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Sep 14, 2023

Troubleshooting brake imbalance in Class 8 trucks

[This article was originally published in 2007 by Trucks, Parts, Service. It has

[This article was originally published in 2007 by Trucks, Parts, Service. It has been updated to include more timely information.]

[This article was originally published in 2007 by Trucks, Parts, Service. It has been updated to include more timely information.]

What is brake balance? A vehicle's brakes are said to be in balance if all brakes apply and release at about the same time, with each brake developing the appropriate braking force for its weight load.

On the other hand, brake imbalance, such as when a tractor brakes faster and more aggressively than its trailer, causes rapid lining wear on the hardest-working brakes, bumping of tractors by trailers, jackknifing and panic stops with more panic than stop.

Torque imbalance, pressure imbalance and different tire sizes can contribute to what typically is called brake force imbalance.

Stopping distances on a vehicle equipped with drum brakes will increase when all brakes on a heavily laden combination vehicle are cool but maladjusted. And, when these maladjusted brakes become hot, causing drums to expand, linings to fade and brake-chamber stroke to increase, a 75 percent or more increase in stopping distance easily can be expected.

Automatic slack adjusters don't always solve this problem, because worn parts and lack of maintenance may cause them to stop working properly.

To muddy the water even more, brakes with properly-functioning automatic slacks are constantly working, promoting faster friction material wear than with manual slacks, which only are adjusted periodically. Therefore, a mix of automatic and manual slacks can unbalance the braking performance of a tractor-trailer combination that originally had no problem with over-braking or premature friction material wear.

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Torque imbalance, or lack of uniform friction-material to friction-surface contact, means that some of the brakes can have more or less stopping power, because of maladies such as oil or grease on the friction material; polished drums or rotors; glazed friction material; linings or pads with mixed friction capabilities at one or more wheels; out-of-spec drums or rotors; incorrect adjustment; different size brake chambers; improperly installed automatic slacks; inoperative or maladjusted antilock (ABS) wheel speed sensors; or improperly speced axle GVWR.

Pneumatic or air pressure imbalance occurs when the tractor-trailer's system delivers improper air pressures to brake chambers on a combination vehicle.

The most common causes are incorrect or malfunctioning relay valves. Quick-release valves also can have characteristics that upset pressure balance. Other causes include air leaks, air system contamination, a front-axle-limiting valve and excessive use of the trolley valve.

Timing imbalance occurs when some brakes receive air faster than others. Common causes include: oversized control lines on pre-1991 trailers, which slow brake application; poor plumbing design or improper installation; and failure to use booster valves, where appropriate.

"Maintaining good service system response, without adversely impacting the pneumatic balance across all the axles of a combination vehicle, is important," says Bendix Commercial Vehicle Systems. "Maintaining good pneumatic balance is crucial in improving brake response." Bendix engineers say that ideal pneumatic balance is achieved when the air delivered to each axle doesn't vary by more than 2 psi during a 10 psi to 40 psi application.

An exception to this rule would be the ill-advised mating of an S-cam-equipped tractor with a wedge-braked trailer. Because wedge brakes have smaller chambers and require more psi than S-cams to make linings contact the drum, the wedge-braked trailer would require higher air pressure than the tractor for balanced braking during low-pressure applications.

Brake system engineers say about 95 percent of braking involves application pressures below 25 psi. And approximately 84 percent of braking is done at application pressures of 15 psi or less.

When Federal Motor Vehicle Safety Standard 121 took effect in 1975, it required trailers to be compatible with a tractor simulator delivering a massive slug of air. To achieve compatibility, trailers needed 1/2-in. OD (3/8 in. ID) control lines instead of the 3/8-in. OD (1/4-in. ID) lines previously used.

During normal braking, a real-world tractor doesn't deliver enough air to fill the trailer's oversized control line. Consequently, trailer braking is delayed, especially with multiple-trailer combinations.

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In some cases, the delay treats drivers to a disconcerting bump as the trailer pushes the power unit. If bump comes to shove when braking through a slippery curve, for example, a jackknife is possible.

Seeking to eliminate the delay, the National Highway Traffic Safety Administration (NHTSA) modified the tractor simulator and changed maximum application/release times for trailers built on/after May 3, 1991; and also specified air-delivery times for control-line gladhands at the rear of tractors, trailers and dollies built on/after May 3, 1991.

Trailer bumping can be eliminated by retrofitting a smaller control line to the trailer and by making changes to the tractor that would speed gladhand timing. This would cause trailer brakes to apply faster during normal brake applications. And there shouldn't be any degradation of stopping distance during panic stops.

Alternatively, tractor brake application can be slowed by a plumbing change. But slowing tractor brakes has the potential to adversely affect stopping distance, NHTSA warns. So, making a change in the power unit's brake system is an ill-advised, but conceivable move.

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For example, trailer brake release time can be cut drastically by adding a quick-release valve at the tractor protection valve. It must be strongly emphasized, however, that any brake modification should have engineering approval by the vehicle OEM as any deviation from OEM plumbing has the potential to cause more problems than it cures.

If a tractor's brakes do most of the work, a combination vehicle can't slow down very quickly without the driver pushing harder on the brake pedal.

Should the tractor continue to brake more aggressively than the trailer, the risk of locking the drive axle (on non-ABS-equipped units) and jackknifing is substantial, especially on a slippery road with an empty or lightly-laden trailer in tow. While ABS prevents over-braked wheels from locking, it's not a substitute for balanced braking.

Sustained high-pressure braking of an ABS-equipped tractor is discouraged because a non-ABS-equipped trailer (or one with non-functional ABS) may receive enough air to lock its brakes, perhaps causing it to swing rapidly out of its traffic lane.

Further, a tractor with non-functional ABS would be inclined to jackknife as a consequence of full and sustained braking. For those reasons, NHTSA admonishes drivers not to change their normal braking habits when piloting ABS-equipped combination vehicles.

The addition of air disc brakes to the equation complicates things even more. Because of the increased capability of air disc brakes to control braking on the tractor, the drum brakes on a trailer tend to develop heat fade faster, transferring most of the work to the discs. This has prompted some manufacturers to suggest that there is an inherent value to specifying air disc brakes on trailers, avoiding many of the problems associated with mixing and matching.

"Imbalance will also exist between a disc-brake-equipped tractor and a drum-brake-equipped trailer unless the crack pressure on the trailer is adjusted," says Meritor. "If not, the air disc brakes on the tractor will apply first, and they’ll be doing most of the work."

By design, some tractors display a lightning-fast draw when shooting air to their own brakes before passing it along to the trailer. Further, some trailers resist accepting air from a tractor because they have a relay valve with a relatively high crack pressure. Depending on the setup, tractor brakes can be applied without the trailer relay valve ever approaching its crack pressure.

Crack pressure, expressed in psi, is the air required to force a valve open. All valves presumed to have identical crack pressures are not necessarily created equal. Even a new, high-quality valve rated at 4 psi crack pressure may open at anywhere from 3.5 psi to 4.5 psi. In contrast, a new and high-quality valve rated at 7.5 psi crack pressure may require anywhere from 4.5 psi to 10.5 psi before opening.

A valve's crack pressure largely is determined by the stiffness of its piston-return spring. If a valve is replaced with a remanufactured unit, or with other than an original-equipment valve, crack pressures can vary because of differences among springs.

The moral of this story is that pneumatic balance is most easily achieved by using low-crack-pressure valves, maintained with OE replacement valves. It doesn't take much of a disparity in crack pressures to cause a problem.

Some tractors are over-braked because tractor axles have been over-spec'd for the loads typically carried, and brakes are sized according to the gross axle weight rating. Benefits of greater-than-needed weight capacity include higher resale value, plus the fact that axle durability is enhanced by larger gearsets and bearings. But the tractor always will over-brake if axle loading is substantially less than rated capacity. This caveat applies equally to trailers with over-spec'd axles.

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The solution to overspec'd brakes may be to switch to less-aggressive linings or (if possible) to attach chamber push rods to a different slack adjuster hole to reduce braking force. Keep in mind though, that spacing between slack adjuster holes varies by make. So it's wise to ask the vehicle OEM or brake component supplier for technical advice before making changes.

An onsite, brake-torque-balance test is also advised before modifications are made. A suitable procedure is offered by Recommended Practice (RP) 613 Brake System Torque Balance Test Procedure, available from the Technology & Maintenance Council (TMC) .

Although relay valves ultimately govern when the air is delivered to the foundation brakes, there is a way to speed the signal to the relay valves by using an electronic braking system, or EBS. This system sends an electronic signal to the relay valves, telling them to open immediately. Brake response is enhanced, and air pressure is sent on its way to the foundation brakes more quickly.

"EBS can have a positive influence in the reduction of possible tractor/trailer brake pressure imbalance," says WABCO. "The technology is capable of estimating the deceleration contribution of each axle of the tractor/trailer combination, and crack pressures can be electronically adjusted to ensure brakes apply in a balanced fashion."

The company adds, "While the optimum solution is to equip both tractor and trailer with EBS, pressure balance will improve if just the tractor is fitted with EBS."

The fact that a combination vehicle provides equal air pressure to all brakes at the same time isn't the only concern with respect to balanced braking.

As mentioned earlier, torque imbalance results from some brakes having better adjustment, bigger chambers, longer slacks and more aggressive friction material than others. Most of these disparities are immediately evident.

Differences between linings are most likely to sneak up and present you with a nasty surprise. That's because lining friction, fade and recovery characteristics at various temperatures can vary widely.

In the early days of non-asbestos linings, certain formulations were prone to swell excessively with heat. Because of this, the swelled linings often caused tightly-adjusted brakes to drag after the treadle was released.

After the lining cooled down, the linings might not shrink to previous dimensions. In some instances, significant and permanent lining growth required slacks to be backed-off before the brakes could be released. While that type of scenario has virtually disappeared, other problems are painfully persistent. Consider edge codes, as a prime example.

Designed to indicate a lining's aggressiveness, edge-code markings often wear off. That's no great loss, however, because even within the same edge code, friction can vary by as much as 40 percent.

To maintain some degree of consistency, spec the same brand and type of lining on tractors and trailers and use the same material for relining.

Even if tractors and trailers initially are well matched, torque balance can degrade over time. For example:

Achieving brake balance is not difficult, as long as you’re aware of what to look for that may cause the imbalance to occur. Once the cause is known, inform your customer and let them know what needs to be done to help prevent jackknifing and ensure a safe ride.