Chapter 31 — Antilock Braking Systems¶

Updated Markdown edition of the HVE User's Manual (HVE Version 5, Seventh Edition, January 2006), Chapter 31, pages 31-1 through 31-22. Verified against the current HVE application source (HVEINV-64/ABSSystemDlg.cpp, ABSDesignerDlg.cpp, BrakePage.cpp).

Overview¶

Most vehicles built today are fitted with anti-lock braking systems (ABS). Accurate simulation modeling of these vehicles during braking, as well as combined braking and steering maneuvers, thus requires the effects of the ABS to be included. This is especially true for the simulation of complex crash avoidance maneuvers.

This chapter describes the ABS model implemented in the HVE user environment. It is a general purpose model, available for use by any HVE-compatible vehicle simulation model. The model is applicable to the design of ABS systems, as well as the study of loss-of-control crashes of ABS-equipped vehicles.

This chapter covers the following topics:

ABS Methodologies — A basic overview describing how ABS works
ABS State Variables — A list of the parameters that determine the current state of the ABS system
Typical Hardware — A description of the hardware components common to most ABS systems
HVE ABS User Interface — A detailed description of the parameters included in the HVE ABS dialogs and how the parameters are used to model various ABS systems
Current Models — An overview of the ABS models currently implemented in HVE
Procedures — A step-by-step overview of the procedures for simulating an ABS system on a vehicle

ABS Methodologies¶

The basic concept behind ABS is quite simple and can be demonstrated by the graph of normalized braking force vs longitudinal tire slip shown in Figure 31-1. This graph is traditionally called a mu-slip curve. It defines the relationship between longitudinal tire slip and the resulting longitudinal (braking) force. A key observation is that the maximum braking force occurs at the peak braking coefficient, \(\mu_p\), in the vicinity of 10 to 15 percent longitudinal tire slip (this varies somewhat from tire to tire). Also, as the tire slip continues to increase to 100 percent, the available braking force falls off. The region of tire slip between \(\mu_p\) and \(\mu_s\) (slide friction coefficient, or 100 percent longitudinal tire slip, associated with locked-wheel braking) is a region of dynamic instability. As slip begins to increase beyond \(\mu_p\) it quickly increases to 100 percent (i.e., the tire locks) with a commensurate loss in available braking force.

The goal of an ABS system is simply to prevent the tire slip from increasing significantly beyond \(\mu_p\) — regardless of how much brake pedal force is applied by the driver. By keeping the longitudinal slip in the stable region below the peak (see Figure 31-1), the tire continues to roll and, therefore, maintains directional control capability (i.e., the driver can steer the vehicle). In addition, the available braking force is larger than for a locked tire and, therefore, braking distance can be reduced.

Figure 31-1 — Typical graph of normalized braking force vs longitudinal tire slip, also called a mu-slip curve.

ABS simulation takes advantage of the simulation model's wheel spin degree of freedom, wherein the braking force is calculated from first principles, rather than simply specified as a force at the tire-road interface. To truly simulate ABS, the algorithm must modulate the simulated brake pressure at the wheel, just as it does on an actual vehicle. The procedures for accomplishing this task are described below.

All ABS methodologies work by controlling tire longitudinal slip. This is accomplished through the use of wheel sensors that compare the tire circumferential velocity to the current reference velocity, \(V_r\), normally calculated using the current spin velocities of two or more wheels (see reference 4.42 for a detailed discussion of the calculation of reference velocity). On a vehicle, the longitudinal tire slip cannot be measured directly. Instead, slip is calculated:

\[Slip = \frac{V_r - \dot{\Omega}_w \times R_{tire}}{V_r} \qquad (\text{31-1})\]

where

Symbol	Description
\(V_r\)	Reference velocity
\(\dot{\Omega}_w\)	Wheel spin velocity
\(R_{tire}\)	Current tire rolling radius

State Variables¶

To accomplish the required control of longitudinal slip, the following state variables are monitored or calculated by the vehicle's ABS control module:

Vehicle Velocity — Linear velocity of the vehicle sprung mass
Wheel Spin Velocity — Angular velocity of each wheel (or axle on some systems)
Tire Longitudinal Slip — Relative velocity between the tire and road, expressed as a fraction of vehicle velocity
Wheel Spin Acceleration — Angular acceleration of each wheel
Tire-Road Surface Friction — Ratio of the maximum braking force to the normal tire force
Brake System Pressure — Pressure produced as a result of the driver's brake pedal application (input variable)
Wheel Brake Pressure — Pressure supplied to the wheel brake assembly (output variable)

Typical Hardware¶

To monitor or calculate the above state variables, the typical ABS system includes the following hardware components:

Electronic Control Unit (ECU) — This is the vehicle's microcomputer. It is programmed with the algorithm that reads the current state variables, determines the required pressure at each wheel and sends the appropriate signals to the brake pressure modulator (see below).
Wheel Speed Sensors — These components directly measure the wheel spin velocity of each wheel using a wheel-mounted pulse rotor (a notched metal ring) and a fixed, magnetic sensor that measures the rotation of the pulse rotor.
Brake Pressure Modulator — This component (or components, depending on the system) controls the wheel brake pressure according to the control conditions specified by the ECU.
Brake Master Cylinder/Air Compressor — This component provides the fluid pressure source.
Wheel Brake Caliper/Cylinder/Chamber — This component applies the specified braking force at each wheel according to the wheel brake pressure.

The basic hardware requirements are generally the same for all vehicle types, ranging from passenger cars to on-highway trucks. Reference 4.42 provides a detailed description of these required components.

ABS User Interface¶

The HVE ABS user interface allows the user to select an ABS algorithm and to enter and edit the independent parameters required by the selected ABS algorithm. The interface includes numerous options; thus, various algorithms may be supported. The interface is divided into two sections:

System Variables — Variables that are applicable to the entire vehicle
Wheel Variables — Variables that may be specified independently for each wheel

The interface, dialogs and associated variables are described below.

Figure 31-2 — ABS System Data dialog. These parameters affect the entire vehicle (compare with the ABS Wheel Data dialog).

Figure 31-3 — ABS Wheel Data dialog. These parameters affect the selected wheel (compare with the ABS System Data dialog).

Table 31-1 — HVE ABS System variables. These variables apply to the entire vehicle (compare with Wheel Variables in Table 31-2).

Parameter	Unit Name	Description
Control Source	UtNone	Location of ABS controller (This Vehicle or Tow Vehicle)
Algorithm	UtNone	ABS algorithm selected from a list of available algorithms
Control Method	UtNone	ABS control method (Vehicle-Based, Axle-Based or Wheel-Based)
Cycle Rate	UtFrequency	Determines the time required for a complete ABS cycle
Threshold Brake System Pressure	UtBraPress	Minimum system pressure for ABS activation
Threshold Vehicle Velocity	UtVehVelLinear	Minimum vehicle velocity for ABS activation
Low Friction Threshold	UtNone	Tire-terrain surface friction threshold
Delay Method	UtNone	ABS delay method (Vehicle-Based, Axle-Based or Wheel-Based)
Apply Delay	UtBraTime	Time delay for start of controlled output pressure increase
Release Delay	UtBraTime	Time delay for start of controlled output pressure decrease

Table 31-2 — HVE ABS Wheel variables. These variables apply to the selected axle or wheel (compare with System Variables in Table 31-1).

Parameter	Unit Name	Description
Wheel Minimum Velocity	UtVehVelLinear	Minimum wheel forward velocity for ABS control
Tire Slip, Minimum	UtBraPercent	Minimum tire longitudinal slip for pressure modulation
Tire Slip, Maximum	UtBraPercent	Maximum tire longitudinal slip for pressure modulation
Spin Accel, Minimum	UtVehAccelAngular	Minimum wheel angular acceleration for pressure modulation
Spin Accel, Maximum	UtVehAccelAngular	Maximum wheel angular acceleration for pressure modulation
Cycle Rate	UtFrequency	Determines the time required for one complete ABS cycle
Apply Delay	UtBraTime	Time delay for start of controlled wheel output pressure increase
Apply Rate, Primary	UtBraPressRate	Primary rate of controlled wheel output pressure increase
Apply Rate, Secondary	UtBraPressRate	Secondary rate of controlled wheel output pressure increase
Release Delay	UtBraTime	Time delay for start of controlled output pressure decrease
Release Rate	UtBraPressRate	Rate of controlled wheel output pressure decrease

System Variables¶

The ABS System variables included in the HVE ABS model are included in the ABS System Data dialog. The variables and a brief description are shown in Table 31-1. A detailed description of each variable follows.

Controller Source — This selection defines the location of the ABS control unit (ECU; see Typical Hardware, above). For unit vehicles (e.g., passenger cars, vans), it is always This Vehicle. However, the controller for a trailer may be on This Vehicle or the Tow Vehicle. Selecting Tow Vehicle causes the remaining parameters on the ABS System dialog to be disabled; the ABS system parameters are assigned according to those selected for the tow vehicle.
ABS Algorithm — This is the ABS algorithm, selected from a list of the various ABS algorithms available to the user. The algorithms currently available are Tire Slip and Bosch Version 1. This list is updated as new algorithms become available.
Control Method — This option determines if all wheels are controlled by a single controller or if the individual wheels or axles are controlled separately. Vehicle-based control uses the same control cycle (see below) for all wheels; Axle-based control allows different control cycles for each axle (typically only the rear axle is controlled); Wheel-based control allows different control cycles for each wheel.
Cycle Rate — If the Control Method is Vehicle-based, this parameter determines the maximum time required to perform a complete ABS cycle. It is the same for all wheels.

Specifying the cycle rate is a way an ABS algorithm can enforce a specified periodic cycling of the brake pressure. However, most ABS algorithms do not use the Cycle Rate parameter. Instead, the cycle rate is the natural result of the Apply and Release Delays and the Apply and Release Rates (see below).

NOTE: Cycle Rate is disabled if the Control Method is Axle-based or Wheel-based. Instead, the Cycle Rate is set separately for each wheel (see ABS Wheel Data dialog).

Minimum ABS Pressure (Threshold Brake System Pressure) — This parameter provides a minimum system pressure threshold for ABS actuation. ABS is bypassed when system pressure falls below this value.
Minimum Vehicle Velocity (Threshold Vehicle Velocity) — This parameter provides a minimum vehicle velocity threshold for ABS actuation. ABS is bypassed when vehicle velocity falls below this value.
Low Surface Friction Threshold — This parameter sets a threshold defining low friction surfaces. Algorithms can use this parameter to invoke friction-dependent modulation behaviors. For example, the Apply Delay or Release Delay (see below) may be reduced for low friction surfaces.
Delay Method — This parameter determines if control pressure delays are Vehicle-based, Axle-based or Wheel-based. Vehicle-based delay uses the same delay period for each wheel; Axle-based delay allows different delay periods for each axle; Wheel-based delay allows different delay periods for each wheel.
Apply Delay — If the Delay Method is Vehicle-based, this parameter determines the delay period for all wheels before output pressure is increased. (See Wheel Variables for Axle-based and Wheel-based delay.) This value defines the period of constant pressure before pressure begins to increase (see Figures 31-5 through 31-7).
Release Delay — If the Delay Method is Vehicle-based, this parameter determines the delay period for all wheels before output pressure is reduced. (See Wheel Variables for Axle-based and Wheel-based delay.) This value defines the period of constant pressure before pressure begins to decrease (see Figures 31-5 through 31-7).

NOTE: Apply Delay and Release Delay are disabled if the Delay Method is Axle-based or Wheel-based. Instead, the Apply and Release Delays are set separately for each wheel (see ABS Wheel Data).

Figure 31-4 — Flow chart showing integration of the HVE ABS model into a vehicle simulation.

Wheel Variables¶

The ABS Wheel variables are shown in Table 31-2. A brief description of each variable follows.

Minimum Wheel Velocity — This parameter specifies a minimum wheel velocity threshold for ABS actuation. ABS is bypassed when the wheel velocity falls below this value.
Minimum Tire Slip — This parameter may be used in an algorithm to establish a lower threshold for tire longitudinal slip.
Maximum Tire Slip — This parameter may be used in an algorithm to establish an upper threshold for tire longitudinal slip.
Minimum Wheel Spin Acceleration — This parameter may be used in an algorithm to establish a lower threshold for wheel spin acceleration.
Maximum Wheel Spin Acceleration — This parameter may be used in an algorithm to establish an upper threshold for wheel spin acceleration.
Cycle Rate — If the ABS System Control Method is Axle-based or Wheel-based, this parameter determines the maximum time required to perform a complete ABS cycle for the selected axle or wheel.

Specifying the cycle rate is a way an ABS algorithm can enforce a specified periodic cycling of the brake pressure. However, most ABS algorithms do not use the Cycle Rate parameter. Instead, the cycle rate is the natural result of the Apply and Release Delays and the Apply and Release Rates (see below).

NOTE: Cycle Rate is disabled if the Control Method is Vehicle-based. Instead, Cycle Rate is set in the ABS System Data dialog.

NOTE: If the Control Method is Axle-based, the specified cycle rate is applied to both wheels on the selected axle.

Apply Delay — If the Delay Method is Axle-based or Wheel-based, this parameter determines the control pressure delay period before output pressure is increased at the selected wheel. This value defines the period of constant pressure before pressure begins to increase (see Figures 31-5 through 31-7).
Primary Application Rate — This parameter determines the initial rate of pressure increase. This value defines the slope as pressure increases (see Figures 31-5 through 31-7).
Secondary Application Rate — This parameter provides an alternative rate of pressure increase, usually substantially lower than the Primary Application Rate. This value defines the slope as pressure increases (see Figures 31-5 through 31-7).
Release Delay — If the Delay Method is Axle-based or Wheel-based, this parameter determines the control pressure delay period before application pressure is reduced at the selected wheel. This value defines the period of constant pressure before pressure begins to decrease (see Figures 31-5 through 31-7).
Release Rate — This parameter determines the rate of pressure decrease (see Figures 31-5 through 31-7).

Selection of the values for the above parameters effectively determines the cycle rate for an ABS system. Using this approach (instead of assigning a specific Cycle Rate; see earlier description of the Cycle Rate variable) has the advantage of varying the cycle time according to the current operating conditions.

NOTE: Apply Delay and Release Delay are disabled if the Delay Method is Vehicle-based. Instead, the Apply and Release Delays are set in the ABS System Data dialog.

NOTE: If the Delay Method is Axle-based, the specified Apply Delay and Release Delay are automatically assigned to both wheels on the selected axle.

The ABS System and Wheel Data variables are provided as a palette of parameters available to the designer of an ABS system algorithm. The selection of individual variables and their effect on the simulated characteristics of any specific ABS system are algorithm-dependent.

Implementation¶

To integrate the ABS model into a vehicle dynamic simulation requires that the simulation have the following features:

A brake system model, including a pressure source and optional proportioning, lag times and rise times
A wheel brake model that computes brake torque ratio as a function of current wheel brake pressure
A spin degree of freedom for each wheel

A flowchart showing the integration of the ABS model into an existing vehicle simulation is shown in Figure 31-4. See also reference 4.43.

Current Algorithms¶

Two ABS algorithms are currently implemented. These are Tire Slip and Bosch Version 1. These algorithms are described below.

Tire Slip Algorithm¶

This is a simple and straight-forward ABS algorithm. It is designed based on the fundamental goal of an ABS system, that is, to maintain tire slip in the vicinity of the peak friction coefficient (refer to Figure 31-1).

On brake pressure application, once ABS is invoked (that is, the minimum vehicle velocity and brake pressure thresholds are exceeded), the algorithm incorporates switching logic depending on the current tire slip:

Tire Slip ≤ Minimum Tire Slip — Under this condition, the status of the ABS during the previous sample determines how pressure is modulated for the current cycle. If the ABS modulation status was off, the output pressure is set equal to the input pressure and the ABS system parameters (delays, etc.) are reset. Otherwise, brake pressure will be controlled. One of two possibilities exists: (a) Input pressure is decreasing — in this case, output pressure is set equal to input pressure and the ABS status is turned off; or (b) Input pressure is constant or increasing — in this case, the output pressure is maintained constant for the specified Apply Delay, after which output pressure is increased according to the Primary Apply Rate.
Minimum Tire Slip < Tire Slip < Maximum Tire Slip — Under this condition, pressure will be controlled. One of two possibilities exists: (a) Input pressure is decreasing — in this case, output pressure is set equal to input pressure and the ABS status is turned off; or (b) Input pressure is constant or increasing — in this case, the output pressure is maintained constant for the specified Apply Delay, after which output pressure is increased according to the Secondary Apply Rate.
Tire Slip ≥ Maximum Tire Slip — Under this condition, pressure will be controlled. The output pressure is maintained constant for the specified Release Delay, after which the output pressure is reduced according to the Release Rate.

Figure 31-5 — Typical control cycles for the Tire Slip algorithm (wheel pressure vs time during a hard brake application).

Figure 31-5 shows a typical pressure vs time history for a few cycles of a hard brake pedal application (i.e., enough system pressure to lock the wheel). The parameters used by the Tire Slip algorithm are shown in Table 31-3. Additional details for the Tire Slip algorithm, including a flow chart, are provided in reference 4.43.

Table 31-3 — Variables used by the Tire Slip ABS algorithm

Parameter
Control Method
Threshold Brake System Pressure
Threshold Vehicle Velocity
Tire Longitudinal Slip, Minimum
Tire Longitudinal Slip, Maximum
Delay Method
Apply Delay
Apply Rate, Primary
Apply Rate, Secondary
Release Delay
Release Rate

HVE Bosch Version 1 Algorithm¶

The HVE Bosch Version 1 ABS algorithm is based on the algorithm described in reference 4.42. The Bosch ABS system is used on many passenger cars. The algorithm is based on wheel spin acceleration and a critical tire slip threshold.

Figure 31-6 — Idealized control sequence for the Bosch algorithm (reprinted from reference 4.42). Compare the lower portion with the pressure cycle displayed in Figure 31-7.

Figure 31-7 — Simulated control cycle for the HVE Bosch Version 1 algorithm. Compare with the lower portion of Figure 31-6.

Upon brake pressure application, once ABS is invoked (i.e., the thresholds are exceeded), the current brake pressure application is divided into eight phases (see Figure 31-6):

Phase 1 — Initial application. Output pressure is set equal to input pressure. This phase continues until the wheel angular acceleration (negative) drops below the Wheel Minimum Wheel Spin Acceleration, \(-a\).
Phase 2 — Maintain pressure. Output pressure is set equal to the previous pressure. This phase continues until the tire longitudinal slip exceeds the slip associated with the Slip Threshold. At this time, the current tire slip is stored and used as a criterion in later phases. This slip corresponds to the maximum slip; the tire is beginning to lock.
Phase 3 — Reduce pressure. Output pressure is decreased according to the Release Rate until the wheel spin acceleration again becomes positive (this is a slight modification to the sequence shown in Figure 31-6, in which the pressure is decreased until the wheel spin acceleration exceeds \(-a\)).
Phase 4 — Maintain pressure. Output pressure is set equal to the previous pressure for the specified Apply Delay, or until the wheel spin acceleration (positive) exceeds \(+A\), a multiple (normally 10×) of the Wheel Maximum Spin Acceleration (signifying the wheel spin velocity is increasing at an excessive rate).
Phase 5 — Increase pressure. Output pressure increases according to the Primary Apply Rate. This phase continues until the wheel spin acceleration drops and again becomes negative (this is a slight modification to the sequence shown in Figure 31-6, in which the pressure is increased until the wheel spin acceleration drops below \(+A\)).
Phase 6 — Maintain pressure. Output pressure is set equal to the previous pressure for the specified Apply Delay, or until wheel angular acceleration again exceeds the Wheel Minimum Spin Acceleration (negative).
Phase 7 — Apply pressure. Output pressure increases according to the Secondary Apply Rate, normally a fraction (1/10) of the Primary Apply Rate. This achieves greater braking performance while minimizing the potential for wheel lock-up at tire longitudinal slip in the vicinity of peak friction. This phase continues until wheel angular acceleration drops below the Wheel Minimum Angular Acceleration (negative), indicating wheel lock-up is imminent.
Phase 8 — Release pressure. At this point an individual cycle is complete, the process returns to Phase 3 and a new control cycle begins.

As stated, some minor differences exist between the HVE implementation and the Bosch description provided in reference 4.42. These differences reflect some inconsistencies between the acceleration and pressure profiles shown in Figure 31-6. For example, unless the throttle is applied, it is physically inconsistent that the wheel spin acceleration would be positive, let alone increase (as shown in Phase 5), in the presence of increased brake pressure (and, therefore, brake torque).

Each of the above phases begins with a comparison between the current tire longitudinal slip and the value stored during Phase 2. If the current slip exceeds this value, the normal logic is bypassed and resumed at Phase 3. This effectively allows the algorithm to "learn" the wheel slip associated with wheel lock-up on the current surface. This is referred to as adaptive learning, and is a key to the success of this ABS algorithm. As the tire travels onto surfaces with differing friction characteristics, the ABS model is able to maximize its performance accordingly.

Default parameters used by the HVE Bosch Version 1 algorithm were developed through the evaluation of numerous simulation runs. See reference 4.43 for typical parameters applicable to a P195/75R14 passenger car tire.

Figure 31-7 shows a typical pressure vs time history for a single cycle of a hard brake pedal application (i.e., enough system pressure to lock the wheel). The parameters used by the HVE Bosch Version 1 algorithm are shown in Table 31-4. Additional details for the HVE Bosch Version 1 algorithm, including a flow chart, are provided in reference 4.43.

Table 31-4 — Variables used by the HVE Bosch Version 1 ABS algorithm

Parameter
Control Method
Threshold Brake System Pressure
Threshold Vehicle Velocity
Tire Longitudinal Slip, Minimum
Wheel Spin Acceleration, Minimum
Wheel Spin Acceleration, Maximum
Delay Method
Apply Delay
Apply Rate, Primary
Apply Rate, Secondary
Release Delay
Release Rate

Other ABS Algorithms¶

The ABS model implemented in HVE is not restrictive in terms of the algorithms it can support, other than its need to provide the parameters required by the algorithm. Endless tweaking of an algorithm is possible, resulting in different ABS system characteristics, each with its advantages and disadvantages. Thus, it is certain that new ABS algorithms will be developed and implemented in HVE over time, both to develop and to model new ABS systems.

Procedures¶

The steps for using the HVE ABS model are:

Display the Brake System Information dialog.
Click the ABS Installed checkbox.
Optionally, edit the vehicle's ABS System parameters.
Optionally, edit the vehicle's ABS Wheel parameters for each wheel.

These steps are described below.

Figure 31-8 — Vehicle Brake System Information dialog. Click on ABS Installed to invoke the ABS model. Click on Edit to display the ABS System Data dialog for the selected vehicle.

Invoking the ABS Model¶

Vehicles in the EDC Generic Database (Generic.db) and the EDC Custom Vehicle Database (EDC.db) are currently created with the ABS model turned off.

To model an ABS system, perform the following steps:

Choose Vehicle Mode. The Vehicle Editor displays the current vehicle.
Choose the desired vehicle from the list of active vehicles. The vehicle is displayed in the viewer.
Click on the vehicle's brake system icon. The Brake System Information dialog is displayed (see Figure 31-8).
Click the ABS Installed checkbox. The Edit button becomes enabled.

The vehicle's ABS parameters may now be edited.

Editing ABS System Parameters¶

After invoking the ABS model, as described in the previous section, the ABS System parameters may be edited. To edit the ABS System parameters, use the Brake System Information dialog (see Figure 31-8), and perform the following steps:

Press Edit. The ABS System Data dialog is displayed (see Figure 31-2).
Edit the various ABS system parameters as desired.

Editing ABS Wheel Parameters¶

After invoking the ABS model, as described earlier, the ABS wheel parameters may be edited. To edit the ABS wheel parameters, perform the following steps:

Click on the desired wheel and choose Brakes from the Wheel Data pop-up menu. The Brake Assembly dialog is displayed.
Press the ABS Designer button to display the ABS Wheel Data dialog for the selected wheel.
Edit the ABS parameters as desired.
Press OK. The Brake Assembly dialog is still displayed.
Press Copy to Other Side if you wish the left-side and right-side wheels to have the same properties.

NOTE: If the Control Method and/or Delay Method are Axle-based, those parameters are automatically copied to the other side.

Press OK to assign the ABS Wheel parameters for the selected wheel(s).

Any event that supports the HVE ABS model (e.g., SIMON) will now include the effects of the ABS system when the event is executed.

Tips and Suggestions¶

The following tips and suggestions are helpful when using the HVE ABS Model:

Although the ABS Model will be executed each trajectory integration timestep, the simulation will output results according to the currently specified output time interval (default value 0.1000 sec). Therefore, if you wish to evaluate detailed results from an ABS event, you should reduce the output interval (see Simulation Controls dialog) to 0.0100 seconds (or less). Otherwise, the (rather large) output interval will act as a filter, and the details of the pressure modulation will be lost.
To monitor results from an ABS simulation, use the Variable Selection dialog and choose the desired output parameters. The most interesting results for an ABS simulation include the following parameters:
Tire Group — Fx', Fy', Fz'; Long Slip; Skidmark
Wheel Group — Spin Velocity; Spin Accel; Brake Pressure
Driver Group — Brake (System) Pressure

Using Variable Output to graph these results is most helpful. - Two conditions are required to cause a tire to leave a skidmark. First, the tire must be skidding (as determined by the value of the Skidmark output variable), and second, the radial load on the tire (Fz') must be sufficient to leave a mark (see Minimum Fz for Skidmark; this value is assigned in the Vehicle Editor, Tire Information, Physical Data dialog). Otherwise, even a locked tire may not leave a black rubber tire mark on the road. - In early versions of HVE, the skidmark was either ON or OFF. HVE now has the capability to vary the skidmark intensity according to the current level of tire longitudinal and lateral slip and vertical tire load. This capability is particularly useful for ABS simulation, wherein the skidmarks are faint, rather than pure black. HVE simulates this effect. - To illustrate the presence of faint skidmarks on a textured road surface, remove the texture (right-click on the road surface and choose No Texture). You can actually see the faint skidmarks on the untextured surface.

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