Doug,

The point of short cranks is to go faster and with less wear on you knees.  Spinning will help to an extent, but short cranks are proving to be a better choice than long in some situations.  If you aren't racing, or having trouble with you joints then there is little point in spending the money.  Long cranks seem to work better on the upright bikes they were designed for.  You take rules of thumb from the upright world and apply them to recumbent riding at your own risk.  The folks I know that have tried them are all trike riders, and they swear by them.  One guy even tried 75mm cranks, but settled on 100mm for racing (woth full fairings).  The main goal there is to drop the front down and reduce frontal area for better top speeds.  With long cranks the driver's heels would start to hit the ground without shorter cranks.  Going from 175mm to 100mm theoreticall allows the front of the trike to drop by 6 inches.

Mike

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Michael Ross
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Cycling in Central North Carolina
Schwinn Voyageur 11.8
Linear LWB, Greenspeed GTO, BikeE CT, AT