Trial By Dirt is a side mission in Grand Theft Auto: Vice City which is triggered when jumping on the parked Sanchez on the dirt track in Downtown. It is a checkpoint mission, but it has no time limit. To complete this mission, you must clear each checkpoint without destroying your bike or dismounting too long from your bike. There is a health pickup nearby on the track, in case Tommy falls off and gets injured. If you enter the Landstalker instead, a similar mission entitled Test Track will start, the reverse of this mission but is a bit harder since it will fail you if you flip the car or bail.
The first four times around the track, a few bikers in unique black Sanchez dirtbikes will chase you. The fifth time, there will be a security guard in a black Rancher.
You can keep playing this mission once you've completed it. Each time you beat your time record, you receive a higher prize (in money).
Due to a bug in the game, the initial scores of both Trial By Dirt and Test Track will have the same time results once both scores are listed in stats. Once you play the missions again and beat each time, the scores will be individual as they should be.