Another kid's bike question - crank length and gearing?

Last year I built a 26" wheel bike for my 10 year old, I didn't think much about the cranks and just used the 170mm ones that came with the bike. He doesn't seem to have any trouble at all and certainly hasn't complained about them. For gearing he has 1x9 with a 36t chainring and 12-32 (ish) cassette, which seems to be plenty for him especially round here where it's flat (I used to run while he rode, but he's too fast for that now, I need a bike to keep up with him!)

This year it's time to build something similar for my 8 year old. I initially thought about 170mm cranks but from what I've read they might be way too long for him so started looking for something shorter. There are a few options, but most of them are a lot more than standard 170mm cranks. The cheapest I've found are the stronglight impact kid cranks, which are 38 GBP plus postage at spacycles. They're 110 BCD double, so I'd be limited to a 34t ring.

Thorn do a 104/64 BCD 4 bolt triple crank in 150mm for 50 quid which would give better options I think

Either way, gearing would probably be 1x9 with a 12-32 ish cassette

Just wondering what others who have built "proper" bikes for their little ones have done about cranks? and gearing? Any options out there? Any problems arising from cranks that are too long? or am I thinking it all too much? I don't want them to have injury problems are anything, but they don't even ride every week, let alone race or anything

thanks in advance for any opinions or advice!

I think you are right to focus on crank length. Too long and the knee over-articulates - in extreme cases the thigh ends up with the knee higher than the saddle. Certainly there is no power until quite late in the stroke, quite apart from the risk of injury.

My kids were never keen to ride anything that required super-low gears ("Dad I hate this"). Something like a 1:1 ratio was fine at the bottom. For longer rides and touring we have the luxury of a tandem with specially shortened cranks at the back (by Highpath Engineering). IIRC they were 140mm at first, then 160 as the kids grew. We are now about to make the final move back to 170.

I used a 150mm Shimano for my sons bike.

Thanks for the answers! I will look at the knee articulation of my 10 year old next time we ride and see how much he has.

Bullpup, where did you find 150mm shimano cranks? Do you have a model number? shortest I've found for shimano is 165

Good morning. It was an ebay find.

OK, thanks, will keep my eyes open for one...

Does anyone know if the usual methods for setting saddle height as you would on an adult's bike apply to kids too?

I've been googling, and the only option I can find in 104 bcd is the thorn cranks. There are several options for 110 bcd 5 bolt chainrings, but they would limit chainring size and I'd prefer to have the option of installing more chainrings later on (current plan is 1x9 with a cheap NW chainring to keep things simple)

One option is the suntour XCT chainset, which is available in a junior version with 152mm arms. It's a 4 bolt triple chainring design, but on the suntour page BCD is not specified, and I've found some reports that the BCD isn't 104 but is something proprietary that is close to that so I'm not too keen on them... anyone have a set and able to confirm BCD?

I would defer to them in the know and IslaBike are one of them
https://www.islabikes.co.uk/product/bikes/beinn/
They list their sizes.
152 for the Beinn 24 Small, that's the 8+

I see they've eventually fallen for some expensive top end bikes and suspension with Pro series
At least it's decent stuff.

I used a kids impact single from spa with an 8 speed cassette on a 14" frame kona for my kids and it's been fantastic for them. I then built a 16" kona with slightly longer cranks from Paul milne cycles, still 1 x 8 which has been equally as good.