The Importance of 'Rollout'

The first few inches of a drag strip.
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Should the owner of one of these performance meters want to compare its performance against the authoritative equipment used at drag strips, he or she will need to deal with "rollout," which is the distance a vehicle can move before triggering the timing lights.

A little drag-strip primer: As a car creeps toward the two light beams of the starting area, its front tire will eventually block the first beam and trigger a "prestage" warning. This informs the driver that the official starting line, where the "stage beam" shines across the track, is only seven inches ahead. When the front tire triggers that second beam, the vehicle is properly staged for a run. However, the vehicle's position can still vary by well over a foot while its front tires are blocking the stage beam. This is critical, because the drag-strip clock does not start until the second light, or stage beam, is uncovered. It is this distance that is the critical rollout.

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The length of the rollout depends on the diameter of the tire and where the driver chooses to position the car at the start. Although the rollout distance is typically only a foot or so of a quarter-mile, it can affect the elapsed time by as much as 0.3 second, and to the serious gearhead, that's an eternity. Therefore, we needed to make sure every run in our test started at precisely the same point because of that 0.3-second issue.

The meters presented some problems in this area. They didn't all deal with rollout in the same way. Most had adjustable distance rollout. One used speed rollout that triggered the meter's clock when the car reached a speed threshold, and one had a nonadjustable 12-inch rollout. Another had no rollout at all. To keep things even, we positioned the cars on the track with a 12-inch rollout for all the tests because that distance matched up with most of the meters. Also, a car accelerates to 3 mph in about 12 inches, so that distance worked well for the meter with speed rollout.

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