Understanding NASCAR: Mid-Race Adjustments

Understanding NASCAR: Mid-Race Adjustments

Separating winners and losers
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What separates winners from losers in NASCAR? One answer: mid-race adjustments. During a NASCAR Cup Series race, it's not unusual for fast cars to slide to the back while others gain speed. The difference: changes made during pit stops. (Also: changes not made and the wrong changes.) This fine-tuning-which, once the green flies, is limited to tire pressure, wedge, track bar height, spring rubbers, tape, and brake bias-can turn an evil-handling car into a lap-leader, or keep a fast car at the front despite evolving track conditions.

In Sync

No one reaches the NASCAR Cup Series without being an extremely skilled driver. I know: I've raced against Davey Allison, Ken Schrader, Ron Hornaday, Jerry Nadeau, Mark Martin and Jeff Gordon in the NASCAR Southwest Tour Series, the Rolex 24 at Daytona, and other races. Among the things separating the merely great from race winners and champions is the ability to work with the crew to make beneficial adjustments during a race. The most successful drivers accurately describe how the car is behaving, while top crews understand their driver's nuances and know how to make the right changes at the right time.

Even those with racing experience have difficulty figuring out from TV and radio reports why some mid-race adjustments are made. My assessment: Crew chiefs regularly lie to confuse eavesdropping competitors. (It wouldn't be surprising if some installed left-hand threads on the wedge and track bar adjustments to further confuse the opposition. And me.) Equally likely: The reporter has little clue as to what's being described. There's another explanation: aerodynamics. Not that long ago, Cup cars employed much stiffer suspension springs on the front axle. On the faster tracks, that's been reversed. In an effort to keep the front splitter close to the ground to produce maximum aero downforce, the hot setup has changed to very soft front springs with stiffer rear springs.

Allowed Adjustments

Here are the adjustments allowed during a race, what they do, and why they're made.

Track Bar Adjustment

You're incorrect if you think that this is what unsuccessful drivers do after a bad race. The short answer: "Up on the track bar" reduces push by increasing the rear roll center, while "down on the track bar" increases push by lowering the rear roll center. ("Loose" means the car is trying to spin out. "Push" means the front tires lack desired grip.)

The track bar (also known as a "Panhard rod") is a long steel tube that locates the rear axle laterally. Look under many road-going pickups or SUVs with solid rear axles and coil springs and you'll likely find a track bar. Without a track bar, the rear axle would move from side to side during cornering, which would make the car undriveable. On Cup cars, the track bar is fixed to the axle near the left rear spring and connected to the chassis near the right rear spring with an adjustable mount. Changing track bar height is accomplished by inserting a very long ratchet extension through a hole in the right rear corner of the rear window and cranking on the ratchet.

When a driver complains about needing more "forward bite"-the ability to accelerate off the corner-the team may lower the track bar. When the driver complains about needing more "side bite"-the ability to get through the corner quicker-the team may raise the track bar. When the driver complains about lack of side bite after the team has adjusted to compensate for lack of forward bite, the team may tell the driver that the easiest thing on the car to change are the seatbelts; another driver will correctly adjust them with no help from the crew.

Tire Pressure

This is the easiest and quickest mid-race adjustment. That's because it's done before the tires are fitted on a pit stop. Easy and quick it is, simple it's not. The short answer: Increasing tire pressure reduces mechanical grip (assuming the team didn't start the race with too little pressure), but may hurt aero grip. (This is not true for your street car: Its tire pressure should never be lower than that recommended by your vehicle's manufacturer.) Since a tire is an air spring, raising tire pressure is the same as increasing the strength of the suspension spring. Increasing spring rate beyond the optimum reduces the mechanical grip of that corner of the car. Loose is often corrected by adding pressure to the right front, while push is quelled by increasing the right rear. Due to the asymmetric nature of oval racing, the opposite is true for the left-side tires.

Wedge

Also called "cross weight," this is the amount of the car's weight carried by the left rear and right front tires. Increasing wedge helps the left rear tire's grip. This allows the driver to put power to the ground exiting the turn, but hurts corner-entry response. If the car is "loose off," the team may add wedge.

Racers express cross weight as a percentage (i.e. "54 percent wedge"), but this is rarely heard on broadcasts. When crews add or remove "rounds of bite," they are changing diagonal weight bias by adjusting the ride height. Similar to a track bar adjustment, a wedge change is accomplished by inserting a long ratchet extension through holes in either the left or right side of the rear window. The crewmember turns the ratchet, which turns a bolt that raises or lowers the perch on which the suspension spring rests.

Spring Rubbers

These inserts are placed between the coils of the suspension spring to increase the stiffness of the springs. "Removing a rubber" softens the overall rate of the suspension spring, which increases the mechanical grip of that corner of the car. Ignoring aero influence, "removing a rubber" from the right rear or left front increases push, while removing it from the left rear or right front reduces push. Often teams will start the race with rubbers in place in order to provide more adjustment possibilities. "Inserting a rubber" is a drastic move usually done in an attempt to shut up an overly high-paid driver.

Brake Bias

With a knob in the car, drivers can change the force between the front and rear brakes. On tracks where the drivers employ a significant amount of braking, moving the brake bias toward the rear may help the car better turn into the corner. Adding front brake may quell corner-entry looseness. The reason: Additional braking force reduces tire grip on that end of the car. However, too much rear brakes bias can cause the car to spin if the driver has to brake hard to avoid an incident or enter the pits. If you've ever pulled the parking brake in the middle of a turn, you know what too much rear brake bias feels like.

Tape

Tape (similar to duct tape) on grille openings increases front grip and reduces push. It does this by reducing lift-producing air from flowing into the engine compartment. But there's no free lunch: Limit air to the radiator too much and the engine will overheat.

Without skilled communication between driver and crew, none of these adjustments can be made accurately. This need for communication is arguably the biggest difference between NASCAR and open-wheel racing: With Indy, Champ and Formula One cars, on-board data-acquisition and real-time telemetry makes driver input almost unnecessary. When I tested Indy cars for magazine articles, the crews knew how the car was behaving without a word from me. In open-wheel racing, the driver needn't even speak the same language as the crew chief. That's not true in NASCAR.

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