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Dr. Diandra: Turn to turn experts for speed and passing

I always encourage fans to listen to driver-crew chief communications. I think you appreciate a race more when you understand a team’s strategy and struggles. A lot of the discussion focuses on the turns.

Drivers and crew chiefs break turns into at least three parts: entry, middle and exit. Some break the corners down even further, but let’s start simple.

Rule one: Grip is proportional to the force pushing down on the tire.

The total force pushing down on a car is the car’s weight plus the aerodynamic downforce. I’ll ignore aerodynamics to simplify the explanation.

That leaves just the car’s weight; however, the total weight isn’t the relevant factor. We’re interested in the weight pushing down on each tire. That’s why weighing a racecar doesn’t involve a giant car-sized scale, but rather four smaller scales, one for each tire.

In the absolute simplest case, equal weight pushes down on each tire. Each tire thus has the same grip.

Rule two: You can only go as fast as the least grippy tire.

Anyone who has driven on ice or snow knows this rule. If the rear tires of a rear-wheel-drive car are on an ice patch, the car won’t go. If one drive tire is on a slick surface, the car may move, but probably not the way you’d prefer it move.

That’s an extreme case, but you get the idea. The least-grippy tire limits a car’s speed. You can have lots of grip in your rear tires, but without grip in the front, you won’t be able to turn.

I’ve posited a perfectly balanced car, with the same force on all four tires. But even then, the forces are equal only when the car is sitting still.

Imagine yourself driving. You engage the brakes as you approach a stop sign. What happens?

Your body shifts slightly forward as the car stops.

So does some of your car.

Everything supported by a vehicle’s suspension (the chassis, body, engine, driver, etc.) can move relative to the wheels. Engineers refer to this as sprung mass because it’s attached to the wheels with flexible components like springs and shocks.

When you brake, the car’s sprung mass shifts from the rear toward the front. That means:

  • More force pushes down on the front wheels
  • Less force pushes down on the rear wheels.

Rule one tells us that the car now has more grip on its front wheels than its rear wheels. Rule two tells us that the car will be slower because, while the front wheels have more grip, the rear wheels have less grip. The rear wheels limit the car’s speed.

The opposite happens when you step on the gas. The sprung mass shifts from the front toward the back, and the car now has more rear grip than front grip.

Since a driver brakes coming into a turn and accelerates coming out of the turn, this shift in grip has a major effect on speed and handling.

And there’s more. There’s also load transfer when a car turns. You’ve no doubt seen the tippy truck sign, right?

The highway sign showing load transfer

A truck turning left could tip because of how its sprung mass shifts. Intuition might suggest that the truck should tip in the direction it’s turning, but that’s not how it works. In a left turn, load transfers from the left wheels to the right wheels.

And if you’re performing some combination of turning and braking or accelerating, load transfer happens in all directions.

Rule three: All the effects of load transfer add up.

Let’s combine all three of the phenomena to understand what happens when a driver turns a racecar.

  • She brakes as she approaches the turn.
  • While still braking, she starts to turn.
  • The car rolls through the corner at constant speed, with all the load transfer from left to right having taken place.
  • She accelerates as she starts to complete the turn.
  • As she leaves the turn, she accelerates while going straight.

The table below summarizes how the loading of the wheels changes during this process.

A table showing how the load changes as a car turns

The magnitude of the shift in force on the tires is proportional to the height of the car’s center of gravity. Semis and SUVs have a high center of gravity. That’s why they’re more likely to tip on a turn than a low-to-the-ground sports car.

One of the Car of Tomorrow’s vexing points was that its center of gravity was quite high for a race car. That made load transfer a much bigger effect. The video below shows the then-brand-new Car of Tomorrow at Martinsville Speedway. You can see as Robby Gordon comes out of the turn and gets on the gas that there isn’t even enough force on the left-front tire to keep it on the ground.

A video showing how significant load transfer can be

Because teams have access to detailed data about how their drivers — and all the other drivers — navigate corners, they can analyze a turn in exquisite detail. That’s why they don’t stop at entry, middle, exit, but specialize to late entry or early exit, for example.

William Byron’s crew chief Rudy Fugle explained how teams use this data on NASCAR America Motormouths.

“We break the track down into segments,” Fugle said, “and say, entry, early entry, middle… wherever, these parts of the corner make the lap time. All the fast cars that are making laps make it here.”

Making a fast lap is important for qualifying, but there’s more to racing than speed. You have to be able to pass other cars, and that frequently happens in the turns as well.

“And then we also have some analysis on the most passes are made in this part of the corner,” Fugle said. “So you kind of know, like, OK, to go fast I need to be good here and to make passes, I need to be good here, because sometimes they’re in different spots.”

The challenge of dealing with a car whose grip constantly changes is, in my opinion, one of the most underappreciated aspects of racing.

When I lived in Nebraska, I put bags of sand in my pickup truck’s bed to increase its rear grip. That additional force made the truck less likely to get loose. Racing is like having a bag of sand on each corner of the car, but the weight of each sandbag changes when you speed up, slow down or turn. The best drivers are those with an expert feel for how far they can push their car without exceeding its traction limit.