umm on the rear you have to run a chainring on your cranks (anything between 22t and 24t) and on the front you run a rear chainring (12t) also i think im right in saying a rear freewheel would last longer than a front as there's less stress on the freewheel, but some frames wont allow a rear free to be used (zona's, zoo!s. i think montys and frames like that will allow the use of rear free) as the chainline causes the chain to rub away at the frame. I believe a rear free would increase the ep's? might be wrong but its that or they feel stronger.