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From the pages of Velo: Getting the most from your post

Deflection and Damping: A Quick Lesson

Deflection, or flex, and damping, or vibration absorption, are both aspects of suspension. Flex is a static quantity based on load. With a rider’s weight applied, a seatpost flexes so that the saddle moves in an arc, which we measured on both X and Y (vertical and horizontal) axes. The bike’s fork was mounted to a steel test table and the rear tire, pumped to 120psi, rested on top of the table. We used dial indicators to measure the postion of the saddle with and without a rider; no additional force was applied; the difference between the two measurements is the amount of flex.

Damping is related to the speed of movement, or how quickly the post flexes. When riding, the amount the seatpost and saddle are flexed has little relevance over the small bumps on a rough road, but vibration damping insulates the rider from the harshness of the road. On a completely smooth surface, the stiffest seatpost would be most efficient; it would move the least in response to pedaling motion. When hitting big bumps, though, the total amount the seatpost and saddle can flex will protect the rider from a potentially damaging jolt, while the damping decreases how much it flexes back and forth after the hit.

Damping was measured while riding a set of rollers with 1/4-inch tall ridges, welded longitudinally along the front roller and the forward of the two rear rollers. This produced a high-frequency vibration similar to riding fast on a bumpy road. An accelerometer mounted to the saddle rail took 1,000 readings per second (1,000 Hz) and measured the magnitude of the up-and-down acceleration of the saddle rail, which was then converted into a G-force measurement; 1 G is equal to the acceleration due to gravity. We measured for 30 seconds, garnering a total of 30,000 data points per vibration test for each seatpost. A lower number of Gs for any given seatpost would indicate more vibration damping by the seatpost and saddle, and therefore less transfer to the rider.

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