Primary materials: Some old frame tubing from the scrap metal bin. Looks like it used to be a column of some sort.
Quick layout to get a sense of what the frame will look like and how high off the ground it will be. Please pardon my messy garage!
The rear end will probably be from a suspension MTB. I'd prefer to use a 20" wheel, but this will be convenient for brazing to the main tube, since it has a pair of tabs that fit nicely on either side. Also, the 26" rear triangle may have enough room in it for the 27" wheel that's currently on the old upright tandem, which has a 7-speed internal gear hub and a drum brake.


I'm using fleettrikes style kingpins - a large bolt and some bronze bushings - but where he uses 1/2 inch bolts in a tube with 3/4" inside diameter, I ended up using 3/4" bolts and 1 1/8" steer tubes, which happen to have a 1" ID for most of their length. This is mostly because I couldn't find any tubes with 3/4" ID, but the beefier kingpins may be a good idea anyway on a tandem.
This is the crossbar, with 15 degree wedges cut out of it to (1) compensate for the kingpin inclination and (2) lower the front end of the trike a little.
The front of the main tube is cut at a 12 degree angle, since most builders seem to agree that 12 degrees is about right for the caster angle. It will also angle up the front boom a little. I left a tongue coming out of the bottom that will extend across the bottom of the crossbar and onto the front boom so the joint covers a larger area - I don't like brazing butt joints.
The crossbar fits against the front of the boom, like so...


Well, I didn't photograph a few steps here, but... On the left is the end of the crossbar. I shaped it to fit the kinpin tubes by drilling through it with a 1 1/8" hole saw a little short of the end and cutting wedges out of the top and bottom on either side, so I'll be able to bend the sides in to meet the kingpin tubes. On the right is one of the kingpin tubes, and underneath is a rough jig: Plywood with a 1 1/8" hole drilled in it.
Put the kingpin tube in the hole, which should (1) hold it in place and (2) make sure there's about 1/4" sticking out the bottom, so the assembly that holds the axle doesn't hit the crossbar or anything. Also, because I think it would look odd to have it flush with the bottom, and because if it were flush I would probably end up with brazing filler inside the kingpin tube and have to ream it out with a dremel tool AGAIN.
When I'm brazing this together I'll hold the crossbar to the jig with a clamp of some sort instead of using my hand. Speaking of which, I should be able to start brazing tomorrow or the next day.
Finally got to the Craft Center for a few hours today and brazed up the crossbar. I'm afraid I warped the kingpin tubes some in the process, so tonight I'll have to ream them out and try to get them back into their proper shape.


The kingpin tubes were a pain to ream: I looked into buying a 1" chucking reamer (or hand reamer, for that matter) but they're expensive and nobody in the area stocks them. I finally used the sanding attachment on a dremel tool, which took some time but got the job done. I also tried a rotary file, but the dremel was more effective. Afterwards someone suggested a flap wheel, which I'll probably use if I run into this problem again. The folks at BentRiderOnline were quite helpfult, though I didn't take any of their suggestions exactly as given. On the worst of the tubes, I heated it with the torch and jammed a 1" steer tube in to bring it back into shape before sanding.

I ended up using T fittings for 1/2" pipe to hold the axles. They had to be drilled out quite a bit, but the result was good. I ground them down on a belt sander and stick-welded them to the kingpins.

Here's the main frame. This is a very convenient stage for working on it, since I can pull it around like a trailer if I tighten the kingpins enough.
I'm trying a(nother) new type of seat clamp, since this is my first build with square tubing. It's a simple sandwich of plywood layers around a square U-bolt. Three layers are shown here, but I had to make a copy of the top one, since one layer of 1/4" plywood wasn't enough to be flush with the top of the U-bolt. The bottom layer has 2" cut out of the middle to fit snugly around the 2" frame tubing and keep the seat from twisting.

The resulting sandwich is clamped while glue dries. I made a simple plywood seat (just two pieces of 1/4" plywood with a hinge between them) and attached the base of the seat to this clamp with wood screws. It seems to be holding up well so far, though the struts that hold the seatback up are temporary. Photos to come...

I spent some quality time with the angle grinder today (blech) and got things sanded for attaching the boom and making one of the clamp assemblies for the bottom brackets. As with the seat clamp, I'm making extensive use of U-bolts.


These arms will hold the brake calipers, the handlebars, and the attachment points for the steering linkage. They're brazed to the plumbing fittings that serve as axle tubes... I hope they're strong enough to hold up under braking forces!
Rear frame, fully triangulated. It's for a 26" wheel, but fits the 27" just fine.
Well, it's now a push-trike. The frame seems stable enough.

Skateboard wheel -> idler pulley. I first tried putting a bolt through the wheel, putting a nut on it, and putting the end of the nut into the drill chuck to create the assembly at right, but the bolt has a tendency to slip out of the chuck, so I switched to the old wood lathe I picked up at a garage sale last year. The only problem with the wood lathe is that it has a dead center, so I had to put a bearing in one side of the wheel and turn a piece of wood down to fit between the dead center and the bearings.

I switched back and forth between using a chisel and using a file. The chisel tends to get jammed in the rubber of the wheel, so a 1/2" triangular file ended up being the best tool.

Since last taking photos I've also attached the front boom, built the steering linkage, some of the idler mounts, the second bottom bracket, and a few other things. More on those later...