I am building a bike for my nephew soon, he’s about 10 and he’s getting to be a bit of a shredder. He has never had a proper bike though, just low to mid tier kids bikes with the standard rusted solid forks and perished tyres etc.
He currently stands 144cm (4’9") tall with a 60cm (23 and 5/8") inseam. He often jumps on his mother’s size small Marin san quentin and hoons around on that, but its still quite a bit too big. Basically I am thinking of doing what would be a size XS hardtail with a moderately slack (65ish) hta, and short stays to try keep the wheelbase compact. Probably go as short on the seat tube as I can maybe somewhere in the 300-350mm range. It looks like even with a 300mm seat tube and a 100mm dropper at full extension it would be too long currently, but he will get bigger.
Front centre on my current draft is looking to be around 695mm. Bear in mind it will be a bike for him to grow into, and his sister will inherit it once he grows out of it.
Since I have only made bikes for myself and my wife I have a few questions about fit
My questions are this:
-what sort of numbers should I be looking at for saddle to handlebar grip.
-what sort of numbers for crank to saddle (or seat tube angle)
-some folks here have made bikes for kids, anything I should know/have not considered for a kids bike that you can tell me about?
Anyways here’s the draft:
Lemme know what you think!
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Based on that drawing, you’re going to have to consider how you’re going to get the seatstay to clear the tyre and come back in to get to the seattube without it either being some kind of segmented stay or a crazy tight bend.
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I haven’t designed a kid’s bike, so take all of this with a grain of salt. I am, however, planning a hardtail for my 4’11" partner. A bike that your nephew can grow into is an interesting challenge.
I think there’s a lot of merit to keeping the chainstay short, but 415mm might be a bit too short? I think bumping it to 420 or 425mm might help balance things out as they grow?
I imagine a kid will feel most comfortable with a more upright position than an adult, but good seat height is still super important. Maybe slacken the STA to 74deg or so? As your nephew grows, the dropper can be slid up in the tube and will bring the saddle rearward a bit.
I think it also makes sense to increase the chainstay length a bit. Maybe 425 or so? I think it will do a lot to decrease chatter and increase stability. If you’re worried about keeping the wheelbase where it is, maybe steepen the head angle to 65.5 or 66?
Again I am far from an expert, and these are just potential considerations that came to mind.
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I agree with @drwelby that he’ll need a steeper HTA with bars proportional to his size.
If I were making a bike for my 10 year old nephew (I have one too), I’d use a rigid fork. He doesn’t weigh enough (maybe 75lbs) where I think the fork would provide any benefit. I mean just look at the drawing. His hands will be so high there’s no chance he’ll be able to get his weight over the front end anyways.
Here are my random suggestions:
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With a rigid fork you can dramatically lower the stack. Maybe place his hands an 1" or so above the saddle.
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Maybe a 70º HTA with a 600mm wide bar
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Lower the BB to 280mm with his what 150-160mm crank arms.
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Use a dropper post like the Fox Transfer SL that has a metal spring instead of an air spring so he can easily lower the saddle with his body weight.
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Use smaller tubes. 1" TT, 9/8" ST, 9/8" DT. Smaller tubes are a little more dent resistant. The bike is going to be stiff for him regardless.
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I would also use v-brakes instead of disc because it’s only a matter of time before a rotor gets bent.
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I’ve done lots of kids mountain bikes but I’m teaching kinder today so I’ll try to post something this evening. There are a number of useful tricks I can prattle on about.
-Walt
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A very cool project. We have done quite a lot of testing with riders ~5’2-5’4 range, and built several bikes for customers which they loved.
I think the hardest part is the equipment constraints (wheels, tires, forks, etc…)
What are the wheel size, fork, and fork travel that you are planning on using?
It’s 27.5". I think 26 would be a better fit now, but this will probably be his bike until he’s about 14.
The drawing shows with a 120mm fork but I had a look and it would be easy to spin up a spacer to bring it down to 60-80mm. The fork is a rockshox recon, I aired it down to just enough to maintain full extension and I think it will work for him. But at his current weight he would never bottom it out because it ramps up too much.
@manzanitacycles thanks for the suggestions I will reply point by point a bit later, but I have some ideas 
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Here’s the print on the frame (full suspension, 120mm travel) that my 10 year old just outgrew. He’s almost 5’ even now and it’s too small. It would fit your nephew perfectly, probably (but it’s going to my 8 YO daughter, sorry!)
Keep in mind that the “toptube” shown here is not quite what I ended up building because clearance for the shock was really tight. It ended up being smaller diameter and mounted a little higher.
Pictures of the actual bike (not any good ones, sorry):
https://www.instagram.com/p/CPOg-4Dh1fe/
https://www.instagram.com/p/CjiVgxxMDAY/
And just for fun because it’s so silly, my daughter’s 24" full squish:
https://www.instagram.com/p/CPCUFTrFgfB/
https://www.instagram.com/p/CPDiphGB20b/
You are not going to be able to make something that will fit until he’s 14, unless puberty is really delayed, so IMO you want to shoot for something that is comfortable to ride but just at the bleeding edge of too big right now.
Some thoughts on your design:
-Standover is going to be horrible. You could think about a super-v style frame like I typically do, or at the very least do a curved toptube.
-You’d be better off with 26" wheels, probably.
-BB is really high unless he has long legs and will run long cranks. My kiddo uses 155s (or did) and the bike was running ~30-40mm of sag, so the equivalent BB height on a hardtail for him would be 280 or so. If using shorter cranks go even lower.
-You’ll need to bend or otherwise reconfigure the seat tube, probably, but that’s going to limit dropper insertion.
-I’d probably do longer chainstays if I built my son’s bike again, and you may need to here for functional/fabrication reasons regardless. Again 26" wheels will help a lot.
-Stack seems crazy high. What’s the A-C on the fork? Can you reduce travel/A-C?
-Clearance for saddle/pants/butt is not sufficient when dropped.
-Design around a 35mm Flatforce stem and flat bar. Bars can come up/forward with stem swaps for a few years this way.
Kids bikes are super fun to build!
-Walt
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Agreed, we considered doing a 26/27.5 mullet for riders under 5’2, but the tire options are so limited and outdated.
Any insight on this? We tried super short 410mm chainstays on small bikes but we ended up going longer for ride quality. I am still not sure if it was because of rider expectations, or actually something about small bikes.
For reference, that is a 620mmm saddle height.
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That’s a print of a full suspension bike that doesn’t show any of the rear suspension bits so probably not comparable. Kids also don’t care about “ride quality” like adults for a variety of reasons.
I have lots of thoughts on and experience with bikes for small adults (and dwarfs but that’s a whole other topic) but kids bikes are totally different so imo that would be better in a different thread.
-Walt
For a production bike this is a real problem. For a kids bike there are piles of really nice 26” parts you can get for practically free to build up a one off.
-Walt
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Ok I think I have a plan.
First of all I will address some things:
@manzanitacycles
-Rigid fork would lower stack, but I don’t really want to make one at this point and theres not affordable off the shelf options that I know of. I will however make custom spacers to bring the fork travel down to a much more manageable length
-I think 70* would be good with a rigid but I’m sticking with suspension, but I am going a bit steeper than planed as will be seen shortly
-bb has been lowered, I think I will go with 140mm cranks as theres some aliexpress jobbies I have my eye on that should work nicely
-I dont think there’s budget for a fox post, they cost a fair bit, But I will find something with a schraider valve so I can do some tuning to optimise for his weight a bit
-I have some 1" floating around that I can use for the TT. I was going to go with 1 1/4" for DT but I will have a look in the shed and see if I have anything smaller. Any thoughts on CS diameter? I have 5/8 and 3/4 available, I was going to use 1/2" for the SS.
-I don’t have to tooling for V brakes, but my brother is a bike mechanic so He can take care of rotor straightening duties
@anon91558591
yeah 14 years old might have been pushing it. I can’t say I’m a kids person so I’m just not that aware of growth rates at given ages if that makes sense hahaha.
-re standover, I will try make a banana top tube. that should be a fun little side project!
-see plan below about 26" wheels
-BB has been lowered
-CS have been lengthened a bit to make it a bit easier to accomodate straight seat tube
-stack for fork will be lowered to 474 @80mm of travel. It was 514mm for 120mm of travel in the drawing. I can make more spacers and lower it some more if he struggles at first and take em out later as he grows.
-hmmm butt clearance. I will think on this one a bit more.
-Will look into flatforce stems and see if I can find any locally
So here’s the plan
The bike can have a early stage and late stage version. Early stage is this:
-Fork is set to 80mm travel, to compensate for the handlebars further from saddle the stem is backwards PVD styles, and its mullet 26/27.5 which helps level things out a touch.
Late stage version:
-Stem has been flipped around to get a bit more length, fork is up to 120mm of travel and I can relace the rear hub to a 27.5" rim
It’s challenging trying to make something to suite a moving target. Anyways, Thoughts? could probably go another 5mm on cs length and a little longer on seat tube length, its currently showing a 300mm seat tube, going up to 320mm might solve the butt clearance problem.
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Oh, side note. Worst case scenario and he can’t fit as is I have a set of 24" wheels in the shed so we can just chuck some tyres on those and he can ride it til he’s big enough for the big boi wheels if need be.
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5/8x.035 would be fine for the chainstays and 1/2x.028 on the seat stays would work well. Heck, you could probably use 3/8 seat stays no problem.
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Well the frame is done now, still need a bunch of parts to build it up. I’m quite excited to see how it all goes together, I think I will probably go with 26" wheels and size up to 27.5" when he gets a little bigger.
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Sorry, it’s time for me to bail on the forum.
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It was more for butt to rear tire clearance until his legs get a little longer. Though I saw him over the weekend and he’s pretty lanky so might not be required. Will have to see how it goes!
All in one piece now. No dropper yet because my nephew has a bit of growing to do before he will get any benefit out of it as the seat height is approximately correct for his current leg length. I have ridden it around a bit and its a fun bike, super planted with the 280mm bb height.
Weather permitting he will be getting a nice surprise this weekend, though the forecast is currently looking terrible hahaha. So it might be a few weeks yet til handover day.
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