Joining Tubes

The basic requirements for joining the main tube are strength both in bending and in torsion, plus guaranteed separation even after an extended period in place. And besides the main tube there's the extending booms, reclining seats, sliding seats, you name it. I've had my TriSled boom pushed hard in for a week and that was a real problem to pull back out again. In the end it took five of us! So with much bigger tubing on the tandem I can see a possible problem.

Ken suggested a non-tube joint using a couple of bits of angle iron bolted together. This would key well, and be quite strong. It would also be easy to modify to get whatever strength is required just by extending the angle iron.

One obvious modification to this idea is to key it vertically by welding an old bolt on one side and making a matching hole on the other. But if you're doing that, why not drill a hole big enough to take the main tube and gain the additional strength? But it's a bit of a tricky thing to build then, you need to precision drill a 40-70mm hole to match the tube. The basic idea is good, but I think it makes point forces at the joins which is not good, and it seems a bit ugly to me. Maybe not.

I asked the trikes list about this and got some useful ideas. Which I can't remember, but I'm sure they fed into my thoughts below.

The traditional solution to joining tubes is to put a sleeve over the two tubes or make one large enough to slide over the other, then cut a slot and put a seat post style clamp on it. This works really well for a lot of applications, and by adding more clamps you can get quite solid joins.

The sleeve solution lacks only a way to stop the tubes rotating, because I'm not sure I want to have so many clamps done up so tight that the beam doesn't rotate when the trike has a touring load on and one wheel in the air. Trice seem to use a diagonal cut in the sleeve, which is simple and should work well. It will also help separate the tubes once the clamps are undone. Doing that means bulking up the angles part to get strong edges, but 50mm of 3mm wall tube on each joint is not going to weigh a lot.

One high-tech solution is to use S&S couplings, which are basically a thread on one side, a lock ring on the other and some interlocking teeth in the middle. And only $300 each. I'd need probably four of them, plus design the frame so that I can pair them up. Oh, and they're only available to approved frame builders. I could fake something similar up out of stainless steel dairy couplings but they're heavy and tricky to join using a gas welder.

I thought of using hinged bolts to hold the thing together, much as trucks use on their bonnets (the wing nut style, not stretchy fasteners!)

For a joint that is not stressed there's the hole and pin joint, but those are not good where there is vibration or stresses close to the limits of the materials.

For the sliding seat I settled on the clamp, since I can use a fairly long bit of tube with as many bolts as necessary to make it stick. As compared to the Windcheetah which has one pair of bolts on a narrow, cast clamp. Since I'm also going to be sticking the trike together with these, if they don't work the seat could be the least of my concerns. So I'm going to use at least three, possibly as many as five clamps of on main tube joints, and three or four on the seats.