I always thought shorter cranks make the bike more responsive to lift the front up for shorter run ups, but that was just a theory haha ? cause i had long cranks on my 24 and it always felt a bit "loose" as if it would have been better with 170mms
i found how to do it on forum somewhere, cut bent end off a spoke then put nipple upside down on it and hammer it in, you need to make a clamp though to put in vice, I just got two bits of wood and drilled down centre of them