From their first appearance over 20 years ago, the question of stability systems has come from a good idea to a system being mandatorily fitted to vehicles. Diesel Workshop has a look at the basic idea and gets answers to some basic questions.
A lot of workshops have experience with stability control systems and a lot of workshops have none. The fitment of such systems has been sporadic in the past. This situation is about to change with the mandatory fitting of some form of stability control making its way through to law. Now anyone dealing with truck and trailer maintenance has to be up to speed with just what the implications are of fitting these kinds of systems onto a vehicle.
Diesel Workshop would like to thank and acknowledge the Australian Trucking Association’s (ATA) Technical Industry Council whose Technical Advisory Procedure (TAP) is the reference on which this article was based. The aim is to provide trucking operators with key information about the fitment and operation of stability control systems for improved vehicle safety.
Anti-lock Braking Systems (ABS) were designed to prevent wheel lockup during braking thus allowing the driver to maintain directional control of the vehicle. ABS was not designed to reduce stopping distances and under some circumstances may increase it, however in emergency stops, ABS has proven to be highly effective and is widely accepted as an invaluable safety feature.
Since then, ABS systems have been considerably enhanced with the development of Electronic Braking Systems (EBS), which integrate with Automatic Traction Control (ATC) and key stability control system features to deliver the next generation of braking control. A further development has been the inclusion of the roll stability function, often referred to as RSS, RSC or other similar acronyms.
EBS has become the de-facto term for all stability systems. The major advantages of EBS include quicker braking response times, improved brake distribution/balance and a feedback system that modulates braking force to maximise effectiveness. The type of components and therefore the level of functionality varies, however, once EBS is fitted. This is because EBS is also a technology enabler, providing a platform for other advanced braking features to be added.
However, the laws of physics still apply and a stability control system cannot prevent all rollover crashes. The driver must drive to the prevailing conditions and not rely on enhanced safety systems to manage inappropriate speed.
The roll stability control (RSC) function is an active vehicle safety system that continuously checks and calculates the lateral acceleration of the vehicle and compares it to the critical threshold at which point rollover may occur. When the critical threshold is exceeded, the RSC intervenes to slow the vehicle. Depending on the system, this could be by reducing engine torque, engaging the engine retarder (for the prime mover) and automatically applying the braking systems (prime mover and/or the trailer). Frequently, system activation takes place before the driver is aware of the need.
There are two levels of stability control:
- RCS which controls a vehicle’s roll. It is the only option for trailers, but may be fitted to powered units as a low cost retrofittable solution.
- Electronic (Enhanced) Stability Control (ESC) which includes a steering angle sensor and capability for braking wheel groups independently to provide direction control of the vehicle. This feature is only found on powered units such as prime movers and rigid vehicles.
- ESC, with its greater capability, provides vehicles with the most capable stability control system.
There are currently three suppliers of trailer stability systems – Haldex, Knorr-Bremse/Bendix and Wabco. Currently, EBS platform-based systems capable of Controller Area Network (CAN) communications between units are available. These systems provide for full system functionality including brake activation and faster braking force requests via wire. These systems are generally called Trailer EBS or TEBS. Stability control is available on a trailer without the CAN communication as long as the fitted TEBS unit has a power supply, however the control signal is pneumatic.
There are currently two suppliers of truck (rigid and prime mover configurations) stability systems – Knorr-Bremse/Bendix and Wabco. Today, they supply either ABS or EBS platform-based systems. The EBS platform on the truck provides a direct CAN output of key parameters to support the full functionally of a TEBS-equipped trailer unit. The ABS platform-based stability control systems does not, but can be fitted with a separate and optional Trailer Response Management (TRM) system which will produce a simple one-way brake activation signal to the TEBS unit.