WO2009055841A1

WO2009055841A1 - Stability control for a vehicle

Info

Publication number: WO2009055841A1
Application number: PCT/AU2008/001590
Authority: WO
Inventors: Raymond Andrew Munday
Original assignee: Kinetic Pty Ltd
Priority date: 2007-10-29
Filing date: 2008-10-28
Publication date: 2009-05-07

Abstract

A method of providing an input to at least one control system of a vehicle, by determining a) that a substantially vertical force at the contact patch between the ground and at least one wheel assembly of the vehicle during cornering is approaching a zero value or is at a substantially zero value, and b) providing a signal as an input to the at least one control system derived from the value determined at step a). The method can include controlling a dynamic limit of a vehicle, including the steps of; i) determining that a substantially vertical force at the contact patch between the ground and at least one wheel assembly of the vehicle during cornering is approaching a zero value or is at a substantially zero value; and ii) providing a signal derived from the value determined at step as an input to at least one control system of the vehicle; and iii) controlling, operating, initiating or varying operation of the control system to return the vehicle within the dynamic limit.

Description

STABILITY CONTROL FOR A VEHICLE FIELD OF THE INVENTION

The present invention relates to vehicle dynamic handling characteristics, such as detection, monitoring or control thereof. BACKGROUND TO THE INVENTION

If the point at which the tyre contact patch vertical force of one of the wheels of a vehicle reaches zero is known or estimated, then it can be used as an indicator of the approach of the dynamic limits of the vehicle and therefore a trigger for one or more vehicle safety systems to operate. When a vehicle having four or more wheels is in a turn it may, in extreme, have the vertical force at the tyre contact patch of one of the inside wheels reduce to zero, at which point the wheel may begin to lift off the ground. At this point, the roll moment distribution (RMD) characteristic of the vehicle begins to change. The wheel with zero ground contact force and the laterally adjacent wheel then provide a maximum fixed roll moment with increasing roll angle, whereas the other wheels of the vehicle can continue to provide an increasing roll moment.

There are known methods for predicting the loads on the wheels of a vehicle by using base vehicle data, such as roll centre heights, position of the centre of mass, track width, suspension stiffness characteristics including the RMD, as well as additional measured data such as lateral g, vertical g and preferably an estimate of the load in the vehicle and the centre of mass of that load and other effects due for example to roll displacement of the vehicle body such as the offset of the centre of mass and the migration of the roll centres. Using such inputs the vertical force at each tyre contact patch can be calculated. The accuracy of these calculated forces can be highly dependent on the estimate of the position of the vehicle load, most specifically, its height.

These known methods require a large number of inputs in addition to the base vehicle data and the lateral and vertical g accelerations. For example, sensors can be required to determine the additional load in the vehicle, and ride height information can be required to find instantaneous roll centres, and to use for suspension roll angle calculations in predictions of the height of the additional load centre of mass. The accuracy of the contact patch force calculations is dependent on the accuracy of each input and in particular the height of the additional load, which is itself an estimate.

Once one of the contact patch forces is reduced to around zero, indicating that the vehicle dynamic limits are being approached, various safety devices or counter-measures can be triggered, or varied if already actuated. However, if the calculation of the contact patch force is not accurate, then the safety devices or counter measures will either be intrusive through being triggered too early and/or at inappropriate times, or ineffective through operating too late to be of significant benefit. It would therefore be desirable to provide improved determination for an input to be used to enhance control of vehicle handling characteristics in such situations. SUMMARY OF THE INVENTION

With the aforementioned in view, the present invention provides in one aspect a method of providing an input to at least one control system of a vehicle, including the steps of a) determining that a substantially vertical force at the contact patch between the ground and at least one wheel assembly of the vehicle during cornering is approaching a zero value or is at a substantially zero value; and b) providing a signal as an input to the at least one control system derived from the value determined at step a).

A further aspect of the present invention provides a method of controlling a dynamic limit of a vehicle, including the steps of; i) determining that a substantially vertical force at the contact patch between the ground and at least one wheel assembly of the vehicle during cornering is approaching a zero value or is at a substantially zero value; and

H) providing a signal derived from the value determined at step i) as an input to at least one control system of the vehicle iii) controlling, operating, initiating or varying operation of the control system to return the vehicle within the dynamic limit. Thus, advantageously, a reduced number of parameters, such as vehicle roll centre heights, vehicle centre of mass, vehicle track width and/or suspension stiffness values can be excluded to provide an improvement in the balance between accuracy and complexity by determining that the vertical load at any single tyre contact patch is close to zero. Also, some of those parameters would traditionally need to be estimated values in the beginning, leading to inaccurate results. By reducing the number of parameters, such degrading factors are avoided. One or more forms of the present invention may include evaluating a sum of the vertical components of a number of forces at a contact patch between said at least one wheel assembly and a ground surface during cornering of the vehicle.

Preferably the contact patch force (FCP) may be determined utilising an algorithm:

where Fcp is the vertical component of the sum of the forces at the tyre contact patch

Fus is the weight of unsprung mass of the vehicle at that tyre Fss is the support spring force at that wheel assembly and FAR is the anti-roll force (acting in opposite direction).

In addition or alternatively, determining that the substantially vertical force at the contact patch between the ground and at least one wheel assembly is a substantially zero value or is approaching a zero value may be performed without factoring in vehicle roll centre effects.

The vehicle mass, the centre of that mass and/or the track width may be excluded from determining that the substantially vertical load force is at the substantially zero or approaching zero value.

Vehicle load weight, load height, or combinations thereof may be excluded from determining that the substantially vertical load force is at the substantially zero or approaching zero value.

Positions of the wheel assemblies with respect to the vehicle body may be used in determining that the substantially vertical force at the respective contact patch between the ground and at least one wheel assembly is at the substantially zero or approaching zero value. In some cases the suspension stiffness values may be required, but in other cases the suspension stiffness values can be excluded when determining said force. The vehicle may include a fluid suspension system and/or a fluid roll control system such that fluid pressure in at least one fluid volume thereof may be used in determining that the substantially vertical force at the contact patch between the ground and the respective wheel assembly is at the substantially zero or approaching zero value.

Damping forces may be measured and included in the calculation of the instantaneous substantially vertical component of forces at the contact patch. Alternatively the damping forces may be estimated from measuring one or more different variables such as relative position, velocity and/or acceleration between wheel and body and using the measure(s) and the known damping force specifications of the wheel cylinder (or shock absorber).

The measured or estimated damping forces may be filtered through a low pass filter to reduce false triggering of the at least one control system of the vehicle. For example, a sustained damping force due to a whole body motion could affect the actual contact patch vertical load sufficiently over a long enough period to initiate a wheel lift, so the input of low frequency (for example <5Hz) damping loads into the contact patch force algorithm would be beneficial. However for higher frequency (primarily wheel control) damping forces, the damping force may not be sustained for long enough to have a significant effect on the likelihood of wheel lift, so can be filtered out to prevent false triggering of the at least one control system which may otherwise be detrimental to comfort and handling.

The control system, such as a vehicle dynamic control system, may include at least one of a braking system, transmission system, torque control, front and rear wheel drive proportioning system, engine control, suspension characteristics control, and an electronic stability control system.

In a vehicle dynamic control system, there is provided in a further aspect of the present invention a method for prompting a vehicle dynamic system to actuate, including the steps of; generating a signal corresponding to at least one ground contact member of a wheel assembly during cornering of the vehicle being at or approaching a point of lifting from contact with the ground; and utilising that signal as an input to the vehicle dynamic system. In such a case, the vehicle dynamic system may include processing means that evaluates whether the inputted force value prompts or modifies activation or control of one or more of a vehicle braking system, front and rear wheel drive proportioning system, engine, engine control, suspension characteristics control, vehicle driveline, electronic stability system, electronic stability program or electronic stability control system, vehicle transmission or torque control system, or combinations thereof.

Preferably the signal may be determined at least in part by a contact patch force between the respective wheel assembly and surface of the ground. The vehicle dynamic system may include at least one of a braking system, transmission system, torque control, front and rear wheel drive proportioning system, engine control, suspension characteristics control, and an electronic stability control system.

In a vehicle dynamic control system, in a further aspect of the present invention there may be provided at least one control system, at least one detection means to signal that a ground contact member of the vehicle is at or approaching a point of lifting from contact with the ground surface during cornering of the vehicle, processing means receiving said signal, and at least one control means operating, initiating, controlling or varying operation of the control system dependent upon an output of the processing means derived from the signal.

The control system may include one or more of a vehicle braking system, front and rear wheel drive proportioning system, engine, engine control, suspension characteristics control, vehicle driveline, stability control system, vehicle transmission or torque control system, or combinations thereof, operated, initiated, controlled or varied in operation dependent upon the signal relating to the (or at least one) ground contact member approaching or at the point of lifting from contact with the ground surface.

The control system may be provided by or include, for example, a dynamic safety system, such as, electronic stability control, ESP, drive proportioning, electronic braking system and/or braking control such as ABS.

It will be appreciated that if, say, a front inside ground contact member (e.g. front inside tyre) of the vehicle is determined as being close to lifting, one or more counter measures by a control system is called for, such as increasing roll stiffness at the rear of the vehicle, or applying one or more wheel or transmission brakes, or reducing or completely cutting the vehicle throttle. However if a corresponding rear inside ground contact member (e.g. rear inside tyre) is detected as having a close to zero contact force with the ground surface, braking could reduce the stability of the vehicle, and thus actuating a control system to reduce braking effort would be beneficial, or the safety system applying a greater proportion of braking force, preferably all, to the outside front wheel to make the vehicle understeer, or increasing the front roll stiffness relative to the rear roll stiffness, would be beneficial to safely control the vehicle. Therefore the signal may: i) relate to whether one or more wheels have contact patch forces at or approaching zero, i.e. the actual force value between the ground contact member and the ground surface can be disregarded in utilising the present invention; ii) may include the wheel(s) at which the contact patch forces is/are at or approaching zero; and/or iii) may include a magnitude or relative magnitude of the forces at the wheel(s) at which the contact patch forces is/are at or approaching zero.

At least one sensor may be included to determine that a substantially vertical component of forces at a contact patch between the ground contact member and the ground surface is at or approaching zero. Possible sensors include wheel position sensors, fluid pressure sensors and force sensors. Force sensors can be used to measure support spring forces, anti-roll forces in bar drop links and bushings such as actuator mounts. They can also be used to measure damping forces, as can pressure sensors.

One or more forms of the present invention may incorporate a valve or restriction to restrict or prevent fluid flow in a conduit or element of a suspension or associated system, such as a roll control system. BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 exemplifies an embodiment of the present invention in relation to a double wishbone type suspension and associated wheel assembly of a vehicle. DESCRIPTION OF PREFERRED EMBODIMENT

In an independent suspension of a vehicle, cornering force on a tyre acts along a line from the tyre contact patch through the roll centre, so is thus usually acting at an angle to the horizontal e.g. an imaginary line between the contact patch produced by the tyre on the ground and extending through the roll centre. So, in addition to the lateral component, there is usually a vertical component to this cornering force, also known as a "jacking" force.

When the vertical load at the contact patch of each tyre is being calculated during cornering it is essential to factor in the jacking force to get an accurate result. However, in the case of many dynamic controls, particularly those concerned with the stability of a vehicle in extreme situations, it has been realised that it is only important to know when the vertical load at the contact patch of one tyre reaches zero, thereby, calculations can be reduced in complexity. This is because the jacking force effect of the roll centres is produced directly from the lateral force at the tyre contact patch, which is in turn related to the vertical load at the contact patch. When the vertical load is reduced to zero, the lateral force must be nonexistent, and the jacking force must be zero. That means that the roll centres are not required to determine when a single wheel reaches the point of zero vertical load. Therefore, assuming that velocity and acceleration is low making the damping and inertial mass forces negligible, then vertical force at the tyre contact patch can be simplified as it tends towards zero such that it is equal to the addition of the unsprung mass gravitational force and the forces from the suspension springs. The unsprung mass gravitational force Fus is the force produced by gravity acting on the wheel assembly.

The forces from the suspension springs usually consist of a support spring force Fss from the respective spring supporting the vehicle body above the wheel assembly (acting in the same direction as the unsprung mass gravitational force) and anti-roll force FAR from a resilient anti-roll device (e.g. anti-roll bar) providing a roll couple on the body (and in the case of the wheel approaching zero load, in the opposite direction). i.e. As the vertical force at the tyre contact patch (Fcp) tends to zero: Fcp ⁼ Fus ⁺ Fss - FAR where FCP is the vertical force at the tyre contact patch

Fus is the weight of the unsprung mass Fss is the support spring force and FAR is the anti-roil force (acting in opposite direction)

So when the vertical force at the tyre contact patch (Fcp) reduces to zero, the equation can be written:

0 = Fus ⁺ Fss - FAR However, when the tyre is not in contact with the ground the resultant of the forces may be less than zero, so the aforementioned equation can be rearranged:

FAR ≥ Fus + F_Ss The gravitational mass of the wheel assembly Fus should be a constant so only the anti roll force F_AR and the support spring force Fss need to be calculated. These can be calculated by measuring displacements of conventional mechanical spring suspensions (with limits for travel and non-linear characteristics for bump and rebound stop inclusion). Alternatively, if air spring supports are provided, the pressure in the air spring can be used to help determine the support spring force. Similarly if an hydraulic roll control system is provided, one or more pressures in the hydraulic roll circuits can be used to calculate the forces. For example, in an active roll control system such as that described in US 5,597,180 one of the two hydraulic lines is pressurised in dependence on the direction of roll moment being applied to the vehicle and the pressure indicates the magnitude of roll moment being provided by the roll control system therefore permitting the anti roll force to be calculated. Alternatively the front and rear anti-roll bar actuators can be individually controlled such as described in DE 4337765 and/or the actuators may retain some pressure in the other circuit as described in GB 2235168 in which case two or more pressures would be required to calculate all the anti-roll forces contributed by the anti-roll system.

In US 6,302,417 by the present Applicant, details of which are incorporated in its entirety herein by reference, the two hydraulic conduits/circuits are pressurised so the pressure in each conduit/circuit can be used to calculate the anti-roll forces.

Another example of a type of hydraulic anti-roll system is described in the Applicant's US Patent number 7,384,054, which is incorporated in its entirety herein by reference, in which the two hydraulic volumes both contribute to the anti-roll force at each actuator/wheel, so two hydraulic pressures are required.

A further example of a hydraulic anti-roll system is described in the Applicant's International Patent Application number PCT/AU2003/001637, which is incorporated in its entirety herein by reference, in which several pressures are required to calculate the anti-roll force as the anti-roll system also provides a pitch function.

Other interconnected hydraulic systems can provide both support and roll control (and pitch) such as the Applicant's US Patent number 6,270,098, which is incorporated in its entirety herein by reference. In this case, as the support and anti-roll functions are combined in the one system, if several hydraulic pressures are used, the combined support spring force and anti-roll force can be calculated together. Of course there are many different types of suspension system as the above selection indicates, so there are correspondingly many different ways in which the present invention may be applied (calculating the loads on each wheel primarily from displacements and/or pressures rather than from modelling the effects of measured accelerations on the body and then calculating predicted wheel loads).

Figure 1 shows as an example, a double wishbone type suspension in which the wheel 1 is located to the body (not shown) by an upper wishbone 2 and a lower wishbone 3. Where the tyre contacts the ground is known as the contact patch and the centre of the contact patch is marked P. During cornering, a lateral acceleration is present on the vehicle body which is reacted by lateral forces such as FL at the tyres. As the lateral force is applied to the vehicle body through the wishbones, the reaction point R is defined as where construction lines through the centre of each wishbone joint meet. So the force at the tyre contact patch P due to the lateral acceleration has a line of action which passes through the reaction point R and therefore through the roll centre C. This gives a vertical component known as the "jacking" force Fj in addition to the lateral component FL. The jacking force is relevant to independent suspensions generally, not just the double wishbone example shown. Therefore the calculation above can be used in any independent suspension to accurately predict when the vertical force at the contact patch is close to zero. The present invention is particularly applicable to independent suspensions, and whilst it does not precisely predict normal wheel loads, embodiments do accurately calculate vertical force at the contact patch when the vertical force is close to zero, which is a significant advantage. It is accurate because measuring the suspension spring forces F_Ss and FAR inherently takes into account the RMD, height of the centre of mass, the load in the vehicle and its height, the lateral acceleration on the vehicle body, etc.

Further refinements to the calculation are possible and envisaged. For example, shock absorber or damping force can be estimated from one or more of wheel position, velocity and acceleration. Similarly an inertia force of the unsprung mass can be included (preferably with the shock absorber or damper force as the damping force counters the inertia force to a greater or lesser extent).

As some forces can vary greatly with motion of a wheel relative to the vehicle body, yet be undesirable or inappropriate to include in any stability calculations, it can be beneficial to provide one or more filters (such as for filtering one or more signals into the controller, or within the controller, or control algorithm). For example, damping forces can be significant for relatively small amplitude wheel motions, but when the frequency of these motions is greater than typical whole body motions these damping forces are unlikely to be producing any significant effect on the vehicle roll over stability for example. Therefore while including further refinements, such as damping forces into the calculation can provide increased accuracy in some situations, low pass filters can be required in respect of these additional forces to prevent false triggering of the vehicle dynamic control system due to higher frequency (say over 5Hz for example) forces at the wheels. One or more actions can be triggered once it has been determined that the vehicle is approaching a dynamic limit. For example, power or torque of the engine can be limited, reduced or cut. The vehicle's transmission can be controlled or the brakes controlled either using the vehicle's existing Electronic Stability Program (ESP) controller or any other similar or dedicated controller. One or more stiffness parameters, settings or characteristics in the vehicle suspension can be changed (e.g. switched or continuously controlled) to alter steering characteristics of the vehicle (such as understeer-oversteer characteristics) and/or to stiffen one or more modes of the suspension. Similarly, suspension damping can be controlled. Alternatively or additionally the vertical forces at the tyre contact patches can be changed by controlling one or more forces in the suspension system, such as by active control of the RMD of the suspension system, preferably by using independent control of the front and rear anti-roll bar forces. While reducing engine power can be effective when low load on a wheel is due to a combination of cornering and acceleration, it may not be sufficient to control vehicle stability where the low load is due to lateral acceleration alone or combined with braking. Therefore, more than one system may be required to effect control. The use of individual braking control, such as provided by an ESP system, can be used to maintain load on the lightly loaded wheel and also control the path followed by the vehicle using a stability system already provided on many vehicles.

There are many possible ways to change the stiffness in a suspension system. For example, there are roll stiffness switching devices which vary the stiffness of the suspension dependent on conditions such as speed, driving style, terrain, vehicle acceleration(s) and/or driver selectable switch position. There are also modal stiffness systems which provide a low warp or other modal stiffness.

For example, in US 6,217,047 and PCT publication WO2006/092012 (both by the present applicant and incorporated herein in their entirety by reference thereto) the longitudinal hydraulic connections between the front and rear anti-roll arrangements can be blocked to retain roll stiffness while adding warp stiffness. Also in US 6,217,047, lateral connections between the air spring supports can be blocked adding both roll and warp stiffness. Support spring interconnections of any type (mechanical, fluid connections between laterally, longitudinally or diagonally spaced wheels) can be locked regardless of whether the support spring system provides all modal stiffnesses or is combined with other arrangements. There is shown in the applicant's PCT publication WO 2006/092013 (details of which are incorporated herein by reference) a roll control system comprising cross connected double-acting wheel rams at a front of a vehicle and at a rear of a vehicle. While the inclusion of resilience provides a high roll stiffness and low heave stiffness, a central device is required to interconnect the two front volumes and the two rear volumes to permit the warp stiffness to be removed by becoming decoupled from the roll stiffness. In this arrangement warp stiffness can similarly be added whilst maintaining roll stiffness (and potentially changing roll moment distribution) by blocking the operation of this central device which can be accomplished in a variety of ways.

As discussed above, increasing the roll stiffness and/or warp stiffness can improve stability once the dynamic limits of the vehicle are approached and exceeded. Similarly increasing the damping of the suspension can provide some improvement in stability once the dynamic limits of the vehicle are approached and exceeded. Damping can be increased by a wide variety of methods depending on the particular suspension system, either by increasing the damping force at each wheel or by increasing modal damping where possible. For example modal damping can be increased where dampers are provided which have their primary action in modes such as roll. Examples of such roll damping can be found in US 7,384,054 and WO2006/092012 (which as already noted are by the present applicant and incorporated herein by reference) where roll damping is provided by dampers for the accumulators. Pitch damping is additionally provided in the applicant's PCT publications WO 2004/076211 and WO 2006/010226 details of which are incorporated in their entirety herein by reference, as well as the applicant's previously referenced WO 2004/052667 also incorporated in its entirety herein by reference. Warp damping can be provided in all of these modal suspension systems, for example in WO2006/092012 by dampers in the longitudinal lines also incorporated in its entirety herein by reference.

Claims

CLAIMS:

1. A method of providing an input to at least one control system of a vehicle, including the steps of; a) determining that a substantially vertical force at the contact patch between the ground and at least one wheel assembly of the vehicle during cornering is approaching a zero value or is at a substantially zero value; and b) providing a signal as an input to the at least one control system derived from the value determined at step a).

2. A method of controlling a dynamic limit of a vehicle, including the steps of; i) determining that a substantially vertical force at the contact patch between the ground and at least one wheel assembly of the vehicle during cornering is approaching a zero value or is at a substantially zero value; and ii) providing a signal derived from the value determined at step i) as an input to at least one control system of the vehicle iii) activating or controlling the control system to return the vehicle within the dynamic limit.

3. The method of claim 1 or 2 including evaluating a sum of the vertical components of a number of forces at a contact patch between said at least one wheel assembly and a ground surface during cornering of the vehicle.

4. The method of any one of the preceding claims, including determining the contact patch force (F_Cp) utilising an algorithm:

where FCP is the substantially vertical component of the sum of forces at the tyre contact patch

5. The method of any one of the preceding claims, wherein determining that the substantially vertical component of forces at the contact patch between the ground and at least one wheel assembly is a substantially zero value or is approaching a zero value is executed without factoring in vehicle roll centre effects.

6. The method of any one of the preceding claims, wherein vehicle roll centre heights, vehicle centre of mass, vehicle track width and/or suspension stiffness values are excluded from determining that the substantially vertical component of force at the contact patch between the ground and at least one wheel assembly is at the substantially zero or approaching zero value.

7. The method of any one of the preceding claims, wherein vehicle load weight, load height, or combinations thereof is/are excluded from determining that the substantially vertical component of forces is at the substantially zero or approaching zero value.

8. The method according to any one of the preceding claims, wherein positions of the wheel assemblies with respect to the vehicle body are used in determining that the substantially vertical component of force at the respective contact patch between the ground and at least one wheel assembly is at the substantially zero or approaching zero value.

9. The method according to any one of the preceding claims, wherein the vehicle includes a fluid suspension system and/or a fluid roll control system whereby the fluid pressure in at least two fluid volumes is used in determining that the substantially vertical component of forces at the contact patch between the ground and the respective wheel assembly is at the substantially zero or approaching zero value.

10. A method according to any one of the preceding claims, wherein the vehicle control system includes at least one of a braking system, transmission system, torque control, front and rear wheel drive proportioning system, engine control, suspension characteristics control, and an electronic stability control system.

11. In a vehicle dynamic control system, providing a method for prompting a vehicle control system to actuate, including the steps of; generating a signal corresponding to at least one ground contact member of a wheel assembly during cornering of the vehicle being at or approaching a point of lifting from contact with the ground; and utilising that signal as a control input to the vehicle control system.

12. A method according to claim 11 , wherein the vehicle dynamic control system includes processing means that evaluates whether the inputted signal prompts activation, control or change in control of one or more of a vehicle braking system, front and rear wheel drive proportioning system, engine, engine control, suspension characteristics control, vehicle driveline, electronic stability system, electronic stability program or electronic stability control system, vehicle transmission or torque control system, or combinations thereof.

13. A method according to claim 11 or 12, wherein the signal is determined at least in part by a contact patch force between the respective wheel assembly and surface of the ground.

14. A vehicle dynamic control system, including at least one control system, at least one detection means to signal that a ground contact member of the vehicle is at or approaching a point of lifting from contact with the ground surface during cornering of the vehicle, processing means receiving said signal, and at least one control means operating, initiating, controlling or varying operation of the control system dependent upon an output of the processing means derived from the signal.

15. A system according to claim 14, wherein the vehicle control system includes one or more of a vehicle braking system, front and rear wheel drive proportioning system, engine control, suspension characteristics control, vehicle driveline, stability control system, vehicle transmission or torque control system, or combinations thereof, operated, initiated, controlled or varied in operation dependent upon the signal relating to at least one said ground contact member approaching or at the point of lifting from contact with the ground surface.

16. A system according to claim 15, at least one sensor to determine that a substantially vertical component of forces at a contact patch between the ground contact member and the ground surface is at or approaching zero.