Noun. In bike racing, the sag wagon is a support vehicle that picks up riders unable to complete a road race because of injury, mechanical problem or other mishap.
__Oww, man, Lookit my bike! I'm not gonna be able to finish the race on this potato-chipped front wheel.
__Too bad man, but don't worry. The sag wagon will pick you up in a few minutes and give you a lift to the mechanic.