How to Test Vehicle Stopping Time

Calculating Vehicle Stopping Time

Brake testing methodology has evolved quite a bit over the years. The old days of start lines and measuring tape long ago gave way to the fifth wheel, which later lost ground to pavement-reading laser systems. Today, a mix of accelerometers and global positioning systems dominate testing, but even the cheapest of these instruments costs thousands of dollars, so the budget-conscious tester will want to either purchase one used or utilize a start-line-and-measuring-wheel approach instead.

Most reviewers, including "Car and Driver," test braking from 70 miles per hour (112.7 kilometers per hour) to zero. "Motor Week" and "Consumer Reports" test from 60 miles per hour (96.6 kilometers per hour), partly to stay in step with older tests made when the national speed limit was 55 miles per hour (88.5 kilometers per hour). Yet whatever devices or speeds you choose, you'll want to follow a few essential guidelines and strive for a uniform methodology.

Braking characteristics fluctuate over time due to a combination of heating characteristics, tire traction and brake fade, so experts suggest doing multiple tests close together. "Motor Week," for example, performs six straight-line, dry-track tests in quick succession, tossing the best and worst results (if the data spread exceeds tolerances) and averaging the other four. To gather their data, they employ a windshield-mounted gadget with built-in accelerometers and dynamometers that measure braking force, speed, distance and time, along with g-force and friction characteristics. According to host John Davis, a competent car's braking performance will not vary much over four to six stops.

Ideally, you should also put your brakes to the test under both wet and dry conditions. "Consumer Reports" performs some of its checks with one set of wheels on a wet track and the other set on a dry track, to test ABS versus brake performance. Wet track testing can be tricky, however, because consistent wet conditions require a good irrigation system.

Brake testing encompasses nuances beyond just stopping time and distance, however. "If it pulls to one side or the other, then the braking is improperly balanced," says Davis. "There shouldn't be any corrective steering needed, and the back end should not swing out."

Pay attention to the car's nosedive, too, he adds. Note the feel of the brake pedal, which should be firm but not hard, providing the driver with an instinctive feel for what the brakes are doing. The pedal and brakes should engage right as you step on them and furnish good feedback as you continue to push down.

Follow these tips and you'll soon have brake testing down to a science. Safe motoring!

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