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  1. #1
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    Modifying Tandem frame - conversion to cargo

    Hello all-

    This is my first post on BikeForums, but have been getting great info for a long time now. So much so that I have yet to create a thread.

    Anyways, my question regards making a tandem into a cargo bike. I've inherited a Schwinn Twinn Deluxe, and me and my wife do not enjoy tandem riding very much, but thats another story. So I want to fix it up and make it into a gear hauler for picnics/camping/etc. My question is, can I remove the rear seat post and seat stays to make more room for a good cargo platform? Will the frame remain safe? I have very minimal welding skill, but am able to patch up gaps left from removing tubes. I also have a cheap powdercoat guy, and will have the paint redone after modifications are made.

    I've photoshopped a picture of someone else's twinn to illustrate my idea:

    twinn2.jpg

    Thanks!

    Dan

  2. #2
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    I'm not an expert on this, but it seems like it should work. The parts you're removing are mainly serving the purpose of supporting the weight of the stoker, and you're not removing anything that supports the captain's weight. I considered a similar modification for purposes of fitting kids on the back of a Raleigh tandem we have, but it turned out not to be necessary because the kids are growing so fast. If you try it, definitely post the results here.

  3. #3
    rebmeM roineS JanMM's Avatar
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    Don't know what the market for a clean Deluxe Twin is, but have you considered selling it and getting a trailer? Oh, that's not yours.......is yours in good shape?
    RANS V3 (steel), RANS V-Rex, RANS Screamer

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    Looks workable to me but I would consider adding a horizontal "top tube" bar between captain's seatpost and head. As is that area is a parallelogram instead of a triangle and would thus be more susceptible to bending.

    BTW the modified frame is much like the Co-Motion Periscope if you want some validation of the concept.

  5. #5
    Senior Member Mauriceloridans's Avatar
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    What is your vision for the cargo carrying structure? Before I would make irreversible changes, I'd look at how the seat post and it's stays might be needed to support a rack (perhaps made out of a crutch) that also ties in with the stoker stem. This rack could support lower units like a big dummy or extra cycle. You would have to clear the cranks and remove the pedals down there.

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    Quote Originally Posted by qspencer View Post
    I'm not an expert on this, but it seems like it should work. The parts you're removing are mainly serving the purpose of supporting the weight of the stoker, and you're not removing anything that supports the captain's weight.
    this was my assumption, too.

    Quote Originally Posted by JanMM View Post
    Don't know what the market for a clean Deluxe Twin is, but have you considered selling it and getting a trailer? Oh, that's not yours.......is yours in good shape?
    They are posted for about $300 on craigslist search in the area in similar condition, but mine will need a tune-up to get to that point. Its from a family member, though, so I'd rather fix and use it then sell it and buy something new.

    Quote Originally Posted by vrooom3440 View Post
    Looks workable to me but I would consider adding a horizontal "top tube" bar between captain's seatpost and head. As is that area is a parallelogram instead of a triangle and would thus be more susceptible to bending.

    BTW the modified frame is much like the Co-Motion Periscope if you want some validation of the concept.
    thanks for the info on the Periscope. that design seems oriented toward a child in the rear, making me think it definitely cant support as much weight as a full adult. don't think i want to have to add tubes, either, but i think youre right that it would probably help.

    Quote Originally Posted by Mauriceloridans View Post
    What is your vision for the cargo carrying structure? Before I would make irreversible changes, I'd look at how the seat post and it's stays might be needed to support a rack (perhaps made out of a crutch) that also ties in with the stoker stem. This rack could support lower units like a big dummy or extra cycle. You would have to clear the cranks and remove the pedals down there.
    i envisioned cutting the seat tube down but leaving an inch or so to drill holes to bolt the main support of the rack there. then, a bracket in front connected to the captains seat post, and some struts to the rear axle.

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    Quote Originally Posted by dmak_ View Post
    ...
    thanks for the info on the Periscope. that design seems oriented toward a child in the rear, making me think it definitely cant support as much weight as a full adult. don't think i want to have to add tubes, either, but i think youre right that it would probably help.
    ...
    Not true at all. The Periscope design is about the maximum flexibility. The top tube is dropped much like your photoshop picture so that it runs in a straight line from head to rear axle. This was done on the Periscope to allow the very low seat position needed for young/short riders. It was combined with a double section telescoping seat post (which is where the name comes from) to allow for normal height adult riders too.

    There may be a small reduction in rear stiffness with the lowered seat stays. In theory as the seat stay gets closer to the chain stay you lose the triangle affect and are left with just the bending strength of the chain stay. But this is at the extreme limit of lowering and with the angles you have shown I would not worry about it so long as the loads are backed up well from the seat stays up into the forward frame.

  8. #8
    coprolite fietsbob's Avatar
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    you dont even need the rear crank at all , just take it out. drive it all with 1 long chain..

    and One Can probably make a way to drape some oversized panniers without sawing off the rear seat post.

    Using the seat tube to support a extended beam from the back of the front seat post over the rear wheel .

    getting it all moving and stopped remains to be sorted out.

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    Quote Originally Posted by vrooom3440 View Post
    Not true at all. The Periscope design is about the maximum flexibility. The top tube is dropped much like your photoshop picture so that it runs in a straight line from head to rear axle. This was done on the Periscope to allow the very low seat position needed for young/short riders. It was combined with a double section telescoping seat post (which is where the name comes from) to allow for normal height adult riders too.

    There may be a small reduction in rear stiffness with the lowered seat stays. In theory as the seat stay gets closer to the chain stay you lose the triangle affect and are left with just the bending strength of the chain stay. But this is at the extreme limit of lowering and with the angles you have shown I would not worry about it so long as the loads are backed up well from the seat stays up into the forward frame.
    thanks thats some good info, thanks

  10. #10
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    Quote Originally Posted by fietsbob View Post
    you dont even need the rear crank at all , just take it out. drive it all with 1 long chain..

    and One Can probably make a way to drape some oversized panniers without sawing off the rear seat post.

    Using the seat tube to support a extended beam from the back of the front seat post over the rear wheel .

    getting it all moving and stopped remains to be sorted out.
    trying to be miserly, so will probably just chop the crank arms off.
    the extended beam idea sounds, good but with the tube sawed off, i can get it lower.

    i will be integrating a mount for a rear caliper in some fashion, but thats an easy project. getting it going? i'd better start working out more!

  11. #11
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    Interesting idea. I have thought of a cargo tandem conversion before, too. I have a Trek Transport and a Burley Nomad cargo trailer, so I haven't felt the need to pursue it yet, though, and I don't have the heart to chop up my nice old Burley RockNRoll tandem. But yeah, this thought has occurred to me before, basically as a way to make a poor man's Big Dummy of sorts.

    Can't wait to see what you come up with. If you Google a phrase like "tandem bike cargo conversion" or something like that, and click IMAGES tab, you'll see that some other people have played around with this concept, too. Might give you some mental fodder.

    One thing that occurred to me is that if you got a tandem with a frame size where the stoker position was as small as possible, might make it easier to keep the center of gravity lower with your build. The lower the stock frame was in the rear, the easier it would be to keep your cargo area COG low.

    One idea I saw, a guy used the stoker seat tube. He welded a platform to a seat post and inserted his new cargo platform in the tube. Problem with this idea is it keeps that load awful high up. In my experience hauling a fair bit of crap by bike, you want the load as low as you can get it.

    Alternately, you could just remove the rear seat, bars, and crank arms and load it like a pack animal with bags on either side of the frame.

    Basically a bicycle version of this. lol


    AlphaDog, U.S. Marines Robot Pack Animal - Legged Squad Support System | AiirSource - YouTube
    Last edited by syncro87; 06-11-14 at 01:51 PM.

  12. #12
    Senior Member zonatandem's Avatar
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    Saw one a few years ago that installed solar panels in the stoker area to provide electric power . . .
    However, no sun/no power!

  13. #13
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    The tubes being removed are all pretty close to the rear wheel so I don't see much space reclaimed by removing them. How do you plan on mounting your payload? I see the tubes as a means of keeping your cargo away from the rear wheel.

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