In the mid-60's, motocross took off big time; we even had a professional track down here where I live, the local Yamaha dealer built it to showcase their bikes, and the son of the owner (my age) was a regionally-ranked competitor.
The big brands were Maico, Penton, CZ, Bultaco, Yamaha, and the BSA B50. Used to love going to the track and watching the boys run.
Never will forget (must have been about 1973) when the first Yamaha YZ-250 came out, and the dealer who ran the track got one of the first ones. Their rider (Lee Williams was his name) lined up in the front row, the gate dropped, he out-dragged the whole field to the first turn by 10 lengths, and with his adrenaline going wide open, he accelerated hard (like he would have done on his old DT-1-based racer) toward the whoop-de-dos.
He hit the first one and must have gone 25 feet into the air .. the bike turned over, landed on top of him, and hammered him into the track like a tent-peg. He was carried off with a badly broken leg and a couple of broken ribs, hospital time even for a young man.
Wild men, all of us ... !