A friend of mine, John Brost, recently asked me if I had noticed that there were two types of the new tread links out there. Surprise surprise, it seems LEGO has modified the mold slightly. The new tread links will work fine with the old ones - there's no difference in the parts that connect link to link. However, some small new "bumps" on the new treads do change a few things for robot builders. First, they tend to make the side-to-side stability a little better (a good thing). Second, it means that using them "backwards" to form large wheels now works only with still larger diameters. Mixing them in an alternating fashion makes small "old style" (?) tread link wheels work... but you'd have to pay very careful attention to make sure multiple "new style" links aren't adjacent, or you might end up with flat places in your wheels. As to the why for this, it might have bee
n to increase the side-to-side stability, or it might have been to allow better flow in the molds and less rejects due to bubbles (suggested by Bryan Bonahoom, who knows much more about these things).Now, if they would just come out with a rubber "shoe" for these links...