In its long-awaited proposed rulemaking, the U.S. National Highway Traffic Safety Administration (NTHSA) concluded that stopping distances for heavy trucks can be improved by 20 to 30 percent using brake technology that’s readily available; specifically larger S-cam-actuated foundation brakes on steer axles.
The announcement silences any notion that new trucks will need the fatter torque output and fade resistance of air-disc brakes to meet stopping standards.
It also means you should expect to hear a lot about the development of wider, drum brakes and friction materials for steers, often seen as an underutilized source of brake torque. And to most truck owners, this is good news.
Current Canadian and American rules call for a tractor and unbraked trailer with a gross combination weight of 52,000 lb to be able to come to a full stop from 60 mph in no more than 355 feet. Brake engineers had widely anticipated that NHTSA would propose a 30-percent reduction in stopping distance for tractors, to 249 feet, a target only all-disc-brake configurations had comfortably achieved during testing. A tractor equipped with larger-capacity S-cam drums on the steer axle and standard S-cams on the drive axles registered an average braking distance of 269 feet. The same vehicle with disc brakes on the steer axle also failed to make the cut, with an average stopping distance of 263 feet.
By proposing a 20-percent cut in stopping distance instead of 30 percent, NHTSA is giving the S-cam a financial cushion, too. The average incremental cost to equip a three-axle tractor with air-disc brakes all around would be $1,308 US, NHTSA says, compared to $108 US for larger S-cam drum brakes.
More than 95 percent of commercial vehicles have some type of S-cam brake, in sizes ranging from 12.5 in. all the way up to 18 in. in diameter. They’re simple to understand, there’s plenty of good aftermarket parts, and they also do the job.
But with brakes on tractor drive axles already at their limit, the most logical place to find more brake torque is on the front axle, where the 12,000-lb static load doesn’t demand a bigger brake.
Most over-the-road tractors in North America mate 15 x 4-in. S-cam brakes with 5.5-in. slacks and a Type-20 air chamber on the front axle. The front axle may take on a dynamic load as high as 21,000 lb in during a heavy brake application or panic stop, easily enough to load up a 16.5 x 5-in drum brake, which provides 65 percent more wearable lining volume than a 15-in brake.
Roughly 10 percent of air-braked tractors have high-output drums on the steer axle. NHTSA’s tests of larger S-cam drums focused on two steer-axle brake packages: 16.5 x 6-in S-cams with a Type-30 air chamber and 5.5-in slacks; and Meritor 16.5 x 5-in Q-Plus brakes with a Type-20 chamber and 5.5-in slacks. Neither could reliably produce stopping distances that would comply with the 30-per-cent reduction.
As surprising as it may be to see NHTSA reduce the stopping-distance target to 20 percent is how the proposal doesn’t account for truck speeds greater than 60 mph. The legal speed limit for heavy trucks exceeds 60 mph in 42 states, and several states post a 75-mph speed limit for trucks.
While NHTSA tests at 60 mph showed relatively little difference in the stopping distance of a truck equipped with larger-capacity drums on the steer axle versus the same truck with discs, raise the speed to 75 mph and discs present a more compelling advantage. During tests conducted by ArvinMeritor, S-cam drum brakes stopped a NHTSA-spec test vehicle in 517 feet on average. The same tractor with disc brakes stopped in 345 feet, a 172-foot decrease.
Why? Heat will expand a brake drum about 0.010-in per 100 F. Stopping from 60 mph can increase drum brake temperature by about 600 F, causing the drum to grow by 0.060 in. Expanded drums require linings to move out further to meet the drum. This, in turn, requires more push-rod travel and more air needed in each brake chamber.
With disc brakes, a spinning cast-iron disc with air vents to shed heat is held on both sides by friction material. The iron heats and expands into, not away from, the calipers applying the friction material. Heat builds quickly, but is just as quickly transferred to the air that is cooling the disc, making it far less sensitive to fading.
NHTSA said no decision is forthcoming on electronically controlled air brake systems (EBS), although the agency is studying their performance. EBS uses air to actuate the brakes but delivers the commands signal electronically instead of pneumatically. The signals move to electro-pneumatic control valves much faster than pneumatic signals flowing through brake tubing, providing quicker brake application and release times and reduced stopping distances. Since brake force is evenly applied to each wheel, your brake linings and tires wear more evenly, which prolongs component life and reduces maintenance costs.
Also, EBS can be integrated with an electronic stability control system to apply the brakes and reduce speed when instability conditions are detected through on-board sensors and processors.
But FMVSS 121 applies to air-brake systems only and makes no accommodation for EBS. As a result it only can be used when the vehicle is equipped with a redundant pneumatic system as a backup. When the day comes for NHTSA and Transport Canada to consider EBS, it will require a completely new regulation.
The full text of NHTSA’s proposed rulemaking was published in the Dec. 15, 2005, edition of the Federal Register (www. gpoaccess.gov/fr). You can submit comments online at http://dms.dot.gov; by fax at 202/493-2251.
NHTSA says a two-year lead-time after the final rule is issued will be sufficient for manufacturers to comply. Following the comment period, the agency expects to announce a final rule by the end of the summer with an implementation date two years later, or mid- to late-2008.
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