Title drifting pic


by Billy Johnson

Braking late, hitting apexes, and powering out of corners:  The goal of driving a car quickly requires keeping the tires at their limit at all times.  Having good car control is a crucial skill in the development of a driver to not only drive a car fast, but also to have the ability to get up to speed quickly in a new car and on new tracks.  However, without self control and discipline, having good car control can actually work against you.

For those of you who don’t know me, my name is Billy Johnson.  I have raced professionally for close to 10 years now and was an instructor at the Skip Barber Racing School before privately coaching drivers ranging from beginner HPDE students on up to drivers in the IMSA and NASCAR Sprint Cup Series.  The following article is a result of what I have learned in over a decade of racing cars ranging from spec Miatas to prototypes and stock cars, and from instructing drivers in all of these fields with various backgrounds and driving disciplines. 


Billy Pic


This is going to be a long, in-depth analysis of driver development and I may lose some folks right off the bat, however if you care to improve your driving craft and understand where you are as a driver, it may be worth hanging in there.  Whether you are a beginner or a pro, I hope the following information helps you on your quest to become an even better driver.  Now on to the article.

During the development of a driver, learning to approach – and mastering the craft of balancing the car at the limit of grip is the name of the game.  Having the comfort and confidence (or lack thereof) greatly affects the process of learning and the ability to execute driving a car fast.  To simplify things I will separate the car control abilities of drivers into three categories: Under developed, Over developed, and Refined.  However, before talking about these three types of drivers, we first need to get a crash course on understanding how tires behave.


3D Tire ModelSince speed is taken into account, a 3D tire model is a more accurate depiction of the relationship between grip and slip angle than a 2D graph.  The graph above shows when speed is increased, the slip angle of the peak coefficient of friction is reduced.  For simplicity’s sake, we will stick with 2D models.

We could talk about tire deflection, aligning torque, mechanical and pneumatic trail, tire load sensitivity and a bunch of technical details of how a tire works, but for driving purposes we will focus on themes and abstract concepts related to driving without getting too far off on a technical tangent.   


Slip AngleThe slip angle of a tire is the difference (in degrees) between where the wheel and tire is aiming, and the path the tire actually takes.  This deflection at the contact patch is very small and is a result of the actual tread twisting within the tire as it rolls over the ground.

First off we need to be aware that “SLIP ANGLE” and “YAW ANGLE” are different.  Generally speaking, SLIP ANGLE refers to the difference between the direction the tires are aiming and the path that they actually take.  For example, if a car encounters a heavy cross wind, the force pushing on the side of the car will cause its path to change even though the steering wheel is held straight.  This happens because the crosswind generates a slip angle on all four tires which is different from where the steering is pointed, causing the heading to change.  When cornering, tires will always generate a slip angle regardless if the car is pushing off the track while understeering, sliding the rear tires while oversteering, balanced through a corner, or simply turning the car in a parking lot at 5mph.


Yaw AngleThe yaw angle of a car refers to the difference (in degrees) between where the centerline of the vehicle is aiming, and the actual path the vehicle is traveling.

YAW ANGLE refers to the difference from where the front of the car is pointed and the path that the car is traveling in.  In drifting, an important judging criteria is how large of a yaw angle a car can achieve.  In some cases the car can be perpendicular (90-degree yaw angle) or even slightly past perpendicular from the direction it is traveling.  Yaw angle is somewhat of a separate but related concept from slip angle.  For example:  If a car hits a patch of ice and the driver adds full steering lock, the front tires could have significant front slip angles, zero rear slip angle, and zero yaw and the whole car would just go straight.  Likewise, a car that is drifting could have significant yaw (sideways) and rear tire slip angles, but a much lower front tire slip angle since the front tires are not sliding very much and controlling where the front of the car is heading.  When we discuss slip angles, we are talking about the deflection at the tire and not the yaw angle of a car during over or understeer.

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