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!
For more information about vehicle stopping time and other related topics, follow the links below.
- 5 Signs That You Need Your Brakes Checked
- How Brake Failure Works
- How Brake Lines Work
- How to Check Brake Fluid
- How to Check Brake Pads
- Is brake flushing really necessary?
- How should your brakes feel under foot?
- Is it bad if your brake pedal goes to the floor?
- What tests work for diagnosing brake problems?
- What do the brake warning lights mean in my car?
- ASTM International. "ASTM E1337 - 90(2008) Standard Test Method for Determining Longitudinal Peak Braking Coefficient of Paved Surfaces Using Standard Reference Test Tire." (Oct. 21, 2010)http://www.astm.org/Standards/E1337.htm
- ASTM International. "ASTM F1649 - 96(2003) Standard Test Methods for Evaluating Wet Braking Traction Performance of Passenger Car Tires on Vehicles Equipped with Anti-Lock Braking Systems." (Oct. 21, 2010)http://www.astm.org/Standards/F1649.htm
- Consumer Reports. "How Consumer Reports Tests Cars: Braking." (Oct. 19, 2010)http://www.consumerreports.org/cro/cars/how-we-test/braking/braking.htm
- Davis, John. Host of "Motor Week." Personal correspondence on Oct. 22, 2010.
- Federal Motor Carrier Safety Administration. "Part 571.135: Standard No. 135; Light Vehicle Brake Systems." (Oct. 27, 2010)http://www.fmcsa.dot.gov/rules-regulations/administration/fmcsr/fmcsrruletext.aspx?reg=571.135
- International Organization for Standardization (ISO). "TC 22/SC 2 - Braking Systems and Equipment." (Oct. 21, 2010) http://www.iso.org/iso/iso_catalogue/catalogue_tc/catalogue_tc_browse.htm?commid=46734
- Martin, Elly. Spokeswoman for the National Highway Traffic Safety Administration. Personal correspondence on Oct. 26, 2010.
- National Highway Traffic Safety Administration. "Federal Motor Vehicle Safety Standards; Hydraulic Brake Systems; Passenger Car Brake Systems." Federal Register. Sept. 5, 1997. (Oct. 20, 2010)http://www.federalregister.gov/articles/1997/09/05/97-23318/federal-motor-vehicle-safety-standards-hydraulic-brake-systems-passenger-car-brake-systems
- National Highway Traffic Safety Administration. "Light Vehicle Antilock Brake Systems (ABS) Research Program." (Oct. 22, 2010)http://www.nhtsa.gov/Research/Light+Vehicle+Antilock+Brake+Systems+%28ABS%29+Research+Program#task4
- Vericom. "VC3000 Performance Testing Computer / Braking Test Computer / Data Acquisition System /On-Board Dynamometer." (Oct. 27, 2010)http://www.vericomcomputers.com/Support/18_page_brochure_Dec_06.pdf
- Webster, Larry. "The Power to Stop - Tech Stuff." Car and Driver. August 2008. (Oct. 20, 2010) http://www.caranddriver.com/features/08q3/the_power_to_stop-tech_stuff