The plank ride is a basic skills test.  A plank is selected.  Then each contestant rides onto one end of the blank and off of the other end.  A two by four or two by six that is ten feet long is usually adequate depending on the skill level of your contestants.  Anybody who rides off the side of the plank is disqualified.  Usually only a couple of rounds is needed to eliminate your competitors.  If need be the plank can be covered with motor oil to make it more slippery.  If you do this please caution your riders to take it easy riding home with oil on their tires

The balancing plank ride is a variant that is played the same way except the two by four is layed across another two by four in the middle.  As the bike rides over the center point the plank will rock on the fulcrum.  This is enough to mess up a few riders who can ride the level plank easily.  Again if your contestants are too good motor oil can be added to the plank to add to the degree of difficulty in later rounds.

I have also seen plank ride final rounds where contestants have been totally uneliminatable done as a side by side slow race competition.  Very rarely is it necessary to go this far.  Usually just a couple more runs on the level plank and somebody will mess up.