Brake System, including ABS
In order to understand exactly how a 4×4 brake system, or any brake system for that matter, works and why a 4WD vehicle stops when the brakes are applied, it is necessary to understand some of the physics involved. If Isaac Newton had not formulated his laws, an off-road vehicle in South Africa, or anywhere else in Africa for that matter, would still have stopped, but the mechanics of brakes as it relates to energy conversion and conservation would not have been understood as well, and brake systems may not have reached the high state of development they have.
In short, the working of brake systems can be explained because Newton (paraphrased) stated that: “An object at rest, or in motion, will maintain that state until an unbalanced force is applied to that body, which is when one of two things will happen: the body will either be slowed down, or it will be accelerated, depending on the velocities and relative directions of motion of the object and the unbalanced force.” However, in the case of 4WD vehicles, the energies involved in this process are rather large and this is where the First Law of Thermodynamics comes in. This law states that: “Energy cannot be created or destroyed; it can only be converted from one form to another.”
So, how does all this relate to why a 4WD vehicle stops when the brakes are applied? To fully understand this, there are a few more terms, principles, and relationships that need mentioning and clarification since they have a direct bearing on the brake effectiveness of particularly 4WD vehicles, especially those 4×4 vehicles that are subjected to the harsh off-road driving conditions of Southern Africa, or the whole of Africa, for that matter.
Clamping Force
This is the force exerted on the disc by the calliper (or in the case of drum brakes, the outward pressure of the brake shoes), and is the sum total of all the forces and pressures generated in the system, which is relative to the initial force applied to the brake pedal. Wheel lock-up occurs when the clamping force exceeds all other forces.
Brake Torque
Since torque is defined as force multiplied by distance, a rotating wheel can be seen as possessing torque, which is determined by the distance between the contact point with the road and the centre of the axle. However, since the torque is imparted to the wheel from the centre of the wheel outwards by the movement of the vehicle, the weight and momentum, or inertia, of the vehicle as well as the friction coefficient, or traction of the tyre, must be brought into play when a brake system is designed to overcome the combined brake torque and inertia of the vehicle.
Brake disc and drum diameters have a direct bearing on how well, (or not), brake torque is absorbed, or converted into another form of energy. Since torque is defined as force multiplied by distance, distance can be imagined as a rotating lever extending from the centre of the axle to the point (disc diameter) at which the clamping force is applied. The amount of clamping force required to absorb the brake torque is directly relatable to the length of the “lever” (disc/drum diameter), and the inertia of the 4WD vehicle, wheel diameter, tyre traction, contact area between friction surfaces, and the value of the friction coefficient.
Increasing the length of the “lever” by fitting larger diameter discs, decreases the required amount of clamping force required to achieve the same effect, however, since the space occupied by discs and callipers on the front axles of 4WD vehicles is often limited, fitting large discs is often impractical. Manufacturers overcome this obstacle by increasing the contact area between disc and brake pads by using larger pads: however, this leads to the generation of large heat values which can only be absorbed by ventilating the discs, allowing for improved airflow over and through the disc to aid in cooling the disc.
Friction Coeficient
This is the value of the friction between two or more surfaces when they are brought into contact. As it relates to braking systems, especially those on 4WD vehicles, this value can be manipulated (or negatively impacted), by the materials in question: since cast iron (the accepted material for brake disc and drum manufacture), exists in various degrees of hardness, depending on composition and hardening, the amount of friction generated can be severely influenced. The formulation of the friction materials on brake pads and shoes also differ greatly: thus, the combination of the grade of the cast iron of the discs, drums, and pads and shoes has a critical effect on the overall efficiency (or not), of the entire system.
Thermal Efficiency
This refers to the ability of the brake system to absorb the heat generated by friction without suffering damage of any kind. Thus, thermal inefficiency would refer to a system in which brake discs and drums are prone to cracking, warping, brake fade, or glazing because of the heat generated by friction.
How does it all work?
From all of the above, it should be clear that quite a lot happens when the brakes on a 4WD vehicle are applied, but to summarise, the following happens: assuming that the system is in perfect working order, applying pressure to the pedal generates a clamping force, the value of which is dependent on the:
• Brake pedal length, bore of the master cylinder, multiplication of the pedal pressure by the booster, and the differences between the bore of the master cylinder and the diameter of the calliper pistons:
Since the evacuation of the booster results in atmospheric pressure multiplying the pedal pressure by the area of the booster diaphragm, the vastly increased pedal pressure causes pressurised brake fluid to exert pressure on the calliper pistons. The internal volume of the callipers is immaterial since a brake system that is under pressure is effectively a closed system, which means that the pressure is equal at all points in the system. However, the amount of force generated by the calliper pistons is dependent on the relationship between their diameter(s) and that of the master cylinder.
Similar to the relationship between gears, where torque can be multiplied by the difference in the diameters of two different sized gears, the pressure generated by a small piston can be multiplied by the difference in the diameter of a larger piston. For instance, if the effective diameter of the master cylinder is 20 mm, and that of the calliper piston is 60 mm, the increase in force will be three times greater than that developed by the master cylinder. This relationship holds true irrespective of the relative diameters of the two pistons, and is of critical importance when modifications to the brake system of any off-road vehicle is contemplated.
As a direct result of this relationship, the distance the calliper pistons have to travel before a force is applied to the discs, is reduced by the same amount that the force is increased. This is important to bear in mind when pedal travel suddenly increases: a binding or sticking calliper could mean that the pedal has to be fully depressed (or even a few times), before the calliper pistons have extended enough to exert a force on the discs.
Clamping force and heat
Again assuming that the brake system is in perfect working order and that applying the brakes has brought about a clamping force, the following happens:
• Heat build-up:
The amount of heat build-up is directly related to the friction coefficient of the friction material, contact area of the friction surfaces, value of the clamping force, brake torque, traction, inertia of the 4WD vehicle, and the duration of the clamping force.
Nevertheless, since the kinetic energy possessed by the hurtling 4×4 cannot be destroyed, but only converted into another form, the heat that is generated by friction serves to absorb this kinetic energy, bringing the vehicle to a stop, thus satisfying the First Law of Thermodynamics.
Brake systems involve a lot of physics, and it may be asked what happens to the heat energy that was used to absorb the kinetic energy of the 4×4? Simple, really: it was not there to begin with. It only came into being when the brakes were applied and continued in existence only for as long as was needed to absorb the kinetic energy of the 4WD, which when it stopped, had no more kinetic energy to be converted into heat, thus the build-up of heat energy stopped as well, which means that the two forms of energy had cancelled each other out. No energy was lost or destroyed; one form was merely converted into another.
ABS or anti-lock brake system
First introduced by Mercedes-Benz on their passenger cars in 198
5, Anti-lock Brake Systems work on the principle of differing rates of rotation of individual wheels. Sensors, controlled by a micro-processor, measure the rotational speed of individual wheels, and should a wheel be found to be rotating at a slower rate than any other(s), the brake pressure in the affected channel(s) is momentarily reduced until the wheel rotates at the same rate as all the others.
The mechanics of these systems involves a modulator that divides the brake system into separate channels, one per wheel. Design specifics of modulators differ somewhat but their principal, shared characteristic is their ability to increase the volume, thereby reducing the pressure in each individual brake channel in rapid succession. It is this ability that is felt as a rapid pulsation of the brake pedal when the system is operation.
• Why ABS?
All drivers, off-road drivers included, depend on the traction of the tyres fitted to their 4×4 vehicles to maintain control of their 4WD vehicles. The loss of traction for whatever reason means a driver is in imminent danger of completely losing directional and braking control of his 4WD vehicle.
Maintaining control is of critical importance, especially in the sometimes extreme off-road driving conditions in Africa, and it is here that ABS systems are the most valuable. By preventing wheel lock-ups, and therefore maintaining traction, at least as far as braking goes, an off-road driver is able to steer around obstacles, even while maintaining maximum pressure on the brake system. At least, that is the theory: the reality as it relates to ABS systems and off-road driving is somewhat different.
• 4×4 Driving and ABS
While ABS systems have proven their worth by preventing many accidents and fatalities, they are widely considered to be dangerous in the off-road driving context.
Off-road driving in South Africa or Southern Africa, in fact, the whole of Africa can be extremely treacherous, just as it can be anywhere else, with sand and gravel coated mountain passes, vast expanses of thin gooey, greasy mud, and lots of powdery sand. It is sometimes difficult enough to bring a heavily laden off-road vehicle to a safe stop in a reasonable distance in these conditions, since traction is marginal at best and wheel lock-ups are a very real danger.
Since the primary function of an ABS system is to prevent wheel lock-ups, in these conditions the system will release the brakes every time a lock-up occurs, which is very often, leading to a situation where the off-road vehicle effectively has no brakes. In these conditions it will be virtually impossible to bring a 4WD vehicle to a stop, which is why almost all serious off-road drivers have devised means to deactivate the ABS systems on their 4WD vehicles in conditions where the continued operation of the system could be dangerous.
EBD, or Electronic Brake Distribution
This refers to a system that monitors inter alia, the weight distribution of a vehicle, (important in the case of off-road vehicles), road conditions, suspension movements on all wheels, and steering angle in some systems.
The main function of the system is to autonomously determine the correct amount of braking force that needs to be applied to each wheel under varying conditions in order to prevent possible skids and loss of control.
These systems work in conjunction with the ABS, and can correct incipient dangerous conditions of which an off-road driver intent on negotiating a tricky mountain pass, may not even have been aware existed.
EBA, or Electronic Brake Assist
Inexperienced off-road drivers often underestimate the amount of braking force required to bring a 4WD to a safe stop, especially in difficult driving conditions. EBA systems have the ability to determine whether a 4×4 vehicle is in an emergency stop situation by examining the speed of the off-road vehicle, the amount of brake pressure applied, and in some cases, the steering angle and suspension movements as well.
When the system determines that a 4WD vehicle is in an emergency stop situation, it will in conjunction with the ABS and other safety systems apply maximum available braking force, irrespective of the initial pressure applied by the driver until a safe condition, (as determined by the system) has been reached.
Are 4×4 Brake Systems Safe?
4×4, and 4WD vehicles have generally speaking, reached a very high state of development, however, whether the refinements, especially those on brake systems, are safe to use in the often treacherous off-road conditions of Southern Africa, or the rest of Africa, are always safe to use in all conditions, depends largely on the level of skill possessed by off-road drivers, as well as their expectations of these systems.
While systems like Electronic Brake Force Distribution may have definite advantages in the off-road driving context, other like ABS and Electronic Brake Assist, may only be of value on hard, paved roads, of which there are many in South Africa, but not in the rest of Southern Africa, and points north. Africa is notorious for its extreme off-road conditions, and any off-road driver contemplating a long and difficult overland expedition through wild but beautiful Africa, would be well advised to first get fully acquainted with all the brake related safety systems and the sometimes unpredictable handling characteristics they can impart to his 4WD vehicle.
