Thursday, January 5, 2012

@stevefleck @rappstar Crank Call: Ectomorph, Endomorph there’s the rub

I needed a few more than 140 characters to make a point that often gets overlooked in this talk about crank length/cadence, etc (and many other position related issues). I called out Steve and Jordan in the title in part because they are tall, lean riders, who might not appreciate what your options reduce to if you are short. They can ride a 15 cm or greater tri drop, and probably 10 cm on a road bike with no problem. Hence, changing crank length is possible in their generous fit box* allowing them even more room to explore. This freedom does not exist for small people with extremely tight fit boxes.
Consider a small endomorph like myself. If you use Dan Empfield's average drop for a person my size riding at 80 degrees you'll find I'm on the low end of drop (e.g. aggressive for distance tri), 7.5 cm, and probably about half (or less) of what tall folks can ride and still be in a fairly relaxed aero position.
This situation is a sense related to the seat angle issue: for short people we can move our butts a small amount on the saddle to change our seat angle significantly (a few degrees) while that adjustment is not practical for the big folks as the distances are to far.

Don't believe me? Find your favourite short person hop on your road bike and go for a spin. Play around trying difference drops/positions as you roll along. You'll quickly realize as a tall person the issue the small people face: tall people can imitate all the pictures you see of top road and tri riders, while the short people can't be in many of the positions you see (and I want to note here on a scale of flexibility for an AG triathlete I would put myself on the high end, I stretch religiously, this is not a flexibility issue it is just a question of space). Put another way a 5'3" tall person isn't going to be able to flatten their back like Dave Zabriskie.

While going to shorter cranks for a tall rider who has sized up to a 180 mm crank for TT they may be able to drop their position enough to significantly reduce their CDA** using a relatively short crank. But I am highly skeptical that significant CDA changes are attainable for small riders who are already in the middle of their fit box. In my case I have ridden as short as 160 mm cranks and now ride 172.5 mm ones with basically the same CDA. Our fit boxes are just not as sensitive to these changes.
Riding in the real world on real hills there are real advantages and disadvantages between shorter and longer cranks, but for smaller riders in a TT position crank length is not likely to have a significant effect on CDA. For taller riders the differences may indeed be significant.

*fit box refers to the area one can adjust a rider's position and still be in an acceptable aero position, e.g. changing seat angle versus dropping bar height.
**CDA is short for the product of the drag coefficient times the rider's frontal area; the drag force on the rider is directly proportional to CDA, so a smaller CDA means your position is more aerodynamic.

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