Over-driving often occurs when trying to catch up to a leading car or when pushing to eke out those last tenths in qualifying. As such, over-driving can be counterproductive.

In the last article, I discussed techniques for identifying driving below one’s limits and methods to remedy that. This article discusses the other end of the spectrum: over-driving. In both situations, the driver is getting less than the optimal performance out of the vehicle, even reducing the peak capability of the vehicle. When under-driving, the tires may be slow to get up to temperature and may never reach optimal temperature. When over-driving, oil, coolant, and tire temperatures may exceed their optimal range leading to an overall loss of power and grip. It’s also rough on equipment, which causes increased use of consumables and greater likelihood of mechanical failure.

It’s important in a discussion of overdriving to understand slip-angle: the difference between the direction a tire is pointing and the direction it is moving. Tires are elastic, so even small steering angles cause the tire to twist at the contact patch. The larger the side load on the tire, the larger the twist and the larger the slip angle. There is a point where the tire reaches its maximum side load capability and the slip-angle increases, but the grip actually drops off. Beyond the optimal slip-angle, the tire begins to slide, which creates extra heat and excess wear. I could dedicate an entire article to slip-angle, but the important thing to understand is that when a driver asks more from a tire than it is capable of giving, the driver ends up getting less than optimal results.

Over-driving often occurs when trying to catch up to a leading car or when pushing to eke out those last tenths in qualifying. As such, over-driving can be counterproductive. In many ways, overdriving can be easier to identify than under-driving. Slamming through gears, bouncing off the rev-limiter, sliding through corners and putting wheels off are all obvious signs of over-driving. More subtly, pushing harder and harder until lap times peak and then fall off can also indicate over-driving. In this case, back off a bit and see if the lap-times improve. However, there can often be factors that influence lap times that are beyond a driver’s control. The difficulty in identifying over-driving is that it just feels faster.

The graph above shows the optimal level of grip. When a tire is over or under pressure or temperature, grip would be lower along the full length of the curve.

The most effective way to identify over-driving and optimize a vehicle’s performance is to notice when the tire has reached the optimal slip angle and resist the temptation to steer or accelerate more. Unfortunately, it can often be easier for a driver to notice when the grip falls off rather than when it reaches the peak. Finding this peak takes experience and practice. It comes from an awareness of what you’re feeling as well as what your hands and feet are doing when the car is at the limit.

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Steering force is directly related to cornering force. As we increase the steering input, the steering effort increases up to the point of maximum grip — and slip-angle. Then there is a point where grip — and steering force — plateaus even as slip-angle increases. This plateau can be fairly large or very tight depending on the tire’s design. A wide plateau makes a car easier to drive at the expense of ultimate grip. A very narrow peak can have higher overall grip but becomes more unpredictable at the limit. The driver’s job is to get the tire up to peak grip with the minimum slip-angle.

Finding peak grip is one of the biggest challenges that a driver faces. Instead of driving in the same way that you always do, practice changing your input when you feel the tires are at their limit. Notice the way the steering force loads up mid-corner. Do you get any more cornering force if you add a bit of steering? What if you reduce the steering angle slightly or very slightly? Does the cornering force increase or fall off? How does the steering force feel when you increase or decrease steering angle? Do you unconsciously tend to add more steering when driving at the limit? These are the questions to ask yourself while driving and while reviewing video and data.

Other important factors in over-driving are vision and mental bandwidth. A driver with very busy hands and feet may be driving faster than he is able to process information, or right at the limit of what he can process. In this case, it might be enough to look farther down the track to allow for more planning and anticipation rather than reactivity and operating from a survival mind-frame.

The fastest drivers that I’ve ever seen seem to have an effortless, smooth quality where their inputs are slow but precise. That is what it looks like to bring a car right to the limit and not an inch nor a mph beyond.

Image courtesy of Brett Becker

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