Main Frame Layout

Frame design provided hours of entertainment. I played with bits of the frame, the whole frame, multiple frames. Hey, if you've got one removable middle section, why not add another to make a triplet? Or perhaps add the option of a rearward facing seat? At the front or the rear? Arrrgh, it's Lego tricycles time... This is also where I decided I really must use the PC. I hand-scrawled diagrams, did an attempt ant and engineering drawing and decided that copy and paste was essential. Two seats? No worries. Two wheels? Ditto.

First I had to decide on a basic frame layout. One main tube or two (three... Moulton here we come)? Since one tube has better torsional resistance than two or more for the same weight, that's not too hard. But I could run a second, smaller tube or box section along the top for better vertical strength. Ian Humphries is a big fan of that idea, so we'll see how his tandem comes out. I think it's better to just use bigger tube in the first place, because multiple tubes are also harder to join using sliding sections. If I was using screwed couplings (like S&S) then three tubes would be a better option, but I'm not. So one tube, about 200mm off the ground, running the full length of the trike. Bits on top of this to attach pedals and seats etc to.

The next question was where to break the frame. I need enough length away from other joints to avoid stressing them, and enough overlap to give reasonable flex resistance. Basically under the front or back of each seat. This decision can be left for later, because the joints are "just" 500-800mm long sleeve tubes over cuts in the main tube.

How big should that tube be? The obvious answer is "as big as I can afford". I'm basing the design on about a 70mm tube, with the requirement to be able to get a metre or so of tubing that sleeves over it. So maybe the second largest tube I can get :)

The diagram above shows mainly how I intend to split the trike into sections. The red rectangles represent the 300mm long tubes that hold the seats, and allow them to slide back and forth. The two blue rectangles are joints in the main tube, the blue parts being sleeves and the black rectangle being the main tube. In both joints the rearwards (right hand side) part of the blue tube is fixed to the main tube, and the forward (left hand side) part of the main tube slides in and out of the blue tube.

And this is what happens when I reassemble it as a solo. Dismantling is going to require playing with chains (dirty), and making the solo will mean breaking chains and playing round quite a lot.