How to Build a Tall Bike, one that you can stop safely, and could be construed as street legal in most places. With this method, you'll be able to stand over the bike, allowing you to hop down easily, have two brakes, a full complement of gears, And have a better riding position than two bikes stacked on top of eachother. You'll only destroy one bike, The top portion is bolted to the bottom bike, so if it breaks, or you get tired of it, the bottom bike can be reverted to normal. All in all I think this makes for a suprisingly sane tall bike. Not that you'll look any less the madman riding it through traffic.
Step 1: Gather two bikes, extra bits
You'll need a pretty functional bike for the bottom, and a bike for the top that just needs a good bottom bracket, and a frame that isn't rusted away. I prefer cheapish old mountain bikes.
As far as parts;
"steel seatpost to fit the bottom bike
"full length brake housings/ one normal cable front, extra long cable rear
"handlebar/stem cobo to reach 28-32"
If you want to get multiple gears running you'll need
"two farly well matching drive side cranks
"extra long gear cables
Step 2: Strip the top bike of parts
first, to make things easier, you're gonna want to strip all components off the back half of your top bike, cranks, derailleurs, bottom bracket, brakes, seatpost, back wheel
Step 3: Cut the rear trriangle off
Now, our first cut, slice the down tube and top tube off flush with the seat tube, leaving a nice clean rear triangle.
Step 4: Attach top rear triangle to bottom bike's seatpost
So now, take your steel seatpost for your botttom bike, remove the saddle from it, and chop the tip off. now cut a slot in the end to fit the chainstay bridge of your top bike. error on the tight side so you can just stick it on there and let it hang while aligning it.
Now you'll want it nice and centered, parallel to your seat tube, use a ruller to line them up ,like in the second picture. To make sure it's lined up the other way, and centered in the middle of the chainstay bridgeyou can put two straight eges on each side of the BB shell.
Now tack it, i used a flux core welder for tacking, MIG is way better, but at this time i only have gas for stainless, so i'm tacking with flux core and fillet brazing to finish. Tack everything before you weld anything, and you will be much happier.
Once you've got your seatpost tacked, doublechecked for alignment, and tacked some more, drop the post into your bottom bike.
In hindsight, you could also do this by cutting the seatpost at an angle, welding it to a plate, such as a kickstand plate, tahen welding the plate to the bottom of the chainstays, this would allow you to move the top triangle lower and further froward
Step 5: Align top triangle with bottom bike
Now get your seatpost into your bottom bike, just tight enough that it soesn't slip on it's own. Use a straight edge against the BB shell to line the assembly up with the bottom bike. clamp it tight, now eyeball it from different angles, look good? the seat tube of the top bike should be parrallel to the bottom one, the bottom brackets of the two should line up nice.
Step 6: Use fork as new wishbone seatstays
Now take the fork out of the front half of your top bike. take the brakes off, this will be the wish bone stay for the tall bike.
Put the fork up against the top triangle. overlap the steer tube with the seat cluster, and place the blades so they poin to the dropouts of the bottom bike, now mark the angle you'll cut the steer tube to pit the seat tube, and the length you'll cut the seatstays to but up against the back of the fork blades.
Step 7: Cut the fork and top triangle
Cut the steer tube of the fork at the angle you marked, as well as cutting the fork ends off. then cut the seatstays of the top triangle flush, and the chainstays where you marked.
Now put the fork up there, you'l probably have to fine tune the stear tube and chainstay cuts for better fit and even legth.
tack the fork on there.
Step 8: Set up drive train
now if you just want to do one gear, you can just make up a big long chain and run it from the top BB to the back wheel, using the derailleur as a tensioner,
If you're keeping the gears, you'll want to set up th drive train before finishing the seat stays,
Make up your top bottom bracket, and put on of the pair of drive cranks on the left side. pull the non drive crank from the bottom bike, and replace it with the other drive crank. make up a chain long enough for to reach, then adust the seatpost to tension the chain. while adusting chain tension you'll probably knock the alignment off, ust get it back as well as you can.
Step 9: Attach seat stays
Now with your drive train settled, you can finish the seat stays. i just attached them to the bottom bike by the rack mounts. If you wanted more strength and had a long enough rear axle, you could bend them around and use the dropouts.
So, cut apart the seatstays from eachother, drill out the rack mount threads, and bolt them to the droupouts of the bottom bike, tack the top ends to the fork blades, and voila, seat stays.
Step 10: Handlebars.
Now the most solid way to go is a four bolt clamp stem and ape hangers. This is what i have on the prototype. Procuring Ape hangers ;larger than 22" seems impossible, so they will have to be built from scratch, and i have yet to find a proper stem.
For Now i have built a 18" rise stem and have some 14" mini ape hangers, This setup feels like it may break off.
Step 11: So far so good
And here we have the rideable bike, hooked up a brake, hopped on lightly, everything seemed okay so i brazed the joints. Shortened up the stem and added rebar struts to take out the flex.
Step 12: Ride!!
Now you're ready for a test ride, almost everyone i meet first tries to get on with one hand on the bars and one on the seat, this doesn't work. Both hands on the bars, then either swin a leg through and put your foot on the pedal and go, or run and jump onto the top tube.
Step 13: For the Future
So For the next tall bike, i'd put the center of gravity a little farther forward, by welding the bottom seatpost further away from the bottom bracket. Also, this bike doesn't shift all that well, so i may make it single speed, and move the sync cranks on to a new bike. If i build any more tall bikes, I'll post a follow up.