From the Today’s Trucking April 2000 issue.
Auto slacks and long-stroke chambers and a bunch of other brake bits seem to attract a lot of misinformation. So with the help of ArvinMeritor’s brake
engineering people, and Prakash Jain in particular, here’s a look at some of the myths – and the matching realities – surrounding your truck’s foundation air brakes.
MYTH #1: Wider brake blocks and drums mean shorter stopping distances.
FACT: Better stopping distances are the result of higher torque, which is primarily determined by brake application pressure, lining formulation, air chamber size, and brake diameter, but it’s independent of block width.
Wider brake blocks do wear longer under demanding high-temperature operating
conditions. They dissipate heat more readily than narrower blocks, since they
have more surface area. This results in maintaining brake torque that would have
been reduced otherwise due to fade on a narrower block. The advantage is negligible under normal highway operating conditions when brake temperatures are in the 250-300 F degree range, but can be beneficial under extreme-duty conditions, such as mountain descents and on refuse packers.
MYTH #2: Wedge brakes and the newer air disc brakes perform better than cam brakes, which is why both are widely used in Europe.
FACT: There are different features and advantages unique to cam, wedge, and air disc brakes that have appeal to different users. None of the three is inherently superior in all situations and applications. Europe has been switching to the air disc and away from cam and wedge brakes on heavy trucks and trailers.
The principal differences among these brakes, other than matters of fundamental
design, relate to fade resistance, size, ease of maintenance, and parts
Cam brakes are much easier and more economical to maintain than either disc
or wedge brakes, and parts are more readily available worldwide.
Disc brakes feature superior fade-resistance characteristics as well as less speed sensitivity, resulting in constant torque.
Wedge brakes are smaller and lighter than air disc and cam brakes, while delivering comparable performance. A wedge brake can represent a 10% reduction in brake diameter and nearly 30% less weight than a cam brake.
MYTH #3: Anti-lock braking systems eliminate the need for brake adjustment.
FACT: ABS is an electronic braking system that senses impending wheel lockup during emergency stops and cycles the brake actuation to avoid the lockup. It has nothing to do with sensing brake-block clearance with the drum or air-chamber pushrod stroke – the two primary mechanical means for activating automatic adjusters – so it has no effect on the operation of the adjusters.
MYTH #4: Automatic slack adjusters eliminate the need for brake maintenance.
FACT: Slack adjusters are a mechanical device designed to maintain a pre-determined stroke or clearance between the brake drum and brake block under most operating situations. However, like all critical components of heavy
trucks, ASAs should be periodically inspected and maintained, including regular
lubrication, to assure proper operation. They are not maintenance-free.
MYTH #5: Automatic slack adjusters can be manually adjusted.
FACT: No. After the initial installation and setup, automatic slack adjusters should not be adjusted except when re-lining or doing major maintenance work (eg. drum
or seal replacement) when they should be set up again. They should not be touched at any other time. If you have reason to believe that a brake is out of adjustment despite the presence of an auto slack, make four or five hard, full-pressure brake applications. If the brakes do not come into adjustment, there is something fundamentally wrong with the automatic slack adjuster and it should be replaced – not manually adjusted.
MYTH #6: Steer-axle brakes should be disconnected because they cause loss of control; or at the very least automatic brake-pressure limiting valves (ALVs) should be used.
FACT: This is perhaps the most widely held misconception about truck brakes. It also can be traced to the early days of U.S. FMVSS 121 when some trucks were equipped with very aggressive front brakes required to meet very short stopping distances. In fact, it is now Canadian and U.S. law that tractors must have
The facts are that heavy trucks equipped with full front brakes can stop the vehicle in shorter stopping distances and have better control than trucks with no front brakes or limited front brakes in virtually every situation. This has been
demonstrated time and time again, and was substantiated by the U.S. Department of Transportation in a 1991 report on “Improved Braking Systems for Commercial Motor Vehicles”.
National Highway Traffic Safety Administration tests of loaded, empty, and bobtail
tractor-trailer combinations have demonstrated significant increases in stopping
distances for vehicles equipped with limited front brakes compared to the same
vehicles with full front brakes.
The tests also noted at “ALVs increase the chance of jackknife or trailer-swing
because drivers must apply brakes at higher pressure to stop the vehicle, thereby
increasing the likelihood of lockup of the tractor drive or trailer-axle brakes if
unloaded or on reduced surface coefficient.”
The report also noted that “ALVs exacerbate problems associated with downhill braking situations.”
In Europe, where heavy trucks normally operate with even larger front brakes than
their North American counterparts, front-wheel brakes are universally used. In fact, while North American tractor-trailers normally have only 20% of the braking effort on the front wheels, with 40% each on the rear tractor and trailer wheels,
European trucks divide the effort equally among the three axle sets. Since ABS is
now mandated on all vehicles here, this concern should not be an issue.
MYTH #7: The brakes are out of adjustment on a significant percentage of the trucks checked in government inspections. Even though the rate has been steadily dropping over the years, this means these vehicles are dangerous since they probably couldn’t stop in an emergency.
FACT: Inspections do sometimes reveal that the brakes on vehicles checked are out of adjustment at 80- or 90-psi applications. But studies have shown that 80%
of brake applications are made at just 20 psi or less, which in most cases means
the driver simply needs to apply higher pedal pressure to generate the same
braking force to stop safely. It does not mean the vehicle’s brakes were
inoperable or too difficult to operate successfully. During a mountain descent,
however, the driver may notice performance degradation under this condition.
Inspections do show that vehicles with automatic slack adjusters have a much
lower incidence of out-of-adjustment notices than manual adjusters, proving that ASAs do work.
The adjustment standard was established as an arbitrary limit with manual slack
adjusters on the amount of stroke required for the rod in the air chamber to initiate braking. This range is narrowly defined, assumes a “worst case” braking situation, and does not take into account technical changes on brake assemblies
incorporating automatic adjusters and air-brake actuation systems that give
chambers with longer stroke increased braking reserves.
MYTH #8: The slack adjuster should be at a 90-degree angle to the pushrod (fully applied) when the brake is properly adjusted.
FACT: The 90-degree angle provides the maximum input force to the brake.
However, deviation from this angle only affects torque output by about 2%.
MYTH #9: It’s okay to mix automatic and manual slack adjusters on the same rig.
FACT: No. It’s important to note that when automatic slacks are adjusting constantly for wear and manuals are not, balance will suffer, sometimes with severe consequences if a panic stop is required. It’s also recommended that different brands of automatic slack adjusters should not be mixed on the same axle since they maintain different clearance.
MYTH #10: When a counterman tries to sell you new return springs and cam bushings at reline time, he’s just trying to increase parts business.
FACT: No. Failure to replace these parts is frequently the cause of short lining life.
Most automatic slacks adjust in the return stroke. If the shoe spring is weak it will
not allow the automatic adjuster to function properly. Replacing them at reline time will ensure proper performance of both brake and slack adjuster and improve lining life.
MYTH #11: You can lower your cost of operation by shopping carefully for the very lowest parts prices.
FACT: You get what you pay for, as always. Using quality parts means lower parts cost per mile, and less expense in the long run by eliminating for unnecessary downtime.
In the world of brake components, cheap-to-buy parts such as drums and linings can in some cases compromise safety severely.
MYTH #12: The use of long-stroke chambers will tend to create brake imbalance.
FACT: ArvinMeritor supports the use of long-stroke chambers because there will be no change in the timing or braking force with the adjusted brake. The system will remain in balance. Long-stroke chambers maintain higher force output compared to standard chambers.
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