Comparing S-Cam and Air Disc Brakes
Many U.S. Commercial Drivers Are Still Spooked About Aggressive Front Brakes.
Jim Winsor
Executive Editor
The venerable S-cam air brake has been around for more than 65 years and has undergone many, many technical and performance improvements. In North America, it's the standard air brake - cost effective, reliable, easy to repair and offering amazingly high mileage between relines.
By contrast, air disc brakes are standard on virtually all medium and heavy duty trucks, tractors and trailers in Europe. These brake preferences continue to be among the principal equipment differences between the two continents.
This doesn't mean that U.S. manufacturers are asleep at their computers. Far from it. North America's truck manufacturers and the braking industry all have aggressive air disc brake programs. In fact, several U.S. truck builders offer optional front air discs brakes for about $1,200-$1,400. A number of low-volume specialty vehicle manufacturers in the fire/rescue fields use air discs on all axles as does one top-of-the-line coach builder.
Air discs are expensive compared to S-cams, and price is the main reason we haven't seen more of them. A second reason is that the brake industry continues to improve its S-cam products, making the change to air discs less appealing.
As you'll see below, the use of larger or wider front S-cam brakes and high-performance linings for all wheel positions have significantly reduces stopping distances. Bigger front brakes (producing higher front-brake torque) is the principal gain to shorter stopping distances because rear brake torque is just about maxed out in most applications.
If you're interested in better brake performance at a relatively small premium, look t to the availability of 16 1/2 x 5-inch S-cam front brakes with 24-square-inch chambers. Where available, the option usually is in the $200-$300 range.
If you're interested in performance and longer lining life, look at the 16 1/2 x 8 5/8 drive axle and trailer axle brakes. They're also available with so-called high-performance linings, which are more aggressive.
NHTSA has a goal of getting big trucks to stop in about 30% shorter distances than currently mandated, in order for big rig stops to be more in line with passenger car stopping distances.
Europe already has high-torque front brakes (air discs) compared to the U.S. Many stateside commercial drivers are still spooked about aggressive front brakes. I think much of it is based on horror stories from drivers 30 and 40 years ago, who recall locking up on wet pavement when braking hard, losing steering control and piling up.
Most of you reading this are probably unaware that "delete front brakes" were a popular option on three-axle tractors and were perfectly legal well into the '70s. This made drivers happier - even if it took longer to stop. And for rigs with front brake, many drivers in earlier years would back off front brake adjustment so brakes couldn't lock up. That just wasn't legal.
With today's braking systems, anti-lock brakes and future optional stability systems coming on the market, there's no reason for drivers to fear S-cam front brakes. The only exceptions might be brake fade during severe braking at high speeds, or front-wheel pull from wet brakes when driving on flooded highways. In both cases, air discs have a clear advantage.
Data from tests done by ArvinMeritor engineers compares the stopping distances between standard S-cams, all-wheel air discs, and air disc fronts with standard S-cam rears with performance linings. Tests were on stops using tractor brakes, no trailer brakes at 60mph. Gross vehicle weight: 56,500 pounds. Maximum distance allowed: 355 feet
• With 14 x 4-inch fronts w/20 chambers and 16 1/2 x 7-inch rears w/30 chambers, stopping distance was 267 feet average, 263 feet best.
• With air discs front & rear, stopping distance was 204 feet average, 199 feet best.
• With air discs front and 16 1/2 x 7-inch rears w/performance lining, stopping distance was 214 feet average, 209 feet best.
Note what air discs up front do to shorten stopping distances.
