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Recumbent cycle: "blue sky" design query Answered

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I am not one of nature's cyclists. I didn't actually learn properly until I was in my twenties. My bike is very old, very rusty and very, very heavy.

However, I feel that I "ought" to cycle more, for shopping trips and the like. I quite like the idea of switching to a recumbent, but they are expensive (current available funds: zero).

So, I'm playing with the idea of rebuilding my mountain bike into a recumbent (and learning how to weld on the fly?).

In the sketch below;
  • The black section is the back of the old bike, turned upside down.
  • The blue section is the front of the donor bike.
  • The red part would need fabricating somehow.

I haven't thought properly about steering mechanisms, but probably under the seat, via cables to arms welded to the forks of the will-be-the-back wheel.

Would the bike, as I have sketched it, in principle, work?

I'm bothered about stability, especially at take-off and landing. Am I worrying unnecessarily? I have access to a second donor cycle. Is there any mileage thinking about "doubling up" the blue section, to make a trike? Not massively wide, but I could add luggage space, or an extra rear-facing chair-style seat.

Brakes: before I cut up the bikes, am I right in thinking that I just remove the cables and levers from the handle-bars and add them to the under-seat steering?

Gears: the donor bike is a mountain bike, but I never used all the gears anyway. This area is pretty flat - can I get away without any gears?

Coasting: although the back of the donor bike ends up upside-down, it is still turning the same way, so I can still coast OK.

I'm sure I've missed something obvious...

Anyway, even though I've not decided to do this for certain, I'd appreciate any comments or thoughts.

14 Replies

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bumpus (author)2008-08-02

Interesting concept Kiteman, If you do plan to make it like in your second picture, add another support bar like this (see attached photo) and never wear shorts when riding a recumbent bicycle :D

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DrWeird117 (author)bumpus2008-08-22

Oh my god, I NEVER do anymore.
although encourage women to wear skirts....

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Kiteman (author)bumpus2008-08-02

Ah, that is what I was missing - strength.

I'm planning to make the joint at the front of the seat stronger by using the shaft of the current saddle inside the joint, like an internal ferrule. Sound feasible? (Because maybe then I could make the joint by drilling the tube and putting bolts through, so no welding...?)

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DrWeird117 (author)Kiteman2008-08-22

Yeah, I'm gonna be getting one of those KMX tadpole designs.

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bumpus (author)Kiteman2008-08-02

That would be pretty cool, maybe a few "creative cuts" to the tubes.. I say go for it!

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Kiteman (author)bumpus2008-08-05

I've just realised - if I turn the back wheel+frame over, I will probably have to weld, since the slot the wheel is in will end up as a U - open at the top.

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bumpus (author)Kiteman2008-08-05

Hmm, yeah I think you would have to weld it...

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Kiteman (author)bumpus2008-08-06

Which means I'll have get hold of something to weld it with, and learn how to weld it...

I think this project just moved a notch or two down the list. It's still on the list, though.

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mdj817 (author)Kiteman2008-08-18

I've recently learned how to weld as well, however, from the little I do know, I believe aluminum is fairly difficult. You are probably going to need to TIG (Tungsten Inert Gas) weld it, which is regarded as one of the more difficult types to learn. Definitely start on steel, and no sheet metal until you're ready. When you're welding, make sure to wear a long sleeve shirt, because the arc can be so bright that it can sunburn you (and if you're a pale-skinned geek like me, that's a real possibility!). Also, try to get your hands on one of the new digital masks that is only slightly dark, but goes dark when you initiate the arc. TIG welders are expensive, so its probably out of the question for you to buy one. For nice aluminum welding, you will probably need one of the higher quality TIG welders, so try to get your hands on the ones with more features (that cost around $6000, or about 3210 pounds.).

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Kiteman (author)mdj8172008-08-18

Fingers crossed, I won't have to weld, but, if I do, it will definitely be steel - the donor bike is nearly two decades old - built of steel and scaled to survive a crash with a tank. If I need a welder, though, there's a tool-hire centre round the corner, so I won't have to buy one.

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whatsisface (author)Kiteman2008-08-02

I'd love to see a no-weld 'bent.

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PKM (author)2008-08-09

AFAICT, the most important thing will be to sort out the angle of the head tube so that the steering wheel has positive trail (contact point is behind the point where the steering axis hits the ground). Rear wheel steering is usually frowned upon for stability reasons, but I've seen BMXers ride their bikes backwards so it can be done, albeit over short distances. If the seat back isn't load bearing, the point where the red frame meets the blue frame (below/behind seat) would be a highly stressed point and need heavy reinforcement. The intricacies of steering geometry on regular bikes makes my brain hurt, so this is a nightmare for me- I'd love to see how it works out if you do build it, though. To address your other points, though: Brakes are just as simple as you said. Gears are optional if you have strong legs, but if the back half of the bike frame is being kept intact anyway you should be able to keep the gears (maybe attach the shifters to the frame between your knees like a 10-speed) and coasting will be fine. Thinking about it, you might be able to get away with not welding anything on this frame design, just using a lot of overlaps and bolts.

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gmoon (author)2008-08-05

Moving the seat closer to the rear wheel would convert the rear section of the frame (now a quad. polygon) into a triangle....with greater strength and rigidity. Beats me whether extending the front tube would be necessary at that point for stability...I know next to nothing about recumbents.

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KentsOkay (author)2008-08-05

dang that would be cool... I might try later with some of my spare bikes...

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