My son and I are new to Muni.
I’m 6’/200# riding a Nimbus 26 with the dual position KH cranks (137/165).
My son is 12 yo, 5’6"/115# riding a Nimbus 24 with the inexpensive QuAx 145mm cranks.
I prefer the longer crank (165) for riding around, learning tricks. My son prefers the 145 over the 125 on our Sun 20". I want to upgrade his cranks so he has more durability, so should I go longer for more dual purpose or dual position?
I would just stick with whatever your son likes and/or is used to. Most people ride 150 mm cranks on 24" MUnis around here. I’ve been using 137s, and I really like them. I don’t like 165s at all, but I know some people really like them.
Times were when everyone rode 165 or 170s on a 24", these days 150s are far more popular and with good reason. Unless you intend to ride the most technical of trails 150s will suit you well, and 125s will offer a better turn of speed on the flat.
Because the length of the crank will give you mechanical advantage, if you go longer you’ll substitute it for speed. If you want control of the wheel on a smaller scale say for hopping on uneven rocks and stuff i’d say longer in theory would be better.
I’ve only ever had 150 myself but my friend with 165’s i think, has no trouble at all with fine control hopping and all that so i dont think it’ll matter HEAPS.
I’m a long-crank guy. My first two MUni’s had 170 mm cranks on them, as I started in the days that 170 mm was the norm. Last April I bought a KH24", with both 150 mm (the default) and 165 mm cranks (the longest available in the KH moment series). I tried both and will stick for now with 165 mm.
I should add that when I ride it’s not so much for speed, rather for ability to clear certain sections. Especially steep ups are challenging but I like to earn my gravity karma by riding rather than walking hills up.