Truck News


Stopping power

The National Highway Traffic Safety Administration (NHTSA) has left no doubt about the fact that the stopping distances for trucks need to change. Today's unloaded tractors are designed to stop from s...

Collision warning systems such as Meritor Wabco's OnGuard can help reduce the risk of rear-end collisions, which account for about 20% of heavy truck crashes.
Collision warning systems such as Meritor Wabco's OnGuard can help reduce the risk of rear-end collisions, which account for about 20% of heavy truck crashes.

The National Highway Traffic Safety Administration (NHTSA) has left no doubt about the fact that the stopping distances for trucks need to change. Today’s unloaded tractors are designed to stop from speeds of 60 mph within 335 ft.. In August 2013, that allowable limit will shrink to 235 ft.. Their loaded counterparts that once had to stop within 355 ft. will need to do the job somewhere between 250 and 310 ft., depending on the number of axles and Gross Vehicle Weight Ratings.

The interest in altering brake performance is a matter of saving lives. While large trucks are involved in fewer fatal crashes per 100 million vehicle miles in the US, they continue to account for about 13% of all fatal crashes. When NHTSA unveiled its rule, it suggested that the shift to shorter stopping distances will save 227 lives, 300 serious injuries and US$169 million in property damage per year.

The question that remains is how truck makers will meet the new requirements.

“Right now the OEMs don’t really have a game plan,” Jeff Spitzer of Eaton said during a presentation at this year’s Technology and Maintenance Council meetings. “They don’t have a full understanding of what the parts manufacturers are going to do.”

Stopping distances can be met by using additional friction material, more aggressive linings or air disc brakes, but it is still unknown which will become the most popular options.

“What will the fleet do? That’s the question,” said Mike Colaccino, commercial vehicle safety manager at TMD Friction. “The options are there and they are many.”

The changes will largely influence brakes at the front of the truck. A brand new tractor will place a static load of about 12,000 lbs on the steer axle, but the loading and extra torque during a stop will increase that load up to 34,000 lbs as the axles to the rear begin to lift, said Jim Clark of Vehicle Brake System Consulting. The 15-inch brakes on a typical steer axle simply do not produce enough torque to stop the vehicle in time.

“It’s only when you come to the panic stop where you get the tremendous surge to the steer axle,” he added, noting how a heavy truck experiences about one panic stop a month. Engineers need to focus on the changes that will address these situations. “And any time you change anything, something is going to come up and bite your butt.”

The engineering challenges can certainly have an effect on experiences at the fleet level. “The vehicle will feel more aggressive. The drivers might be more sensitive to that,” Clark said. The added torque could also present some potential maintenance challenges such as premature tire wear or suspension damage.

“We think we can stop the truck and still not cause an unacceptable amount of degradation to the drum,” Colaccino observed, referring to one potential issue. Fleet tests over the next year will be measuring noise, lining and drum life in real world settings to address potential challenges just like that. But suppliers will also want to ensure their products do better than NHTSA’s absolute minimums. “Brake manufacturers are requiring an additional margin of 10%, making the real distance 225 ft.,” he added.

Future regulations

Nobody knows exactly what NHTSA will eventually require in terms of stopping distances for trucks that are already in service, but panelists at the recent meeting of the Technology and Maintenance Council expect some sort of change in this regard as well. Research by Battelle Memorial Institute shows why these regulations might be justified. Of 23 randomly selected trucks tested in New Mexico, 13 did not meet the minimum stopping distance requirement of 40 ft. from a speed of 20 mph. Of those 13, three failed under the watch of the Performance Based Brake Tester (PBBTs), and only two were taken out of service after a visual inspection.

“The visual inspections really don’t tell you how that vehicle is going to stop,” Clark noted, referring to the limits of measuring push rod travel.

While there are in-service requirements for stopping distances, they are virtually never enforced by NHTSA, Clark says. But they could be measured with roller dynomometers or other PBBTs.

It is easy to see why there is an interest in regulations governing trucks that are in service. While today’s standards govern trucks rolling out of the factory, there are no standards governing the quality of aftermarket brake linings. Colaccino may be understating the issue when he suggests some offshore suppliers have “not fared very well” in terms of performance.

The question is how the stopping distance would change if the equipment is fitted with a lower quality of aftermarket components. “You want to be more sensitized to your OEM brakes and make sure the stopping capabilities are maintained,” Clark said.

Of course, the regulations that affect vehicle stopping characteristics may not end there.

Rear-end crashes accounted for about 20% of all heavy truck crashes in 2004, and the heavy vehicles hit the vehicle ahead of them in 60% of these cases. In 26% of cases, the lead vehicle had stopped, while 14.5% were decelerating and 13.3% were moving at a constant speed.

There are technologies that could make a difference here.

An autonomous cruise control, intelligent cruise control, or active cruise with braking can automatically accelerate or slow a vehicle. Other options including a collision warning system could warn a distracted driver to take action.

A third form of technology comes in a variety of names such as a collision mitigation, dynamic brake assist or automatic brake application.

“This is one where there is a lot of confusion,” says Mark Melletat of Meritor Wabco. “This function is on regardless of whether there is cruise control or not.” But it does not activate until the moment there is an imminent collision.

Most of the studies on these systems have been limited to light vehicles, but research is underway to test their use in heavy-duty counterparts.

Everyone continues to look for the combinations that will offer the best possible stopping power.

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