Vehicle Tuning Guide

This section describes the effect of editable vehicle parameters.

Original text (please note that parameter names can differ):

http://docs.nvidia.com/gameworks/content/gameworkslibrary/physx/guide/Manual/Vehicles.html

Max RPM (~Physx: mMaxOmega)

This is the maximum rotational speed of the engine expressed in revolutions (rotations) per minute.

Max Torque (Physx: mPeakTorque)

This is the maximum torque that is available for the engine. This is expressed in Newton metres. A starting value might be around 600.

Curve Graph (Physx: mTorqueCurve)

This is a graph of peak torque versus engine rotational speed. Cars typically have a range of engine speeds that produce good drive torques, and other ranges of engine speed that produce poor torques. A skilled driver will make good use of the gears to ensure that the car remains in the "good" range where the engine is most responsive. Tuning this graph can have profound effects on gameplay.

The x-axis of the curve is the normalized engine speed. This means that the engine speed is divided by the maximum engine speed. The y-axis of the curve is a multiplier in range (0,1) that is used to scale the peak torque.

Tire/Dump (Physx: mDampingRate)

This value describes how quickly a freely spinning wheel comes to rest. The damping rate describes the rate at which a freely spinning wheel loses rotational speed. Here, a freely spinning wheel is one that experiences no forces except for the damping forces that arise from the wheel's internal bearings. Higher damping rates result in the wheel coming to rest within shorter times, while lower damping rates result in the wheel maintaining speed for longer. Values in the range of 0.25 and 2 are sensible values. Experimentation is always a good idea, even outside this range. Always exercise some caution with very small damping rates. In particular, a damping rate of exactly 0 should be avoided.

Spring Strength (Physx: mSpringStrength)

This value describes the strength of the suspension spring. The spring strength has a profound influence on handling. It does this by modulating the time it takes for the vehicle to respond to bumps on the road and on the amount of load experienced by the tire.

The key to understanding the effect of spring strength is the concept of a spring's natural frequency. Consider a simple spring system such as a pendulum swinging back and forth. The number of trips per second that the pendulum makes from full left to full right and then back again is called the natural frequency of the pendulum. A more powerful pendulum spring will result in the pendulum swinging faster, thereby increasing the natural frequency. Conversely, increasing the pendulum mass will result in a slower oscillation, thereby reducing the natural frequency.

In the context of a suspension spring supporting a fixed portion of vehicle mass, the strength of the spring will affect the natural frequency which is the rate at which the spring can respond to changes in load distribution. Consider a car taking a corner. As the car takes the corner, it leans into the turn, putting more weight on the suspension outside of the turn. The speed at which the spring reacts by applying forces to redistribute the load is controlled by the natural frequency. Very high natural frequencies such as those on a racing car will naturally produce twitchy handling because of the load on the tires, and therefore the forces they can generate, can vary very rapidly. Very low natural frequencies, on the other hand, will result in the car taking a long time to straighten up even after the turn is complete. This will produce sluggish and unresponsive handling.

Another effect of strength and natural frequency is the response of a car to a bump on the road. High natural frequencies can result in the car responding very strongly and quickly to the bump, with the wheel possibly even leaving the road for a short while. This does not only create a bumpy ride but also periods of time when the tire is generating no force. Weaker springs will result in a smoother trip over the bump with weaker but more constant tire forces. A balance must be found to tune the car for the expected types of turns and terrain.

The natural frequency of the spring presents a challenge for computer simulations. A smooth and stable simulation requires that the spring is updated at a frequency much greater than the spring's natural frequency. An alternative way of expressing this is to consider the period of the spring relative to the timestep of the simulation. The period of the spring is the time the spring takes to complete a single oscillation, and is mathematically equal to the reciprocal of the natural frequency. In order to achieve a stable simulation, the spring must be sampled at several points during each oscillation. A natural consequence of this observation is that the simulation timestep must be significantly smaller than the period of the spring. To discuss this further, it is helpful to introduce a ratio that describes the number of simulation updates that will occur during each spring oscillation. This ratio is simply the spring period divided by the timestep:

alpha = sqrt(mSprungMass/mSpringStrength)/timestep
where sqrt(mSprungMass/mSpringStrength) is the period of the spring.

An alpha value of 1.0 means that the chosen timestep and spring properties only allow a single sample of the spring during each oscillation. As described above, this is almost guaranteed to produce unstable behavior. In fact, the argument presented so far suggests that a value of alpha significantly greater than 1.0 is essential to produce a smooth simulation. The exact value of alpha at which stability emerges is very difficult to predict and depends on many other parameters. As a guide, however, it is recommended that the timestep and spring properties are chosen so that they produce an alpha value greater than 5.0. This is a minimum of five simulation updates per spring cycle.

When tuning a suspension spring it can be very useful to use manufacturer data to discover typical values used across a range of vehicle types. This data is not always readily available. An alternative strategy would be to think in terms of the natural frequency of the spring by imagining how quickly the car would oscillate up and down, if it was dropped onto the ground from a height of, say, 0.5m. The springs of a typical family car have a natural frequency that is somewhere between 5 and 10. This means that such a car would make 5-10 oscillations per second, if gently dropped to the ground. If the mass supported by the spring is already known, then the spring strength can be calculated from the following equation:

mSpringStrength = naturalFrequency * naturalFrequency * mSprungMass

Note

To achieve a spring that is theoretically correct, the values for mSprungMass, mSpringStrength and mMaxDroop should be chosen so that they obey the equation:

mSpringStrength*mMaxDroop = mSprungMass*gravitationalAcceleration

When this equation is satisfied, the spring is guaranteed to provide exactly zero force at maximum elongation and also to support the sprung mass at the rest pose (the rest pose is defined by PxVehicleWheelsSimDta::setWheelCentreOffset).

It is often the case, however, that the visual requirements of the car are in conflict with its handling requirements. An example might be a visual requirement, imposed by an artist, on both the rest pose and the suspension travel limits. In order to satisfy this visual requirement and achieve a theoretically correct spring, the value of mSpringStrength must be equivalent to mSprungMass*gravitationalAcceleration/mMaxDroop. If this value of mSpringStrength does not meet the handling requirements of the game, then there is a conflict that cannot easily be resolved.

For this reason, the PhysX Vehicles module does not require the spring to be a theoretically perfect spring. The consequences of an imperfect spring are that the spring either stops providing upward force before it hits maximum elongation or that it still provides a non-zero force at maximum elongation. The effect on handling or on the visual appearance of the vehicle is often quite difficult to spot. In particular, tire load filtering, discussed in Section PxVehicleTireLoadFilterData, further disguises any imperfection.

Shock Absorber Strength (Physx: mSpringDamperRate)

This describes the rate at which the spring dissipates the energy stored in the spring.

The key to understanding the damper rate is by understanding the concepts of under-damping, over-damping, and critical damping. An over-damped pendulum displaced from rest is unable to make a single back-and-forth trip before it dissipates all its energy, while an under-damped pendulum would be able to make at least a single back-and-forth trip. A critically damped pendulum makes exactly a single back-and-forth trip before expending all its energy.

For vehicle suspension springs, it is typically important to make sure that the spring has a damper rate that produces over-damping, but not by too much. When cornering, for example, it is important that the spring doesn't over-respond by shifting the weight from the left suspension to the right suspension and then back again. If this happens, the tire load, and the forces generated, will become extremely variable, resulting in twitchy and uncontrollable handling. A very heavily over-damped spring, on the other hand, will feel sluggish and unresponsive.

The concept of critical damping can be used to help tune the damping rate of the spring. It is helpful to introduce a value known as the damping ratio, which helps to mathematically describe the under-damping, critical damping, and over-damping regimes:

dampingRatio = mSpringDamperRate/[2 * sqrt(mSpringStrength * mSprungMass)]

A dampingRatio with a value greater than 1.0 produces over-damping, while a value of exactly 1.0 generates critical damping, and a value less than 1.0 is under-damped. It can be useful to first think about whether the spring will be under-damped or over-damped, then think about how far it will be from critical damping. This process allows a number to be subjectively applied to the damping ratio. From here, the damping rate can be directly computed by rearranging the previous equation into the one below:

mSpringDamperRate = dampingRatio * 2 * sqrt(mSpringStrength * mSprungMass)

A typical family car is probably slightly over-damped, having a dampingRatio with a value that is perhaps just over 1.0. Values that are very far from critical damping are likely to be unrealistic and will either produce sluggish or twitchy handling. It is difficult to put an exact figure on this but somewhere between 0.8 and 1.2 seems like a good starting point for the damping ratio.

mSprungMass

In our case, this roughly equals one quarter of the chassis weight.