20" and 24" kids MTB frame geometry- Mtbr.com
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  1. #1
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    20" and 24" kids MTB frame geometry

    A frame builder friend offered to build a few MTB frames for my friends son. He needs to purchase the BB shell and drop outs, and provide geometry, the builder will do the rest. I am wondering if anyone out there has thoughts on kid bike geometry? Can we just scale down a favorite adult bike? If not, any suggestions for a more modern, slacker geometry?
    Thanks in advance for the help.

    Regards,
    Wardo

  2. #2
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    You do not need modern or slack for kids who fit on 20" wheels, unless they are the next coming of Hans Rey.

    Here's the 20" my son rides. Note that it's ~70mm of trail (very low by modern standards) with super high handlebars and tons of standover. These are all things you want, because they will bail a lot, they won't be going super fast, and they will have the wheels on the ground pretty much 100% of the time unless they're jumping on plywood in the driveway.

    Plus tires are key. Kenda Slant Six 2.8s are money.

    20" and 24" kids MTB frame geometry-screen-shot-2018-06-22-8.45.23-pm.png

    -Walt

  3. #3
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    Quote Originally Posted by Walt View Post
    You do not need modern or slack for kids who fit on 20" wheels, unless they are the next coming of Hans Rey.

    Here's the 20" my son rides. Note that it's ~70mm of trail (very low by modern standards) with super high handlebars and tons of standover. These are all things you want, because they will bail a lot, they won't be going super fast, and they will have the wheels on the ground pretty much 100% of the time unless they're jumping on plywood in the driveway.

    Plus tires are key. Kenda Slant Six 2.8s are money.

    Click image for larger version. 

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    Ward

  4. #4
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    I've often thought little kids would benefit from the stability of longer stays. My kid's old 24" Rockhopper had long stays, there was a fistful of room between the tire and seat post. That seemed like a good idea.
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  5. #5
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    Yes, long (proportionally, anyway) stays are a good thing for kids.

    Other stuff I've found to be good:
    -As mentioned before, plus tires. Specialized and Kenda make 20" and 24" plus tires.
    -Short cranks (100s for smallest riders, once they hit 4' or so, they can move up to 120mm or so) and low BB.
    -Tons of standover.
    -Wellgo KC008 flats are appropriately sized for kids feet and have good pins to keep their feet on. Wear shinguards initially!
    -Put the coolest/loudest/most colorful bell or horn on. Let the kid pick their favorite.
    -Minimalist foam grips (ESI 30mm/racer's edge) are easiest to hold for kid size hands.
    -Shimano hydro levers can be adjusted nicely for small hands.
    -Aim for long wheelbase for stability, but low trail for low speed climbing. If they are descending at 20mph through rock gardens suddenly, you can make them a more "aggressive" bike, but the most frustrating thing for kids is the climbing.

    -Walt

  6. #6
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    why not just go look at commencal, spawn or flow bikes website for proven 20"/24" geos for a baseline.

  7. #7
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    Quote Originally Posted by dc40 View Post
    why not just go look at commencal, spawn or flow bikes website for proven 20"/24" geos for a baseline.
    There are some things that actually sort of suck about those bikes, because they are required to adhere to CPSC requirements about BB height and such. They tend to have horrible standover (compared to what you can do on your own) and they sit up too high/use too long of cranks for smaller kids.

    But basically, sure. If you go look at the Spawn geo for the Yamma Jamma you'll see it's quite similar to what I have here in a lot of ways. Long WB, low trail.

    -Walt

  8. #8
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    Quote Originally Posted by Walt View Post
    they won't be going super fast, and they will have the wheels on the ground pretty much 100% of the time unless they're jumping on plywood in the driveway.
    This all depends very heavily on the individual kids.
    Some are ground-bound slowpokes, but there are also plenty of little rippers out there.
    I guess with the 20", you'd be talking about ~4-6 year olds, so they're probably a bit rarer at that age, but by the time they get on 24's, lots of kids are killing it, and many of those enjoy a livelier ride. I know for my son, jumping, manuals, drops, skinnies, etc have always been the highlights of a ride
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  9. #9
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    Quote Originally Posted by slapheadmofo View Post
    This all depends very heavily on the individual kids.
    Some are ground-bound slowpokes, but there are also plenty of little rippers out there.
    I guess with the 20", you'd be talking about ~4-6 year olds, so they're probably a bit rarer at that age, but by the time they get on 24's, lots of kids are killing it, and many of those enjoy a livelier ride. I know for my son, jumping, manuals, drops, skinnies, etc have always been the highlights of a ride
    Yes, good point. I am talking about younger kids on 20" wheels who are relatively new to riding. I assumed that was what the OP was interested in.

    Remember, too, that even BMX bikes have pretty "long" chainstays. They're most commonly 350+mm. They manual well because they have pretty high BBs and lots of room in the cockpit to move your weight around, not because their chainstays are crazy short (the wheelbases are, but it's because of the ~75 degree HTA and 20" ETTs they run). For the kid who wants to get crazy, a 350mm chainstay length won't hold them back.

    The real issue when kids get more aggro/bigger is more steering trail, IMO. With gravity on their side, there's nothing preventing them from going just as fast as an adult - so if they're really killing it on the DH, you'll want to bump up to 90+mm of trail like an adult mountain bike.

    -Walt

  10. #10
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    I would say that kids should have proportionately higher bars, shorter top tubes, and longer stays than an adult bike. However, I still think chainstays on most kids' bikes are too long. For cheaper bikes, this is so they can fit a kickstand. For less cheap bikes (Spawn, etc.) they're probably still longer than they need to be in order to sorta approach complying with Shimano (or whomever's) standards about chainline. One way to get around this is run 9 speeds of a 10 speed cassette (e.g. take out the 15T and 17T and put in a 16T as if running a giant cog). There's some company that does this though I can't recall who. Another way to get around it is to not worry about it--It's a kid's bike.

    That said, even though Walt built his kid with the same length stays and he'd use on a 29"+ bike*, I'd say he's more/less on the money.

    *Calm down, internet. This is a joke since Walt talks about using square tapered bottom brackets so he can cut out the shell and get maximum tire clearance.**

    **Again, not really.

  11. #11
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    Quote Originally Posted by Feldybikes View Post
    using square tapered bottom brackets so he can cut out the shell and get maximum tire clearance.**
    I like it.

    20" and 24" kids MTB frame geometry-img_3407.jpg

    Here's a 20 and 24 my buddy made for his son.
    Pretty sweet.

    20" and 24" kids MTB frame geometry-20.jpg20" and 24" kids MTB frame geometry-24.jpg
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  12. #12
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    SO many cool points for cutting up old Instigator dropouts and welding them to breezers!

    I think that beats any frankendropout I've ever constructed pretty handily.

    All those bikes look super fun, though to be fair they're all basically BMX/DJ rigs. Not many kids will ride up any sort of decent climb on a singlespeed.

    Getting my 6 year old balanced enough to feel safe while actually having the saddle high enough for full leg extension is my biggest challenge. I have to sneak the saddle up in 2-3mm increments so he doesn't notice.

    -Walt

  13. #13
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    I did these 20" and 24" frames for my daughter. Can't really say how perfect they were or weren't, but they rode just like bikes and she got a lot of use out of them. One key for me was shortening the cranks and removing the bigger rings. The 24" frame seems kinda short now that she's on the verge of bigger wheels, but it works.
    Attached Thumbnails Attached Thumbnails 20" and 24" kids MTB frame geometry-mia-bike-1.jpg  

    20" and 24" kids MTB frame geometry-9053444457_4bbec274e1_b.jpg  


  14. #14
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    Those bikes remind me: 1x is GREAT for kids. My 6 year old is on a 32t front ring (20" wheels) and 11-34 rear cluster (9 speed with a cog removed for chainline reasons). That's enough gear for him to ride up anything he has the technical skill for, and also enough that he basically never spins out. And it's easy. He much, much preferred gripshift to a trigger.

    YMMV, of course.

    -Walt

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    Since we're talking kids bikes... I'm working on one for an almost-5-yr-old, using 16-inch wheel standard, 115mm cranks. If I were to size this for "normal" pedal clearance, the BB would be so high as to be ridiculous. What sort of bottom-of-the-pedal-stroke clearance do you all use when designing the micro-machines?

    BTW -- this would be the first pedal bike, and will be used initially sans crank/pedals/BB as a kick bike.

  16. #16
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    Walt has hit most of the important points. Short cranks are the number one fitting issue. Easy to use brakes and shifting is imperative. Get new cables and housing. Small grips and adjust the reach on the levers.
    Light tires like rocket ron's 24's helps them get up to speed.
    Bells, horns, stickers spoke beads..... anything to make them excited about riding.
    I like 1X to keep it simple and light. Most kids don't want to tackle a mountain or have the endurance too. Gotta keep it fun!
    I prefer plastic pedals without pins. Shredding your shins would turn off my kids fast.
    The shorter cranks move the saddle up and give the bike a high BB. I was able to lower the rear on this bike and restrict top out to compensate.

    My boys bike.
    20" and 24" kids MTB frame geometry-20170923_175201.jpg20" and 24" kids MTB frame geometry-20170923_152454.jpg
    Making shit harder than it needs to be isn't awesome, it's just...harder.

  17. #17
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    Quote Originally Posted by briderdt View Post
    Since we're talking kids bikes... I'm working on one for an almost-5-yr-old, using 16-inch wheel standard, 115mm cranks. If I were to size this for "normal" pedal clearance, the BB would be so high as to be ridiculous. What sort of bottom-of-the-pedal-stroke clearance do you all use when designing the micro-machines?

    BTW -- this would be the first pedal bike, and will be used initially sans crank/pedals/BB as a kick bike.
    I think it depends on what the kid is going to be riding and how comfortable you are with the potential for a pedal-strike wreck, which as you probably know can be very bad.

    The frame from upthread that I posted has a 215mm BB height with 102mm cranks. That was me being paranoid, it's probably too high. His new bike is 225mm height with 125mm cranks.

    I guess a good rule of thumb would be not to go under 100mm of ground clearance, but you could probably push that lower if you really wanted to.

    -Walt

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    Quote Originally Posted by alexbn921 View Post
    The shorter cranks move the saddle up and give the bike a high BB.
    ??

    BB is unaffected by crank length.
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  19. #19
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    Quote Originally Posted by slapheadmofo View Post
    ??

    BB is unaffected by crank length.
    I'm guessing he meant high COG? You do have to move the saddle higher relative to the BB with shorter cranks (but you can put the BB lower in the first place). COG is pretty much the same either way in that scenario.

    If you're just slapping shorter cranks on an existing bike, then yes, the saddle and COG will both have to go up.

    -Walt

  20. #20
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    Quote Originally Posted by Walt View Post
    I think it depends on what the kid is going to be riding and how comfortable you are with the potential for a pedal-strike wreck, which as you probably know can be very bad.

    The frame from upthread that I posted has a 215mm BB height with 102mm cranks. That was me being paranoid, it's probably too high. His new bike is 225mm height with 125mm cranks.

    I guess a good rule of thumb would be not to go under 100mm of ground clearance, but you could probably push that lower if you really wanted to.

    -Walt
    I think going at least a little on the high side is a good idea. It can take kids awhile to grasp the concept of riding with level pedals and not pedaling through corners.

  21. #21
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    Thanks for all the input! For some reason, I stopped getting notices on new posts, but I stopped by and WOW! So much info. I'll work to take it all in and apply your hard won lessons.
    Wardo

  22. #22
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    Quote Originally Posted by slapheadmofo View Post
    ??

    BB is unaffected by crank length.
    You are correct. I was more talking about relative BB height not an actual measurement. My kids like to be able to touch the ground while seated or they don't feel safe stopping. I do sneak out and raise there seats as they grow.
    Making shit harder than it needs to be isn't awesome, it's just...harder.

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