The big bikes started sweeping into Downville, Md., on Thursday night, headed down Route 632 toward McCahon's Mill in Williamsport.

BIG bikes.

And they were stopping at the Downsvill stop sign.

If you're thinking Hondas or Kawasakis, those two-wheelers that impress little brothers and sisters, stop right there. Forget Harley-Davidsons too -- "Hardly-Davidsons," these folks call them. We're talking big bikes. We're talking BMWs.

"The only thing BMWs have in common with other bikes," declared one rider's t-shirt, "is the road."

A different kind of motorcycle. A different kind of motorcyclist.

"There's an old joke in BMW culture," said George Brosseau, a 50ish National Science Foundation administrator who participates in Washington area BMW activities. "A Kawasaki pulls up alongside a BMW and the driver says, 'Wanna race?' And the BMW driver says, 'Yaeah, to Miami.'"

BMW stands for Bayrische Motoren Werk, Munich manufacturer of the high-priced bike ($4,200 to $7,250 new) that has launched a thousand long-distance trips. Six years ago, the motorcycle division moved to West Berlin, but no one would dare touch the acronym. Folks believe in BMWs.

This weekend, 675 believers converged on William McMahon's log-cabin lodge and campsite for the 1980 International BMW Rally. A few hundred yards from the muddy Potomac they parked their bikes, hung out hammocks, and set up tents. Because of the price, BMW owners tend to be, in the words of Reston resident Brosseau, "a little older, more affluent and more managerial-class or skilled blue-collar" than the stereotype of a motorcycle rider. As a result, blue and white collars mix regularly at BMW rallies.

"That kind of background just fades into the background at a rally," he explained, sporting a full salt-and-pepper beard and jeans from neck to toe, his jean vest decorated with patches from past rallies.

Some business types immediately began peddling -- mostly cycle accessories like "Hippo Hands," to protect gentle hands from handlebar abuse. Other attened technical demonstrations run by "Oak" Okleshen, the man from Chicago who, BMWers say, can tell you everything about your bike except where it's been.

In a field full of striped shirts, sunglasses and beer bellies, with a CB specialist comparing engine size with a computer programmer at one tent and a lawyer shaking his head at a tall tale with a machinist at another, not a bad word could be heard about anyone there. "I've taken a stack of checks like this," said Bob Bates, a Vermont sheepskin dealer who sells at BMW rallies, holding his thumb and forefinger wide apart, "and I haven't had one bounce."

Unlocked bikes abounded. "The really good thing about BMWs," added Mike Vardy, who restores homes in Baltimore, "is that the kinda people who ride 'em aren't the kinda people who steal 'em."

Like the polite motorcyclist who waited patiently for "Oak" to finish his elaborate answer on what to do when "you're two-up and heavy in hot country" and then asked, "Would you care to make a comment about oil filters?"

Mostly, they let each other know how they felt about their BMWs. Without anyone asking.

"It's hard to be humble when you ride the best," read the shirt of Howard Sargent ("Just old Sarge, they call me"), a 68-year-old retired foreman from Parkersburg, W. Va. Sargent stretched out in a chair and listened as Jack Bresnahan, himself 62 but looking 50, told tales about BMW travels through three continents and 49 states ("the bottom was too rough" to Hawaii).

"One thousand seventy-eight miles in 20 hours," crowed Bresnahan, red bandanna wrapped around his neck, about a recent trip from New Ulm, Tex., to Knoxville, Tenn. "Cost me $81 dollars in Arkanssas, but who gives a damn." They talked about the old four-cylinder "Indian" motorcycles from the days when they were making $12 a week, and about giants of BMW history like Julius Kegel, killed at age 85 in Illinois when someone went through a stop sign. Kegel used to end all rally competition for the oldest-rider award just by showing up. Naturally, they ballyhooed their bkies.

"We pay a lot and we expect a lot," said old Sarge, nodding.

"We get more than we pay for," said Bresanhan, nodding back.

You can expect to pay a lot for a BMW -- if you can get one. Special U.S. models made to meet E.P.A. requirements are scarce at the moment. Used ones are advertised with signs like "Low mileage -- under 200,000 miles."

Brosseau says BMWs for people who ride seriously, which means on long trips or just about every day. The bikes, engineered to cruise easily at 75 mph "all day," encourage new owners to start thinking far. Once started, Brosseau gives a very scientific analysis of motorcycle culture. For instance, take Hell's Angels. Please.

"Hell's Angels are not motorcyclists," he asserts. "They ride on motorcycles once in a while, but they are not motorcyclists. These people read motorcycle magazines, take motorcycle courses." He admits that BMWers have "a real problem" with the identification of Hell's Angels and rough-tough Marlon Brando clones with motorcycles. "We represent the vast majority," he complains, "but they get all the press."

"That image is certainly being dispelled," agreed George Recker, principal trumpet in the Kennedy Center's orchestra for most operas and ballets. "Bmw drivers, I think, go out of their way to dispel it."

Hell's Angels would have felt out of place at McMahon's Mill, anyway. They'd probably have sneered at Saturday field events like "Two-Up Pin Drop" (passenger tries to drop clothespins in beer and soda cans without feet touching ground) and "Two-Up Unicorn Shower" (passenger with spiked helmet tries to break balloons strung on clotheline). As it was, master of events Walt Drame of Brewster, Ohio, was "scared to death" that there wouldn't be enough balloons.

After Sunday morning services, a fair share of the BMWers set their sights on Oregon, where an even bigger rally will be held shortly. Once on the road, BMWers will be able to rely on the BMW "blue book," a listing of phone numbers of more than 1,000 cohorts around the country who are ready to help out their fellow adventurers who have any mishaps in their territory.

So yesterday afternoon they began to streak out, past the Downsville Church of the Brethren, J. Carpenter, pastor, and its thought for the week: c"Love seeks not limits but outlets."