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"Jerky, violent driving means abrupt accelerations that transfer weight quickly. The tires don’t like that."

 

Why Smooth Is Fast

by Paul Haney

 

Tire Webinar3Anyone who has tried to learn how to drive a car quickly on a track has read or heard someone say, “Smooth is fast.” It seems to be true. Watch any group of cars on a racetrack and you will see some cars that look smooth as they brake, turn and accelerate out of corners. Others swerve violently under braking, careen wildly into corners and snake onto the straights as they smoke the rear tires out of the turns. The latter might look fast but lap times favor the smooth.

Why is that?

The driver has to make the car accelerate to create quick lap times. There is negative acceleration under braking, lateral (lateral to the car) acceleration in the turn and forward acceleration out of the turn and all the way down the straight until the next braking event. A driver who can cause larger accelerations can get around the track quicker and create lower lap times. But acceleration has consequences.

Firstly, tires are load sensitive meaning they generate decreasing amounts of grip with equal increases in contact patch load. That’s a complex topic. I explain it thoroughly in my book but for now we’ll look at another consequence of load transfer due to acceleration.

Acceleration causes weight or load (engineers prefer to use load but I use the two terms interchangeably) transfer among the tire contact patches. The schematic below, from my book, The Racing & High-Performance Tire, shows the forces on a car in a left turn. We’re just showing the rear tires but the front tires are out of sight up ahead and our car has equal weight distribution so the load on each contact patch is the car’s weight divided by four or W/4. When the car is sitting still on level ground, we have W/4 amount of weight on each contact patch.

Looking at the schematic notice the CG or Center of Gravity is above ground level. The CG is a spot that we can consider to contain the total weight of the car. Any forces acting on the car can be considered as acting on the whole car at the CG. The tires are generating forces in their contact patches at ground level. These tire forces, as in braking, turning and driving off corners, accelerate the car but the weight of the car resists that acceleration with an opposing Inertial Force. In the schematic, the tire forces are accelerating the car toward the center of the radius of the arc of the turn, in this case to the left, and the resisting inertial force acts to the right at the CG. The result is the inertial force trying to roll the car to the right which pulls some weight off the inside tires and adds that same weight to the outside tires as the schematic shows. Of course braking and forward drive also cause weight transfer. The car always weighs the same so these contact patch weights must add up to the total weight of the car.

smoothisfast

A fast lap requires getting all the work (acceleration) possible from the tires and that means that the amount of weight transfer you want is the most you can make happen. Going faster through a corner on the same path generates more weight transfer than going slower. Accelerating and braking harder-same thing. More weight transfer is the result of a faster lap.

Back to why smooth is fast. Jerky, violent driving means abrupt accelerations that transfer weight quickly. The tires don’t like that. Why? The short answer (without going into the viscoelastic nature of rubber friction) is that more tread rubber in contact with the track surface and the deeper penetration of the track texture into the tread result in more force from the tire and faster lap times. Drivers refer to this as being “down in the track” vs. “up on the track” which is slower. That’s actually a very good analogy.

Tread rubber acts sort of like a shock absorber-the faster you try to apply force; the more resistant the tread rubber becomes to penetration by track texture. A slower application of the same force results in deeper penetration of the track texture into the tread rubber. And more grip!

When a pro race team starts a race weekend they need to dial in spring rates and shock adjustments trying to optimize balance and grip. Even if they’ve been to that track before, ambient conditions and track surface conditions have probably changed. They usually start soft and go stiffer until the car slides instead of sticking. Then they back off toward softer to find the best setup. A stiffer setup transfers weight quicker and, at some point in their process of stiffening, weight transfer gets too quick for maximum penetration of the tread into the track surface.

But driver style matters also, especially for a club racer or track-day guy who doesn’t have all those adjustments. With a goal of a slower generation of maximum weight transfer what’s a driver to do?

Don’t make any quick movements of your controls. Don’t jerk the steering wheel at turn in. Turn in to a corner firmly but smoothly. Spread your actions over a longer time. I know a driver who makes a small turn away from the corner before the real turn in. He didn’t know why but it worked for him. His car may have some stiction in the suspension that would result in a spike in weight transfer build up before suspension movement. Spring preload does the same thing. His counter-steer/steer maneuver might smooth that out. His tires liked that. Yours may or may not. Experiment.

Don’t stomp on the brakes. Don’t go from full throttle to hard braking resulting in a tire-squealing, twitchy turn in. Find a quick, smooth rate of brake application that results in a controllable turn-in at the exact point that’s right for that corner and your car on that day. Letting off the brakes smoothly is just as important.

Don’t stomp the throttle at exit. Find a rate of throttle increase that gets your car off the apex to the exit edge of the track quickly and smoothly. Some cars like a little throttle at mid-corner before going wide open at exit.

Your driving instructor or coach probably has other tips and tricks for a smoother style.

I’m not saying smooth is easy. Unless you’re a pro dancer you’re probably not used to moving your feet quickly, smoothly and precisely. Ditto your hands on the steering wheel. But it’s fun flogging your toy around the track. Isn’t it? Why not try doing it smoothly? You might be slower at first. As you get better at driving smooth, the lap times will drop.

Have fun!

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