They ride their bikes over and through creekbeds, hills, trails, ponds, and grassy fields. When they run into obstacles they can't pedal through or across, the cyclists dismount and carry their 10-speed steeds up, over, or around. After the race, their bodies are caked with mud head to toe.
The event is cyclo-cross, a newcomer to the American sports scene. One racer described it as a steeplechase on bicycle.
In contrast to conventional racing around a looped course, cyclo-cross competitions are an hour long, with contestants covering about 10 miles over natural obstacles and such man-made hindrances as logs and chains. Racers sometimes carry their bikes as much as 25 percent of the distance.
''With (conventional) bike racing, it's how well you handle your bike within a large group of riders,'' said Matt Smith, 18, the 1982-83 California junior champion who vacated his throne this year to compete in the senior division. ''In cyclo-cross it's more individualistic. You're riding against the terrain.''
Veteran Sacramento, Calif., cyclo-crosser Bob Edwards stressed handling skills as the path to success. ''You have to be skilled in running and riding, plus you have to have a lot of endurance. You run up hills carrying your bike, then you pedal down as hard as you can. If you've ever ridden across a wet, grassy field, you'll understand how tough it is.
''Anybody can ride in hard, compacted dirt. But when you're sliding down a hill with your rear wheel locked up, you've got to know what you're doing or you'll go sliding down in front of the bike.''
An 81-year tradition in Europe where crowds of 30,000 to 50,000 watch races, cyclo-cross has been taken seriously in the United States for only about 10 years.
It was first used by US cyclists as a fall-winter conditioner for their No. 1 love, summer racing. But now, for many, it is becoming the primary form of racing.
From 1,000 to 2,000 spectators cheer cyclo-crossers at the annual nationals, which have been held since 1975 and were contested this year in Plymouth, Mass. Steve Tilford of Topeka, Kan., a top-five finisher in both 1981 and 1982, put it all together this time to win the title of US champion. Second place went to Clark Natwick of Pacifica, Calif., who had also finished in the runner-up spot a year ago after winning the title in 1981.
Although US cyclo-cross racing ends at Christmas, the top finishers at Plymouth will continue to train for the world championships next February in Europe.
Competitors have to get used to looking messy, because mud is a key element in the makeup of any cyclo-cross course. Cyclists don't mind; in fact, they revel in it and refer to themselves as ''mud hogs.''
''The reason most of us like the mud is that it separates the men from the boys,'' Edwards said. ''When you're up to your axle in mud, it comes down to the skill and strength of the rider.''
Mud also turns even the worst-looking spills into comic plops.
''Compared to road racing, crashing in mud is fun,'' Smith said. ''You can have a spectacular crash and fly over your bike, turning head over heels, and just get back up and ride off.''
Cyclo-crossers are lured to the sport by its ruggedness. ''You have to carry your bike up a steep hill when you're caked with mud,'' Smith said. ''Your arms get tired. Your legs get tired. And then you have to ride down the hill. The extreme physical exertion is the reward of the sport.''
Natwick concurred, noting that since much of the season comes in winter, there is the frequent challenge of bad weather.
Asked the most crucial attribute of a cyclo-crosser, Edwards replied, ''Courage - or to be completely crazy. There's a motocross saying we use, 'When in doubt, wind it out.' In our case it means to let go of the brakes.''
Even though the numbers of racers and spectators are rising, both are minuscule compared to those in other sports.
Natwick said cyclo-cross has the potential to become a ''phenomenal'' spectator sport. ''There is one thrill after another,'' he said.
But Smith noted that the elements attracting bikers drive the fans away.
''With criterium racing,'' he said, ''you can put the course right in the center of town. With cyclo-cross, you go off away from everything. It's outside, with no protection from the rain, and it's cold out.''
Self-satisfaction is the cyclo-crosser's biggest reward. Cash awards, if any, are dwarfed by those in other sports.
Natwick, who came in 39th and 44th in the world championships the last two years, says this lack of money makes it hard to compare US and European racers.
''There is as much money in cycling in Europe as in football and baseball here,'' he said. ''I feel we have the capability, but there are no incentives for American athletes to develop cyclo-cross skills.''