2 bones, 3 leg pieces. Middle piece is deforming. Why? I'm trying to rig a robot leg that has 3 different pieces (thigh, knee &amp; shin), but I would like to use the standard biped armature, so the leg has only 2 bones. 

All fine, it moves nicely, but the knee piece is deforming when I move the bones, it gets squished the nearer the bones are:



The middle piece is 0.5 weight for each bone, maybe I shouldn't do like that, but I'm not sure how would be the best way to do that. I tried adding constrains instead of using the armature deform, but it is very mad lol.

Any tips?
Thanks
Could be sufficient to add a small knee-bone between the others, which is the only one to move the knee?