Some of the units we tested have multiple accelerometers capable of taking measurements in a lateral as well as longitudinal direction and can therefore also measure cornering grip. Change your tire size or run too little or too much air pressure, and you'll get inaccurate readings. The problem is that the wheel-speed sensors on new cars are calibrated precisely to the diameter of the car's original tires. Using wheel speeds from the computer, the PDA-Dyno calculates speed and distance. One of the seven performance meters, the Nology PDA-Dyno, relied on this method. Racelogic, the company that makes the VBOX, asserts that its devices are accurate to within 0.06 mph.Īnother way to measure performance involves tapping into a car's onboard computer. By measuring this shift, the VBOX calculates speed, acceleration, and distance.
As a vehicle travels down the test venue, there's a minuscule shift in the arrival time of the radio signals that travel between the satellites and the VBOX's antenna. To work, the VBOX must be in communication with at least four of 24 GPS satellites orbiting the earth. Instead, the VBOX relies on the global-positioning system (GPS) to measure a vehicle's motion. Our sophisticated VBOX system doesn't use accelerometers, precisely to avoid this problem.
However, if you want to determine whether one car performs better than another, you'd better budget some time to calibrate the meter for each vehicle. You can ignore this effect if you simply want to find out whether a modification has made your car faster or slower. Under hard acceleration, for example, a car's tail squats and its nose lifts, so the accelerometer on the windshield is pointing slightly upward rather than horizontally down the road in the direction of the car's motion. The unavoidable pitch and roll motions of a vehicle being driven hard can upset the readings since they alter the plane in which the accelerometer operates. Simply put, if you can monitor acceleration in relation to time, you can calculate speed and distance traveled.Īccelerometers have one big drawback, however. An accelerometer is a device about the size of a wristwatch battery that produces an electrical signal that changes under acceleration. Six of the seven units we bought use internal accelerometers and an electronic timer to measure performance. (For you cynics, we had a nonstaffer buy the units to ensure we got representative examples.) In fact, we wound up making more than 100 quarter-mile passes at a drag strip 1000 miles away in Gainesville, Florida. Our mission was to find out if they work, and it was not easy. Most of these meters simply attach with suction cups to the inside of the windshield, like radar detectors. Because they can get pricey, we imposed a $1000 cap our seven ranged from $90 to $960. So we got our mitts on seven of these performance meters. In the past decade, a number of companies have sprung up that sell relatively inexpensive devices they claim can accurately measure a car's performance. Although testing is the most entertaining part of our jobs, we do take it seriously enough to have forked over more than $30,000 a few years back for three state-of-the-art VBOX testing systems made by Racelogic that provide the performance numbers essential to our road tests.īut perhaps there's no need to spend so much. "What'll it do?" That's the big question heard over and over during the course of C/D 's instrumented testing of some 130 cars a year.