One of the objectives of the trucking industries in the United States and Canada is to reduce the gap between the stopping abilities of passenger cars and those of heavy trucks. Trucks require more space to stop than cars, and when the space isnâ€™t available an accident is nearly guaranteed. The National Highway Traffic Safety Administration (NHTSA) took a step toward this goal in 2009, issuing new braking standards for commercial truck tractors to minimize brakes-related crashes. The main tenant of the new required stopping distance is that a tractor traveling at 60 miles per hour (MPH) must be able to stop in 250 feet. This is 105 feet less than the previous requirement, aÂ 30 percent reduction. The transition will be completed from August 2011 to August 2013, with all vehicles in use before that considered exempt from the standards.
How Will the Trucking Industry Meet These Standards?
Currently, the majority of tractors in the United States use drum brakes. Air disc brakes offer a shiny alternative, but American companies have been slow to accept them. Disc brakes boast many improvements on drum brakes, but the price for their greater efficiency is about $200 more per unit.
Brake manufacturers Bendix Spicer say benefits of air disc brakes include
- Nearly40 percent reduction of stopping distances when compared with drum brakes
- Elimination of brake fade, a common problem with drum brakes
- No increase in the wear of disc pads on a tractor or brake linings on a trailer
Last year, Peterbilt Motors became the first manufacturer in North America to offer air brakes as standard equipment on their heavy truck Model 587. According to Bedix Spicer, more than 80 percent of the commercial truck market in Europe relies on air disc brakes. The U.S. is still a long way behind Europe in terms of accepting disc brakes. However, disc brakes may be the only alternative that will enable trucking companies to comply with the new brake regulations.
Truck Brakes vs. Car Brakes
Trucks have a considerably different braking system than cars. Car brakes rely on a hydraulic system, in which fluid flows through the brake system to engage the brakes.
Trucks have either drum or disc air brake systems. Truck brakes utilize compressed air; the fact that air will never run out like fluid can makes the brakes more reliable. Many newer trucks have a dual air brake system, so that if one set of brake controls fails, the other will work to stop the truck. Although the brake system in a truck is more reliable than a carâ€™s, it does have a downside: brake lag. As a part of their training, truck drivers must learn that the brakes donâ€™t work immediately as they do in a car. Lag time is less than a second, so drivers just learn to slightly adjust their expectations when braking.
How do Disc and Drum Brakes Compare?
Once the drum brakes in a truck are engaged, compressed air moves into a brake chamber through the air lines. The air forces out a pushrod, which pushes the slack adjuster. The slack adjuster turns the camshaft, which twists the S-Cam, effectively pressing the brake linings against the brake drum and stopping the truck. Unfortunately, when the brake drum gets too hot, it expands away from the brake shoes. This can lead to brake failure, an extremely dangerous event.
Disc brakes utilize the same system as drum brakes, except the S-Cam is replaced by a power screw. The power screw is turned, similarly to how the S-Cam was twisted. When it turns, it grasps the disc or rotor between a caliperâ€™s brake lining pads. Since there is no drum in disc brakes, the risk of brake failure diminishes.
NHTSA Support for Disc Brakes
NHTSA believes that using air brakes will result in fewer collisions due to their superior stopping power. As the regulations set in 2009 begin to take shape, innovations will have to be made to the drum brake system or it appears that disc brakes will lead the way towards better brake safety.
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