Bottom Line
Stopping shorter
Braking technologies rise to the challenge of newly mandated reduced stopping distances

By Paul Abelson, senior technical editor

That car that cut in front of you and slammed on its brakes can probably stop from 60 mph in 130 feet or less. If your truck is typical and you did a maximum brake application, you could probably stop in about 300 feet, a bit less if you have an engine brake turned on.

Do the math. You're still slowing down while the car is stopped in front of you. There's a law of physics that states that two objects cannot occupy the same space at the same time. Since your truck is bigger, heavier and stronger, it will move the car out of the way, with obvious and disastrous consequences.

That scenario is, thankfully, a relatively rare occurrence, but it illustrates the thinking that went into the braking regulations that became effective Aug. 1.

The new stopping distances are mandated for model year 2012 tractors, with straight trucks and specialty vehicles to be included in future years. The National Highway Transportation Safety Administration determined that, with the use of existing technology, differences in stopping distances can be reduced. Just as with speed limit differentials, the smaller the differences, the fewer crashes we'll have.

NHTSA does not have the authority to require vehicles to be retrofitted, so your pre-2012 trucks will not be affected.

The laws of physics tell us that energy cannot be created or destroyed. If we want to change an existing condition, such as the speed of a truck, we have to convert its energy from one form to another. The kinetic energy of a truck, its energy of motion, is changed to heat energy through friction.

Brake drums are attached to and rotate with the wheels and tires. They rotate as the truck moves down the road. When we apply the brakes, we allow compressed air to enter the brake chambers, where they push on rods that rotate the slack adjusters.

They, in turn, rotate shafts with cams that act on brake shoes. The shoes, covered with a heat-resistant friction liner, are anchored at one end so they can pivot around their anchor pins. The cam acts on rollers at the other end of the shoes, forcing the friction liners against the rotating drums. That creates the friction that converts kinetic energy to heat.

The great amount of heat generated when a truck is slowed from its maximum speed, especially when going downhill, must be absorbed and dissipated. That's why brake drums are massive. They absorb the heat, and then dissipate it to the surrounding air flowing past the wheels.

Under extreme conditions, the heat can vaporize the surfaces of the friction linings. The vapor is between the linings and drums, where it acts like a lubricant, reducing friction and diminishing the effectiveness of the brakes. We experience this as brake fade.

The more mass there is in the brake assembly, the greater the amount of heat the assembly can absorb and manage. Temperatures at the lining surfaces rise less rapidly as more heat is absorbed into the drums. That reduces fade. And the amount of heat generated varies geometrically with the rate of deceleration. That rate grows as the required stopping distance gets smaller. One way to meet NHTSA's new stopping distance is by enlarging the mass, or size, of the brakes.

Pre-2012 trucks, with few exceptions, have 15-inch-by-4-inch front drum brakes and 16-1/2-inch-by-7-inch drive axle brakes. In the real world, they can stop trucks from 60 mph in a bit over 300 feet. The best stopping distances have been measured at 292 feet for a rig loaded to test conditions. The previous regulation called for 355 feet to stop from 60.

The new requirement calls for the test tractor to stop in 250 feet from the same speed. Because of weight transfer during braking, steer axle brakes will do an increasing share of absorbing the heat generated. But they cannot do the entire job. Drums on steer axles on new trucks will be 16 1/2 inches by 5 1/2 inches, and drive axle brake drums will be 16 1/2 inches by 8 5/8 inches. But larger drums are not the only solution to shorter stopping distance, nor are they necessarily the best.

Just as with cars, pickup trucks and even medium duty commercial vehicles, disc brakes are increasing in popularity for big rigs. They are lighter than drum brakes with better stopping distances, are easier to maintain, and are more resistant to brake fade.

Current regulations measure from 60 mph, but about half the states have speed limits of 70 mph or more. Under similar conditions of load and grade, two things that directly affect stopping, brakes need to convert 36 percent more energy at 70. Increase speed to 75 and the brakes must do 56 percent more work than from 60.

When NHTSA released the revisions to Federal Motor Vehicle Safety Standard 121 calling for a 30 percent stopping distance reduction, the citizens groups claimed that was not realistic enough given the higher speed limits.

Disc brakes convert motion to heat by pressing two friction surfaces against both sides of a rotating disc. As the disc spins, it transfers heat to the air stream and also carries away any gases that would otherwise cause fading. Harder, more fade-resistant friction materials, including ceramics, can be used, improving brake performance and prolonging pad life.

Maintenance requirements are greatly reduced. Some fleets that were involved in air disc brake development report no brake service during their period of ownership. When maintenance is needed, pad replacement can be done in minutes compared to hours for changing shoes on drum brakes.

Another benefit of air disc brakes is that side-to-side variations in braking force are greatly reduced. There is virtually no pull or drift under hard braking.

All manufacturers include the cost of brakes in the price of the truck, but when NHTSA first announced the requirement, they projected cost increases for new equipment. They estimated an increment of about $215 per truck for larger drum brakes and $500 per axle for disc brakes. At the time, Freightliner estimated an increase for the enhanced drums at $222, while disc brakes all around would raise the cost of a three-axle tractor by $1,627.

All truck makers make air disc brakes available on front axles. Most have them optional on drive axles. At this year's Mid-America Trucking Show, Peterbilt announced that disc brakes would be standard on steer axles on all air-braked trucks. Kenworth made disc brakes optional on all wheel positions.

Owner-operators who run one truck or who have their maintenance outsourced will not see any increase in costs, other than for inflation, until they buy a new truck. Then there will be an increase in the price of shoes and linings for drum brakes.

Replacement drums will also cost more. But buying trucks with ADBs will most likely lower total brake maintenance because of reduced parts and labor costs. LL

July Digital Edition