At first, the ball joints seem a little "hard", but it is necessary to wear a reflex! The ball is an indispensable BH1 with !!!
Attention scare that I made (! That'll teach me not to read the instructions), there is an "adapter" of screw 1/4 -> 3/8 which is installed by default on the GP . This means that it must be removed in order to secure the knee-on!