it has nothing to do with friction or weight, it has to do with what the motor likes in a particular chassis.
lets take a motor any motor, brushed CO27 for example, or a Speed Passion 13.5 doesn't matter, each and every motor as a gear ratio that it performs best at. what spins faster and/or longer, a 10lb disk that is 6 inches around or a 10lb disk that is 10 inches around. whats easier to get moving, whats easier to keep moving, what slows down more off power, what accellerates better off the correr. it all matters, IMO
I also believe just because IMO my Mi3 performs best with a 106 64p spur and a Speed Passion 13.5 does not mean that the chassis of your choice (say xray with duo) will be the same, internal ratios, tire size, belt or shaft all make a difference, thats why your buddy running a 4.3 gear ratio is running 20 degrees cooler then you are at a 4.5 gear ratio.
#49, #25a, #91, #7, #12 the ones to watch.... UPDATE. #12 is your new AMA GNC #1
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