ABS Brakes
By Steve Sancricca, ABS Application Engineer, Robert Bosch Corporation
The intent of this article is to explain the basic operating principles of anti-lock braking systems (ABS), discuss the system components, and relate performance of the system to some basic vehicle dynamics principles. The effects that common vehicle modifications such as tires, brake pads, shocks, springs and sway bars have on the ABS system performance will also be discussed.
How Does ABS Work?
Any explanation of ABS must begin with a discussion of the tire mue/slip curve. Tire slip (?) is an indication of the difference between the relative velocity of the vehicle and the rolling velocity of the tire. Longitudinal slip is defined as: ? = (1- r?)/V, where: r is the tire effective rolling radius, ? is the wheel angular velocity, and V is the vehicle forward velocity. Mue (m) is an indication of the "grip" available at the tire/road interface. Mue is defined as F/Fn, where F is the force generated at the tire contact patch (Fb for braking force or Fc for cornering force), and Fn is the normal force (or vertical load) acting on the contact patch. In the graph below, two sets of curves are shown, mb and mc. mb is the grip available with increasing slip in the longitudinal (braking) direction, and mc is the grip available with increasing slip in the lateral (cornering) direction. The curves shown below are for a high mue surface (i.e. dry asphalt or concrete).
Let's start with the top longitudinal mue curve (mb). Notice that there is a linear portion, a "knee", and then the curve drops off. The linear portion of the curve is the stable region, where an increase in braking force produces a predictable increase in tire slip. The area after the knee in the curve is the unstable region. Once the tire passes into this region, a dramatic increase in slip occurs with no additional increase in braking force, resulting in wheel lock. The goal of an ABS system is to control tire slip at the top of the curve just before the knee to maximize braking force, yet prevent the wheel from entering the unstable region of the curve. Controlling wheel slip in the unstable region is pretty much impossible, even for the most sophisticated ABS systems. As you can imagine, controlling all four wheels to this target is no easy task, and things get even more difficult when you throw different surfaces (ice, snow, gravel) as well as different tire loading conditions (cornering, bumps, etc.).
Now lets look at the lateral mue curves (mc). Notice that side force starts to drop off immediately, and quite rapidly as slip increases. However, instead of the wheel locking as in the case above, the result is understeer (front wheels sliding sideways) if the front wheel lateral slip threshold is exceeded first, or oversteer (rear wheels sliding sideways) if the rear wheel lateral slip threshold is exceeded first.
The arrows on the diagram show that as the side force loading of the tire is increased, the longitudinal force capability of the tire is reduced, and vice versa. This simply shows graphically what all the racers already know: braking while cornering at the limit must be performed very carefully! The tire has a limited amount of "grip" available - if you use 90% of the available grip for cornering, only 10% will be available for braking before exceeding the limits of the tire and entering the unstable region of the mue/slip curve. When braking while cornering, the ABS system is tuned to release brake pressure to prevent saturation of the tire contact patch, thus allowing more cornering force and stabilizing the vehicle. ABS "senses" when a vehicle is cornering by detecting the difference in wheel speeds between the inside and outside wheels - the outside wheels travel on a larger radius than the inside wheels, so they rotate faster.
The ABS system calculates wheel slip by comparing the individual wheel speeds to a vehicle reference speed which is a complex calculation using all 4 wheel speeds and some assumptions based on vehicle parameters (mass, tire rolling radius, weight distribution, wheel inertias, mue/slip curves, etc). The first and second derivatives of the wheel speed information (acceleration and jerk respectively) are also used to determine what type of surface the vehicle is on (asphalt, snow, ice, gravel, etc.) and when to enter ABS.
The ABS System Hardware
Every ABS system consists of three main "subsystems":
Sensors: A typical ABS system will have a sensor at each wheel that supplies a square wave signal to the ECU that changes in frequency based on speed. The ECU converts this signal to wheel speed and acceleration information. Wheel speed sensors are typically the Hall Effect type. Electronic stability control systems have lateral acceleration , yaw rate, steering wheel angle and master cylinder pressure sensors in addition to the wheel speed sensors.
Electronic Control Unit (ECU): This is the brain of the system. It takes the information from the sensors, processes it, and sends valve activation information to the hydraulic unit if ABS brake interventions are needed.
Hydraulic Unit (HU): This is the part of the system that does the actual modulation of the brakes. There are 2 valves for each channel (circuit) that control pressure build, hold and release. During normal braking (no ABS), the pressure build valve is open, allowing fluid flow in both directions from the master cylinder to the wheel cylinder. In an ABS pressure release, the build valve closes which prevents fluid flow to the wheel, and the pressure release valve opens to allow fluid to flow from the wheel to an accumulator in the hydraulic unit. When this accumulator fills up, the pump motor starts up and pumps the fluid back to the master cylinder. In an ABS pressure hold, both valves close and prevent fluid flow to or from the wheel cylinder. This process of build, hold, release is what causes the noise and pedal feedback during ABS activation.
ABS System Features
3 and 4 Channel Systems: There are 2 main types of ABS systems, 3 and 4 channel. Most earlier systems were 3 channel to save weight, complexity and cost. In a 3 channel system, both wheels on the rear axle are connected to the same circuit on the hydraulic unit, meaning that they are both controlled to the same slip level. The speed on both rear wheels is monitored for slip, but the controller will reduce pressure on both wheels to the level required to prevent the wheel with the largest amount of slip from locking. In a straight line stop on a homogenous surface, the performance of a 3 channel system is equal to a 4 channel system. But if one wheel is on a slippery surface or while braking and turning, one wheel on the rear axle of a 3 channel system will be underbraked for stability. This is not as bad as it sounds, since the rear axle contributes only about 15 to 20% of the total braking capacity of a vehicle.
Electronic Brake Force Distribution (EBD): EBD is an ABS system enhancement that first became available on some production vehicles in the mid-90's. EBD takes the place of the mechanical front/rear axle pressure proportioning valve installed on earlier ABS systems and non-ABS equipped vehicles. EDB is superior to a mechanical proportioning valve because it can be tuned to compensate for different vehicle speeds, deceleration rates and loading conditions, while a mechanical prop valve is strictly pressure dependent. Vehicles equipped with EBD systems should NEVER be driven with the ABS disabled!! With the system disabled, equal brake pressure is sent to both axles. During braking, weight transfer will reduce the normal force on the rear wheels, thus reducing the available grip and causing them to lock very early because there is nothing to limit pressure. The rear wheels will lock well before the front wheels have reached maximum braking potential, resulting in very long stopping distances and serious stability problems.
ABS Tuning
The ABS software has literally thousands of parameters that can be tuned to tailor the system performance for an application on all surfaces - asphalt, gravel, snow and ice. The system must also be tuned for braking while cornering, surface transitions (dry pavement to ice, etc), split mue (one side of the vehicle on dry pavement, one side on ice), and bumps.
Tuning an ABS system is a compromise between stopping distance and stability - if the system is tuned to allow more longitudinal slip during straight line braking to improve stopping distance, the ability to steer and the stability of the rear axle are sacrificed to some degree. Most vehicle manufacturers tend to bias ABS system performance towards stability.
I've often read "opinions" of people who believe that ABS systems are an unnecessary complication and expense in new vehicles today. These people reason that ABS actually increases stopping distance on ice and gravel, and that a good driver can even beat ABS on asphalt IN A STRAIGHT LINE. The problem with this line of reasoning is that the purpose of ABS is not to provide the shortest possible stopping distance, but to provide the shortest possible stopping distance with MINIMAL LOSS IN STEERABILITY AND STABILITY of the vehicle. Remember the mue/slip curve? If a driver has maximized the braking potential of the vehicle in a straight line, there is no "grip" left to steer around an obstacle. As soon as an attempt is made to steer around an obstacle, the wheels will lock and the vehicle will continue on in a straight path (and with less deceleration too). Even a skilled driver that can beat the stopping distance of an ABS system in a straight line will not be able to beat ABS while trying to avoid an obstacle, especially in a panic situation.
Vehicle Modifications that Affect ABS System Performance
There are many common vehicle modifications enthusiasts make to their vehicles that can have an effect on the performance of an ABS system. When modifying a vehicle with ABS, ALWAYS check to make sure the ABS system performance is not adversely affected!
1) Tires - changing tires can have an impact on ABS performance because the mue/slip curve of the new tire may be significantly different. Usually this is not a problem if a similar type of street tire is used. However, the enthusiast that uses DOT race tires on an ABS equipped vehicle may notice that the vehicle is not decelerating as hard as the tires will allow before the ABS intervenes. This is because DOT racing tires have a radically different mue/slip curve than an average street tire, and the ABS system is not tuned to take advantage of the more aggressive curve.
2) Brake pad lining material - The pad/rotor interface is the 2nd most important interface in the brake system (see above for #1). The pads chosen for OEM applications are obviously a compromise between braking performance, durability and comfort (noise/dust issues). ABS systems are tuned for the OEM pads, not an aggressive track pad.
3) Suspension - stiffer springs, dampers and sway bars affect the rate of weight transfer while stopping and turning. This affects ABS control! Again, the ABS system was not tuned to take advantage of a stiffer suspension. The system is tuned for weight transfer to occur at a faster rate with the softer stock suspension, so the interventions may be earlier than necessary.
4) Wheel inertia - This may not be much of a factor for most racers, since the stock size wheels are usually used with lightweight race tires. But for the enthusiast that installs much heavier oversized wheels and tires this becomes an issue. Again, the system is tuned for a particular wheel/tire combination. More wheel inertia can cause the ABS system to overshoot the wheel slip target, requiring a large decrease in pressure to bring the wheel back into the stable region of the mu/slip curve. This happens for each cycle of control, and the end result is very choppy control and longer stopping distances.
Adding different tires, brake pads, and suspension parts to a vehicle changes the characteristics of the wheel speed information and the thresholds for ABS entry are no longer accurate. Also, the reference calculation may no longer be accurate, which means that the slip calculations may no longer be accurate. The typical end result is that the ECU thinks the tires are operating at slip levels greater than they actually are and begins to release pressure earlier than necessary, thus underbraking the vehicle.
Perhaps an easy way to explain the limitations of the ABS system is to draw a analogy to an engine controller. If you make engine modifications like different cams or bigger injectors the engine controller will have to be re-tuned to optimize performance. The same principle applies with ABS.
Considering that most enthusiasts who drive their cars on the racetrack (racing or schools) have DOT race tires, "track" pads and stiffer suspensions, it comes as no surprise that even an average driver may be able to out-brake the ABS system in a straight line. Please note that this does not mean that I advocate disabling ABS for track events! I still believe that it is safer for the average driver to keep ABS active on the racetrack, provided that modifications have not caused the vehicle to be severely underbraked. At the very least, ABS will prevent flat-spotting those expensive race tires!
ABS and Race Cars
After all this talk about the benefits of ABS, many readers are probably still thinking "If ABS is so great, why don't they use it on race cars?" There are several reasons that I can think of:
1)The shape of the mue/slip curve of a race tire can change dramatically over the course of a race. An ABS tune that works great at the beginning of a race when the tires are relatively cool may not stop the car when the tires are smokin' hot. An adaptive algorithm is needed to correct for this change in tire characteristics with temperature, as well as some sort of tire temperature monitoring.
2) Many drivers prefer to have adjustable brake proportioning to balance the handling of the car. The ABS system would have to be re-tuned for each brake bias adjustment the driver made.
3) Race cars are much more adjustable than street cars. As I explained earlier, changes to tire compounds, brake pads, spring rates, damping rates, and sway bars will all have an effect on ABS performance. The ABS system would have to be re-tuned every time a major setup change was made.
Keep in mind that re-tuning an ABS system is not possible for most racers, and the amount of testing needed for each re-tune would be prohibitive even if it was possible. The above reasons, in addition to the fact that race car drivers are much more skilled at brake modulation than normal drivers, are why ABS is not used in most race cars.
