A step-thru china girl. Almost.

Discussion in 'Motorized Bicycle Welding, Fabrication and Paintin' started by bluegoatwoods, Apr 30, 2014.

  1. bluegoatwoods

    bluegoatwoods New Member

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    Here's a project I've been working on over the winter. It's come along enough that you'll be able to see where I'm going with it.

    I took an old girl's frame mountain bike and cut out the upper down tube.
    (Top tube?)

    This weakened the frame, of course, to the point that I wouldn't even dare to ride it.

    But maybe with some reinforcement.......
    frame mods 01.jpg
    frame mods 02.jpg
    frame mods 03.jpg

    I'm not able to estimate whether or not this is strong enough for the job. My gut feeling is that it is. When I get the bike together the way I want it, then I'll just test ride it carefully for quite some time and keep an eye out for looseness.

    If all goes well, then I intend to build something that will be reminiscent of one of these;
    other__champion_moped_m45s_saxonia_1954_1_lgw.jpg

    Though I can already see that the gas tank up by the stem is not likely to work. There's very little room. It might have to go over the rear rack.
     
  2. maniac57

    maniac57 Old, Fat, and still faster than you

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    I'd be VERY careful....looks sketchy to me.
    The clamps you have holding the reinforcement look like they will slide when exposed to weight and stress. Maybe tack weld them to the frame to make sure it holds?
     
  3. bluegoatwoods

    bluegoatwoods New Member

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    Good point. And I've been thinking about welding it all together once I'm sure of it. It wouldn't even be all that hard.

    But I don't think my clamps will move. They're holding on tight. I custom made each one to it's particular spot on the frame.

    I'm a bit more concerned with vibration loosening the bolts. Which is why I'll likely weld everything together once I'm sure it has the mechanical strength needed. Once I'm certain that I won't snap off the headset, for instance.

    I'm not even gonna bother painting it or making it pretty in other ways until then.

    But I've got patience and I'll test ride it for quite some time before I decide to go for it.
     
  4. greaser_monkey_87

    greaser_monkey_87 New Member

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    Nylon locking nuts will prevent loosening due to vibration. But imho, I would not reinforce a frame with clamps. Even very tight clamps can slip with enough load. I would weld gussets onto the frame to reinforce it. And if those clamps are not made out of very hard steel, they can also bend very easily, which will eventually cause them to loosen. Weld some gussets on it and git-r-done.
     
  5. 2door

    2door Moderator
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    Blue,
    Maybe I'm mising something in the photos but if not I'm a little concerned about this design. Your reinforcements don't address the problem that all of the weight/stress is concentrated on the down tube to bottom bracket joint.

    View the bike from the side then imagine a downward force being applied to the center of the frame. It becomes apparent that the apex of that force will be that weld where the down tube meets the bottom bracket.
    Yes, the engine spans that apex but I'm not comfortable with using the engine as a structural member.

    Just offering a little advice, not putting you down, sir.

    Tom
     
  6. bluegoatwoods

    bluegoatwoods New Member

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    I understand, Tom, and no offense taken. As a matter of fact, this is exactly the sort of feedback I want.

    After all, two heads are better than mine. :)

    My photos are not really properly lit. I'll have to try again.

    If you take another look you should see that there is a brace from the lower end of the downtube to the seat tube. It's under the engine.

    What's more, the total of six clamps (1 seat tube, 3 downtube + 2 headset plus connecting struts) all form one assembly. They're all reinforcing each other.

    Still, you've pointed out something that I might not have given proper attention. That spot on the seat tube where my clamp is located, under the engine, might withstand some shocks that are concentrated just a bit too much for that one little stretch of tube.

    I'll have to give that some thought. And if (not certain yet) I decide to test drive this thing, I'll make it slow and close to home and I'll test it for quite some time.
     
  7. 2door

    2door Moderator
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    Blue,
    As a matter of fact I did see the bracket you mention, on the seat tube but I feel it is too low to add any real support when you think about road shock, rider weight, etc. being applied to the frame and the point that will need to withstand that force. Even with that bracket the downward force, or maybe I should say, the rearward presure of the head tube pushing back will impose a lot of force against the down tube/bottom bracket weld joint. In the original configuration, the seconday down tube you removed was there to take some of that force. With it no longer there everything is concentrated on the lower end of the down tube.

    Whatever, please be careful and keep a close watch on things. Good luck, sir and let us know how things go.
    It would be cool to duplicate that step-through bike you pictured. Neat idea.

    Tom
     
  8. bluegoatwoods

    bluegoatwoods New Member

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    Yup. I'm having some second thoughts about that lowest brace, too.

    Though the brace is there, I can see that it's the weakest point in this rig.

    It would be a simple matter to include another identical strut on the other side. I'll have to think about whether or not I'd feel confident with that. Or I might have to rebuild that brace with something heavier.

    Plus maybe a couple of struts from higher up, widening out around the engine, and down to the down tube.

    I'm gonna have to be careful with this. If I ever ride it, I'll be skating on thin ice.
     
  9. wheelbender6

    wheelbender6 Active Member

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    I've seen some cool builds with step thru frames. I don't think you need to cut out sections of frame tubing to make a nice step thru.
    I had considered a mixte frame for an ebike build, because the frame provides a stable, easily accessible mounting place for batteries.
     
  10. mapbike

    mapbike Active Member

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    I sat here and looked several times at what the erector set looking bracing is actually doing for you there BGW, I'm NOT knocking what you have done at all since I know it took time, work and planning to get done, but I would like to give my opinion on this set up also if its ok with you and if not i'll delete this post as soon as you tell me its not welcome.

    Looking at that frame what I think will work great and be plenty strong enough that would eliminate all the bolt together stuff if you ever wanted to change it is some simple gusseting on both ends of that down tube and then I would weld a piece of flat strap steel the full length of that down tube the full length of the tube on the under side, that would give that down tube serious strength and the gussets will make it bullet proof strong at the weld points on each end of that down tube.

    No doubt in my mind you will have a nice build when you get done, but as far as safety is concerned I can honestly tell you that welded gussets and a good back bone of flat steel on its edge running down the bottom side of that down tube, will be way stronger than the bolted together pieces yo u have on it now, but if you dont have a way to weld things you have to just do the best you can with what you have, been there done that many times myself.

    best wishes BGW..

    Map
    dnut
     
  11. bluegoatwoods

    bluegoatwoods New Member

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    No, no, no Mapbike. Your comments are absolutely welcome. This is exactly the kind of feedback I need.

    On the other hand, I've already given up on this design. Looking at it more I'm convinced that the junction of downtube and seat tube, the bottom bracket in other words, is not strong enough. I think it was 2Door who first pointed that out. And I'm glad he did. It might have taken me too long --think broken frame-- for it to occur to me.

    I do have another idea to address this. It'll have to wait while I build another bike. But I'll get back to it and update this thread.
     
  12. mapbike

    mapbike Active Member

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    Ill be looking forward to waching it all come together, im sure you will have a good solid build when you get done and thabk you for your understanding with us kinda critiquing what you have there so far, I know from my own experiences that sometime it hard to see the forest for the trees and then someone comes along and looks at what Im doing and says, Hey what about this or that and I just think dang why didnt I see that or think of that..... we can all learn something from each other if we're open and willing, best wishes for the build BGW.

    MAP
    .wee.
     
  13. 2door

    2door Moderator
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    You know, somewhere here is a picture of a motorized girl's step-through bike that was very well done. If I recall the builder had rack mounted the fuel tank and the engine was mounted on the secondary down tube. It still offered the advantages of the step-through design even with the engine taking up some of the open area. If I find it I'll post a link here. It might offer some ideas.

    Tom
     
  14. mapbike

    mapbike Active Member

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    Tom didnt Silver bear do a step through girls bike one time?

    Maybe im way off in left field but for some reason that is who is coming to mind.


    Edit: silerbear commented on this one but camlifter buit it....

    Link in next post.

    Map
     
    #14 mapbike, May 13, 2014
    Last edited: May 13, 2014
  15. mapbike

    mapbike Active Member

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  16. mapbike

    mapbike Active Member

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  17. turnofftheradio

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    silverbear's step throughs have been inspirational to me, as I'm sure they will be for bluegoatwoods, and are a great example of motorized step throughs. I'd like to see more builds with them. I'm planning my own some day. Keep us posted, there is no better motivation than failure. I'm sure what you come up with next will be better.
     
  18. 2door

    2door Moderator
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    Darn, Shan, you're right. It was Silverbear's bike. And that's one of the photos I remembered. There was also another one, similar but it seems the bike was maroon and gold (if my memory cells haven't died off too bad).
    Thanks for finding that. I hope Blue can use it.

    Tom
     
  19. mapbike

    mapbike Active Member

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    Just happy to help Tom.... surprised I could find anything since I was at work on my "smart" phone...LOL!

    Map
     
  20. bluegoatwoods

    bluegoatwoods New Member

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    I gotta admit, those were some good looking bikes.

    I might or might not try to do it that way. The only thing that could stop me is that I'm trying to work out something that my wife could ride. She's only 5 ft tall and needs something pretty low.

    Still, not modifying the frame has obvious benefits and I'd better not dismiss it out of hand.
     

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