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Sunday, August 31, 2008

Fork In The Design Road, And New Sketch

I was too worn out to remember to post the main sketch of the ideas in the previous post until now, but here it is:

As usual, click the image to bring up a full-size version. You might still not be able to read the notes on it, but they're better read in the previous post, anyway.

Still no pics of the actual parts to be used, as we've had more storms and rain keeping me from doing it when I get home at night (most of the parts I need to take pics of are too big to keep in the house right now), and I forgot to do it yesterday during the day before the storms came in, while I was actually working on them.

I did get to sorting thru what I have, and found I'll probably need to use the shock-fork off the 24" bike instead of the 20" one I'd rather use, because the 20" does not operate--the springs are either broken, stuck, or not mounted correctly inside. Possibly even missing--I can't tell because I can't open them up without first making a really long (12" or so) allen-head wrench to undo the bolt inside. Since I can't tell what size that is for sure (probably 5mm), I may have to make more than one, out of some old metal rods that used to be laptop-display-locking-bars, plus some cut-off allen-wrenches, welded together, with some sort of t-handle to turn it by. Easy enough to do, once I have a couple extra hours to dig up all the pieces and make them.

Using the longer fork means either the front end is going to be several inches higher than I wanted it, or else is going to be at a more severe angle than is desirable for correct steering angle/trail. I am not sure, but I don't think I could successfully modify the 24" fork to be shorter and still work right, and even if I do, I'd have to make the same tools to take that one apart as I would the 20", *and* I'd have to weld on new brake mounts a couple of inches lower, in order to use them with the 20" wheel. The 20" also has a bit of offset to it's wheel mounts, where the 24" is perfectly straight, requiring near-vertical installation to get the originally-designed trail from it (which will mean a very tall frontend for the bike, either blocking part of my forward central vision or forcing me to make a higher seat, which I also don't want for mostly COG reasons).

Yet, the 24" fork is almost exactly 1 pound lighter, at 4 vs 5 (for the 20" fork). Part of it is materials, as I think the steel in the 24" is a lighter but stronger one, and part of it is construction--the 24" is a pair of single shaped-pinched tube for the wheel-mount part, with standard welded-on U-bracket with brake mounts, and the 20" a thicker, heavier material that is basically just tubes of different sizes welded together, with flat endplates inside the bottom largest ones, and plates cut to shape for wheel mounts welded onto the bottom largest ones. If I could just make the 24" shorter, it'd be ideal.

Now, there *is* one other way to fix this, which I thought of while typing this post up: Mount the 24" fork *backwards*, so the brake mounts are on the back, and weld on new mounting points for the wheel, as slotted tabs, on the front edge of the 24" fork, in such a way that they are higher up and far enough away from the tube to both allow the brake mounts to be used on the 20" wheel, and also to put that wheel's contact patch at the correct trail point for the angle of the fork I will have to use to compensate for it's length. That compensation will be less *because* of this modification, but it's still a taller fork above that new wheel-center, and will still need a greater forward angle to keep the front end of the bike as low as I'd like. This is going to take some careful measuring and sketching before I actually try anything, as I will only get one chance to weld it right--screw it up, and the fork will probably be useless.

Oh, and that magnetic shock idea is probably worthless--I realized there will be nothing to limit the rebound speed, so it'll likely be bouncy as all heck. :( It'd probably need a spring or something to limit the rebound, and if I have to use a spring, I might as well just use only a spring, for weight. We'll see what I can find in my scrap piles and do this all as simply as I can. Right now, I think the rear shock is going to end up being an adapted front fork shock, or rather, one side of one, as I have a damaged fork with one trashed bent-up side and one alright one that could be cut apart and used for this.

You know what? I didn't really think it would be all that much fun to try designing a *bike* around the limitations inherent in the parts I have laying around, though it certainly has been fun doing that for the motor on my Columbia. It's more of a challenge than I thought it would be, but having already read about the construction and design of many other homebuilt recumbents (almost none of the ones I've seen made of scrapped bike parts, and none *entirely* from them, though I'm sure someone *has* done that that I haven't yet seen), I find modifications of ideas others have used for their projects popping readily into my head whenever I run into problems with something--sometimes too many possible solutions, and I have to narrow down to just one!

Some ideas I thought were original (like my chain/sprocket/cable steering idea) actually aren't, and I probably thought of them because of having seen something vaguely similar on others' projects--I'll never really know if anything I'm coming up with is actually original thought, after all I've already read about, but at least I know in most cases the ideas are already road-tested and work. :)

I'm hoping to have a testable bike by Saturday. It'll need lots of improvements, I'm sure, but this ought to be interesting!

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