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What effects can a replacement brake pad have on the ABS, traction and stability systems?
ABS, TCS and ESC
Everybody who installs or orders brake pads should be asking themselves the following questions:
- Will installing replacement brake pads from any source impact the performance of the safety systems that use brake control?
- Also, are all these new safety systems capable of adapting to different brake pads?
To understand these questions and evaluate the potential answers, it is first important to have a basic understanding of the various chassis control systems the ?wheel brakes? support and what are the operating principles and control strategies they employ. There are essentially five general chassis control systems available on today?s vehicles. On some vehicles they are standard and on others they are options available to the car buyer.
The Five Chassis Control Systems
- ABS (Anti-Lock Brake System)
This is the basis for all the other technologies. The primary function of ABS is to manage the slip (lockup tendency in engineering speak) of each tyre and control the amount of slip each tyre is allowed. This is done to optimize the ability of the tyre to generate longitudinal tyre forces and lateral tyre forces. This is engineering jargon which means how much stopping power and cornering power the tyre, and ultimately the vehicle, can generate. In other words, the system allows the driver to brake and turn at the same time.
- EBD (Electronic Brake Force Distribution) is the next extension of ABS
This system replaces the function of the traditional proportioning valve and manages the front to rear brake balance of the vehicle across all vehicle loading conditions and road surface conditions.
- TCS (Traction Control System) is essentially ABS in reverse.
TCS works to allow the tyre to provide the maximum longitudinal tractive force during acceleration and also match the engine output to the level the road surface can absorb.
- ESC (Electronic Stability Control).
This technology takes the path the vehicle is really travelling and compares this to where the driver really wants to go. If the two do not match, the vehicle is either understeering or oversteering. Once one of these conditions is identified, the brake system is activated to provide a rotation force on the vehicle to get it back on track. It is kind of like sticking a paddle in the water of a canoe to force a turn.
- Roll Stability Control (RSC).
The most recent technology emerging on some vehicles.
This system evaluates the tendency of a vehicle to roll over and once the onset of a roll is identified, the wheels on one side of the vehicle are heavily braked in order to reduce the amount of lateral adhesion. The goal is essentially to put the vehicle into a side slide to prevent the role tendency.
The control algorithms for all these systems are extremely sophisticated and contain literally thousands of individual parameters that are tuned specifically to the particular vehicle line and brake system used on the vehicle. These parameters are developed, refined and tuned on a large variety of manoeuvres, speeds and road conditions.
It is certainly reasonable to wonder, after all the work to develop a fully integrated set of systems, what happens when a different set of friction material is put on the car, or more commonly, on one axle of the car during service?
To understand the potential effects, we must go a bit deeper into how these systems generally operate. In all of these systems, the computer controller (the brains of the system) is monitoring the vehicle watching for an indication that an undesirable operating condition starts to appear.
These systems closely monitor individual wheel speeds, vehicle deceleration, engine output, steering wheel angle, yaw rate, lateral acceleration and roll rate among many other secondary indicators. The controller uses all of this information to determine very precisely what the vehicle is doing compared to the desired behaviour of the driver. Then a corrective action plan is determined. The brain then commands a ?torque change? (or ?torque? in engineering shorthand) to the desired wheel brake. The system cannot directly control torque. It can only control hydraulic pressure.
The desired correction may be a torque increase, decrease or hold. An individual torque command is developed for each wheel independently. The hydraulic pressure in the brake system is multiplied by the friction level of the pad resulting in a torque being applied to the wheel.
- Always choose a high quality replacement pad that mirrors the original friction characteristics, like Bendix. Bendix pads are designed in the largest friction laboratory in the Southern Hemisphere and are designed for Australian road conditions.
- Always chose a material with the same friction rating. (EE, FF) etc. and change it on all four wheels together. This will at least ensure that the braking ratio from front-to-rear is maintained as much as possible.
- Never replace only the Rear Brakes with a pad that has a higher friction level than what is specified for the vehicle. This will increase the amount of rear braking and increase the potential for a rear over brake condition under some road surface and loading conditions. Bendix has researched friction and worked with the OEMs to define the best friction for the vehicle and the application of the vehicle. Some of the cheaper pads on the market may have a ?one size fits all? approach which can be detrimental to the safety of the driver. These cheaper pads are not designed to ?sync? with the electronics that are built in to your customer?s vehicle.
In the modern vehicle, the wheel brake is challenged to contribute to a great many vehicle driving conditions extending way beyond just stopping the car. The friction material is a very key element in this. To maintain the optimum performance, it is important to understand the implications of the choices that are being offered to consumers and help them make an educated decision.
Links:
www.bendix.com.au
www.bendix.com.au/performance
