Originally Posted by
Roadiedvm
A longer crank arm requires more force to turn the crank and if the arm is too long or you're not used to it, the result can be stress and femoral-patellar pain in the quadriceps tendon at the patellar insertion.
Actually, just the opposite. A longer crank requires
less force to turn under the same conditions. A crank arm is a lever and a longer lever requires less force.
What a longer crank does require is that your feet travel further per rotation and the flexion of your knee is greater at the top of the stroke if you adjust the seat height to keep the extension at the bottom the same.
BTW, your problem with the FSA crank isn't unique. Apparently FSA had a run of badly made cranks that caused the shifting difficulties you experienced. A local bike shop had a number of new bikes that came with the FSA cranks and had the manufacturer replace them with Shimanos after a raft of customer complaints.