SOMERVILLE, MASS. — TIRE treads along nature trails are a common sight today, an unimaginable situation 10 years ago. Knobby-tired ``mountain bikes'' are taking to the trails at warp speed. Roughly half of the 10 million bicycles sold in America today are mountain bikes, says John Fritz, marketing director of the League of American Wheelmen (LAW) in Washington. Nearly eighty percent of the sturdy vehicles never see terrain more rugged than a pothole, he says.
But that other 20 percent has some hikers, horseback riders, and environmentalists protesting their ``invasion'' - while mountain bikers defend themselves and set out to repair an image of recklessness.
According to LAW, there were 250,000 mountain bike riders in 1982 - and 7.5 million six years later. Competitions are popping up throughout the United States, and popularity has grown in Europe as well.
``Never before did a lot of hikers have to share trails with mountain bikers, and on top of that you had some bikers not being so careful,'' says Chris Ross of the National Off-Road Bicycle Association in Colorado, explaining how mountain bikers first got a bad rap.
Environmentalists, too, feared the activity would cause extra erosion, a claim mountain bikers universally reject. ``No one has proven that mountain biking is more detrimental than hikers and equestrians,'' says Scott Nichol, a master frame builder at Ibis mountain bikes in Sebastopol, Calif. ``When properly used,'' that is, when not skidded down trails, ``they don't cause erosion,'' says the long-time biker, who has a degree in soil science.
We're not ``high-speed, hormone-enraged adolescents bombing down the trails,'' says Heidi Davis Fulk, a mountain bike racer and organizer of the New England Mountain Biking Association (NEMBA). Ms. Fulk, who works as an environmental analyst in Boston, doesn't see a problem with erosion with mountain-bike use in general. If certain areas are prone to erosion, such as in wetlands or when ground water is high, she's in favor of limiting access.
Tom Lennon, national trail coordinator with the United States Forest Service, says he approaches mountain biking like any other other activity. After listening to both sides, ``we've tried to assess the degree of conflict and whether or not they are creating resource problems ... and overall, it does not appear to be a serious problem,'' he says. Some restriction is needed, he adds, and there have been complaints from intensively used local areas. But ``it seems to be people who feel very strongly about not sharing `their' trail with these bikes,'' says Mr. Lennon.
Mountain bikers do need to adhere to etiquette on the trails, says Mr. Ross, and not ride in inappropriate areas. On the other hand, environmentalists, hikers, and horseback riders can realize that mountain bikers want to enjoy the trails, too, he says.
But highly populated trails still have problems with ``two-wheel terrors,'' as Newsweek called them in 1987. Sally Reid, vice- president of the Sierra Club, refers to ``huge conflict'' areas in California, for instance, especially along steep, well-used trails. She's heard ``hair-raising stories'' of hikers being run off trails by packs of mountain bikers, sometimes as many as 12 in a group. Trail maintenance workers complain about the gross erosion in such areas, she says. And bikes zooming by at high speed can ruin hikers' aesthetic enjoyment, she adds.
On Jan. 1 of this year, California - regarded as the birthplace of the mountain bike - adopted a policy stating that bicycles are not allowed on trails in state parks without written authorization from a district superintendent. The policy goes into effect in September.
Mountain bikers, while agreeing that some trails should properly be off limits, see other trail restrictions as discriminatory. They have organized land access lobbies to fight back.
``There are people going out [mountain biking] that have a less-enlightened sense of the environment,'' says Mark Langton, associate editor of Mountain Biking magazine and organizer of CORBA - Concerned Off-Road Bicycle Association, in Van Nuys, Calif. Those riders can be reached and educated, he says. But even so, the problem won't completely disappear: Just as you have irresponsible highway drivers, you're bound to have irresponsible trail riders, he says.
CORBA and about 35 other organizations across the US hope to establish responsible behavior among bikers and spread goodwill.
One such effort is the code of ethics adopted and adapted by mountain bike organizations that includes such statements as: ``I will be courteous to other trail users, stay on designated trails, wear a helmet, maintain control of speed, respect property, and ... not litter. People judge all cyclists by my actions.''
IN Marin County, Calif., mountain bike volunteers organize roadblocks at which they caution other bikers about speed limits and remind them to dismount and walk by horses so as not to spook them. ``It's been effective in a self-policing way,'' says Nichol, the frame builder. In many places riders are also asked not to exceed eight to a group.
But responsibility doesn't stop at trail use. Mountain bikers are offering their services in trail building, cleanup, and maintenance, which puts another spoke in their wheel of goodwill.
``We're one of the major users, and we should help,'' says Heidi Davis Fulk matter-of-factly. Mountain bike patrols have been formed; park rangers have begun riding them, too, as a quicker way to travel trails.
Mountain biking needs ``a little tolerance, and more effort on the part of mountain bikers to slow down and be conscientious of the big picture,'' says Langton of CORBA. ``Time is on our side.''
NEMBA's Marc Abrams says, ``If people were educated to have fun, but be cautious, it could be a perfect thing.''