Mass

Mass is a property of any physical body. In terms of vehicle dynamics, the main effect of mass is to change the inertia of the vehicle which affects the way the vehicle accelerates and rotates. The position and amount of weight of the vehicle also affects vehicle handling and stability.

Center of Mass/Center of Gravity

Main page: Center of Gravity

The center of mass is the point that describes the average of the mass and position of every part of the vehicle, while the center of gravity is the point that describes the average of the weight and position of every part of the vehicle. For the purposes of this article, center of mass and center of gravity will be used interchangeably.

Inertia

According to Newton's 1st Law, an object at rest stays at rest and an object in motion stays in motion. We are familiar with this concept in the form of momentum. As a general rule, engineers try to reduce the inertia of the vehicle in order to increase the maximum acceleration of the vehicle.

Polar Moment of Inertia

The Polar Moment of Inertia is defined by how far the mass is from the pivot point. When the mass is closer to either end, you have a high polar moment of inertia, and when the mass is centralized close to the pivot point, you have a low polar moment of inertia. Mid-engine cars are considered the best layout for Road Racing because of their low polar moment of inertia, since both the Engine and Driver are in between the wheel axes.

Weight Transfer

Weight transfer is a result of the vehicle accelerating. When a vehicle accelerates (for example when turning or braking), the tires create a moment that causes the vertical force applied to each tire to change. This can be seen by drawing the forces applied to a vehicle. Using Newton's 2nd Law we can calculate the forces on each tire in a steady state situation.

Unsprung mass

The unsprung mass is the mass that, as the name implies, is "not sprung", i.e. not suspended by the vehicles suspension. For example the tyres are unsprung mass. There are parts of the car that can not be described as purely sprung or unsprung mass. Parts like the push rod can be partially sprung.

Unsprung mass is worse for the car's performance than sprung mass, as the unsprung mass negatively affects the tire's bump-following ability. This is further compounded by the fact that a significant portion of the unsprung mass consists of the wheel assembly, which is also rotating mass. Because of this we need ways to measure the unsprung part of the vehicle's mass. There are three main ways of doing this.

English method

Measure the mass of all suspension components and assume that only a third is sprung while the rest is unsprung.

Scale method

Make sure the car is in ready to race condition with tyres (at least when it comes to wheel and suspension). Raise the car and put a weight scale under the tyre. Make sure the raise the car just enough for the wheels not to be touching the scale but as close to touching as possible. Then disconnect the pushrod from the chassis side. Let the wheel fall onto the scale and the mass you measure is the unsprung mass of that wheel. Do this for all four wheel to obtain the total unsprung mass.

Rotating mass

Rotating mass is the portion of the vehicles mass that rotates as the car travels, e.g. wheels and drivetrain. The negative effects of rotating mass is amplified compared to non.rotating mass, as in addition to requiring force to be linearly accelerated (F=ma) it also requires torque to change the angular velocity of the mass propotional the the mass' moment of inertia (τ=Iα, where I is the moment of inertia andα is the rate of change of the angular velocity).