ROADBOSS’s tech guru’s guide to in-service brake inspections, test methods, and equipment
In recent years, Australian heavy vehicle braking regulations have expanded to mandate life-saving technologies such as Electronic Stability Control (ESC) and Autonomous Emergency Braking (AEB).
These developments are thoroughly supported for good reason – safety technologies are an effective tool in combatting the tragically increasing road toll.
Despite this, it would be a major folly to place these systems on a pedestal so high that they transcend basic regard to the foundational aspects of heavy vehicle braking safety. This is true for any safety technology.
With the exception of exhaust brakes, the only part of the vehicle that produces braking power is the friction material (ie the shoe or pad) when it contacts the brake drum or disc.
And it cannot produce the right level of braking power if the systems that control it are not adjusted correctly. If there are deficiencies at that basic level, no amount of electronic and sensor wizardry can compensate.
So how do we ensure that we’ve got the basics right? ROADBOSS’s Adam Ritzinger looks at in-service brake system inspections, test methods and equipment, to demonstrate what ‘braking best’ looks like.
Braking basics
Let’s start with the brakes themselves. The brakes fitted to truck and trailer axles, whether they are drum or disc brakes, are fundamentally simple mechanical devices.
Drum brakes operate by pushing two static brake ‘shoes’ onto the inside of a circular brake drum, which rotates with the wheel. Disc brakes are similar but replace the shoes with pads, and the drum with a rotating disc. In both cases, the brake converts the rotational energy of the wheel into kinetic energy (heat), via the friction between the shoe/pad pressing onto the disc/drum.
The brakes are actuated by devices called brake chambers. On drum brakes, the chambers are mounted on the axle, and control the brake via a pushrod connected to a slack adjuster. The purpose of the slack adjuster is to automatically adjust the gap between the brake shoe and the drum as the components wear through normal use.
On disc brakes, the brake chamber is mounted directly on the caliper, which do not need a separate adjustment mechanism due to their design.
The brake chambers operate via compressed air. This means that the air compressor system, including the tanks that hold the stored compressed air to operate the brakes and the auxiliary components such as gauges and dashboard indicators, all become important system components.
Rounding out the set of basic components is the load proportioning system. These are critical for one fundamental reason. Trucks and trailers need to brake safely when empty, fully laden, or anywhere in between. Without load proportioning, if the brakes were set up when the vehicle was fully laden, they would lock the wheels instantly when the vehicle was empty. In the opposite case, brakes set up for an empty vehicle would be ineffective when fully laden.
A load proportioning system compensates for this. It measures the load on the axles and delivers the right amount of compressed air to the brake chamber when the driver pushes the pedal. On early vehicles, this is achieved via a load proportioning valve which monitors airbag pressure and axle position. Most vehicles fitted with advanced electronic brake systems incorporate the load sensing function directly into the system’s control module.
Inspect your gadgets
In-service brake safety starts with regular inspections, in the form of a daily or ‘pre-start’ check performed by the driver. Luckily for the driver, these need not be onerous, and can be limited to checks that the driver can readily perform without tools, and without needing to crawl under a truck or trailer. Examples of pre-start tasks include:
- Locate and operate the air tank drain valves;
- Confirm that the brake failure indicators are operational;
- Confirm that the pressure/vacuum gauges are operational; and
- Confirm operation of park brake before moving, and service brakes when underway.
The next level of inspections are the workshop checks performed by suitably trained and qualified automotive mechanics, using the correct equipment and tools.
The National Heavy Vehicle Regulator’s (NHVR) National Heavy Vehicle Inspection Manual (NHVIM) provides a nationally consistent approach to heavy vehicle compliance and vehicle inspections, and should be considered as the primary resource for conducting in-service workshop inspections.
Inspecting vehicles to the standard of the NHVIM is the best way operators can ensure their vehicles comply with the heavy vehicle standards, operate safely on the roads, and avoid non-compliances.
The brake section of the NHVIM is an extensive resource and covers the following:
- Brake system components (eg pedals/controls, cables/hoses, air tanks, chambers, drums/discs, shoes/calipers, compressors, and filters);
- Brake system adjustment (eg adjustment push rods and slack adjusters);
- Brake system operation (eg warning devices, pressure levels, system integrity); and
- Brake performance testing.
An example of a simple diagnostic check that a workshop can perform is to jack an axle and rotate the wheels to check for brake drag. Trailer brake systems can have many unique valves and circuits, and it is critical to check that they are all functioning correctly.
Brake performance testing, however, is a crucial element of a regular brake inspection and maintenance regimes and needs careful consideration as each method has advantages and disadvantages.
Testing times
The NHVIM specifies four possible in-service test methods:
- Decelerometer – using a device mounted in the vehicle to measure vehicle deceleration during an on-road brake test, conducted at or above 35 km/h;
- Stopping distance – measuring the stopping distance achieved by the vehicle during an on-road brake test, conducted at 35 km/h;
- Skid plate tester – using a floor-mounted device to measure brake torque (and other parameters) during a stop conducted at walking pace; and
- Roller brake tester (load simulating or otherwise) – using a floor-mounted device to measure brake torque (and other parameters) while the vehicle is stationary, and the wheels are spun on the rollers.
The first two methods require the vehicle to be driven during the test, most likely on public roads unless a suitable off-road test site is readily available. In either case, this can present logistical challenges and introduce safety hazards. Nonetheless, they are readily achievable using simple and cost-effective equipment, and may be attractive for some workshops/fleets.
The third and fourth methods require the purchase and installation of dedicated test equipment. This carries additional cost and requires a suitable off-road location but solves the above logistical issues and removes the associated hazards.
Additionally, they are the only methods which can measure the braking force at each wheel, enabling a more direct diagnosis of potential braking issues. Roller brake testers may offer more repeatable results as they remove vehicle travel speed as a test variable.
There are many makes and models of skid plate and roller brake testers available in the market, each with varying capabilities. Irrespective of which method is chosen, it is important to consider testing the brakes in all a vehicle’s load conditions, for the reasons explained earlier. The NHVR’s advice on this matter is that:
“In-service brake tests conducted on an unladen or a lightly laden vehicle only provide an indication of the vehicle’s braking performance in the configuration it was tested. Testing a vehicle, fully loaded or equivalent (simulated load) is the only test level that assures that brake force is sufficient for all other loading conditions.”
It is acknowledged that this presents additional logistical difficulties that vary with vehicle type and test locations. Vehicles designed to carry perishable commodities and dangerous goods are two such examples. That factor, however, increases the attractiveness of some examples of dedicated brake test equipment available in the market, which can simulate a laden axle, thereby allowing brake torque on a laden vehicle to be measured.
Notwithstanding, all testing methods carry advantages and disadvantages, and it is important that workshops take all considerations into account before making a decision. Interested persons should also read the NHVR’s NHVIM Fact Sheet on Brake Testing methods.
Finally, it is important to remember that in-service brake tests are not an absolute indicator of brake performance. They are screening tools used to verify that a vehicle’s braking system is functioning, and to diagnose any potential issues.
