Adjustable swb

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Feb 3, 2017
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I made a short wheel base recumbent for my first recumbent build and now i want to make another better one.
I want it to be adjustable so different height people can use it, but this is proving to be a difficult task. I actually think adjusting the BB and chain each time might be easier than a seat moving mechanism..
And I feel all heights of people will want to sit as far back as possible.
I've struggled to get good images or info on commercial recumbent's adjustment details and they don't sell them in my city so i can't look at or try one.

I've stripped a frame and picked wheel sizes and now I'm confused how I'm going to pull this off haha. Any help or tips would be hugely appreciated.
 
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You'll need to get two sizes to tube that sit nicely in each other, weld a long nut or two to the end of the larger one then split the nut(s) and tube to a depth of 2" with the grinder. Drill out one side of the long nut(s) and you've created a pinch joint to hold the thinner tube. Weld the bottom bracket to the thinner tube and you have an adjustable boom.



That's only half the solution though as a regular derailleur is only going to allow so much adjustment (not much) before it runs out of ability to take up the slack or the chain gets too tight to cope. If there's a wide range of rider heights you'll need to add another derailleur mechanism (ideally a long cage) into the chain return to assist in the effort. This second derailleur ought to have a weaker tensioner than the one changing the gears so the one at the rear can retain some semblance of it's proper orientation to do it's job. You could opt for an internally geared hub thus removing the need for a derailleur there leaving the one in the middle free to do it's job without worrying about any gear change issues. It will help if the main derailleur is also a long cage as a long one offers more adjustment range than a short one.
Only using one front cog will give the rear derailleur less range to have to take up for gears leaving more room to allow it to take up slack on bottom bracket adjustment. If you don't have a massive range of rider heights one front cog and a long cage may suffice without a central extra mchanism.



It doesn't have to be a derailleur in the middle as a means of taking up the slack, you can use a manual means of diverting the chain return but they all work on the same principle of diverting the chain to take a longer or shorter route.

 
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Thanks a lot for that.
i hadn't thought of a pinch joint, that's better than drilling lots of holes. Although i guess u have potential to fasten it misaligned that way.
...hmm i wonder about a bit of pinch joint seat adjustment as well..
That last pic was sort of what i was thinking in my head n it seemed bulky or messy. I will see if i can put a derailer somewhere good.
Thanks again for the reply.
 
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Build log: I was so close to getting stuck into it today, but then... while looking at what parts and steel tubes i have, i ended up trying to reorganize the whole worshop. I found an interesting vacuum part that i will try to use, but mostly i just made junk piles neater. I'm hopeful for tomorrow.
 
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I look forward to seeing your build unfold! I am now "done" with mine, so I need to tear apart my workshop and clean it. It's got a lot of metal shaving all over the bench and the floor, and no semblance of organization anymore. Part of the problem is that it's quite small. But, that means less to clean up, too! :)
 
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Once you clean/tidy it up, don't go in again
Unintentionally i actually took this advice.

I've moved the project into a better workshop. Was pretty excited to use the notcher, and a jig table..that turned out to be not as life-changingly helpful as expected.
It doesn't seem to offer a hugely better way to get the frame and head tubes lined up straight with all the correct angles. I don't want build a full wood jig. I guess i still just have to eyeball it and tack&adjust.
But this scour pad for paint removal instead of a wirebrush attachment might be a game changer. Love it.
 
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Frustrating day. I was mocking it all up for welding and the lower frame tube is now at least 5cm too short somehow. I'm so confused, i swear it fit yesterday. Anyway, onward, i cut a new peice of pipe, tacked it all and tomorrow I'm going to try to use a laser guide to check its straight. This is my best opportunity to use fancy tools that i would never bother buying, so I'm making an effort to see what I'm missing. Cannot wait to use an expensive welder.
 
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Using a laser level for alignment is pretty great.

Using an expensive welder is great too, but i still should've practiced a little on thin tube before starting. I did some tests on straight flat steel and thought it was too easy.
 
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Frustrating day. I was mocking it all up for welding and the lower frame tube is now at least 5cm too short somehow. I'm so confused, i swear it fit yesterday. Anyway, onward, i cut a new peice of pipe, tacked it all and tomorrow I'm going to try to use a laser guide to check its straight. This is my best opportunity to use fancy tools that i would never bother buying, so I'm making an effort to see what I'm missing. Cannot wait to use an expensive welder.
 
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I made the extended handlebar stem today.

I feel it looks like it might be a bit too close to the rider's chest (more in real life than the photo). I assumed i had to copy the angle of the headtube... which, to be fair, i thought looked a bit too angled when i did that as well. I also coulda/woulda/shoulda mocked it up before welding. Sigh. It might be fine, gotta keep going.
I am wondering if i can bend the handlebar stem a bit or if that would screw up the steering... does anyone know ?
 
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My own head tube and steering tube are not at the same angle, and I can't tell that it makes any difference. As for the angle, just grind out the bottom half of the weld attaching the steerer to the stem, clamp it down to bend it forward a bit, and weld it back up without even having to remove it :)
 
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Handlebars can be at any angle within reason. The more you deviate from the headtube the more tiller effect you'll put into it. ie instead of twisting the bars you'll end up swinging them left and right in an extreme case. You can get a fair way away from the headtube axis before this becomes an issue.
 
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Thanks guys. Okay now I'm actually thinking it's bad without any tiller effect in the steering in the semi recumbent position. So i will change it, i have access to a pipe bender so that might be quicker than halfcutting and rewelding.

As for the chain, it looks like the derailer can't pick up the slack for my planned 20cm of front bb movement.

foto make

So i will have to do the pulley wheels attached to the boom tubes thing.
It looks like it will complicated to make all the parts and have them all avoid everything in that area so I'm not looking foward to it. And i can't find any decent pulley wheels at the hardware.

I have to have the bike done, painted and polished by november 😳💀
 
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If you just lengthen the cage of that derailer by a few inches, you'll get all the pickup you need. Or, just use a second one a little further down. As for getting it done by november, just focus on getting it rideable as soon as you can so you can focus on any bugs that crop up.
 
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If using pulleys you want flat I_I shaped grooves. These are not common. Get nylon U groove pulleys and using a bolt, some penny washers and a drill as a poor mans lathe sand, grind or ream the groove flat. Generally the bigger the pulley the better from the chains viewpoint.
 
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