The following was published in CCJ's Air Brake Book, 11th edition, sponsored by SilverbackHD. CCJ's Air Brake Book is a complementary industry resource, courtesy of our partnership with SilverbackHD, the Technology and Maintenance Council and Commercial Vehicle Safety Alliance. You can download the entire Air Brake Book here.
A balanced brake system is one in which all the vehicle’s brakes apply and release at about the same time, with each brake developing the appropriate braking force for its respective load.
Visualizing this concept, it’s not hard to imagine the safety and maintenance problems that imbalanced brakes can cause for a commercial vehicle.
If a tractor, for example, brakes more aggressively than the trailer it is towing, uneven brake wear will be the most obvious consequence as the linings on the hardest-working brakes will wear faster than the brakes not doing their fair share of the work. Worse, this condition could lead to a trailer bumping into the braking tractor, jackknifing or unintended panic stops as brakes lock up.
The frequency of brake imbalance situations has decreased somewhat thanks to improved brake technology. Today’s brake systems are comprised of a myriad of components that must all work properly and in the correct sequence to guarantee safe, efficient stopping. If just one of those components fails to perform, the vehicle’s brake system will begin applying in an unsynchronized manner.
Brake imbalance is an early warning sign that a brake system is not functioning like it’s supposed to. Torque imbalance, pressure imbalance and different tire sizes are all common problems that can lead to a brake imbalance problem.
Each of those circumstances can be complicated further by the extreme temperatures under which foundation brakes operate. For many technicians, tracking down the source of a brake imbalance problem can be like a forensic investigator trying to figure out a crime scene. Many possibilities have to be examined and discarded before the culprit is identified.
When examining brake imbalance causes, it’s worth noting that stopping distances on a vehicle equipped with drum brakes will increase when all brakes on a heavily laden combination vehicle are cool but maladjusted.
When these maladjusted brakes get hot, their drums naturally expand, causing linings to fade and brake-chamber stroke to increase. In these instances, it can take the vehicle up to 75% more distance to come to a complete halt. Automatic slack adjusters are supposed to combat this problem, but don’t always respond correctly due to worn parts.
Maintaining torque balance
One of the most common brake imbalance scenarios is caused by torque imbalance – a lack of uniform friction material coming into contact with the vehicle’s brake drum or rotor.
This is common sense: A brake shoe or pad with thicker friction material on it will engage sooner and more aggressively than a shoe or pad on another wheel end with a thinner layer of material.
“Even shoe or pad wear means balanced brakes,” said Keith Roth, vice president of operations for SilverbackHD. “Uneven wear means unbalanced brakes.”
Other factors can degrade the friction material’s stopping power as well. These include oil or grease on the pad or shoe, glazed friction material, polished drums or rotors or linings and pads with mixed friction capabilities at one or more wheels.
Out-of-spec drums or rotors, incorrectly adjusted brakes, different size brake chambers, improperly installed automatic brake adjusters, inoperative or improperly adjusted antilock brake wheel sensors and incorrectly spec’d axle gross vehicle weight rating all can cause brake imbalance problems, too.
Differences between linings are most likely to sneak up as a nasty surprise when you least expect it. That’s because lining friction, fade and recovery characteristics at various temperatures can vary widely.
In the early days of non-asbestos linings, some friction materials would swell excessively when exposed to high temperatures. Because of this, the swelled linings often caused tightly adjusted brakes to drag after the treadle was released. However, after the lining cooled down, it could not always be counted on to shrink back to its previous dimensions. In extreme cases, this permanent lining growth required slacks to be backed-off before the brakes could be released.
Other friction-induced imbalance problems remain. Edge codes, for example, which offer easy identification of a lining’s aggressiveness.
But edge-code markings often wear off as friction material is worn down during the braking process. That’s no great loss, however, because even within the same edge code, friction can vary by as much as 40%.
To maintain some degree of consistency, always spec the same brand and type of lining on tractors and trailers, and use the same material for relining. A good guide is TMC’s RP 628C “Aftermarket Brake Lining Classification,” which provides an ever-changing laundry list of various brands and types of FMVSS-121-compliance linings and their torque ratings.
Understanding torque degradation
Even if tractors and trailers initially are well-matched, torque balance can degrade over time. Friction material can be contaminated by leaky, improperly installed wheel oil seals or ill-advised and overgenerous greasing of the cam assembly on drum brakes.
Leaking oil seals on new equipment demands a spot check of other units on the tractor or trailer, since assembly line errors could be responsible. Leaking seals on older vehicles and equipment could indicate a need to spec higher-quality seals or retrain technicians in proper lubrication procedures. Another option is to consider spec’ing “unitized” (sealed) hubs on new equipment.
Over time, brake drums can become deeply scored or bell-mouthed, and disc brake rotors can become “dished” and prevent even contact with the friction material at one or more wheels. Never assume new or replacement drums are automatically good to go, either. They can come from the manufacturer with flaws, like being bell-mouthed or eccentric, requiring that they be turned true in a lathe. In some cases, radius grinding of linings may be required for a good fit. While mild heat checking is acceptable, any drum or rotor with deep cracks should be scrapped.
Remember that brake shoe return springs can stretch or even break over time. That’s why it’s a good idea to replace springs every time brakes are relined, even if they look good. The same rule should apply to rollers that have become flat-spotted.
S-cams can wear down to the point that brake torque is severely affected as well. Likewise, worn camshafts and their splines and bushings also can degrade stopping performance. Pay special attention to the condition of the bushing, as it’s responsible for centering the cam and shoe assembly in the drum. Just like springs, bushings should be replaced every time the brakes are relined.
More than one anchor pin has been removed over the years with heat and hammer. But doing so may warp the spider, and bent spiders degrade lining-to-drum contact. Use only light taps, or better yet, use specially designed pullers to remove anchor pins when servicing brakes.
Sliding disc brake calipers can seize, causing accelerated wear of the inner disc brake pad. To combat this problem, make sure caliper pins and sliding surfaces are lubricated properly to assure proper function of the disc caliper. Gum and carbon buildup from air contaminated with oil and water can, over time, clog valves and cause them to slow down or fail. There’s an easy way to avoid this malady: Make sure air tanks are drained routinely, and spec an air dryer if you’re not using one already.
Retrofitting brake chambers or slack adjusters of the wrong size will change performance and compatibility.
Mixing two brands of automatic slack adjusters on the same axle also is not recommended because they will not perform identically and will create uneven brake wear. And although they are highly reliable components, automatic slack adjusters can malfunction or wear out over time. Lubricate them properly, and measure for excessive push rod stroke as brakes are applied. Inspecting the assembly for excessively worn holes in the yolk and slack adjuster, worn clevis pins and general looseness should be a standard maintenance procedure as well.
To ensure proper performance, ABAs must be mounted at the correct angle, as determined by use of installation templates that vary by application and brand. In the real world, however, the mounting angle may be compromised by clearance problems experienced by the OEM. For that reason, clearance should be checked before making a change in mounting position.
Low-profile tires can save you money at the fuel pump, but not matching low-profile tires on a tractor and trailer can cause brake compatibility issues. And retrofitting tires without reconfiguring the vehicle’s brake system is a mistake.
A vehicle or tractor with low-profile tires having a radius 18% smaller than original-equipment tires can cause a vehicle to be over-braked. In fact, an 18% reduction in rolling radius can result in an 18% increase in braking force, resulting in the lockup of lightly laden non-ABS-equipped trailers in the course of normal braking.
And because smaller tires rotate faster at a given road speed, linings will engage the drums at higher rpm and run hotter, especially when braking on downgrades. If you want to spec low-profile tires, consider spec’ing the next smallest chamber on the vehicle, which will reduce torque by about 20%. And changing to a less aggressive lining – or, with engineering approval, placing some sort of pressure modifier in the system – will help resolve an over-braking issue as well.
Consistent overheating, localized wear from lack of uniform friction material contact or exposure to abrasive material all can damage drums and rotors. Always inspect rotors and drums during relining jobs. Any friction surfaces with a mirror-like finish should be roughed up with 80-grit emery cloth and, if accompanied by a glazing on the shoes or pads, should trigger a search for a more suitable friction material.
Foreign abrasive materials also can cause excessive wear along the edges of the trailer lining contact area, or in areas coinciding with lining rivet holes. If this is happening, remove the lower dust shield (if equipped) to provide an exit for the foreign material. When checking a drum for excessive wear, its inner diameter shouldn’t be more than .12-inch more than the original spec.
When resurfacing drums, the finished ID shouldn’t be over .08 inch beyond original spec, and runout shouldn’t exceed .01 inch. The same goes for disc brake rotors. When checking rotor thickness, they shouldn’t be more than .12 inch less than the original spec, and don’t resurface more than .08 inch less than the original spec. Lateral runout shouldn’t exceed .01 inch. In any event, it is important to follow individual manufacturer turning and finish requirements.
Pneumatic (or air pressure) imbalance occurs when a tractor-trailer’s air system delivers air pressure to the vehicle’s brake chambers improperly. This is often caused by incorrectly spec’d or malfunctioning relay valves, although quick-release valves also can upset air pressure balance. Other common air pressure imbalance causes include air leaks, air system contamination, a front-axle-limiting valve and excessive use of the trailer hand control valve. (Refer to SAE J1505 and 1860 for further information.)
Timing imbalance occurs when some brakes receive air faster than others. Common causes include oversized control lines (found on pre-1991 trailers), which impede brake application; poor plumbing design or improper installation; and failure to use booster valves where appropriate.
Most manufacturers say that maintaining good pneumatic balance is crucial to improving brake response. Ideal pneumatic balance is achieved when the air delivered to each axle doesn’t vary by more than 2 psi during a 10- 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% of braking involves application pressures for linehaul applications below 20 psi, and approximately 84% 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 timing requirements, trailers needed 1⁄2-inch OD (3⁄8-inch ID) control lines instead of the 3⁄8-inch OD (1⁄4-inch ID) lines. But during normal braking procedures, a tractor doesn’t deliver enough air to fill a trailer’s oversized control line. As a result, trailer braking is delayed and the problem is magnified on multiple-trailer combinations.
In some cases, this delayed air delivery gives drivers a noticeable thump from behind as the trailer pushes the power unit forward. In extreme cases, that bump quickly can become a full-blown shove when braking in slippery conditions or in a curve, leading to jackknife.
Seeking to eliminate the delay, the National Highway Traffic Safety Administration modified the tractor simulator and changed maximum application/release times for trailers built on or after May 3, 1991, and has specified air-delivery times for control-line gladhands at the rear of tractors, trailers and dollies built on or after May 3, 1991, and should, theoretically, be a rare occurrence today.
But if you’re working with older tractors and trailers, 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 causes trailer brakes to apply faster during normal brake applications without any degradation of stopping distance during panic stops.
Conversely, if a tractor’s brakes are doing most of the work, a combination vehicle can’t slow down quickly without the driver applying heavy pressure on the brake pedal.
And while ABS prevents over-braked wheels from locking up, it’s not a substitute for a properly balanced brake system. Sustained high-pressure braking of an ABS-equipped tractor is not advisable since a non-ABS-equipped trailer (or one with nonfunctional ABS) may receive enough air to lock its brakes, perhaps causing it to swing out of its traffic lane. Worse, a tractor with a nonfunctioning ABS is likely to jackknife during full and sustained braking. For those reasons, NHTSA advises drivers not to change their normal braking habits when driving ABS-equipped combination vehicles.
Air disc brakes provide more efficient braking on tractors, which could be a problem when paired with trailers still equipped with drum brakes. Trailer drum brakes have a tendency to develop heat fade faster when paired with tractor air disc brakes. As a result, the tractor brakes work harder to slow down the rig, resulting in imbalanced brakes.
The key to solving this high-pressure imbalance lies with the crack pressure on the trailer. Crack pressure, expressed in psi, is the air required to force a valve open when air brakes are applied. Some tractors fire air quickly to their own brakes before passing it along to the trailer(s) behind them. But some trailers resist accepting air from a tractor because they have a relay valve set with a relatively high crack pressure.
As a general rule, valve character, including crack pressure, has minimal effect on high-pressure braking and primarily influences low-pressure braking and wear.
Remember that retrofitting remanufactured or aftermarket air valves can destroy pneumatic balance because the crack pressure of relay or quick-release valves fitted with aftermarket springs can vary considerably. Just because a valve “looks right” or “will fit” doesn’t mean it’s a suitable replacement for original equipment. Even where valves of the same make and model are used as replacements, crack pressures and pressure differentials may vary because of differences in bore size and manufacturing tolerances. (Refer to SAE 1860 for more information.)
Correcting overspec’d brakes
A final yet relatively simple problem causing brake imbalance is a tractor with axles that have been overspec’d for the loads it carries. Some fleets do this to boost the tractor’s resale value or to extend axle life by using larger gearsets and bearings. But the safety aspects far outweigh any payload or durability gains because the tractor always will over-brake if axle loading is substantially less than its rated capacity. The same rule applies to trailers with over-spec’d axles. This can lead to compatibility, wear and maintenance issues.
One option to resolve an over-spec’d tractor or trailer is to switch to less-aggressive brake linings. Another option (not always possible) is to attach chamber pushrods to a different slack adjuster hole, thereby reducing braking force. Keep in mind, though, that spacing between slack adjuster holes varies by make. It’s wise to ask your vehicle OEM or brake component supplier for technical advice before making changes.
It’s also a good idea to conduct an onsite brake-torque-balance test before making any fleetwide modifications. A suitable procedure is offered by Recommended Practice (RP) 613 “Brake System Torque Balance Test Procedure,” which is offered by the Technology and Maintenance Council of the American Trucking Associations.