Review for LEGO set 6539 Victory Cup Racers from Brickipedia
“Each go-cart has a transport frame in which it can be placed to carry it on the trailer. The tractor unit contains a "sleeper" at its back, which didn't provide enough room for a minifigure though. The set includes four carts but only one dedicated race pilot. But there are... More enough helmets to refit the remaining minifigures for a race.”
Full review at brickipedia.fandom.com