For part 2 in this Car Control Fundamentals lesson series, we’ll explore the physics of driving a vehicle at the Understeer limit. We’ll learn different ways that a driver might try to sense the limit as well as how they can test for the limit by using driver inputs to check the vehicle’s response. We’ll then explore this testing process further by looking at an understeer training exercise from the Academy lesson series. Finally, we’ll finish up by learning how driving smoothly is achieved in different ways as a vehicle approaches and then reaches the limit. | 1. The Limit Defined 2. The Understeer Limit 3. Slip Angle & Rotation 4. The Oversteer Limit 5. Load Transfer |
In the Racing Line Fundamentals lessons, we looked at several examples where a car was always perfectly at its 1.2 g limit. While using a vehicle’s theoretical optimum is useful when learning how the lateral vs longitudinal forces determine the ideal line, now we’ll begin looking at how a real world driver can work toward getting as close to this ideal as possible by examining the limit from their perspective.
The first driving cue we will discuss is tire noise. As a typical street tire approaches the limit, it will often start to produce a squealing sound, but this will vary from tire to tire and under different conditions. The tread pattern, carcass, surface, etc… all combine to produce different sounds. In comparison however, most track tires with little to no tread often make almost no discernable sound until well past the limit. So while the tire noise cue is typically not very useful for track-oriented tires, it can sometimes be helpful for a driver on street tires, as hearing tire squeal means they are at least somewhere near the limit. It doesn’t offer precise enough information to Identify when the tire actually reaches the limit however. While there is often a change in pitch as more and more of the contact patch starts to slide, a tire doesn’t produce a specific sound at the point it reaches maximum force. We’ll take a deeper look at the contact patch and tire slip in the next lesson. |
For example, even if a very skilled driver were riding as a passenger around a skidpad, they would be unable to determine if the car was precisely at the limit or not. While as a passenger they would be able to see and feel the movement of the vehicle just as well as the driver, only by gauging how this movement is responding to input changes can someone accurately find the limit. This process is called testing for the limit. |
To test for the limit, a driver makes an input change while checking the vehicle’s response. Continuing with the skidpad scenario as an example, in order to test for the understeer limit, the driver could increase steering while paying attention to the vehicle’s movement. If the front tires respond to increased steering by moving inward and tightening the vehicle’s line, then it was not yet at the understeer limit, as the front tires still had some capacity remaining. If on the other hand, the vehicle did not tighten its line, then the front tires had already reached the limit and continuing past this will increase induced drag, causing it to slow down. In order to avoid this, the driver would need to reduce steering until the front tires just start to move outward again, indicating they are once again below the limit. At this point, the driver can test for the limit again by increasing steering once more.
Being smooth is an important element of car control, but a driver must achieve this in different ways as a vehicle approaches and then reaches the limit. For example, although in the skidpad exercise video we see constant steering wheel movements, the associated changes in vehicle movement are subtle and can be hard to see.
If a driver were below the limit however, those same back and forth steering motions would cause the car to move side to side noticeably. The reason for this difference is the progressive nature of tire response. This diagram shows how a typical tire has an initial linear increase in force, but then begins to taper off to the peak before steadily dropping. The result of this is that a steering wheel movement done below the limit in the linear and early transitional area has a much greater effect on tire force and therefore vehicle movement then that same steering movement done near the limit. |
Up Next - Slip Angle Explained I hope you enjoyed this second installment in the Car Control Fundamentals lesson series, and if you have any questions, please use the comments section below. Up next, we’ll move on to take a closer look at slip angle and how it relates to vehicle rotation. After that, we’ll dive into oversteer and then finish up our final lesson by learning about the role load transfer plays in car control. If you are interested in a complete guide to the physics of racing, we also offer The Science of Speed book series, available through our bookstore or at popular retailers such as Amazon. Adam Brouillard | 1. The Limit Defined 2. The Understeer Limit 3. Slip Angle & Rotation 4. The Oversteer Limit 5. Load Transfer |