Planning for #2

Ok, I haven't even started #1 but know what it's going to be (simple, single speed, probably somewhat askew) but then i'm thinking #2 will be for my younger son who is 6 and about 50 lbs.

I've spent the past 3 hours surfing the forums and archives reading tons of interesting stuff but haven't really found a thread that correlates height/weight to tube diameter/wall thickness (and then at the extremes). I'm guessing there's a sort of range for Rider X who weighs 180lbs, 6" tall, hard charging road racer (lots of watts). I'm guessing there's a set of tubes 'he could get away with' and then a 'preferred' set and then a 'whoa, that's a stout frame' set. The pros here probably have all of this in their heads and just know what will or won't work well (knowledge born from doing it a lot and getting feedback and doing it some more).
It seems most current kids bikes - even the nicer ones - are built with fairly heavy aluminum frames and forks and I'm wondering if it might be a good challenge to try to build something lighter with steel with smaller tubes. His bike is used mainly to get 4-5 blocks to school in the morning and then some easy dirt trail riding 5-10 times per year for 20 - 30 minutes per outing. We have one of these( 2014 Conquest 24 | Redline Bicycles ) from my older son still in the garage ready to go but thought it might be fun to build something.

Would it be ill advised to build a 24in wheel cx bike with (for example)

• 7/8th .028 wall thickness DT,
• 3/4 .028 TT
• 3/8 .028 ST
• 1/2in .028 or .035 CS
• HT and BB shell would be standard items

I need to do the math to see if it would actually be any lighter - the tubes aren't butted. Are there things that could be tweaked (eg thicker CS) to make the above make sense or is it just a bad idea all 'round. I suppose the lightness is how I'm rationalizing doing it so, if it wasn't actually lighter, I wouldn't be that bothered. It would still be fun and he'd love helping with the sanding and finishing and assembly of the bike.

I'm just thinking of early 'mini' BMX bikes which typically had pretty small tubes.

Thanks for any advice. Just be warned that if this is a silly idea and you discourage me, I will just spend tomorrow dreaming up another plan that will likely be just as silly.

Here's the math for the weights and strengths: https://docs.google.com/spreadsheet/...rive_web#gid=4

Yes it'll be lighter than any butted standard diameter tubing you can get, and a hell of a lot easier to work with than the fancy alloys needed to draw thinner tubes. Going light will be of benefit not just for the weight, but also keeping it from being too stiff for the little guy

Might want to bump the diameters up a step on the front triangle though to keep it from being too extreme

Also how are you going to manage the seat tube? I'd stick to something with a 7/8" ID.

this isn't really an answer to your question, but something to consider. the kid will grow realllly quick, so if you want to consider building one that can grow with him for 5 or so years, check out what Nao did for his kid, and Ezra for his nephew.
Tomii Cycles bike
Fastboy Cycles bike

We had a Conquest 24" CX for my daughter age 10-13 then I built her a mtb bike and got a road bike. Great combo bike, good for walking paths.
I'm thinking 7/8" is pretty wimpy for a DT, bump it to 1" would be my suggestion. Even then, it will be lots lighter than the conquest.
It may look better visually, I'm guessing it may look thin when it's all done. And it may age well when he grows and you're putting longer stems, higher seat post etc.
cheers
andy walker
Flickr: afwalker's Photostream
Walker Bicycle Company | | Walker Bicycle Company

I was more worried about the rear end but a standard top tube as the downtube wouldn't be bad.
Thanks!