Join Date: Aug 2005
Location: Pittsburgh, PA
Bikes: '''96 Litespeed Catalyst, '05 Litespeed Firenze, '06 Litespeed Tuscany, '12 Surly Pacer, All are 3x8,9 or 10. It is hilly around here!
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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.