“Supercross” is like a porno where all the sex scenes have been replaced with motorcycle scenes. People talk awkwardly and stiffly, the plot consists only of obvious things and foregone conclusions, and every couple minutes they stop what they’re doing to go ridin’. (I’ve never seen porn, of course. I’m basing this on the Wikipedia entry on the subject.)

Evidently, there is a real-life sport called supercross, similar to motocross except that it emphasizes jumps, turns and other xxxtreme accouterments. Calling a movie “Supercross,” then, is like calling a movie “NASCAR” or “Soccer,” i.e., it’s stupid.

Speaking of stupid, our heroes are the Carlyle brothers, K.C. (Steve Howey) and Trip (Mike Vogel), both barely post-adolescent and living on their own in the California desert town of Palmdale. Dad was a supercross racer but now he’s dead; I don’t know what happened to Mom. Both boys are racers, but they have different outlooks. K.C., the older one, is safer and more responsible. Trip is reckless and impetuous. Their day job is cleaning pools, though sometimes Trip likes to take off his shirt and jump in instead. Tee hee! It’s fun!

The brothers are competitive when they race, but you can tell there’s a lot of love between them because they always call each other “dawg,” “dude” or “bro.” Still, Trip is jealous when K.C. gets a spot on a factory team, riding as the No. 2 man for frequent winner Rowdy Sparks (Channing Tatum). K.C. is told that he’s not going to win any races; his job will be to get in the way of the other riders so that Rowdy can win. But hey, it’s a factory team, with money and stuff.

Trip, meanwhile, acquires an over-freckled, too-blond girlfriend named Piper Cole (Cameron Richardson), one of the few female racers on the circuit. Her brother (singer Aaron Carter, who you will notice is not an actor) is a hotshot racer, too, as was their father (Robert Patrick) back in the day. The Cole family offers to sponsor Trip so that he can race against his brother and Rowdy. The ultimate goal is to reach the finals in Las Vegas, which Mr. Cole says is “the Super Bowl and the World Series rolled into one,” which makes no sense when you hear it and even less sense the more you think about it.

And through it all, there are motorcycles. Whether on supercross tracks or on private practice runs or in illegal street races, the boys will always find an excuse to give the audience what they want by riding fast and loud. The racing scenes, directed by stuntman-turned-auteur Steve Boyum, are not especially exciting, but they do have lots of motorcycles in them. Of course, if you really love watching motorcycles in action, there are probably DVDs you can rent that feature such activities exclusively, unburdened with plots or characters.

So intent is this movie on showing motorcycles riding and jumping and sliding that it sometimes forgets what meager plot it does have. In one scene, K.C. is pining for a girl he likes named Zoe (Sophia Bush). In the next scene, with no discernible time having passed, they are boyfriend and girlfriend.

The acting is laughably dull, to the point that I assumed some of the characters were being played by real-life supercross stars. That would have explained the stilted, ungraceful delivery of the hackneyed dialogue (written by Bart Baker and Ken Solarz). But no, nearly every role is played by an actor, not a motorcycle star, which leads me to my second theory, which is that everyone is just retarded.

D- (1 hr., 15 min.; PG-13, plenty of profanity, a little mild sexuality.)