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-   -   Cutting down adult lugged frames to kid size? (https://www.bikeforums.net/framebuilders/960594-cutting-down-adult-lugged-frames-kid-size.html)

1987cp 07-21-14 08:33 AM

Cutting down adult lugged frames to kid size?
 
Is this something that people do or have done with, say, a damaged or unwanted adult frame with lugs and straight-gauge tubing? (Assuming a useful and comfortable kid frame could be produced with all or mostly the same angles ... something I haven't tried to find out quite yet ... though it just occurred to me that the BB drop at least might likely be wrong.) Is it likely to be at all worth the trouble of stripping, blasting, and de-brazing an old bike to have the benefit of (maybe) saving some bucks on frame components and (I'm guessing) only having to miter one end of some of the tubes? Or would it be more likely to end up as an expensive and frustrating exercise just to save some old frames from being melted down?

unterhausen 07-21-14 08:37 AM

I'm sure it has been done by hobbyists, but I think that people that have built frames realize that it's a lot better idea to start from scratch. Lots of angles would be way off, for one thing.

Andrew R Stewart 07-21-14 09:29 PM

First thing I think of is the long TT reach, made for adult torsos, wheel sizes and crank arm lengths. Andy.

Smudgemo 07-22-14 02:12 PM

I agree with unterhausen. If you want to build a bike for a kid, buy plain old round straight-gauge from Wicks or Spruce, and a BB shell, head tube and dropouts from Nova. Mitering isn't the hard part (Nova has a free program on their website), but brazing or welding might be. Don't forget crank length is going to be way too long in most cases, but they can be re-drilled shorter.

Dave Kirk 07-23-14 10:17 AM

It of course can be done but in the end it's a poor use of time and energy. Making something new from scratch will be faster and in the end be better...........and safer.


dave

1987cp 07-23-14 11:38 AM


Originally Posted by Smudgemo (Post 16964178)
I agree with unterhausen. If you want to build a bike for a kid, buy plain old round straight-gauge from Wicks or Spruce, and a BB shell, head tube and dropouts from Nova. Mitering isn't the hard part (Nova has a free program on their website), but brazing or welding might be. Don't forget crank length is going to be way too long in most cases, but they can be re-drilled shorter.

Good info there, thanks! I was thinking of mild steel for the main tubes, but I notice the 4130 pages have a much larger selection of diameters and wall thicknesses in diameters such as 1.125". I imagine a decent framebuilding class would provide a starting point for choosing wall thicknesses?

I've already worried a little over crank length, since sourcing cranks and wheels look to be the most weird and annoying aspects of building out a kids' multispeed bike. I see at least one brand offering modern crank arms at least as short as 115mm, but can also imagine it could be cost-effective to cut down an inexpensive crank such as a Shimano Altus.

unterhausen 07-23-14 03:22 PM

Just use .035 wall. You are better off buying head tube material from a bicycle tube supplier

Smudgemo 07-24-14 06:00 AM

$5 head tube (enough for 3-4 bikes w/ 1 1/8" steerer) He also has some suitable straight gauge tubes. Seat tubes will be another issue. Either buy a bicycle tube set for a normal post, or buy a shim to use with whatever tube you buy.
I normally buy the $35-40 175mm crankset and re-drill it to 125mm or so (I can't recall what I did exactly, and the kids are growing so that number will change.) Look for one that allows you to replace the rings (some are riveted) because small kids don't need a a big ring, and you don't want those teeth exposed. Once I ground the teeth off and left the ring in place as a guard of sorts.
I use a milling machine and get the new holes pretty close to square but you could probably use a drill press with some careful setup and drilling. It's just a kid bike - they'll never notice a pedal a tiny bit off from square. Park makes a tap set that is really only intended for thread clean-up, but I've used it for making threads about four times without trouble. I figured by the time the taps went dull, my kids would be on available adult cranks.
Good luck and hook 'em young.
-Ryan

1987cp 07-24-14 10:22 AM


Originally Posted by Smudgemo (Post 16969400)
Seat tubes will be another issue. Either buy a bicycle tube set for a normal post, or buy a shim to use with whatever tube you buy.

Maybe I'm not envisioning something correctly from never having built a frame, but how is a 12" or 15" seat tube that different from a 21" or 25" seat tube? Unless of course it's made of butted tubing and the thin bit ends up smack where the seat lug or binder clamp should go ....

It had crossed my mind to wonder how a builder chooses what size seat post to use, but since posts can be had in so many different diameters I figured that wasn't a concern until planning is much further underway?

Smudgemo 07-24-14 10:34 AM

It's not length of the tube. The inner diameter of your seat tube either needs to be just a bit larger than your chosen seat post, or somewhat larger if you use a shim. Could be a bit of a soda can, could be a variety of production shims from Problem Solvers. I buy cheap 27.2 seat posts for the kid bikes and cut them shorter. I think I used an externally butted bmx seat tube for one frame and a non-butted tube sized for 27.2 posts for the others. Wicks and Aircraft Spruce list inner diameters, and you can find shim sizes online. It's worth looking before buying. I had one very large seat tube for a tandem I made that shims weren't available (that I could find), but I have a lathe and had some suitable scrap to make a shim.

fietsbob 07-24-14 11:22 AM


Is it likely to be at all worth the trouble of stripping, blasting, and de-brazing an old bike to have the benefit of (maybe) saving some bucks on frame components

Bending tubes and bending the lugs for a different angle joint

is usually done before they are assembled into a frame .. sounds like a lot of work for the results .
but it's your time.. & $


Or would it be more likely to end up as an expensive and frustrating exercise just to save some old frames from being melted down?
I'd guess This may be your outcome. YMMV

GrayJay 07-24-14 12:22 PM

Good thread on kids bike construction, discussion of tube sizes and sources;
kids bike 20"

1987cp 07-24-14 09:42 PM

1 Attachment(s)
I see, so the worry about a seat tube would be related to a tendency to use a small-diameter seat post. Cutting down a post of a common size, say 25.4mm, 26.4mm, 27.2mm, whatever, makes fine sense to me.

FWIW, here's an idea of a bike I might try to copy if I were to make a frame. Maybe make a unisex stepthrough version a la Rivendell Cheviot since the horizontal top tube is pretty high. It's pretty heavy for its size, and being a cheap bike from the '80s means steel rims and cranks, but otherwise seems (to me) to be superlatively desirable for families like mine who use bikes primarily for in-town errand running - and no one seems to be making one like this these days, thus the point in maybe going to the bother of making one. :)


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