# kids bike 20"



## ktm520 (Apr 21, 2004)

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.


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## adarn (Aug 11, 2009)

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.


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## ktm520 (Apr 21, 2004)

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.


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## ktm520 (Apr 21, 2004)

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?


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## afwalker (Apr 26, 2012)

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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## GrayJay (May 16, 2011)

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.















Race Tested!

more conversion info on this 20" fork in thread;
http://forums.mtbr.com/families-riding-kids/novara-pixie-20-project-853041.html


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## GrayJay (May 16, 2011)

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.


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## ktm520 (Apr 21, 2004)

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.


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## afwalker (Apr 26, 2012)

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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## ktm520 (Apr 21, 2004)

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???


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## GrayJay (May 16, 2011)

ktm520 said:


> 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!


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## GrayJay (May 16, 2011)

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.


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## ktm520 (Apr 21, 2004)

GrayJay said:


> 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.


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## Eric Malcolm (Dec 18, 2011)

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


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## ktm520 (Apr 21, 2004)

I was thinking shim stock??


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## Eric Malcolm (Dec 18, 2011)

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


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## _alain_ (May 25, 2010)

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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## afwalker (Apr 26, 2012)

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


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## ktm520 (Apr 21, 2004)

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.


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## RCP FAB (Jun 15, 2011)

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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## GrayJay (May 16, 2011)

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?


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## ktm520 (Apr 21, 2004)

RCP FAB said:


> 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.



GrayJay said:


> 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.


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## ktm520 (Apr 21, 2004)

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.


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## ktm520 (Apr 21, 2004)

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


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## GrayJay (May 16, 2011)

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


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## ktm520 (Apr 21, 2004)

Had to try it. Not really digging it. Looks like a low mount caliper should work though. I hate adjusting them, but they look so much cleaner.


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## RCP FAB (Jun 15, 2011)

Unless you can get a solid rotor cut, I would never put a disc brake on a young child's bike, but that's just my opinion.


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## ktm520 (Apr 21, 2004)

Can you elaborate?


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## RCP FAB (Jun 15, 2011)

It honestly makes me sick to even type it, but there have been many cases of horrific finger injuries to children (and adults) due to bicycle brake rotors. Even with as little force as a child spinning a wheel on an upside down bike.


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## Smudgemo (Nov 30, 2005)

GrayJay said:


> 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?


Personally, I feel while there might be something optimal, these are kids and they'll ride the crap out of anything reasonably well made. This was my last 20" effort and my daughter does quite well on it. I don't have the fork details, but I recall making it similar to what we normally expect to use for an adult bike.


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## ktm520 (Apr 21, 2004)

This is the fork I'm planning to use.
Echo SL 20" Fork - at WebCyclery|WebSkis|Bend, Oregon


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## G-reg (Jan 12, 2004)

[ive QUOTE=RCP FAB;11000356]Unless you can get a solid rotor cut, I would never put a disc brake on a young child's bike, but that's just my opinion.[/QUOTE]

This is why I'm going with Magura HS33s on my kiddo bike. Give the little guy a real brake, and I can run the lever adjustment all the way in for kiddo hands.


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## ktm520 (Apr 21, 2004)

Ordered materials earlier this week from Nova, Bikelugs, and McMaster. Should have most of it this weekend. Went with the Nova horizantal disc drops, which I'm not crazy about, but they are cheap and will do the job. 5$ ht from bikelugs and straight wall from McMaster. I really hope this frame turns out be usable, but I've accepted the fact that it may end up wall art.

Ordered a few of these to use as tubing blocks. They don't make a 1.13" for the seat tube, but I bought a 1.06" and will bore it out.
McMaster-Carr

Welding practice went pretty well last weekend. I finally did a full joint that I would consider safe enough for a bike frame. Ugly, too hot, too slow, slightly oxidized, good enough . . . maybe. I've been practicing on really short pieces so far. I'm going to do some more practicing before I start on the frame.

I've been knocking around the idea of building a fork also. The advantage would be shorter a-c and offset compared to the 20" trials fork that I've been looking at. But, for 80$, I'm thinking I should just use the trials fork and save the hassle on this first build. The numbers aren't that far off. What do you all think?


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## ktm520 (Apr 21, 2004)

Probably one of my best welds to date, short of the oxidation.















Tubing bender didn't work so well. It's a homemade job and as you can see the dies don't fully support the tube 180 deg. Think I'm just going to pie cut and butt weld for this one.


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## ktm520 (Apr 21, 2004)

Feeling pretty good after this test ht junction I did tonight. This is a copy of the geometry I plan to use, but I need to put more edge distance between the dt and end of ht. That's the first throat I've welded that shallow. This should be the hardest area to weld so I'm feeling confident to start on the front triangle tomorrow . . . if the bb and st show up.

I know its not good the start at the throat of a joint, but when the tt and dt are this close to each other, as far as I can tell there is no way around it. Any suggestions? I don't really want to space them out any more as the ht is already on the high side.


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## ktm520 (Apr 21, 2004)

Did three tests of the bb, st, dt junction. #1 was terrible and I burnt the thin st pretty badly when welding the throat between the st-dt. #2 and #3 went much better. I had a hard time finding a hand/torch position that would allow me to weld across the throat, so my approach was to start a 1/4" bead in the center of the throat and then weld down each side. Is this asking for trouble?

The more I weld, the better I get at heat control and weld color looks better. Probably my biggest struggle is feeding the filler into the middle of the puddle and not hitting the edge or the electrode.


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## ktm520 (Apr 21, 2004)

Made some serious progress today. Fixture is ready. Been bounce back and forth on whether or not to spend the money and I think it will easily pay for itself. Really happy with how it turned out. The dummy axle was the main thing I was unsure of. Cut a shallow slot in the angle with table saw to locate axle. Worked great.















Decided to go with a 22.2 bmx seat/post combo with some plastic shims I found in a 1x.035 tube. Fits well and doesn't budge. Plus it saves almost a lb of weight. The mini seat is tiny.


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## ktm520 (Apr 21, 2004)

What a weekend. Was hoping to get the frame done, but still have the seat stays to go. I'll attach some pics to this thread later, but for now, here's a photo dump.

https://www.flickr.com/photos/marklar12/sets/72157643561516825/


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## ktm520 (Apr 21, 2004)

Still have to finish weld the seat stays and cap the ends. I'm slightly stoked on how well it turned out. I've added more build photo's to the flikr album linked above. Your all's silence is killing me.


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## todwil (Feb 1, 2007)

Frame looks good your welding skills have greatly inproved! Also liken the jig!


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## ktm520 (Apr 21, 2004)

Thanks todwil! Man, the jig was key. I can't believe I ever second guesses spending the money. Fitting the tubes was almost too easy. I just used basic alignment techniques and it was good enough for a production bike after welding. Maybe I got lucky. The worst part was the ht pulled to the left at the top slightly. I tried to coldset it, just to see how hard it would be, and it pulled back over pretty easily.

What do you all use for dummy axle widths to account for spring back? I set it at 136mm, after tacking it pulled in to 134.5, but after fully welding the bb it pulled out to 137mm. Interested to see what happens after welding seat stay and capping. Currently, I cold set the cs's so they are just a hair over 135.


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## ktm520 (Apr 21, 2004)

Quick mock up. He wasn't feeling it. Not the stem or bars I'm going to use.


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## todwil (Feb 1, 2007)

Looks like right saddle position and SNARL to me!!


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## RCP FAB (Jun 15, 2011)

Nice! To answer your question on the dummy axle, I use a 137mm. Its almost never enough. Nothing will really move until you start welding bridges in. I do these on a lot of my mtb frames. Boy do they make things move. Ive got a good procedure down now but it involves removing the 137mm axle and using a 144mm one just to weld the bridge (gusset) in.


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## NYrr496 (Sep 10, 2008)

VERY nice. Wish I had the space to build frames. 
You're doing awesome.


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## svj (Feb 1, 2014)

Lookin good. I'd like to do something similar. Where did you source your tubes and materials for the jig.


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## ktm520 (Apr 21, 2004)

svj said:


> Lookin good. I'd like to do something similar. Where did you source your tubes and materials for the jig.


Jig is all 8020 parts from their ebay store along with a few custom machined parts I made. Bike specific materials came from Nova and bikelugs, and straight wall 4130 came from McMaster Carr.

Got the bike together last night for the first test ride. All went well. He picked up the hand brake in no time and was laying down rubber. I could tell it took a bit to get used the higher trail and longer wheelbase. I'll post pics, build kit details, and geometry shortly. Wife called me on the way to work and said he went to the garage first thing this morning, gave his bike hugs and kisses, and wanted a pre-breakfast ride.


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## afwalker (Apr 26, 2012)

Doesn't get much better than that!
Congrats, now make me an airplane daddy!
cheers
andy walker
Walker Bicycle Company | | Walker Bicycle Company


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## ktm520 (Apr 21, 2004)

afwalker said:


> now make me an airplane daddy!


Ha, never heard that before . . .


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## ktm520 (Apr 21, 2004)

https://www.flickr.com/photos/marklar12/sets/72157643561516825/


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## zipzit (Aug 3, 2005)

Awesome job. Nice welding. So what color for the frame?


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## ktm520 (Apr 21, 2004)

Thanks. Clear powder


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