Antilock Braking Systems (ABS)

Heavy Vehicle Safety Systems breadcrumb separator

Antilock Braking Systems or ABS is acclaimed for providing significant improvement to overall vehicle safety and has typically been fitted to trucks for 40 years. Daimler – Mercedes-Benz trucks began series production with ABS in 1981. It is generally a well understood and accepted feature that is typically fitted to all prime movers equipped with electronic engines. However, historical data shows that ABS in trailers significantly lags prime movers. Although the ABS fitment rate is growing quickly, as of mid-2021, we estimate that only about 30% of the total heavy vehicle trailer population has ABS.

The key benefits of ABS include:

preventing each wheel locking during vehicle braking regardless of road grip conditions,
reducing stopping distances,
keeping the driver safe by maintaining vehicle stability and directionality, and
preventing damage to unsealed road surfaces.

Drivers also need to be aware that ABS has a few limitations, which are:

braking is not as effective when travelling on loose gravel and non-sealed roads and drivers need to allow for greater stopping distance
maintenance and repair of ABS should only be conducted by authorised brake repairers
compatible truck and trailer combination is required for ABS to be most effective.

It is highly recommended that all trailers should have at least ABS when being towed by a heavy vehicle equipped with Electronic Stability Control or Advanced Emergency Braking.

Resources

Glossary of terms

Australian Design Rules

Here is a demonstration of ABS in action.

How does ABS work?

You can see a snippet of ABS in action in the video below.

The braking action of a moving vehicle, the consequent deceleration and stopping distances, depend above all on the level of grip between the tyre and road surface.

When the braking system is perfectly efficient, braking action can only be further improved by adjusting tyre friction properties or road surface quality.

Even when conditions are optimum, absolute braking safety cannot be guaranteed! For example, when the driver faces specific critical situations, such as low grip conditions caused by a wet or icy road surface, performance deteriorates. This forces the driver to moderate the braking action to prevent the locking of one or more wheels.

Preventing wheel lock up is essential when braking, as when the wheels lock and the vehicle starts to skid, the driver will lose all ability to steer the vehicle. The vehicle will continue to travel in the direction it was initially heading at the time of wheel lock up and will not change direction regardless of driver steering input. The vehicle will lose directional control and have a significantly longer stopping distance when the wheels lock under braking.

The ABS ensures vehicle stability (under any braking conditions) by using sensors to detect when a wheel is about to lock up and preventing this by partially releasing the brake on that wheel.

Even during emergency stops, the ABS works to eliminate wheel lock by automatically releasing a small amount of braking effort when it detects wheel lock up. The brake force is reapplied until wheel lock up is detected again. The ABS system does this several times per second during the braking event to ensure the wheel continues to turn. The result of the ABS preventing lock up, is that the driver maintains steering control right through the emergency braking event and the brakes operate at their most efficient point for the given road conditions. Noting that a vehicle with complete wheel lock up will have a significantly longer stopping distance that a vehicle that is braking at close to the point where the wheels are locking up.

For a tri axle semi-trailer, ABS will monitor and control the wheel end on each side of the axle group as one.

ABS is inexpensive to retrofit

If the heavy vehicle axles are equipped with pole rings or at least allow for their mounting, retro fitting TEBS (Trailer Electronic Brake Systems) to a trailer is very cost effective compared to the cost of replacing 12 tyres if the brakes lock. A set of re-tread tyres (recaps) is currently $3,600 or double that amount for new tyres. It can take just one major gross- wheel- lock- event to make the tyres unroadworthy requiring their replacement!

Not only will retrofitting a trailer with a modern TEBS bring the benefits of ABS, but the system will also incorporate load sensing and anti-roll functions, bringing significant additional safety benefits.

Australian Design Rules (ADR)

ABS was mandated on all trucks via ADR35/05 from 1^st November 2017. All trailers were mandated from 1^st November 2019 joining all B-Double combinations which had been mandated to have ABS since 1995.

System modifications

ABS is a mature technology having been introduced over 30 years ago, however the ABS module should not be tampered with or moved. Modifications to the truck or trailer may require the ABS system to have its parameter set updated; this includes changing tyre sizes.

Chain of Responsibility and Issues

Having an advanced safety system above the minimum requirement stipulated in the ADR may assist with meeting your Chain of Responsibility (CoR) obligation. It should not be considered as either a mandatory requirement or the completion of your CoR responsibilities.

Issues

ABS ensures the driver maintains control during “panic” stops, but may actually increase stopping distances particularly on unsealed roads. Note ABS may be disabled if the “cross” or differential locks are engaged with a resulting loss of traction.
6x4 trucks typically use a 4Sensors-4Modulators system. The aim is to sense the wheel most likely to lock up in each side of axle wheel group but you may still experience lock up of other wheel ends in the group! For a semi-trailer typically use a 2S-2M system.

2S-2M	2 Sensors - 2 Modulators	This is where 2-wheel ends have sensors and 2 brake modulator valves. This is typical of a trailer system with 1 to 3 axles within a single group. Each side is controlled together. The ‘sensored’ axle is typically the axle most likely to lock up first.
4S-3M	4 Sensors - 3 Modulators	This is where there are 4-wheel ends which have sensors with 3 brake modulator valves. This is typical of a trailer system with 4 or more axles within a single group or of drawbar trailers with multiple axle groups.
4S-4M	4 Sensors - 4 Modulators	This is where 4-wheel ends have sensors with 4 brake modulator valves. This is typical of a truck, the steer axle(s) is sensored and modulated with each side of the drive/rear axles sensored and modulated. The sensored axle is typically the axle most likely to lock up first.