Wooden cargo bike issues- Mtbr.com
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  1. #1
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    Wooden cargo bike issues

    I have been spending the Spring building a wooden cargo bike. Here is my post from FB to give some background:

    "The cargo bike is "done", the steerer linkage is beefed up and the first ride is in the books. The good news is that it looks nice, the bad news is that it is lacking in torsional rigidity in the frame and will require a major rework to get it sufficiently rigid. I am waiting to hear back from an engineer who I met online on what his thoughts are. My current thinking is to fabricate a steel exo-frame that connects the front head tube to the steerer tube, and then continues to the seat tube. That should in theory tie it all together and stiffen things up. This will likely require ditching the box in lieu of a lighter solution. This is all likely a Winter project, so I may buy a Harry vs. Larry Bullit frame in the interim and switch the components over to it.

    This might seem like a big disappointment or setback for some, but for me it was always about the journey and not the destination. I knew starting out that my original concept might be unworkable, and lacking the ability to meaningfully model the design, I built it. I may still be able to create a workable cargo bike out of the project, but it is safe to say that the original concept can now be classified as a protracted fun learning experience (also known as a failure in some circles). I'll post updates as they occur, thanks for the encouragement along the way!"

    I am wondering what the collective genius on this forum would suggest?

    Thanks,
    Wardo





  2. #2
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    I should add that the box is a stressed member of the frame.

    Wardo

  3. #3
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    I can't offer you any advice but I will say, that's some lovely woodworking.
    Quote Originally Posted by Oh My Sack! View Post
    Remember, there's always quilting and knitting if pedalling becomes too tough.

  4. #4
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    How much is it flexing and where is it coming from? I've only ridden a couple bikes of that style and they definitely felt flexy as well. I wouldn't say that just because it's moving around a bit means that it's not fit to ride.

    One suggestion, perhaps adding a rib from the top of the head tube to the steering tube. Perhaps you could hinge one end and put a bolt on the opposite end so you could lift it up in case you needed to load the box with something large. It wouldn't look great and would reduce the practicality of the bike but it's better than scrapping it and starting over.

  5. #5
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    Can you isolate where and how much flex occurs?

    As an experiment, what if you were to run diagonal cables with turnbuckles from the corners of the box to stiffen up the box portion?

  6. #6
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    A lip or halo added to the top of the box would offer some additional stiffness. Wouldn't be too intrusive into the cargo space either.
    "These things are very fancy commuter bikes or really bad dirt bikes, but they are not mountain bikes." - J. Mac

  7. #7
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    Quote Originally Posted by life behind bars View Post
    A lip or halo added to the top of the box would offer some additional stiffness. Wouldn't be too intrusive into the cargo space either.
    Two guys can't torsionally flex the box, but I think the torsional stress of riding is all rotating around the 3x3 wooden beam, the points of attachment aren't offering enough of a moment to resist it.

    Ward

  8. #8
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    Quote Originally Posted by customfab View Post
    How much is it flexing and where is it coming from? I've only ridden a couple bikes of that style and they definitely felt flexy as well. I wouldn't say that just because it's moving around a bit means that it's not fit to ride.

    One suggestion, perhaps adding a rib from the top of the head tube to the steering tube. Perhaps you could hinge one end and put a bolt on the opposite end so you could lift it up in case you needed to load the box with something large. It wouldn't look great and would reduce the practicality of the bike but it's better than scrapping it and starting over.
    The flex is excessive in that it feels "snappy". It loads up torsionally and then releases in an unpredictable fashion. It is flexing primarily between the head tube and steerer, despite the box being VERY rigid and incorporated into the structure. I originally was going to do bent wood elements connecting the steerer and head tube in lieu of the box. I am thinking of returning to that route and the fabricating the basket out of sail cloth and a marine plywood base. I'd put a top tube between the steerer in seat tubes as well since there is some flex there as well.

    Ward

  9. #9
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    Quote Originally Posted by chazpat View Post
    I can't offer you any advice but I will say, that's some lovely woodworking.
    Thanks, a friend suggested I sell it as an art installation. I'd rather figure out how to make it rideable. Failing that, it gets mighty cold here in Vermont and I heat with wood....

    Ward

  10. #10
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    Quote Originally Posted by dr.welby View Post
    Can you isolate where and how much flex occurs?

    As an experiment, what if you were to run diagonal cables with turnbuckles from the corners of the box to stiffen up the box portion?
    Quantifying the flex is difficult. I have flexed various frames of various bikes in my shop and they all have flex to some degree. Using the same techniques, the bakfiet doesn't seem to be excessive, operationally is another matter. I like your idea of trying to isolate it. My approah might be to remove the box and fabricate temporary reinforcements in various areas and test the results of those potential solutions.

    Ward

  11. #11
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    Quote Originally Posted by life behind bars View Post
    A lip or halo added to the top of the box would offer some additional stiffness. Wouldn't be too intrusive into the cargo space either.
    Definitely worth considering.
    Ward

  12. #12
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    What about tieing the steer/head tube/main beam into the box more substantially with some gussets? It seems like you’ve got the strength there, just need to tie it in more effectively.

  13. #13
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    Quote Originally Posted by Equinox_Bikes View Post
    What about tieing the steer/head tube/main beam into the box more substantially with some gussets? It seems like you’ve got the strength there, just need to tie it in more effectively.
    Agree. Does this bike need to be a step through for the rider? I have always believed that bikes like this need to be linked fully from the fork head tube to the seat tube. Relying totally on the lower member to retain all movement is a big ask. I wonder if a bi-lam metal/wood plate being in the center of the lower member with wood on each side would give you the desired result.

    Eric
    If I don't make an attempt, how will I know if it will work?

  14. #14
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    Quote Originally Posted by Eric Malcolm View Post
    Agree. Does this bike need to be a step through for the rider? I have always believed that bikes like this need to be linked fully from the fork head tube to the seat tube. Relying totally on the lower member to retain all movement is a big ask. I wonder if a bi-lam metal/wood plate being in the center of the lower member with wood on each side would give you the desired result.

    Eric
    Eric and EQX,
    I think that a combination of your suggestions might be the best route. I am definitely going to add a top tube. My wife really wanted a step through, but the torsional qualities of wood don't lend itself to this. I knew this going in, but wanted to try the step through for her. I also originally wanted to make the box removable. Your comments got me thinking that if I added gussets to the head tube from the box, and integrated the box seat tube retrofit, that would really firm things up. I can do all this using wood, so I can keep the aesthetics consistent. I will likely make some laminated wood with carbon fiber cloth sandwiched between layers. Thanks for helping me clarify my thinking!

    Wardo

  15. #15
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    New thought, I sat with the bike and considered my options. I decided to approach the issue incrementally, addressing stiffening the front first and then testing things out. I welded up some brackets this morning that are reasonably light and should tie the box and head tube together very well. I need to get a few scraps of 4" angle for the steerer tube gusset.

    Wardo

  16. #16
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    Stiffen the box by adding knees to all 4 top corners

    Stiffen the box to seattube area, you now have 3 leafsprings. I'd start with boxing the area with two sideplates, if that works you can figure out how to achieve the same effect without spoiling the esthetics.

    Wrapping the wood with glass is hard to see when done properly

  17. #17
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    I think you're on the right path here. You have a ton of potential support in that box, and if you tie it to the head tube well, you might see a big improvement. Keep in mind the box needs to be pretty well attached to the bottom beam to fully take advantage of the support it offers to the head tube. I also really like the idea of triangular "webs" or something similar at the top corners of the box, that would stiffen it up quite a bit.

    Another thing to potentially look at is I'm wondering how much flex is coming from that fork. I don't know what percentage of it is wood, but you may find an improvement by having a steel fork made to replace it. I'm thinking that if there's flex in the fork, it will "wind up" like you've described and when it releases you would really feel it in the steering, which wouldn't be ideal.
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  18. #18
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    Quote Originally Posted by Erichimedes View Post
    I think you're on the right path here. You have a ton of potential support in that box, and if you tie it to the head tube well, you might see a big improvement. Keep in mind the box needs to be pretty well attached to the bottom beam to fully take advantage of the support it offers to the head tube. I also really like the idea of triangular "webs" or something similar at the top corners of the box, that would stiffen it up quite a bit.

    Another thing to potentially look at is I'm wondering how much flex is coming from that fork. I don't know what percentage of it is wood, but you may find an improvement by having a steel fork made to replace it. I'm thinking that if there's flex in the fork, it will "wind up" like you've described and when it releases you would really feel it in the steering, which wouldn't be ideal.
    Your suggestions definitely anticipate my current direction. Corner gussets in the box are on the table, but I am going to first see what happens when I tie the head tube and box together with welded up brackets.

    The fork is definitely a possible concern, but is ridiculously stiff in static tests. A frame builder friend played around with it and said it was more than fine. My major concern with it is durability, wooden forks are notoriously unreliable. I built this one as an experiment but will likely have my friend build me something burly to replace it.

    Thanks,
    Wardo

  19. #19
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    Maybe I missed this in the posts but does it flex equally while loaded and unloaded?

    Gussets are a great plan to stop some of that flex. Instead of building a steel exo-skeleton you could always lay up some carbon layers between light wooden support. If you adjust the layup in the right orientation you can help deal with the flex you're seeing.

  20. #20
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    Quote Originally Posted by jgrano View Post
    Maybe I missed this in the posts but does it flex equally while loaded and unloaded?

    Gussets are a great plan to stop some of that flex. Instead of building a steel exo-skeleton you could always lay up some carbon layers between light wooden support. If you adjust the layup in the right orientation you can help deal with the flex you're seeing.
    I haven't loaded it up yet, it is scary enough as is unloaded, but I can see the sense in at least testing some weight. I have integrated carbon fabric into the frame and fork. It definitely helps, I also use Kevlar cloth to help prevent catastrophic failure. The gussets are in process, and the metal brackets are installed (regrettably with no discernible difference at this point).

    Wardo

  21. #21
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    Hey!
    Why don't you mold external gussets from CF and Kevlar lamination (on key areas), by just wrapping the material over all that lovely woodwork. The epoxy resin will bond wonderfully on your wooden frame and if you design it carefully it will add to the overall good looking appearance...
    Just a suggestion

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