Bicycle Mechanics - Crank arm length question

so I'm building a single speed road bike for my daughter using an old Apollo frame. My question is I know that there are road bikes for kids (my daughter is 9) that are 153 mm but I cant source any that size...however I found the Bulletproof's in 150 and 155 mm. so I'm not sure which to choose from as I fear pedal strike at 155 and 150 may be too short already, but i can't say for sure since I left the bike at the LBS to have the wheel redished.

any suggestion? should i be worried the 155 is too long?

thanks for the help...and yes I'll post a pic once it's done :)

No way to know without seeing the clearance.

9 years old and already converting her to the SS Dark Side.... sweeeeet.... :D

Nothing else to offer since this question can't be answered without the clearance information asked for above.

Keep in mind too that this will also depend on how fast she's riding and how hard she corners. Not much lean doesn't require a lot of clearance. But if she's trying to drag knee on every turn then you'll want lots of clearance.

"dragging knee" being a motorcycle slang term... :D

I expect the difference between 155 and 153 or 150 and a 153 will be completely negligable and your daughter will never feel the difference. Use either.

By single speed, I assume the bike will have a freewheel, not be a fixie. If so, just teach her to keep the inside pedal up while cormnering and she will have no problems no matter what crank length you choose.

BTW, years ago Lennard Zinn did a fairly controlled test on a group of adult riders using crank arms from 165 to 220 mm in length. Much to his disappointment (he sells custom cranks) there were no significant differences despite the huge length variation.

thanks for everything guys...I went 150 just to be safe...knowing she'd have a little more clearance than her other road bike...yeah, other!

and to answer Hillrider, yes it's a freewheel, and I've taught her to keep her inside foot up high on turns. thou she's starting to freak me out lately as she's learning to really push herself on her geared road bike going in circles around the round-a-bout...I just dont want to see her spill at speed.