Collision distance in game engine (2.62)

I am trying to learn about physics and the game engine. I set up a simple file (see attached .blend file) where the yellow weight/armature drops down the blue tracks. At first I did not have the green caps on the tracks and the yellow weights flew up instead of falling down. The green caps fixed that, but now the yellow arm never touches the pink rollers. The rollers rotate as if they have been contacted, but there is no contact. Also the yellow arm should, near the end of the action, rotate right between the green caps, but it stops dead as if it has stuck something.

All these problems are as if there is an invisable shield around objects that won’t allow other objects to make contact - almost like repelling magnets. Can anyone give me a hint where to adjust this collision distance in the game engine? Thanks.

Attachments

TREBUCHET 3.blend (545 KB)