Wheel size changes for wider age/size range?
I know changing wheel sizes on a bike is a touchy topic. So I won't go there lightly.
But in the pursuit of maximizing the options for kids, I'll toss this out there:
How about starting with a disc brake kids bike and building alternative wheel sets to stretch the age range?
I know what the first concern is - pedal strikes due to lower BB. So along with alternative wheels would come shorter cranks. As I understand it, this is a common complaint about kids bikes anyways - cranks being too long.
An intermediate step would be to have a full size front wheel and a smaller rear.
Start with a Specialized Hotrock 20 Pro (20", loaded with hydraulic disc brakes and a suspension fork with lockout). Build a second set of 18in wheels. This will lower the BB by 1in or 25.4mm. Replace cranks with ones that are roughly 25mm shorter to keep the BB height the same. Now you have an additional 1" or 25mm more standover and wheels with less rotational mass. As the child grows, you'd first replace the front wheel with 20in, then the rear as well.
Next step up would be a 24" disc bike with a spare 22" wheel set. I think with the selection of 26" frames out there, there wouldn't be a need for a third.
The idea isn't to undersize too much, no more than 2" or so in diameter, or about 10%. Perhaps different tire widths could also be part of the equation.
Does this seem at all sensible, or is there a major negative that I'm missing? I realize that 18" and even more so 22" are not the most common sizes, and that the extra wheel builds would cost a good fraction of the cost of the bike. It would be worth it to me if it made cycling more accessible at an earlier age, but not if it were just another area of complexity with possible negative results.
If, for example, an Isla Beinn 20" Small was as good a fit or better than a lowered Hotrock 20, that would make this a more expensive and more complicated solution than the Isla, and thereby not worth it.
Kids grow too fast to do all of that work. My daughter out grew her 16inch at the end of 2011. So I search forever and found a great deal on a Kona Shred 20 gave it to her for her birthday last year and I could tell at the begining of this summer she was already out growing it and is more comfortable on her brothers 24.
This is why at this point when people ask about fixing up/ modifying 20 inch bikes I leave it putting a mega range freewheel on for climbing and trigger shifters, maybe shorter cranks.
I think the 4 inch size intervals are enough with the rate kids grow.
Demo9 sell frames and bikes for kids. You could buy a frame and use 16" wheels, then later upgrade to 20"wheels Home Page Bill
Originally Posted by TwoTone
Kids also haven't been sucked into the marketing hype of an 1 1/2" of wheel size making some sort of huge difference. Mine goes back and forth from 18" to 20" to 24" bikes over the course of a few hours and rides the hell out of each. Weight is also not that big a concern to kids - at 70lbs or so, most of my kid's bikes are somewhere around 50% of his body weight, and he gets bigger and prettier air than 95% of the grown-ups who know their bikes' weight in grams. Kids rule.
It has struck me as unfortunate that the 24" tire , 507mm rim size was chosen as the intermediate size between 407mm (20") and 559mm (26") rims. 24" wheels are a +4" jump from 20" but only 2" jump from 26". It would have been a more consistent progression of wheel sizes if the intermediate size had been set closer to 23", kids could get onto a 23" bike a bit earlier and once they outgrow it would be completely ready for a 26". The 550A (490mm) rim size looks like probably the best existing candidate size but MTB capable rims and tires for 490mm are probably non-existent.
Last edited by GrayJay; 09-09-2013 at 08:22 PM.
I was going to do all of the wheelbuilding described above but my son went from a 20 to a 24 and currently, I'm building him a 26" wheeled bike all inside of a few years.
Just like Two Tone said... They grow too fast.
I tend to think 10% is a reasonable amount of difference in size to notice, which is about 2" steps.
I'm actually the kind of maniac that would buy/build a bike for his kid well before he was the right size, so that he would always have the right bike. When he doesn't have any larger bikes to move up to, that's when we start building the next one, up until he's an adult medium or so. Case in point: My firstborn child is 5mo old and I'm already thinking this far ahead.
I'm right there with you man, my son mastered the strider, then to the 12". I had already build the 16". Now I am building the 20" to be ready in a couple years.
Originally Posted by BATRG3
I built a XS 26er disc with 24" wheels and 145mm cranks. When my son gets taller he can step up to the larger wheel and slightly longer cranks.
I am thinking about doing what BATRG4 proposes in relation to a bike with 24" wheels. AFAIK, most 24" wheels are ERTRO 507 - ie diameter if the wheel is 507 mm. You can also get ERTRO 451 wheels (sometimes those are labeled as 20"). So that would be (please correct me if I calculate wrong): 56mm, ie 5.6 cm difference in wheel diameter. If I exchange the standard 507 wheel with a 451 wheel that shouldlower the bike by 2.8 cm aka 1.1 inch.
I think that can make a difference, for example if you want to /need to have your kid ride a bigger bike earlier. There are definetely costs involved, but I assume that the people thinking about this are aware of that. So far, I have no practical experience with it (I have heard that it works fine) and I am curious to hear from people whohave actually done that.
It did occur to me that one could drop the 22" size (I can't seem to find any anywhere) and use a 20x3.0, assuming a frame was available.
You're forgetting the rest of the bike. Throwing smaller wheels on a bigger frame doesn't magically shorten the TT, chainstays etc...
Originally Posted by jplonks
Looking at kids' bikes, lots of them seem to have long chainstays relative to the wheel size. It doesn't seem that the wheel size is what drives the minimum chainstay length, as opposed to say a quick handling 29er.
TT would be the biggest issue, provided standover was already adequate. I would think you would address that with handlebar geometry and stem length (and height). I had the funny notion to extend it out by using road drop bars.
Long chainstays might not be a bad thing...
While you look at long chainstays on your bike and think, that will make it hard to loft the front wheel over obstacles and whip the back end around a switchback. The long(ish) chainstays on a kids bike add much needed stability for a not so experienced rider on downhills and make the bike less likely to endo when going over bumps or off curbs.
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