You have to be aware of the fact that Ultimate Stunts is not made for physics realism. If you want realism, pleas take a look at simulators like Racer. If you just want to have fun with self-made tracks and lots of stunts, then Ultimate Stunts is the game for you. Within the limits of the Ultimate Stunts physics engine, you can make your cars as good or as bad as you want, and as realistic or unrealistic as you want. The philosophy of Ultimate Stunts itself is to give gameplay top-priority. Cars should be fun to drive in the first place. Realistic settings are only used where that contributes to the driving fun. Cars that are too difficult to drive are bad, and too easy cars are bad too. This makes car-tuning something like an art.

- Top speed
- Max. power, max. torque + their RPMs
- Gear ratios + the final drive ratio (= differential ratio)
- The traction type (front wheel / rear wheel / four wheel traction)
- The mass of the car
- The weight distribution between front wheels and rear wheels
- The height of the center of gravity above the ground

Non-SI units | SI units | |

1 km/h | = | 0.2778 m/s |

1 mile/h | = | 0.4469 m/s |

1 hp | = | 735.5 W |

1 kW | = | 1000 W |

1 RPM | = | 9.549 rad/s |

#Symbols: v = speed of the car cwA = aerodynamic drag parameter Fdrag = aerodynamic drag force Pdrag = power required to compensate drag v_max = top speed P_max = maximum power of engine r = radius of wheel ratio_g = rear ratio of highest gear ratio_d = differential ratio (final drive ratio) w_wheel = rotation speed of wheel w_engine = rotation speed of engine w_engine_P_max = rotation speed of engine @ max power point #Calculating the top speed Fdrag = cwA * v^2 Pdrag = Fdrag * v = cwA * v^3 v_max = (P_max / cwA) ^ (1/3) cwA = Pdrag / v_max^3 #Calculating optimal gear ratios w_wheel = v / r w_engine = w_wheel * ratio_g * ratio_d = v * ratio_g * ratio_d / r ratio_g * ratio_d = w_engine_P_max * r / v_maxIf you have a car with a certain amount of power, and you want it to have a certain top speed, then you can calculate the gear ratio / differential ratio combination, and then fine-tune the top speed by changing the

With the right first gear ratio, any engine can generate enormous traction forces. A higher gear ratio means a greater traction force. I can hear you asking: "why don't cars have infinitely high gear ratios, if that gives greater traction forces?". Besides the fact that there are technical limitations to gear box technology, there are several reasons for this. One reason is that with high gear ratios, an engine will very soon reach high RPMs, so that the traction force will start to drop, so you will only have an advantage of the gear for a very short time, and then you'll have to switch to a higher gear. Another reason is that tires can only apply a limited amount of force to the ground. If this force is exceeded, then the tires simply start skidding, instead of generating extra force. So, for fast acceleration it may be smart to match the first gear ratio with the maximum traction force that can be applied by the tires.

The maximum amount of force that can be applied by the tires depends heavily on the vertical
force on the wheel (here called the normal force). In a neutral situation, this is simply a
part of the weight of the car: the weight distribution of the car describes how much of the
weight goes to the front wheels and how much to the rear wheels. This distribution can be
changed in Ultimate Stunts by moving the **centerofmass** forward or backward. When
accelerating, decellerating or turning, this distribution can be different from the neutral
situation. When the car accelerates, it leans more on the rear wheels, so the normal force on
the rear wheels will be higher than in the neutral distribution. As a higher normal force
allows for a greater traction force, this effect is good for rear-driven cars and bad for
dront-driven cars. Aerodynamic downforce from spoilers also has an influence on the normal
force, but for acceleration this is less important because downforce is only generated at high
speeds, and acceleration requires the highest forces at low speeds.

Because the high number of effects, no exact description will be given here, especially because the situation is different for front-wheel and for rear-wheel cars. An exact description is possible, but would take an extra page of formulas. Here is a description for the situation where the acceleration generates no weight transfer, and no aerodynamic downforce is present.

#Symbols m = mass of the car g = gravitation constant (9.81 m/s^2) Fz = gravity force wfrac = fraction of weight on the traction wheels (= percentage / 100), 4WD: wfrac=1 Fn = total normal force on the traction wheels mu = static friction coefficient of traction tires Ftr = maximum traction force a = acceleration a/g = acceleration in G-forces M_engine = maximum engine torque M_wheel = maximum total torque applied to traction wheels ratio_g = rear ratio of first gear ratio_d = differential ratio (final drive ratio) r = radius of the traction wheels #Neutral situation normal force Fz = m * g Fn = wfrac * Fz = wfrac * m * g #Traction force Ftr = mu * Fn #Acceleration a = Ftr / m = mu * Fn / m a/g = mu * Fn / Fz #Neutral situation acceleration a/g = mu * wfrac #Gear ratios M_wheel = ratio_g * ratio_d * M_engine Ftr = M_wheel / r = ratio_g * ratio_d * M_engine / r ratio_g * ratio_d = Ftr * r / M_engine = mu * Fn * r / M_engine

#Symbols M_brake = maximum braking torque on a single wheel F_brake = maximum braking force on a single wheel Fn = normal force on a single wheel mu = static friction coefficient of the tire r = radius of the wheel F_brake = mu * Fn M_brake = r * F_brake = r * mu * Fn

If a car needs to follow a circle-shaped path, then it needs to accelerate towards the center of that circle. This means that the tires need to generate sideward forces. Also, the car needs to rotate, so that it remains aligned with the circle. About the sideward forces, you already guessed it: they are limited by the tires, just like engine and brake forces. Also, if engine or brake forces are applied on a certain wheel, then there is less force "left" for steering. Weight transfer effects are extremely important in corners. Because of the steering, the outer wheels get more weight, and the inner wheels less. Also, if the rear wheels get more grip, then the car will understeer, and if the front wheels get more grip, the car will oversteer. The distribution of normal force between front and rear wheels depends on a lot of things, so it can also be tuned in a lot of ways. The weight transfer due to accelerating and braking can be increased / reduced by putting the center of gravity lower or higher, the neutral weight distribution can be changed by moving the center of gravity forward or backward, the distribution at high speeds can be changed with the front and rear downforce, and the grip can be tuned by changing the static friction coefficient of the front or rear tires a little bit.

Another thing that needs to be tuned is how fast the car rotates. When turning into a corner, the front wheels' rotation is changed, so that a sideward force is generated on the front wheels. As this force is initially not present at the rear wheels, the car starts rotating. How fast it starts rotating depends on many things, one of them being the moment of inertia of the car. The moment of inertia is to rotations what mass is to linear motions. If the moment of inertia is extremely high, then the car will respond very slowly to steering forces: it takes a long time before the car starts steering, and after the corner it takes a long time before the car drives in a straight line again. If the moment of inertia is low, then the car will respond very quickly to changes: small changes in steering, gas or braking will cause an overreaction in the orientation of the car.

The following formulas give some general information on how fast corners can be taken with a given tire quality.

#Symbols R = radius of a corner v_max = maximum speed in this corner m = mass of the car g = gravitation constant (9.81 m/s^2) F = maximum total sideways force of the tires Fn = total normal force on the tires mu = static friction coefficient of the tires F = m * v_max^2 / R = mu * Fn = mu * m * g vmax = sqrt(R * mu * g)