The bicycle, which hasn't changed in basic design since the chain-driven "safety bicycle" replaced high-wheelers 100 years ago, is about to acquire a dramatic new look and capability. Just ahead, say bicycle designers, is an era in which cyclists whisk along city streets at automobile speeds on two-wheelers they pedal as they lie on their backs.

The derailleur, introduced nearly 80 years ago as the key to the 10- speed cycle, may be displaced by the "stepless transmission," a device that offers the rider easy shifting into an infinite range of gear ratios between today's low-low and high-high.

Other modifications may convert pedaling into an action more like running, dramatically improve bicycle braking, increase speed and reduce maintenance to a brief and minor chore.

Here are the major innovations: The Recumbent

If sitting on a bicycle feels like straddling a fence post, riding a recumbent is more like settling back in an easy chair.

But comfort is just one of the many virtues of the recumbent. The hunched-over posture of a cyclist on a conventional bike creates tunnel vision. Switching to a recumbent is like trading in a porthole for a picture window.

The recumbent is also considerably faster than conventional bicycles because the low profile cuts wind resistance, which experts say accounts for more than 80 percent of the total drag at speeds higher than 18 miles per hour.

But safety is the reason that MIT professor David Gordon Wilson is pushing recumbents. On the recumbent, it is virtually impossible to fly over the handlebars, and in the event of a front-end crash, the rider would land on his feet instead of his head, says Wilson.

Other are less sure about the safety of the recumbent. Many observers fear that the low profile would be invisible in traffic. Wilson, who has commuted by recumbent for years on Massachusetts Avenue in Cambridge, Mass., (which is every bit as busy as Massachusetts Avenue in Washington), never rides without a flag attached to his bike.

Wilson, who cofounded a company some years ago that sold recumbents for well over $1,000, plans to start turning them out for the masses in a year or so. Price tag: between $200 and $300. The Moulton

With its tiny wheels, it looks too cute to win races. But it won a criterium last summer in Toronto, an event that emphasizes handling and acceleration. Bicycling magazine called the Moulton "the most versatile bike on the road." That review made it sound like the equivalent of a car with an Audi's engineering, a Mercedes' ride, a Porche's handling and a Toyota minivan's practicality.

Alex Moulton originally designed his bicycle for utility. He shrank wheel size to 17 inches mainly to make space for carrying racks, but the miniwheels turned out to be the germ of the Moulton's exceptional performance. Their lightness eased acceleration and their small size made suspension necessary, says engineer Moulton, who had earlier designed the suspension of Austin Minis, the original "econobox" cars. The result was a bike whose handling Bicycling called "a revelation to someone who has ridden only standard bikes." The Powercam

Some people say the bicycle is unnatural. They claim legs were meant to walk or run, not to go around in circles.

Not me. I've ridden more than 20,000 miles in the 1980s. To me, "spinning," which is what cyclists call pedaling at 90-plus rpm, is the most natural thing in the world.

But Larry Brown, of Houdaille Industries in Texas, doesn't agree. Brown, who has an engineering degree from MIT, has invented a device called a Powercam that offers cycling legs a motion that more closely resembles running.

The cyclists who like it best, as you might guess, are runners. Triathlete Elliot Malach says the Powercam complements his running so well that he has been able to increase his riding from 200 to 300 miles a week, and cut his running from 90 to 35, and still come out ahead in the foot races. Me, I don't run. Improved Shifting and Braking

In a bid to unseat Campagnolo as the Rolls-Royce of bicycle parts, the Japanese manufacturer Shimano has just redesigned its "Dura-Ace" line of components with the help of a computer.

The new brakes feel like power brakes on a car, butter-smooth with a light, sensitive touch, says Richard Nazario, sales coordinator for the Japanese manufacturer Bridgestone.

The new Dura-Ace derailleur system takes the grope and grind out of gear-shifting. A ratchet and pawl unit stops the shift lever at the point that will put the chain precisely on the next gear. A special cable adjusting screw on the derailleur allows the unusually precise adjustment that this requires. The Dura-Ace derailleur has a design that keeps the guide pulley (the mechanism that moves the chain from one sprocket to the next) a precise, short distance, from the sprocket as it moves up and down the gears. This ensures that the chain catches immediately on the sprockets, for a clean shift. Stepless Transmission

Even so, derailleurs could become obsolete in about three years if Bridgestone perfects its stepless transmission.

Instead of 10 speeds, or 15 speeds, the stepless transmission offers an infinite number of gear ratios within a range exceeding that of most 15 speeds. You could shift equally well with the bicycle standing still, or under full power during hill climbing.

The stepless transmission could change the automobile even more than the bicycle, says Bridgestone's Nazario. A car that changed speed by changing gear ratio instead of engine revolutions could have a much simpler engine design than a conventional auto.

The current prototype is a five- pound disc-shaped assembly that sandwiches between chainring and crank. Bridgestone hopes to reduce the weight to two pounds, and to shrink the size enough to fit the transmission into the rear hub, like a three-speed Sturmey-Archer. Belt Drive

Here's an innovation that would do away with hours of maintenance, as well as gear tracks on pantlegs. Bridgestone has developed a cog-driven belt drive to replace the chain. It may actually be slightly more efficient than a chain for transmitting power to the rear wheels, but it won't work with a derailleur. However, if Bridgestone's stepless transmission proves successful, the two could become an unbeatable combination.

But for those of us who enjoy working on our bicycles, the belt drive would take away the fun. We love the ritual of cleaning chains and sprockets the way coffee fiends love to hand-grind their own. Why, just the other night, Jack came over and we undid our chains, removed sprockets from freewheels and chainrings from cranks. Then we cleaned the mess in solvent. By the time we had finished, everything sparkled. The Bridgestone belt drive, coupled with the stepless transmission, would do away with all that. No oil, no soil, no toil. The Faring

When I was an adolescent, I had a recurring dream. I'd take my bicycle onto the highway, start spinning like crazy, and pretty soon I'd be in the fast lane, passing everything in sight.

I never dreamed that anyone would actually have the pleasure of such an experience, but in 1980 a tandem tricycle averaged 50.5 mph for 40 miles on Interstate 5 between Stockton and Sacramento, Calif.

The piece of technology that has made it all possible is the faring. On a conventional bike, at 30 miles an hour, about nine-tenths of the effort goes to pushing the air out of the way. The human-powered record holders have full farings, transparent plastic shells that entirely enclose the tricycle. (Generally, tricycles are used with full far- ings, to insure stability in crosswinds.)

It is unlikely that anything resembling these human-powered record-holders will ever replace the bicycle. Their weight and width make them poor hill-climbers and cumbersome in city traffic. But one company, Zzip Designs of California, markets small handlebar- mounted farings that look sort of like motorcycle windshields. They can cut your wind-resistance by 20 percent, according to the manufacturer, enough to add a couple of miles an hour to top speed. Other farings are being developed that can partly enclose rider and bicycle for further reduction in wind drag.