MOUNTAIN BIKE OPENS VISTAS FOR ROAD RACER

For Canadian Alison Sydor, mountain biking is a welcome mid-course change in her life as a world-class athlete. She's still on wheels, only now they're the fat, nubby variety, not the linguine-like tires she had ridden on as a road racer.

When the mountain-biking world championships are held Sept. 23-24 in Germany's Black Forest, Sydor, who currently ranks No. 3 on the international mountain-bike circuit, will try to defend her women's cross-country title. That, in itself, is quite a feat for someone whose cycling career began shifting gears before the 1992 Barcelona Olympics, where she competed in the road-racing event.

''When you've put a lot of years into a sport and reached the highest level,'' she said during a competition at Mount Snow, Vt., earlier this summer, ''it's discouraging to realize that there's even a higher level men can go to that isn't there for women. In road racing, women don't have a Tour de France.... But in mountain biking I can do the World Cup, the world championships, the Olympics - everything the men do,'' including product endorsements, she adds.

Sydor says she played on many teams in elementary and high school, but was never a star. Road cycling was the first sport she showed real promise in, she says. She started racing in college while competing in triathlons for fun. Within a year she had made the B squad of the Canadian national team, then moved up to the ''varsity.''

A medalist at the 1991 world championships, Sydor was a pre-race favorite at the '92 Olympics. ''There was a lot of pressure, and I didn't deal with it very well,'' she says. She finished a disappointing 12th.

Instead of saying ''yes, yes, yes'' to all pre-Olympic demands made on her, she will scale back in 1996 with the help of a management group.