SHOCK ABSORBERS ON mountain bikes are a must for smooth rides over bumpy terrain. Now, for tenderbottoms and others who want the ultimate in riding comfort, Active Control Experts Inc. in Cambridge, Mass., and K2 Corp., maker of Pro-Flex and K2 bikes, have developed a "smart shock." It features variable electronic damping that can respond instantly, while the wheel is still bouncing, to the size of each rock and bump. The technology is similar to that used in the shock absorbers of Formula One racing cars.

A thousand times a second, a sensor measures the speed of the shock's piston and sends the data to a microprocessor. In turn, this chip uses the signals to control a valve actuator that adjusts the flow of oil in the shock. The intended effect is to smooth out rides over rough terrain, thus allowing bikes to travel faster. The system also alleviates common problems with conventional mountain-bike shocks, notably their tendency to bounce like a pogo stick.

The smart shock weighs less than a pound, and is available on K2's 1998 full-suspension bikes costing $2,000 to $4,000.

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