Sorry to dig this old thread out but I've got a question. Hope you guys can help me. I've got an UM O that is modified here and there, and now I finally want to make this mod. My question is regarding the couplers: Which couplers do I have to use? I always see flexible couplers, but why did nobody try star couplers or even rigid couplers with this mod? Let's assume the motors and axes are nearly perflectly upright and in the right angle to each other, wouldn't a rigid coupler eliminate ringing and backlash even more?