I just watched a local pull the other night. The sled had the "cheater" bars as someone below called them which dropped at around 150ft as the weight box moved up the rails.

There were two larger farm stock classes, 20,000 lbs and 30,000 lbs. This sled couldn't stop the 30k class, so they added a 10ft chain so there was no weight transfer from the sled. That took care of most of the tractors (typically 180 to 230hp mfwd). They also added a secondary rule that if you made a full pull, the tie breaker was whoever hit the highest speed during the run. The winner made 10mph and was able to gear down in time before the bar dropped. One unlucky guy tried too hard for speed, and couldn't powershift down fast enough and stalled out. Too bad, I think he could have placed pretty high otherwise. Two tractors that were looking good ended up spinning out at 299ft. Must have been a change in track condition right at that spot.

The 20k class was won by a 240 factory hp tractor. I'm not sure how they got that thing to qualify.