In a perfect world, truck and trailer brakes would work perfectly, would wear out at the same time, and would never cause driver complaints. Utopia. But don’t hold your breath.
However, even in this imperfect world, there are measures fleets can take to spec and maintain equipment for optimal balance.
First, we need a more detailed understanding of what brake balance is.
There’s torque balance. This occurs when each brake is furnishing the correct amount of torque — or braking power — for the load on that particular axle. Brakes are designed and rated for the maximum axle rating, such as a 23,000-lb single axle or a 34,000-lb tandem.
An empty or partially loaded rig will be over-braked, and lightly loaded axles will be prone to wheel lock-up. That’s usually what causes the black tire streaks on the road near intersections: drivers try to stop quickly, and they lock-up those empty axles, flat-spotting the trailer tires.
Worn cam bushings, oily linings, unequal slack-adjuster lengths, out-of-adjustment brakes, and other maintenance irregularities further contribute to poor torque balance.
An easy way to tell which brakes are working and which are loafing is to feel each brake drum, or take temperatures with a special brake or infrared thermometer. A brake that’s cooler than the others isn’t doing its share. If tractor brakes are hotter than those on the trailer, the tractor is doing most of the braking. With good torque balance, most brake and lining engineers agree that all brake temperatures should be within 20 degrees F of each other.
Air-pressure balance and timing balance are equally important. Good air-pressure balance means getting the same air pressure to each brake chamber at the same time. No more than 2 psi difference at a 20-psi brake application is acceptable.
When there’s greater pressure differential, then all brakes aren’t applying equally. If the tractor brakes come on ahead of the trailer’s, there’s potential for jackknifing as the trailer is pushing, or bumping, the tractor.
In addition, since tractors and trailers are made by different manufacturers, there’s no guarantee you’ll get a perfect brake-timing match. Each may choose to use different brake-valve crack pressures (the amount of air pressure it takes to open the valve to begin letting air into the lines). Crack pressures can range from zero to 10 psi or more, although in recent years tractor and trailer manufacturers say that a 4-to-6-psi range is most common. As equipment gets older, application time to trailers can get longer because of deterioration or mechanical problems.
Sid Williams, an automotive engineer in the American Trucking Associations’ engineering department, says that spec’ing a brake system item by item is the only way to achieve perfect balance. Many large fleets, such as UPS and Ryder Truck Rental & Leasing, do just that.
However, getting perfect timing and balance is unlikely for most fleets, because of the small differences in specifications each tractor and trailer manufacturer uses. To help improve the balance/timing equation, the U.S. National Highway Traffic Safety Adminstration is proposing universal 6-psi brake-valve crack pressures for all new equipment.
One of the most important practices fleets must implement to maintain good balance is always to replace brake valves and brake linings with products having identical performance characteristics. This means linings with the same friction levels, and valves with the same crack pressures.
SAE J1860, issued in 1991, requires all brake valves to contain labeling or tags stating crack pressures. Make sure you know the crack pressure of valves removed from equipment, so you can select a like replacement.
A mechanic should not walk up to a parts counter with a valve he’s removed and say, “give me one of these.” If all the ports on the new valve are in the same position as on the old one, a mechanic could think they’re identical. External appearance may be deceiving, however, since it’s internal springs that determine crack pressure.
Richard Bond, a Ford truck brake engineer, says as little as a 2-psi differential between a tractor and a trailer can indicate the start of problems. If the difference is greater than 4 psi, balance problems are almost assured, he says. During low-pressure braking, tractor brakes will be applied and the trailer’s may not, or may barely be applied, and the rig becomes unstable. Situations like this may invite jackknives.
Brake engineers from Eaton and Rockwell say that about 95% of brake applications are made at less than 25 psi, with 10 to 20 psi being most common. All it takes is a 4-psi differential in application pressures between tractor and trailer to cause significant torque imbalance between axles.
This is enough to cause instability, short turning life, and driver complaints. Ideal pneumatic balance, according to the experts, is no more than 2 psi pressure differential between brake chambers with applications of 20 psi or less.
If your fleet is having air balance/timing problems, contact your truck or trailer dealer, or the manufacturers of air-system valves or brake linings, who may have fleet field-service engineers. Most of them do.
These experts can run timing tests to see what time lag, if any, there is from the tractor treadle valve to any brake chamber on the rig. If it’s excessive, they have fixes to speed up or slow down air timing. Often, a slightly larger or smaller air line in the right place will solve the problem. Keep in mind that this is not a do-it-yourself project. Be sure to consult with the experts.
At the same time, brake engineers can compare application pressures between chambers and the tractor foot valve. If pressures are inconsistent, they’ll troubleshoot the cause. In many cases, troubleshooting services are free.
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