Ever wonder why some drivers at the tracks can take a car with less power and still smoke everyone else’s lap time?
Driving ability aside, there’s a technical explanation to why a car like that can achieve such feats. The car is most likely getting more out of its chassis put to use than its competitors. Most experienced drivers understand where to look to pull out that hidden potential. While every car behaves differently, learning to discern how and why brings you closer to both correctly setting up a car, and controlling it like an expert. That ability is directly linked to how well we understand Vehicle Dynamics.
When diving into what Vehicle Dynamics is all about, a good place to start is two commonly used terms: Oversteer and Understeer.
These terms refer to two opposite ways a car chassis wants to move while cornering; when trying to turn, one wants to turn too much and the other not at all. Too much of either can be deadly on track, but a touch of each can translate to a fast car. While you can adjust to these with driving technique, we need to keep in mind the car has it’s natural limits. Understanding how these two behaviors affect the car will help us make critical decisions when we eventually try to push past those limits by changing and tuning components on your car. Rather than fight them for control of the car, we want to use them to our advantage as we attempt that next fastest lap time.
Oversteer takes place when your car rotates more than the amount of your steering input. For example, if you turn your steering wheel ⅛ of a turn, but the car continues to turn after its completed that amount of rotation - you’re now experiencing oversteer. This occurs when the rear tires are put under too much stress and break grip before the front tires do. Too much oversteer can cause spin outs or the rear slamming the wall after kicking out too far. However, when in control, oversteer allows you the tightest racing line which can give you the fastest way around a corner. A little oversteer is considered a good thing in race car design.
Understeer takes place when the car stops rotating before the intended amount of steering input is completed. If you turn the steering wheel, but the car stops rotating and no matter how much steering you add in that direction, the momentum of the car continues in the same direction - You're now experiencing understeer. This occurs when the front tires break grip before the rears. Too much understeer will make the vehicle feel “lazy” and “unresponsive”, not rotating when you ask it to. However, understeer characteristics will add stability when traveling at high speeds and is more predictable to drivers who aren't accustomed to oversteer. It is actually the standard in most road cars and often considered more safe by car manufacturers.
::Oversteer Drive Line
::Neutral Drive Line
::Understeer Drive Line
(NEO TIP: In most scenarios, we attempt to set up a car to perform as most neutrally as possible. The idea is to find the balance between the two where neither is obstructing the way you take the corner. In race scenarios, we are ideally looking for the car to be both as fast and stable around every corner as possible. A neutral vehicle dynamic makes it the best starting point if adjustments are needed on the track.)
|When we look at what the tires are doing in both of these scenarios, one set or both sets of tires are losing grip between the rubber and the asphalt. As we arrive at a corner and begin to turn, the wheels rotate and the driving force causes the tires to slide (or slip). In this state, the tires are pointed in a slightly different direction than the one they are traveling. The difference between the two is what we call the slip angle. When the tire has lost all traction, turning the steering wheel deeper into the corner will only widen the slip angle. If the rear slip angles are wider than the front, we are inducing oversteer. In the opposite scenario, we are inducing understeer. The closer the front and rear slip angles are- the more neutral the behavior becomes.|
Now that we understand Oversteer and Understeer a bit more, we can look at a couple of the factors, Grip & Traction, and Weight Transfer, that is related to them that will help us understand why they happen. Our focus is on what is actually happening to the tires, and the forces applied to it that cause the dynamics of these two behaviors.
|Traction occurs when the rubber of the tires meet the asphalt of the road at the right temperatures causing friction that keeps the wheels (and the entire vehicle) on the ground while in motion. The rubber has a temperature range where it is at its optimal performance. Too low or too high a temperature will result in the tire not taking grip or breaking grip, causing them to slide. When we overwork or put too much pressure on the tires, we are causing the tires to generate too much heat at which point they will break traction. The goal is to keep the tires at the optimal temperatures for as long as possible.|
Accelerating, braking, and cornering hard are all things that put stress on the tires. By doing this you’re shifting the weight of the vehicle onto two of the four tires in all of the scenarios. This is called Weight Transfer. Every car has a center of mass, (sometimes referred to as center of gravity) that sits between the front and rear wheels. This imaginary point shifts with the way we move the vehicle, and that shift is called the “Moment of Inertia”. To reduce oversteer or understeer, the goal is to keep this moment of inertia as small as possible. Setting up the car correctly can take some of the stress off of the tires during weight transfer. Stiffer springs in the front for example take some of the stress off the tires and onto the springs instead, reducing the amount of understeer that could occur. Most performance tuning done on the chassis is to contribute to a more favorable vehicle dynamic in this fashion.
These are the things that attribute to what we call Vehicle Dynamics. Understanding how the car wants to move or behave is the first step toward learning how to push your car to its limits as a driver. Once we get to the point where the car doesn't feel like there is any more to give- that is when we start modifying or adjusting parts. There are more ways than you count when it comes to changes you could make to your car, but understanding your car’s vehicle dynamics will ultimately make those decisions much easier to suiting your driving style.