The National Highway Traffic Safety Administration (NHTSA) has amended an eight-year-old rule to add braking-in-a-curve testing for medium- and heavy-duty trucks that are required to be equipped with antilock brake systems (ABS) to improve directional stability and control during braking. The braking-in-a-curve tests were conducted by NHTSA on a low friction wetted surface. The test curve had a 12-foot-wide lane with a 500-foot radius of curvature. The results indicated that the braking-in-a-curve test is practicable, repeatable, and safe for single unit vehicles.Testing also showed that as long as wheel lock-up is prevented on at least the rearmost axle and the steer axle, the vehicle would remain stable during the braking-in-a-curve maneuver.The amendment to the brake-testing rule is effective October 10, and petitions for reconsideration will be accepted until September 25. The amendment will affect trucks built after July 2005, and trucks built in two or more stages after July 2006.Alan Korn, chief engineer for ABS brake maker Wabco Automotive Parts Group, said that many manufactures have already been conducting braking-in-a-curve testing though the government hadn't required it."There had been no real performance requirements for straight trucks before that, and NHTSA likes to have requirements as opposed to standards," said Korn. "It is really going to have minimal effect on the way our ABS brakes are made."Korn said the testing is done to analyze the stability and controllability of single-unit trucks with a minimum GVW of 10,000 lb. The trucks are tested at the highest speed that they can go through the 200-foot arc, and then the brakes are applied just as a driver would in a full panic stop. The test is done four times, and the truck must stay within the 12-foot wide lane three times.