We started building practice robots for the 2014 season, and I can say that it helps a lot. Especially this year. Every day we would have our driver practice stacking, and it's one of the reasons we made the improvement that we did from week 1 to week 3. Only modification we made to the robot was a cardboard box with a slanted top for a ramp, but the practice bot helped a lot with driver practice. It was actually so dead from all the usage that at off season events it couldn't make stacks of more than 2 or 3 totes without the lift trolley coming off the guides
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I don't necessarily think it is necessary to compete, but it would be necessary to be really competitive.