Enhanced Braking

Last July, the National Highway Traffic Safety Association (NHTSA) issued new stopping distance standards that effectively shorten the majority of large tractor stopping distances by 30 percent. Federal Motor Vehicle Safety Standard (FMVSS) 121 affects all new air-braked vehicles including trucks, buses, tractors and trailers.

In essence, the regulations have shortened the maximum allowable stopping distance to a minimum braking performance of 250 feet from 60 mph, down from the previous standard of 355 feet. For severe-service tractors, the new stopping distance requirement is 310 feet, also down from the previous 355-foot distance. That is for all loaded vehicle conditions.

In addition, all heavy truck tractors must stop within 235 feet when loaded to their "lightly loaded vehicle weight" (LLVW). The stopping distance requirements were also shortened for other load and system operating conditions in the FMVSS 121 regulation.

NHTSA considers a severe-service vehicle as a three-axle tractor with a gross vehicle weight rating (gvwr) greater than 70,000 pounds or a four or more axle vehicle with an 85,000-pound or greater gvwr.

FMVSS121 will be implemented in phases, says D. Mike Pennington, senior director, global marketing, communications and media relations for ArvinMeritor. Three-axle tractors with a gvwr of 59,600 pounds or less must meet the reduced stopping distance requirements by August 1, 2011. All other tractors must be in compliance by August 1, 2013.

ArvinMeritor is a premier global supplier of a broad range of integrated systems, modules and components to the motor vehicle industry, and serves commercial truck, trailer and specialty OEMs and certain aftermarkets.

The regulation does not apply to retrofitting, as existing vehicles are not affected, or to roll stability or collision mitigation systems, notes Paul M. Johnston, senior director, compression and braking, for Meritor WABCO. Nor does the regulation pertain to air-braked trucks, buses or trailers or to hydraulic-braked vehicles.

Meritor WABCO is the leading supplier of braking systems and controls and active safety systems for commercial vehicles in North America.

Highway crash and fatality data for heavy vehicles show that the majority of crashes involve tractor trailer configurations. The goal of NHTSA and its activity with revising the FMVSS 121 braking regulations was to get commercial vehicles to stop as fast (short) as a car - 195 feet, Johnston says, and the industry has made "significant improvements" towards achieving this.

While single trucks, buses and trailers are not included in this rule, he says future research by NHTSA may impact future stopping distance requirements for other air-braked and hydraulic braked vehicles.


"The truck builder is responsible for making sure its vehicles are compliant with the new stopping distance regulation," says Pennington. "The new rule will not significantly change brake system specifications or brake maintenance and service practices, but it is expected that there will be changes to foundation brake types, as well as sizes."

No change in trailer foundation brakes is necessary.

Today's foundation brakes and air brake systems can meet the new requirements for the majority of the vehicles with some enhancements. However, for the heavier gvwr tractors and two-axle tractors, the consensus is that air disc brakes will grow in use.

The major advantage of air disc brakes is that they don't fade after repeated applications and last much longer than drum brakes. The drawback: they are slightly heavier and more expensive than drum brakes.

In Europe there has been a significant movement away from drum brakes to air disc brakes because the vehicles there are of a different configuration and stop much more frequently over their life cycle than do heavy trucks in North America, Pennington points out.

Solutions available to meet the new requirements include higher performance steer axle drum brakes, which include larger diameter drum brakes, wider brake shoes/linings, air disc brakes or a hybrid combination of disc front and rear drum brake systems.

Manufacturers will offer options, and it will be up to the OEM truck builders to decide, by platform, how they will meet this regulation, says Doug King, marketing manager for Bendix Spicer Foundation Brake. From the work conducted by the industry to date, it is anticipated that there won't be any one solution forced on any fleet.

Bendix Spicer Foundation Brake, a joint venture of Bendix Commercial Vehicle Systems and Dana Commercial Vehicle Products, is a single, complete source for OEM brake system design, manufacturing, hardware and support for all foundation brake components and actuation systems, as well as all-makes coverage of medium and heavy duty aftermarket parts.


The baseline brake today on a three-axle tractor with a gvwr of 59,600 pounds or less is 15-by-4-inch drum brakes on the steer axle and 16.5-by-7-inch drum brakes on the drive axles, says ArvinMeritor's Pennington. Options to meet the braking requirements include:

• 15-by-5-inch or 15-by-6-inch drum brakes on the steer axle and 16.5-by-7-inch drum brakes on the drive axles.
• 16.5-by-5-inch or 16.5-by-6-inch drum brakes on the steer axle and 16.5-by-7-inch or 16.5-by-8.63-inch drum brakes on the drive axles.
• Air disc brakes on the steer axle and 16.5-by-7-inch or 16.5-by-8.63-inch drum brakes on the drive axles.
• Air disc brakes on the steer and drive axles.

Each progressive option provides more and more performance, he says.

There is already a trend to a wider drive axle drum brake for more consistent braking torque (less in-stop fade) and longer life when it is combined with the 16.5-by-5 inch steer axle drum brake, notes Pennington.

Test data supports that the optimum solution for best stopping performance is air disc brakes all around. All major tractor OEMs currently have air disc brakes as an option on many of their vehicles.

From a service standpoint, the new brake systems will not necessarily have components that may be interchangeable with the brakes on vehicles today, he says. The new brake assemblies will be specifically designed solutions from each brake suppliers.

Truck OEMs are in the process of deciding which brakes will be standard and which will be optional to give the required stopping distance, says Chuck Eberling, principal engineer, modules and systems integration, Bendix Commercial Vehicle Systems.

"These decisions are being made by weighing the impact of those choices on their whole operation, including their production line, vehicle weight, vehicle complexity and part numbers in their system," he says. "The OEMs are asking questions like: 'Am I going to change the foundation brake more, or am I going to change the system more?,' and 'How am I going to do this - by slightly touching each system or by performing major modifications to one system?'"

But in general, no major changes are anticipated.

It should be noted that truck OEMs have been using larger drum brakes and air disc brakes over the past five years or so and have had good, positive experiences with their products, says Pennington. They have already made some adjustments to axles and suspensions and their bracketry because of brake packaging requirements, but there is still some more work to be done.

For example, some traditional steer axle knuckles are not particularly air disc brake friendly. Many recently engineered knuckles offer both air disc brake compatibility as well as ease of service and overall weight savings. This allows room for the caliper to slide to compensate for the pad and rotor wear.


It is anticipated that the new braking regulation is not going to significantly impact the amount of drum brake maintenance that is needed, says Bendix Spicer Foundation Brake's King. Better torque balance between the steer and drive axle may better balance the maintenance schedules of all axles. In fact, air disc brake solutions actually reduce maintenance time while extending service intervals.

The new upgraded steer axle brakes will now approach the performance capability of a rear brake, says Ron Plantan, principal engineer, Elyria wheel-end group of Bendix Spicer Foundation Brake. In some applications, because the front brake can be doing more work, that work is distributed over more axles.

"There's an opportunity for longer life if everything is optimized across all axles of a combination vehicle," he says.

With the larger drum and disc brakes maintenance costs should go down, says Pennington. Larger brakes will provide such advantages as increased lining volume to drive longer service intervals, lower operating temperatures, reduced fade and improved performance.

Air disc brakes are internally adjusted and factory-greased so there is reduced maintenance required, he adds. Air disc brakes also have fewer parts to replace, all of which helps reduce unscheduled service requirements.


Tractor and trailer braking system balance will not be affected. The air systems will be balanced like they are today, with no need for pressure hold-off valves or changing chambers, etc., to get compatibility among tractor and trailer brakes systems, says Meritor WABCO's Johnston. However, there may be some algorithm changes on ABS systems because of the performance difference.

The tractor will incur a little more of the braking workload, which will have some impact on the front axle and suspension components, he says. The trailer, meantime, will experience a proportionate reduction in workload.

Another thing that fleets need to be aware of: With trucks that can stop in shorter distances, more attention must be paid to properly securing cargo, says Eberling of Bendix Commercial Vehicle Systems.


The cost of the new regulations, according to the NHTSA's assessment, is that new enhanced foundation drum brakes on a typical three-axle Class 8 tractor will add about $200 to the cost of the vehicle. Having an all-wheel air disc brake solution will add $1,400 to $1,500 to the cost compared to the current baseline vehicles.

NHTSA has justified the added cost by estimating that the new FMVSS 121 will prevent more than 200 deaths and 300 serious injuries each year, plus reduce property damage costs by around $170 million annually.

Brake manufacturers are always working to deliver brake system solutions that help keep the cost of operation in check, Plantan says. "But it is fair to say that there will be a nominal cost increase for new trucks and aftermarket parts.

"What is important to remember, is that if the system is optimized, the end-user will see added value because of it. If the customer keeps the vehicle three to four years, they will likely break even, depending on what choices were made."

Furthermore, he says some options that Bendix Spicer Foundation Brake has been monitoring out in the field - such as the larger diameter and wider drum brake packages and air disc brake packages, for example - "have in some cases actually doubled the life of the braking systems."


Performance-based brake testers (PBBT), used for decades for safety inspections and as part of regular preventative maintenance work, are not going to be affected by FMVSS 121. These machines assess the braking capability of a vehicle through a quantitative measure of both individual brake and overall vehicle performance in a controlled test, done through direct measurements of the brake forces at each wheel end, axle or for the entire vehicle.

PBBTs include roller dynamometers and flat plate brake testers. Each of these devices can determine the brake forces without restriction to the brake type (disc vs. drum) or energy supply (air, hydraulic or electric). PBBTs based on mechanical or electronic decelerometers can assess the overall vehicle braking capability through a stopping performance test in which deceleration and/or stopping distance is obtained, also independent of brake type or application method.

PBBTs have been used worldwide for decades for both safety inspections and as part of regular preventative maintenance work.

As of Feb. 5, 2003, section 393.52 of the Federal Motor Carrier Safety Regulations was amended such that certain PBBTs can be used for enforcement of minimum braking performance requirements on commercial motor vehicles. The Commercial Vehicle Safety Alliance (CVSA) approved PBBT-based out-of-service criteria for roadside enforcement officials effective throughout North America on April 1, 2008.

The primary benefit of PBBTs is that they provide an objective, consistent and standard measure of the as-is braking performance of a vehicle, says CVSA, whereas a visual inspection examines a vehicle for the presence and appropriate functioning of physical components. While a visual inspection can indicate a potential problem with a brake component, in practice the actual brake performance can never be determined by visual inspection.

One type of PBBT combines braking testing with a diagnostic tool for fleet brake and tire management - the VIS-Check Fleet Program PBBT, from service automation provider Vehicle Inspection Systems. Based on its VIS-Check undercarriage diagnostic system, the machine can be used to determine brake system effectiveness, as well as tire pressure, tread profile and wear by individual wheel end, on Class 3 through 8 vehicles.

Link-Radlinski is an independent testing and engineering consulting firm that develops and markets brake-testing equipment and conducts certifications testing for many original equipment manufacturers. Its ABS testing system has become the industry standard for end-of-line testing in truck and bus manufacturing plants in the U.S., Mexico and Canada, and its roller brake testers are used by a number of vehicle and brake manufacturers for research and development work. Link-Radlinski also has mobile and permanent brake and ABS testers and will travel to perform on-site testing.

Several other companies can provide PBBTs, PBBT test service and on-site vehicle inspections, including Hunter, Infinity and Maha.