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Thread: kids bike 20"

  1. #1
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    kids bike 20"

    Here we go. Figured I might as well go ahead and throw it to the wolves before I get ahead of myself. Criticism greatly appreciated. This will be my first frame and it's for my 4yr old.

    I started with the intention of using a 26" 80mm squishy fork. Obviously the front end would be crazy high, but it looked do able. But . . . I started thinking practicality wise, weight, etc, and gave up on that for now.

    This model is based on 20" wheels, 110mm cranks, a trials fork I found for 80$, and some measurements I took off my boy. I really have no idea what to use for tubing size other than a few threads I found. As shown, dt/tt is 1", cs 0.5", ss 0.38", and st 28.6mm. How small can I go on the dt/tt and stays?

    I sized the st length based on his inseam standing on tip toes and the seat post slammed. I'm unsure on bb height and trail. This is at 4" of pedal clearance. Guessed on the trail.

    Tear it up.

    kids bike 20"-20ht_2_side-140211.jpgkids bike 20"-20ht_2_iso-140211.jpg

  2. #2
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    For tube sizes you're probably good with that stuff. The one time I made my own chainstays I used 3/4" but I think 1/2" would be fine for a four year old.

    I've spent a handful of summers coaching kids on mountain bikes. It seems like with the really little ones the biggest struggles were standover and having their handlebars up so high that they couldn't really stand up on the pedals - which is hard enough to get kids to do.

  3. #3
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    Thanks adarn. I'm not trying get crazy light, but don't won't to build a tank. One advantage of the smaller tubing od is that it allows me to use straight tubes for the stays with a 73 bb. The flip side is having to buying multiple sizes of tubing, but I guess that not really a big deal. Maybe 1" for the front and .5" for the rear would be the way to go.

    Here's an image with a stickman. Seat height is set so that he can still just barely touch the ground with his toes. Reach is a little further than I'd like, but it will give him some room to grow.

    kids bike 20"-20ht_2_stickman-140211.jpg

  4. #4
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    I'm also beginning to question if it would even be worth it to bend the tt.

    Are straight cut angle miters versus bends a no-no?

  5. #5
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    Bends just have to kept in phase, so it's one more thing to worry about. But you have to line up the miters with straight tube anyway, so do it if you want.
    What equipment do you access to? Benders, tube rollers?
    Maybe just drop the TT lower on the ST to get some standover height if you decide not to bend.
    Planning brazed construction?
    cheers
    andy walker

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    As you are planning this bike, note that old 90's Tomac era manitou 1/2/3/4 forks are pretty easy to shorten into a 20" kid fork that is significantly lighter than any current available 20" suspension fork and will keep the handlebar height much lower than a 26" fork, I built one for my daughters 20" and it has worked great.
    kids bike 20"-pixie-001.jpg

    kids bike 20"-fall-2013-144.jpg
    Race Tested!

    more conversion info on this 20" fork in thread;
    Novara Pixie 20" project

  7. #7
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    Kids really like to be able to reach feet to the ground while seated in the saddle. For beginning kids on a store-bough frame, I find I usually need to set the saddle height much lower than is ergonomically correct for their leg length. Unless they are riding really rough/aggressive trails, I would definitely error on the side of having a low BB & possible occasional pedal strikes in order to keep the seat height as low as possible. On the 20" storebought frame I used for my daughter, it was actually built with negative BB drop (BB above axle center line) which caused major fit problems.
    I would imagine that 1" tubing for all the front triangle will still be plenty stiff for a kid. Note that you can get decent quality 22.2mm (7/8") BMX seatpost so no need to go all the way up to 28.6 for the seattube. How thin of tubing are you thinking of using? 0.8mm (1/32") strait gauge 4130 is probably plenty strong for a kid bike and easily available, easy to work with.

  8. #8
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    I don't directly own any bending equipement. Probably could gain access to a roller and a draw bender, but not sure on die sizes. If I bend, it would probably be really basic wood forms type process. I'm no newbie to metal fab, just haven't done anything bike specific. In the past, I've always done the use what you have on hand creatively versus the go spend a boat load of cash on special tooling. Frame building is something that I think I will get into, ALOT, but at this point I'm not ready to invest much on tooling.

    afwalker, this will all be tig. I was actually looking through some pictures last night and realized the option of lowering the tt junction with st for stand over. I'm not adverse to trying my hand at simple bending, just trying to keep it simple for this first build.

    GrayJay, planning on stright wall .035 for everything at this point. I have been looking at bmx seat post/seat tubes. Been looking at the vendors posted in the FAQ, but I seem to be overlooking a supplier for bmx seat tube. Would 1"x.058 straight wall be the go to? Nice work on that Manitou.

  9. #9
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    I've got lots of benders if you're near SE Missouri, give me a shout.
    I agree with Adarn, get good standover height however you can.
    4130 is a bear to bend, it's pretty darn tough stuff. But give it a try.
    The harbor freight roller is pretty inexpensive and you can upgrade the dies with ones from swag offroad and have a good tool.
    Search results for: 'tubing roller'
    cheers
    andy walker
    Flickr: afwalker's Photostream
    Walker Bicycle Company | | Walker Bicycle Company

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    Thanks Andy. I'm in southern Indiana. I thought you were located around Louisville? Have looked at the harbor frieght roller and swag dies. Maybe somewhere down the road.

    The draw bender I have easy access to has 3/4" dies. What about a 3/4" tt???

  11. #11
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    Quote Originally Posted by ktm520 View Post
    GrayJay, planning on stright wall .035 for everything at this point. I have been looking at bmx seat post/seat tubes. Been looking at the vendors posted in the FAQ, but I seem to be overlooking a supplier for bmx seat tube. Would 1"x.058 straight wall be the go to? Nice work on that Manitou.
    Propensity for a tube to wrinkle during bending is directly related to the ratio of the tube diameter / wall thickness. A small diameter tube with thick wall will be least likely to wrinkle if you are using wood forms instead of a die bender. Maybe even go with mixte style combine seatstays / TT all the way from the dropouts up to the HT, small diameter ST diameter tubes would be easy to bend with minimal tooling.

    Making the entire ST from 0.058 would be easy for fitting to a 7/8" seatpost but it sounds awfully heavy. Maybe use same 0.035" x 1" tubing as for the TT and DT but then sleeve or shim the ID of the top of the ST down to 22.3mm in order to be able to clamp a 22.2mm seatpost.

    Kids bikes are so heavy and overbuilt in relation to their own weight, I definitively think it pays off to make their bikes as light as possible. Even the fairly light 21 pound MTB I built-up for my daughter would be like me having to ride an 85 pound bike!

  12. #12
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    Bikes typically have the TT as smallest of the 3 main triangle tubes, I would imagine that a 3/4" diameter TT would still be sufficiently stiff and strong for a 50 pound kid, it would probably make the frame look more visually proportionate too.

  13. #13
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    Quote Originally Posted by GrayJay View Post
    Making the entire ST from 0.058 would be easy for fitting to a 7/8" seatpost but it sounds awfully heavy. Maybe use same 0.035" x 1" tubing as for the TT and DT but then sleeve or shim the ID of the top of the ST down to 22.3mm in order to be able to clamp a 22.2mm seatpost.
    Ya, a 1x.058 st would be over 100g heavier than a 28.6 straight wall. A 1x.035 with a .025 shim would work with a 22.2 post. How would you hold the shim in place? Or just let it float? Not like it would have to be very long. Would this eliminate the need to ream the st?

    Not a huge fan of the twin small diameter top tube setup, but I might model it to see how it looks. I do like the frames English made twin tt and used them for internal routing. I would probably just mitre and weld if I can't get the tt bent.

  14. #14
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    Two methods for the Seat Tube shim: machine out of alloy a shim with a shoulder - so it won't slide down into the ST and be lost. Make it about 75mm long. Or, machine the same item out of steel and TIG the rim to the ST.

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

  15. #15
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    I was thinking shim stock??

  16. #16
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    No, not shim stock. Either from a thick walled tube or from solid. Only if you have the equipment or a good machinist friend though.

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

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    My dad used to be a welder in his younger days,
    in desperate times when there was no bender available they filled the tube with wet sand, compressed the sand as hard as possible before bending.
    He told me it worked pretty good for bending tubes without wrinkling.

    You might get lucky ;-)

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    Bond a dis-similar metal sleeve with adhesive like dp420 or hysol.
    If you've got access to a bender, give it a bend and see what it looks like.
    I think 3/4" would look ok on this size bike.
    cheers
    andy

  19. #19
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    Thanks all. I still think shim stock could work for a ghetto seat post shim. Easy enough to test out the theory. The trick would be pre-forming it. Having to machine a shim, not that I can't do it, makes the bmx post less attractive. K.I.S.S. I'd rather just go with a 27.2 post over machining a shim, damn the aesthetics.

    I've had no luck finding a bmx seat tube with a 22.2 id.

  20. #20
    RCP Fabrication
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    I used a 1.125" steerer tube for a seat tube. It was 1.1/1.6 butted. With the 1.6 end up, the tube has a 25.4mm ID. Nearly every BMX post is available in 25.4, and there's tons of 25.4-22.2mm shims available for a few bucks.

    One thing I learned, whats keep the BB as low as possible.

    This picture was before I shortened the cranks 35mm.


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    Just curious if anyone has feeback on appropriate fork trail for a small kids bike. Should the fork trail be same as an adult size bike for similar handling or is there a need to scale the trail measurement portportinal to the wheel size for similar handeling?

  22. #22
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    Quote Originally Posted by RCP FAB View Post
    I used a 1.125" steerer tube for a seat tube. It was 1.1/1.6 butted. With the 1.6 end up, the tube has a 25.4mm ID. Nearly every BMX post is available in 25.4, and there's tons of 25.4-22.2mm shims available for a few bucks.

    One thing I learned, whats keep the BB as low as possible.
    Sounds like a plan. Thanks. I'm working with 110mm crank length and 4" of pedal clearance. This puts my boy just barely being able to touch the ground with the seat slammed on a 260mm st. Does that sound like a good starting point? The 16" Hotrock he is currently riding has about the same pedal clearance.

    Quote Originally Posted by GrayJay View Post
    Just curious if anyone has feeback on appropriate fork trail for a small kids bike. Should the fork trail be same as an adult size bike for similar handling or is there a need to scale the trail measurement portportinal to the wheel size for similar handeling?
    Same here. I haven't been able to find anything on trail numbers for the little guys. I would imagine it needs to be scaled back a little.

  23. #23
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    Hmm, just checked the numbers on a Specialized Hotrock 16, 70 hta, 5mm rake. That's roughly 68mm of trail. Maybe I'm on the low side at 64 and need to be up above 70.

  24. #24
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    Cannondale 20's are quite different, 70 hta, 40mm offset, 50mm trail.

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    my instinct tells me that a smaller wheeled bike is probably fine with less trail.

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