Originally Posted by
NickFit0036
Big time. They definitely do what they can to save money. How short do you think an adult would be to need 140mm cranks? That's the length that comes on 20`` kids bikes.
The whole crank length paradigm is completely out of whack with cranks being just far too long for everyone.
140mm cranks are FAR too long for children's bikes with 20" wheels.
I grew up with and until recently owned 20" children's bikes with 110mm cranks. That's what the standard was. There were 22" wheeled bikes with 5" (127mm) cranks and 24" wheel adolescent bikes with 6" (152mm) cranks. Since small adults are roughly the same size as an average adolescent then 24" wheeled bikes with 150mm cranks is FAR closer to the mark than anything current.
I'm a 155cm adult who rides with 135mm cranks on a custom 650 wheeled road bike.
The other issue that is obscenely wrong with current frame design is seat tube angles. Steep seat tube angles of 74 degrees or steeper are a fudge to make frames with long cranks and large wheels "Work".
My custom 650c frame has a 71 degree seat tube angle. When you work out the geometry using shorter cranks the seat tube angles on small frames need to be relaxed. Not steep.
Its money. If your a production manager in a bike factory trying to keep manufacturing costs down the number ONE imperative is to reduce the number of different sized parts being used. If you can make two different sized bikes where the only difference is a little adjustment to the frame but all the parts fitted to those frames are the same then this is a WIN for keeping your manufacturing costs down.
Smaller wheels on a smaller bike? Forget about it.
Anthony