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Test results support advantages of air disc brakes over drum brakes

Bendix Commercial Vehicle Systems LLC , a member of the Knorr-Bremse group, has released test data that confirms its current generation air disc brake product has performance advantages over S-Cam drum brakes.

The data was part of a presentation given at the Society of Automotive Engineers (SAE) International Truck and Bus Meeting and Exhibition in Fort Worth TX recently.

Eighty-four percent of states in the United States have commercial vehicle speed limits in the range of 65 to 75 miles per hour. In light of this, Bendix has been conducting tests of air-disc-braked vehicles and drum-braked vehicles at a range of legal speeds up to 75 mph.

Results showed that performance advantages of Bendix air disc brakes over drum brakes were even more pronounced at the higher legal speeds.

In the testing sequence, two tractor-trailers were driven side-by-side on a closed test track with simultaneously applied full brake pressure to stop the vehicles at 75 mph. One tractor was equipped with a typical S-Cam brake setup, including 15" × 4" front brakes and 16.5" × 7" rear brakes. The other test tractor had front and rear axle Bendix ADB225 air disc brakes with 17" rotors.

Ron Plantan, principal engineer for Bendix air disc brakes, said, “During the side-by-side testing, data showed the stopping distances for the Bendix air disc brake-equipped truck was within the range of 305 to 325 feet. The drum brake-equipped truck stopped in the range of 450 to 518 feet initially when cold, but as the drums heated up, the stopping distances became progressively longer. Stopping distance for the hot drum brake-equipped vehicle exceeded 750 feet, while the air disc brakes consistently stopped at around 320 feet.

“The performance advantages for air disc brakes at higher speeds are particularly noteworthy when considering that during nighttime driving, low-beam headlights only provide 350 feet of visibility,” said Plantan. “This is within the range of the air disc brakes' ability to stop a vehicle, but is not the case for drum brakes.”

At 60 mph, the air disc brake-equipped truck stopped in the range of 185 to 210 feet in both the hot and cold brake temperature conditions. The drum brake-equipped truck stopped in the range of 255 to 292 feet with cold brakes and more than 425 feet with hot brakes.

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