Kids Bike Frame Geometry 26/24/20
As I look over models of kids bikes, I'm a little puzzled. It seems that kids bikes (at least geared ones) seem to have a chainstay length that's pretty long compared to a proportional one on an adult bike. A 20" is barely shorter than a 224", and a 24" barely shorter than a 26". In fact, some adult bikes (ie a race geometry road bike) have shorter ones.
Then there's BB drop. I think I've figured out that properly designed kids bikes have at least a drop and not a rise, because the rise would be to clear cranks that are really too long for them. And whether a 20 or a 24, it's between 0 and 25mm. Same on a 16.
So I have to beg the question, apart from top tube, are geared kids bikes constrained mainly by chain length, or something else in the rear? And would someone be all that far off to consider building with smaller than standard wheels, with an equal drop in crank length to keep the same pedal clearance?
I have heard of 24" builds on XS 26" frames, and in those sizes I've seen the chainstays and top tubes overlap quite a bit. Not as very much with 24" and 20", but some.
The goal would be to increase frame options and possibly get the child into a better bike sooner.
I went with a Marin 24inch since it was the shortest chainstay of all the kids bikes I could find.
Ibis Mojo 3
Carver 420 TI
Lenz Lunchbox punkass
Many 20" and 24" frames are designed with a kickstand in mind. Though serious enthusiasts (or their kids) rarely ride with them there is extra length built into the CS for their attachment.
Longer wheelbase bike with more straight line stability and the ability to use the same raw CS parts to build various size frames are probably also part of the thought process.
Another thing to consider is chain line. The shorter the CS the fewer usable gears can be placed on the rear because the angle just becomes too great.
I was thinking it had to with the chain line a bit also.
Is there a rule for how long a chainstay has to be for a given chain type and spacing? If we go by 7 per 400mm (the Marin 24") at 7/8spd spacing, then a 370mm chainstay would have about 6 gears, which would be about the same as 5 gears at the old 5/6spd spacing. 370mm seems to be the shorter variety of chainstay for a 20" bike, though it still seems to be proportionally longer than an adult bike.
If we took the same ratio down to the 310mm of a 16", that would leave 5 gears at 7/8spd speed spacing, or more at 9 or 10spd spacing.
Does anyone have better numbers than this? Does it even make sense?
If chain angle proved to be limiting the CS length, you could likely still use short CS and also shorten a cassette by a couple of cogs to stay within acceptable chain angle. For her 20" MTB, my daughter only ever routinely the 3 or 4 lowest rear cogs, having 8+ gears was pretty unnecessary. You could also use a a double or triple crank to reduce chain angle if you can teach then not to cross-shift.
Problem with long CS is that it makes it much harder for them to lift the front wheel over obstacles.
Problem with a high BB is that it makes it much harder for kids to able to reach the ground from the seat. When my kids were first starting to ride a 20", I usually needed to set the seat height fairly low for them so they can reach the ground which then too low of a position for proper leg extension while pedaling.
I seem to recall being torn between getting proper extension and being able to touch the ground from the seat. I wonder if kids bikes should be getting more of a pedal-forward approach? Suppose we should consider using set back seatposts more often, provided we can make sense of the cockpit length?
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