical plane. If they aren't, your body weight will want to "fold" the bikes together instead of transmitting straight down through both of them.

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.