Building Recumbent Trike: bike pedal question

March 25, 2010 10:40 AM

Building recumbent (adult) trike. Is there ANY way to replace the non-driving crank arm with a chain sprocket set or a shaft.

I'm building a recumbent adult trike. I have plans that I bought from Atomic Zombie, but I do not want to weld.

I figured out how to do everything easily, but it all hinges on one thing.

A bike has two pedals, each attached to a crank arm. We're talking only about the crank arm that does not have the chainring (the non-driving arm).

Best would be to replace the non-driving crank arm with a shaft, such that if I rotated this shaft, the shaft would rotate (through the bottom bracket) the chainring, move the chain, and hence the rear wheel. That's the ideal situation.

Second best would be if I were able to replace the non-driving crank with a chainring or single gear that accepts a bike chain.

I'm willing to pay for this bit of welding/machining if that's what it takes.

What I'm trying to do (ultimately here) is to have pedals at the center of the trike (as usual) and have them drive one wheel at the side/rear of the bike. I want to do as little modification to the chopped bikes (I have that worked out.)

So I need the pedal motion moved to the right ( by the shaft) and coupled to the existing chainring.

When (if) done, I will freely publish on my website.

Thanks,
Barry

posted by mbarryf to Sports, Hobbies, & Recreation (11 answers total)

I'm not sure I understand what you're trying to accomplish here. On all bikes, the two crank arms are already mechanically connected to each other through the axle/spindle that goes through the bottom bracket. The "non-driving arm" still drives the crankset. Unless there's something really weird with this bike. If so, just get a normal bottom bracket.
posted by zsazsa at 10:55 AM on March 25, 2010

Can you link to a sketch of what you're wanting to accomplish? I can't follow your description at all.

We're talking only about the crank arm that does not have the chainring (the non-driving arm).

There is no non-driving crank arm. The chainring may only be on one side, but both arms drive it.

Best would be to replace the non-driving crank arm with a shaft, such that if I rotated this shaft, the shaft would rotate (through the bottom bracket) the chainring...

The bottom bracket axle already does this. Do you mean you want to add an extension to the axle?

Second best would be if I were able to replace the non-driving crank with a chainring

So now you want chainrings on both sides of the bottom bracket?
posted by jon1270 at 10:57 AM on March 25, 2010

I need more clarification because it sounds to me like you're thinking that the pedal that isn't on the chainring side isn't doing anything. Each pedal can only drive half a rotation, so you need the both pedals to do one rotation.
posted by bonobothegreat at 11:14 AM on March 25, 2010

I assume this is what you're looking for (more or less):

http://www.jtgraphics.net/images/tandem/full-left-cranks.jpg
http://www.jtgraphics.net/images/tandem/stroker-left-close.jpg

(From here http://www.jtgraphics.net/tandem_bike_project.htm )

I also assume you don't want the cranks at all but you're really just looking for a way to transfer the chain a bit to the side? Sort of like this, though probably not quite a far:

http://mnhpva.org/meetings/Nov_02/Mark.html

In any event, as far as I understand, and with most bottom brackets, you can mount the crank arm on either side. So simple solution is, get two old bikes (with bottom bracket of the same type), take the crank arm with chain ring from each and mount both of those crank arm/chainrings onto either side of one bottom bracket.

Chop off crank arms somehow if you don't want them.
posted by flug at 11:19 AM on March 25, 2010

If I'm understanding this correctly, you're a one-legged rider who wants to be able to add an outside power source on the no-leg side.

The easiest way to do this would be to put a captain's crank (from a tandem bike) on the no-leg side, cut off the crankarm, and connect that to whatever other power source you have in mind via a normal bike chain. Note that you'll still need to pedal, because there's a direct connection between the left crank, right crank, and chainring. There is another product for tandems called an Independent Pedaling System that allows the chainrings to "coast" relative to the pedaling motion, so you could be on powered drive without pedaling.
posted by adamrice at 11:30 AM on March 25, 2010

By the way, one reason the 'tadpole' design (two wheels in front, one in back) is inherently better is because it avoids this whole problem. In addition, two wheels in front/one in back is inherently far more stable.

I don't think the idea of moving the chain over laterally using a simple bottom bracket is going to move it nearly as far over as you need.

Take a look at how far this trike moves the chain over--you'll have to do pretty much the same:

http://mnhpva.org/meetings/Nov_02/Mark.html

You can see some examples of how the Sun EZ-3 trikes solve this problem here, starting on page 24:

http://www.bicycleman.com/file-downloads/Sun-Recumbent-Owners-Manual-08.pdf
posted by flug at 11:41 AM on March 25, 2010

Jackshaft, or tandem captain's crank as noted above.
posted by fixedgear at 11:45 AM on March 25, 2010

Response by poster: I have two legs, and I DO plan to cut the other crank off. I only want the chaining.

What I want to do is to pedal the right rear wheel with my feet in front and to the left (by about 12 inches). That's where my feet would be in a delta recumbent trike.

I want the back two wheels of my (delta) trike to be made up of two rear triangles (with only one driven) of chopped up bikes. Thus once I mount the rear triangles, I will have the full chainset and gears all assembled. The task is only to get my pedals to drive the right bottom bracket assembly.

Perhaps instead of saying "driving crank or non driving crank" I should have said crank with the chainring on it and the other crank.

I want to replace that "other" crank with a shaft. I want to extend the axle in the bottom bracket to do the job of the shaft as shown in Mark.html second picture.

In Mark's second pic, he shows my second best solution. There, I have the pedals and their chainring, driving a chain back to a gear (sprocket) that I put on the end of the shaft. The other end of the shaft has another sprocket that runs a chain back to the existing bottom bracket of the existing rear triangle.

So the question becomes: given a bike, can I take off the crank arm that's on the side OPPOSITE the chainring, and replace that crank arm with a shaft? If not, can I replace that crank arm with a (single) chainring?




Don't give up on me...I'll get some diagrams together.
posted by mbarryf at 12:17 PM on March 25, 2010

Response by poster: Sorry, you answered the part about putting on an additional gearing...it looks like a captain's crank could be used for that. But the shaft extension (to the left) of the existing bottom bracket is mucho preferred.
posted by mbarryf at 12:18 PM on March 25, 2010

Best answer: 1. Join BikeForums.
2. Go to the Bike Mechanics, Alt Bikes, and Recumbent Bikes subforums and search for answers to your question.
3. If no answers, start a thread in one of the forums and see what kind of advice you get. Lots of people there with lots of bike smarts.
posted by Doohickie at 12:35 PM on March 25, 2010

So you're definitely talking about a jackshaft to move the drive outboard. Using conventional bike parts won't buy you the 12" offset that you need. It would make more sense to fabricate a new jackshaft with some fixed 15-tooth sprockets on each end that's affixed to the frame, well, I suppose there would be a few ways to do it. Or you could just punt and buy this conversion kit.
posted by adamrice at 12:43 PM on March 25, 2010