Author:  Kacper Michalski

During the design and the creation of Xpand Rally its developers were aiming towards completely new quality and realism level in a racing game. It’s obvious that the easiest way to achieve it is by realistic graphics. From the very beginning however the biggest attention was put on the realistic simulation of the dynamics of the game’s world. Because it’s most important part from the player’s point of view is the car – its physical simulation had to be designed extremely realistically and with maximum care for details. At the same time though car handling had to be fun and playable.

            Below is a list and description of the most important elements that have decided about achieving the above goals.

Curve editor

            Most of the parameters of the physics simulation has a nonlinear character and can be modeled with the help of curves. To make modifying those parameters easier we have developed a special tool for editing the shape of functions. Thanks to it one can easily and quickly examine and change the behavior of the car. As an example in this way one can modify the functions of friction depending on various parameters of the state of a tyre.

Engine

            Each engine in the game has its own characteristic. The parameters describing an engine are among others: RPM range, the function of the torque, power and internal resistances. All those parameters are dynamically computed during the game. Most of the parameters are modeled by curves (e.g. the curve of the torque which gives the value of the engine’s torque for a given RPM). The amount of internal resistances of the engine depends on lubrication and its temperature. If the cooling system is corrupt the engine’s temperature will increase and it may be damaged. Another example is roofing after which more oil gets to the engine and is burned there – in the game it can be seen as a change of the color of exhaust.

            During the implementation of the simulation of the engine and the transmission one of more important problems is the stability of the discreet (done with a certain step)  integration of the state of such system. Because of mechanical joins between real elements in a car such system is relatively difficult to be accurately and stably simulated. The solution of the problem required a lot of time and scientific mathematics knowledge.

Turbocharger

            Just as in the case of other parts the working of the turbocharger is dynamically simulated – the rotational speed of the turbine and the pressure of the charging are computed in real time during the game.

Differentials

            One of the most important elements influencing car handling is the differential. A differential is mounted between powered wheels and in case of 4x4 cars on shafts powering front and rear wheels. The purpose of a differential is an appropriate change of the rotational speed of wheels so the car “behaves better”. The “better behavior” may mean better turning, stability of the track or better traction on a given surface. In the game there are different types of differentials – from basic free differential mounted in every normal car, through limited slip differential up to complicated active differential electronically controlled. During driving a car in a turn the outer wheels cover a longer distance than the inner ones. If all wheels were turning with the same speed it would lead to skidding and unwanted tyre wearing. The aim of the basic differential is letting the wheels spin with different speeds during a turn. Sport cars have limited slip mounted which limits the difference in rotational speeds between powered wheels which leads to stabilization of the track and better traction in sliding. Active differential allows for defining how the limited slip behaves depending on e.g. current wheel turn, acceleration pedal state, braking, etc. To do this special differential maps are used that were created with our curve editor in the game.

Suspension

            The behavior of the suspension is responsible for the proper behavior of the whole body of the car. The car in Xpand Rally has all elements of the suspension that real rally cars have. All elements are separately simulated and their work depends directly or indirectly on the shape of the surface. Wheels are joined with the body of the car through springs and dampers whose parameters can be controlled by the player which thus can change the behavior of the car.

Also the simulation of the elasticity of the tyres in Xpand Rally has the impact on the behavior of the suspension as the tyres are the first elements that absorb the unevenness of the roads.

Friction on the tyre-surface point

            Each surface has different friction parameters which together with various tyres differentiates the behavior of the car. Additionally the traction is changed by the weather which can drastically change the suitability of some tyres for a surface. To correctly simulate the diversity of the conditions during rally races there are almost 50 surface types and 20 tyres types in the game which results in very accurate representation of the reality. Additionally in the game the weather can change during a race and change the parameters of the friction of surfaces. This increases the realism of the game and decides about how important it is to make good decisions between a race.

            To adequately simulate the contact between a tyre and a surface the game separately computes lateral and longitudinal friction. It allows to diversify the behavior of a tyre during a side slip e.g. between tarmac and gravel (on gravel the tyre cuts into the ground) and consider the influence of the tread.

            In the game a multipoint friction model has been used. This means that every wheel independently contacts the ground and for each wheel forces affecting the car are computed independently. Additionally when the body of the car collides with objects of the environment forces of friction appear at the point of the contact similarly to the tyre-surface contact. Thus the behavior of the vehicle is an ideal replication of the reality.

Air resistances

            Objects moving with great speed are affected with significant air resistance forces. Xpand Rally car physics simulate those forces with regard to car as well as other objects. The forces can be seen when for example the wind detaches the car bonnet. Of course the aerodynamic shape of the cars results in pressing them to the road at high speeds which improves their traction.

Steering model

            Towards the end of implementing Xpand Rally physics it was clear that the created model is very realistic which makes driving the car relatively difficult especially with the use of a keyboard. That’s why to let players that don’t have steering wheels have more fun we have decided to add various “helpers” that are to make controlling the car easier with a keyboard. They do not change any of the physical car parameters but try to simulate the behavior of a driver steering with a driving wheel.

For players who instead of struggling with the road and the car prefer fun racing with great speeds we have created an Arcade physics mode that slightly modifies the physics parameters greatly simplifying the steering.