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Monday, October 20, 2008

Swivel Joint Design

I've been working on a way to get a swivel joint between the front and rear sections of the recumbent project for at least 3 weeks now, and so far all the ones I could come up with either would not fit in the space between the sides of the bottom bracket, or they would require parts I'd have to go out and buy--not allowed since this is a recycled-parts-only project, if at all possible.

Another problem was that all of them except one (which would need me to machine some sleeve parts, probably possible with the lathe, if I figured out how) would put the joint in such a place within the chainline as to cause shortening and lengthening of the chains to that bottom bracket when the shock and joint were in operation (during bumps/dips/etc). That would require more complications to the drivetrain, requiring tensioners (like the one in most common rear derailers) on each chain going to that point from the front half, which would be both the pedal's rear chain and the motor's chain. Don't want to deal with that, so kept thinking, and came up with another idea in a sudden "eureka" moment.

If the swivel point for the frame centers on the shaft of one end of the chains (the bottom bracket shaft), then the chainlines will never change length. So if the swivel hardware actually mounts *around* the bottom bracket, using the bottom bracket tube as the actual swivel joint in back (the "axle"), and the front frame mounts to something that swivels around that tube, as a pair of rings of some type that pinion the seat tube and rear chainstay tubes between them in such a way as to prevent the sideways wiggle of the joint, it will allow the whole bike to swivel vertically as I need it to. It doesn't have to swivel very far, just a few degrees during bumps/dips, so it won't get a lot of rotational friction or wear.

I spent the last week or so looking thru hardware in my boxes and bins for something that would do this without much modification, and found some U-clamps. They were too small for the purpose, but they gave me the idea to go find the right size. Nothing I had was nearly the right size, so I had to start checking thrift stores and other places that might have odd hardware bits to re-use. I finally found a big bag of assorted fasteners, clamps, and other hardware stuff at Deseret Industries for 50 cents that had something similar, but not quite right:

I don't know what it's called, though it appears to be something to use in buildings to hang pipe, conduit, or wires in, and would be screwed to or bolted to wooden beams. The black ring is a nylon insert that happens to be almost exactly the same inside diameter as the outside diameter of the bottom bracket tube-it's about a half millimeter bigger, which is acceptable.

The problem with the galvanized bracket it comes with is that it's too thin to support the entire wieght of the bike around that swivel point--it would just bend totally out of shape in any torsional stresses, *and* it has an open top which means I'd have to weld something across that to prevent the bike from just falling apart at that point.

But the nylong ring is exactly perfect as a friction-preventing ring, because regular U-clamps like bigger versions of the small ones I already had would fit perfectly inside the groove around the ring. I looked and looked and could not find any used anywhere, so I did end up actually buying these new, from ACE Hardware, just so I could get the project moving again:

This is how the bracket without the ring would fit, because these clamps don't seem to come in anything between the 1-3/8" size and the next up 1-3/4" size, at least at any kind of hardware or automotive store in this area. The ones I already had were barely 1" inside diameter, so they definitely wouldn't fit. The bottom bracket is 1-1/2" inches outside diameter, so too big for the smaller and just too small for the big one.

Thus, that nylon ring from the other hardware set above, slipped into the U and then slipped over the bottom bracket tube will fit perfectly, both diameter-wise, and width-wise (fits just within the width of the space between the chainstay tubes/seat tube, and the outer edge of the bottom bracket tube).

You can see the basic idea of it above.

Now I have one more little problem. Since I don't know what that first bracket is actually called, just a vague idea of what it might be intended for, I have yet to locate one like it so I can get another nylon ring like the first. There's no name or anything on either part, just "123458-PS" molded in raised lettering on the black nylon ring. Searches of various kinds turn up nothing related using that or parts of it. I've already looked at a couple of dozen websites tonite, scrolling thru literally thousands of little images of parts they carry, but not found a single thing even resembling it.

If I knew what it was called, I could easily find another one, I'd bet.

So I will probably have to take it to Lowe's or Home Depot or some other building-supply store, and see if they can find me a similar item. Since those are all a few miles and more from me, I've first sent email inquiries to various places to see if they can tell me what it is, and if they can get me one just like it. All I need is the ring, not even the bracket it goes on. If I can't find out that way in the next couple of days, I'll just have to bike to the stores and see.

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