Ever since Surly announced that the Straggler would be available for 650B wheels down to 38 cm frames, I've been scheming to build a bike or two for my kids. Well, my scheme finally came to fruition yesterday.
One concern with using smaller-than-spec wheels is pedal strike. If the bike is lowered too much by using smaller wheels, the pedals can strike the ground in corners, which can cause a crash. Since my 9-year-old daughter is only about 4'2" (125 cm) tall, I decided that typical "short" adult cranks (typically 165 mm) would be far too long for the biomechanics of her kid-sized legs. After consultation with my friend Mark at Bikesmith Design, I decided to follow his rule-of-thumb that crank length should be approximately 10% of the rider's height (or 23% of inseam). Ten percent of 125 cm is 125 mm. Bikesmith has a thriving business shortening cranks, and within 24 hours, he delivered a set of shortened SRAM S600 cranks with 125 mm arms.
Even with the tiny 38 cm frame size, smaller wheels, and short cranks, the bike is still marginally too large for my daughter, who is shorter than average for her age. Standover clearance is tolerable, but the reach to the bars seems pretty long. I found a short stem (60 mm) and a riser bar with a bit of sweep.
She took it for a ride around the block and didn't even slip on the ice. The bike fits great (better than I expected), and she looked pretty fast and confident on it. Happy kid!
Total MSRP for a bike similar to this is in the neighborhood of $1400 plus tax (less for me, because I have insider hook-ups and lots of spare parts). Seems like a lot of money for a kid's bike, but I believe she will get my money's worth. With minor adjustments, she will likely ride this for many years (possibly through high school or longer), and if/when she outgrows it, one of my younger kids can ride it. Or I can sell it, because it's a brand that has good name recognition and resale value. But the best justification is that she will have a high-quality cycling experience that will (I hope) forge a lifelong love of bikes and cycling.