Intentionally Plowing And Sliding

Published: June 24, 2014

Novice and intermediate drivers are generally expected to keep the car under the grip limit at all times. A hallmark of advanced drivers is their ability to keep the car just over the grip limit in some situations where this makes sense, and by extension, to know when keeping a car above the grip limit makes sense.

Fixed Externalities

Consider swimmer crossing a square lake. Wherever they swim is where they will go:

Now imagine the same swimmer is crossing a river. To cross perpendicularly to the shore line the swimmer would swim at an angle like this:

The current would put the swimmer back on the perpendicular line.

The current is an external factor that the swimmer has no control over. The swimmer adjusts what they are doing to achieve the desired result in the presence of the current.

Driving is very similar. It is rare to have a car that is setup perfectly every event in every corner. In fact, such perfection just about never happens at HPDE level. Instead the car might have undesirable understeer in some corners and undesirable oversteer in other corners, or it might understeer everywhere. Street cars in particular typically have a very limited handling adjustment range.

Handling characteristics are thus similar to a river that a swimmer has to cross - a fixed externality that a driver cannot do much about. Much like the swimmer, the driver can adjust the way they drive to compensate for the handling deficiencies of their car.

Understeer

A driver plows their car when the car understeers and the driver maintains the car in the understeering condition.

Novice drivers occasionally plow their cars when they enter corners at too high of a speed, especially in street cars that are set up to understeer. The car exceeds the grip limit of the front tires and starts to understeer, the driver maintains steering angle, the result is a plowing car. Typically the difference between what the driver is asking from the car greatly exceeds what the car is capable of - the car is being visibly overdriven. This type of plowing is not good, and instructors get the drivers to enter turns slower to eliminate it.

An advanced driver in a car set up to understeer might experience understeer on corner exit under power. Consider the choice between understeer and oversteer in this situation:

• An understeering car gets to the outside edge of the pavement quicker but is accelerating the entire time. Such car loses some speed due to front tire scrub compared to a neutral car.
• An oversteering car enters a drift and typically maintains the drift for as long as the driver is on throttle. While the car is in a drift it uses only a portion of its power to accelerate forward, with the rest being wasted by spinning rear tires.
• A third option is to lift off the throttle entirely, which typically stops understeer.

Neutral¹ handling would be ideal, but realistically not always achievable. A drifting car exits corners with less speed than an understeering car. A drifting car also spends more time in the corners due to being sideways. An understeering car, when compared to a neutral car, loses some speed due to tire scrub, but generally an understeering car will be quicker than an oversteering car. A car that is coasting because the driver lifted off throttle is similarly slower than an understeering car under power.

Given the choice, then, the fastest option is to have a car that understeers, hopefully not by much, and plow it out of the corner under power. The less the car plows, the faster it will be. Unwinding steering can make some cars not plow due to reduced lateral force demands and still permit these cars to fit into the track.

Oversteer

Depending on how much power and grip a rear wheel drive car has, how fast it is going, and what the track surface is at the precise moment, when the car enters a slide one of two things might happen:

• The car will naturally regain grip even under full throttle. This happens with lower power, higher grip cars and at higher speeds.
• The car will remain sideways until the driver lifts off throttle. This happens with higher power, lower grip cars and at lower speeds.

The important point here is that cars with sufficiently low power levels can regain grip without the driver lifting off throttle. In such cars lifting off throttle often costs more time than waiting for the car to regain grip. If such a car starts to slide, the fastest way to drive is to remain on full power and wait for the car to regain grip.

A less obvious application of this observation is driving in low traction environments, such as in rain. A car might lose grip driving over a puddle, and enter a drift, but if the puddle is isolated the car might regain grip by itself once its tires are out of the water. Drivers who are used to oversteer and have sufficient car control skills can be a good bit faster in wet conditions because they maintain throttle when less experienced or confident drivers would lift and lose speed as a result.

Tire Wear

Even if the car remains on pavement as it is plowed or slid around the track, a notable consequence of doing either is increased tire temperatures and wear. Plowing a car increases temperature and wear of the front tires, while sliding a car increases temperature and wear of the rear tires.

If a car can overheat its tires in 20 minutes while not sliding anywhere on the track, the same car sliding in one or two corners might get 15 or 10 minutes of track time before tires start to overheat. The lap times during those first 15 or 10 minutes would be lower, but lap times toward the end of the 20 minute session would be higher. Therefore, drivers participating in time trial competitions frequently slide the car, because they want those quicker laps, and non-competitive drivers whose goal is to finish their sessions typically do not slide their car to maintain tire grip level through the entire session.

Racers need to decide whether to push or not based on their position in the field and where they are in the race time-wise. Typically tires will not last the entire race if they are being aggressively used, thus racers need to pick when they push hard and when they conserve the tires.

Drivers who are significantly below the grip limit of their car can get away with sliding the car in a turn or two for a session and not overheat the tires. This works because the tires are cooling down throughout the rest of the lap.

Tire compound affects how much a tire can be slid before it overheats. Drivers who expect to run longer sessions while driving aggressively can opt for harder compounds that tolerate more heat. An aggressive driver on soft tires may overheat the tires before getting the best lap of a session.

Tagged: advanced