Racing 101 - Vehicle Dynamics

July 05, 2019

Racing 101 - Vehicle Dynamics

Key video points:

Vehicle Dynamics - How a car responds to the driver's control inputs.

Control Inputs - The three ways a driver controls a car: Throttle, Brake, Steering.

All cars respond to a driver's input following the laws of physics... Your ability to get a car around the race track is dependent on the capability of the car, but it's even more dependent on your ability to control the car. Having a good understanding of how to control a car, whether sim or real, is essential to being a good driver.

Whether you're racing or simply driving to work, there are three basic inputs a driver can use to make the car perform. The gas pedal, along with the clutch and shifter, go; the brake pedal, stop; and the steering wheel? Turn. Learning how to effectively use the controls is the key to driving well. In fact, it's even more important in the sim, as the feedback you get in the sim is drastically less than in a real car. You have no "seat of the pants forces" to tell you the result of your inputs in the sim, and have to rely on visual and limited steering feedback to assess the effectiveness of your actions. Thus, the more knowledge the driver has, the more efficient they'll be in controlling the sim car. 

All of the commands you give the car with these controls are transmitted into action via the tires. Understanding how the tires work is the first step... There are three directional forces the tire is subject to; acceleration, deceleration, and turning. The tire's grip can be devoted to any one, or a combination of these forces, to accomplish a driver's command. A tire interacts with the surface of the track by distortion of the rubber moving over the surface, causing a small amount of slip. This slip, referred to as the slip angle while cornering, determines the effectiveness of the tire's contact patch. Too much slip, and you lose grip. Not enough, and you're below the potential of the tires.

 

Key Video Points:

Optimum Slip Angle - The slip angle between the tire's contact patch and track surface that maximizes the available traction in the tire.

Load Transfer - The vehicle dynamics resulting from driver inputs, track surface changes, aerodynamics, etc. that change how much load (or downward force) is felt by the tire, and therefore how much traction will be available.

Understeer - A handling property in which the front tires exceed the optimum slip angle.

Oversteer - A handling property in which the rear tires exceed the optimum slip angle.

Trailing Throttle Oversteer - Oversteer caused by load transfer; the driver lifts off of the throttle abruptly, delivering weight from the rear tires to the front tires and resulting in the rear tires exceeding their maximum slip angle.

Trail Braking - A controlled compromise between turning and braking. The driver begins to turn into the corner while using the brakes to decelerate, effectively helping to rotate the car and obtain the optimum slip angle from the tires early into the turning event. 

In a real car, slip is felt via G-forces. As you approach the limit, the driver senses or feels what is happening. In the sim, tire noise provides the feedback as to what is happening, and this is how the sim driver determines how close they are to the limit of grip. Once the optimum slip angle is exceeded, the available grip falls off, or the car begins to slide. Unless more load is put in the tire, or the *speed is reduced* (While it's not really the vehicle velocity that is the problem, it is the simple solution), the tire will not respond as desired. Aerodynamics, also known as aero, can help with loading by putting more downforce on the tires as the speed increases, relative to the cars static weight. 

When you step on the accelerator, the weight of the car is shifted more toward the rear of the car. This puts more force on the rear tires, thereby providing more grip. When you brake, the opposite happens and the load shifts toward the front of the car, putting more load on the front tires. While cornering, you want the car balanced with an equal load on the front and rear. Creating an imbalance with abrupt pedal input will lead to understeer and oversteer. If while cornering there is too much addition of throttle, the car will understeer. If your response to the understeer is an abrupt lift of the throttle, something called trailing throttle oversteer, or TTO, is the result. While cornering, if the throttle is applied to abruptly, and the car has enough horsepower, then oversteer is again the result. This is because the rear tires cannot handle both the cornering load AND the acceleration load at the limit of the tire's grip. During braking, if too much steering is added without releasing some brake pedal pressure, then understeer happens as the front tires cannot do both jobs at the limit. When the brake pedal is released too quickly while initiating a turn, also known as trail braking, the car will tend to oversteer as the front tires have most of the grip, causing the back end to get loose. Slow release of the brake, on the other hand, will produce predictable and manageable rotation that can be used effectively to point the car, thus giving you optimum grip through the turn.

In video two, they go on about some other things, but I did not put it in the written portion. This is because they dumbed down a lot of the ideas so much that the way they explain it actually makes less sense physics-wise than if they took a little more time to explain it better. But in general, the iRacing Driving School videos have done a great job outlining the basics of vehicle dynamics. Weight transfer is extremely important to understand. Vehicle dynamics are a simple product of inputs. These inputs come from bumps, elevation change, track camber, driver inputs, aerodynamics, tire properties, and more. It is important to focus mainly on the inputs that you control. The key aspect of vehicle dynamics is understanding how to effectively use vehicle dynamics to your advantage. Whether it be trail braking, maximizing tire grip, finding the optimum slip angle, the general idea here is that you can take your understanding of these principles to help solve the problems you face as a sim racing driver.

Remember that some of the best things you can do to get faster is to have a better understanding of these basic principles and apply them accordingly. I often sit puzzled at the list of lap times in front of me, and wonder what in the hell I'm doing that has me a half second slower than usual all of a sudden. About 99% of the time, a basic principle of car control or vehicle dynamics has escaped me, and once I realize that I am asking too much out of the front tires and forcing the car to understeer ever so slightly, that half second is no more. 


Example:
Mid-Ohio Sports Car Course, full circuit
Turn 4 - "China Beach"

Problem - You notice that in your practice session (because you are an intelligent racer and spend many laps in practice becoming consistent with your chassis and track combination and learning how to deal with traffic at this particular event), most of your competition can get through turn four and into turns 5 and 6 of "Madness" quite a bit faster than you. Using this first Racer's Bible entry, you consider vehicle dynamics and immediately characterize your issue as understeer; the front tires lose traction and the car wants to push through the corner like a snow plow.

Vehicle Dynamics Solution -

You take what you know and understand to be understeer and consider what causes understeer from the vehicle dynamics point of view. It becomes evident that the front tires are breaking traction because you are asking for more traction from them than the rears, and push them beyond what they are capable of providing. In order to solve this, you simply ask less of the front tires. You brake in a straight line going into China Beach, and you brake earlier; this allows the tires to do one job through the corner instead of splitting the available potential grip between trying to stop the car, and trying to turn the car (which, in this case, resulted in the car doing both jobs poorly) and yields all available front grip for turning the car. You also take a second precaution and reduce your speed through the corner. While sounding counter-intuitive at first (Your problem was that everyone else is getting through this section of the track faster, so you slow down?), you find that you car now easily glides through the corner from entry to exit. The tires aren't screaming in agony, and you can now begin adding a little speed and braking a little later until you've reached that optimum slip angle through the corner without going over it like you were. You're also able to set up for the tough double-left-handers in madness much better. And wouldn't you know it, you've cut your time spent in turn four by almost three tenths in your MoTeC Telemetry data analysis, and time in turns 5 and 6 by two tenths! Slow down to go faster? Yep.

 



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