How NASCAR Wedge Adjustments Work

The pit stop crew performs various tune-ups, not the least of which is wedge adjustment. In the above picture, the crew member reaching over the rear windshield is adjusting the wedge.
The pit stop crew performs various tune-ups, not the least of which is wedge adjustment. In the above picture, the crew member reaching over the rear windshield is adjusting the wedge.
Todd Warshaw/Getty Images for NASCAR

­NASCAR races can be as fascinating behind the scenes as they are thrilling to watch. You don't need to be a car expert to be impressed with how a pit crew works its magic. But knowing more about the complex mechanics behin­d what these folks are doing will give you an even deeper appreciation.

One of the mysterious tricks a crew performs during these frenzied tuneups is called a wedge adjustment. During a race, you may hear an announcer mention that a car is getting one. Given that drivers are willing to dedicate several precious seconds to this tuneup during a high-speed race, you can bet it's important. Wedge adjustment can make a dramatic difference in how a race car handles during a race.

­Specifically, wedge adjustment refers to changing the amount of tension on a spring in the rear suspension. But shouldn't the race team take the time to get the suspension right before a race begins? It turns out that the suspension system is so dependant on changing conditions that it's difficult to get it precisely right until the driver feels the subtle handling effects during a race.

But how can pit crews fine-tune the suspension -- something buried in the inner workings of a race car -- midrace? Actually, they don't even have to pop the hood or climb under the vehicle. If you've ever seen a race car up close, you may have noticed small holes in its rear window. These openings allow the crew easy access to tubes that go directly to springs in the suspension system. We'll find out more about this special shortcut a little later.

­Before we do that, let's learn why this adjustment is so important. To do so, we'll first look at the suspension system, which affects more than just the bumpiness of a ride.

Suspension in a NASCAR Race Car

This is an example of a typical kind of suspension spring that may be used in NASCAR race cars.
This is an example of a typical kind of suspension spring that may be used in NASCAR race cars.
Jon Ferrey Allsport/­Getty Images

Wedge adjustment simply refers to adjusting the amount of force that bears down on a tire's spring. So before we go any further, we should emphasize how springs are used in a NASCAR race car's suspension system.

­What makes jumping on a bed so much fun? Unlike a hard floor, the springs inside a bed make the most out of the downward force you exert when you jump -- in fact, they give it back to you in upward force, allowing you to jump even higher. You can push down on a floor all you want -- it won't push back. Springs, however, do. The reverse happens when you try to pull the ends of a spring outward -- they'll try to pull back. Springs are such stubborn and resilient things, in fact, that the harder you compress or stretch them, the harder they will push or pull back. In physics, this is called Hooke's Law.

Car engineers are able to take advantage of a spring's unique qualities in a car's suspension system. This system is meant to combat the negative affects of bumps on a road. When a car is moving and encounters a bump, that bump spends energy by transferring some of the car's forward force to upward force. It can also make the tires lose their grip on the road. If you ever got a tire stuck in the mud, you know how important grip is. Although NASCAR racetracks may not have any mud, even the slightest loss of grip can affect a car's performance.

Springs are a great way to absorb the energy that tires encounter with bumps. With a spring at each tire, these suspension systems transfer the energy of the jolting tire to its spring, which pushes back. This keeps the tire in constant contact with the road as the springs continue to push wheels downward.

Even using springs, maintaining grip between the road and the tire can still be difficult when taking a turn at a high speed. This is what happens during NASCAR races. If weight shifts off of a tire during a turn, the tire will lose grip with the road and the driver will have less control of the vehicle. It turns out that the amount of tension on a spring can alter how much of the total weight bears down on a given wheel. When adjusted appropriately, the amount of tension on the individual springs can help attain proper weight distribution and grip during a turn.

­But here's how things get tricky: When you adjust the tension of a spring on one wheel, it can affect the other three wheels in interesting ways. Find out how next.

Cross-weight in a NASCAR Race car

If more of the car's weight was added to the right-front wheel, it might not have lifted off the ground as we see here. But that adjustment might also add too much weight to the left-rear wheel.
If more of the car's weight was added to the right-front wheel, it might not have lifted off the ground as we see here. But that adjustment might also add too much weight to the left-rear wheel.
Darrell Ingham/Getty Images

We've seen how important it can be for suspension springs to help tires maintain a sturdy grip with the road. It's especially important on a racetrack where every second counts and accidents can end in disaster. To accommodate such delicate conditions, pit crews perform wedge adjustments to alter the tension on the springs. But if you notice, these crews adjust the wedge only on the rear wheels. Actually, it turns out that adjusting the pressure on just one rear wheel is often all a race car needs. That's because, as we mentioned earlier, adjusting the tension of one spring can affect the weight distribution on all tires.

­To understand why this happens, think of a stable, four-legged table where the weight is evenly distributed among the four legs. If you tuck a small piece of folded paper under one of the legs, the table will wobble. Now the wedged leg and its diagonal leg carry more of the weight than the other two legs [source: Guerrero].

­The same thing happens with a race car.­ Compressing the spring of a left-rear wheel or adding wedge puts more of the car's weight on that corner. This adds pressure to that end of the car just like putting the paper wedge underneath the table leg. As with the table, the corresponding diagonal corner of the vehicle gets more of the car's weight. So if you increase the tension in the left-rear wheel, the left-rear and right-front wheels will hold a larger share of the car's total weight than the right-rear and left-front wheels.

The reverse happens if you reduce the tension on the left-rear wheel's spring or subtract wedge. In our analogy, that would be equivalent to cutting short a table leg. It would increase the weight on the right-rear and left-front wheels. This is why a crew may need to adjust only one wheel when a race car needs to add or subtract wedge.

The diagonally related weight between the left-rear and right-front wheels is referred to as cross-weight or simply wedge. It is often measured as a percentage of the vehicle's total weight. When more than 50 percent of the car's weight is on the left-rear and right-front wheels, the car is said to have more wedge.

­Now that we've learned how an adjustment on one corner of a vehicle can affect the weights on the other three corners, let's take a look at how this uneven weight distribution affects a race car's handling.

How Cross-Weight Affects Handling in NASCAR Races

This illustration shows how high speeds combined with improper weight distribution can lead to oversteer.
This illustration shows how high speeds combined with improper weight distribution can lead to oversteer.
Dorling Kindersley/Getty Images

­Think about what happens when you're making a left turn at a high speed -- the left side of the vehicle seems to lift slightly, and the weight shifts to the right side. In physics, this is called centrifugal force. We can feel how the distribution of weight gets exaggerated on the right side and relieved from the left side.

Let's look at how cross-weight affects handling in NASCAR races. When a race car makes a left turn at a very high speed, this weight shift to the right can dramatically affect any uneven cross-weight that exists. Consider what happens when a race car enters a left turn at a high speed with decreased wedge -- where the left-front and right-rear have more weight than the other two. The left turn evens out the difference in the weight on the two front wheels but intensifies the disparity in weight on the rear wheels. The rear wheels then do­n't grip as well as the front wheels, which can cause the rear to swing out (oversteering).

­The reverse happens when a race car enters a left turn with increased wedge -- where the left-rear and right-front wheels have more weight than the other two. The left turn evens out the difference in the weight on the rear wheels but exacerbates the disparity in weight on the front wheels. The front wheels then don't grip as well as the rear wheels, which can cause the vehicle to push forward despite the turn (understeering). However, the advantage to increased wedge is that it can give the driver more control when coming out of a turn.

­So, how does this relate to real-life racing situations? Find out how pit crews adjust the suspension and weight distribution of the race car in the middle of a race.

Wedge Adjustment during a NASCAR Pit Stop

This pit crew member is adjusting the wedge mid-race to give the driver improved handling.
This pit crew member is adjusting the wedge mid-race to give the driver improved handling.
Stephen Dunn/Getty Images

­The right percentage of wedge depends on current driving conditions and a slew of other factors. Because there's no set formula, a driver might not be able to predict the optimal amount of wedge. During the race, however, he'll feel the car's tendency to swing out or push forward in a turn. When he stops, he can have his pit crew adjust the wedge.

You may be wondering how a stock car can get a wedge adjustment during a pit stop. To make this task quick and easy, engineers fashion a shortcut into every NASCAR race car. Two jack bolts extend from the rear springs in the suspension system up through the car. A member of the pit crew called a "tire changer" uses an extended ratchet that reaches through one of the openings in the rear window to fit onto the bolt. This bolt raises or lowers the post that supports the spring [source: Demere]. By turning the ratchet, the tire changer can add or subtract wedge. The adjustments are measured in turns (of the wrench) or rounds. Often, the car may only need half a round of wedge adjustment.

­These quick-access jack bolts are only available on the rear wheels, however. Getting to the springs on front wheels entails opening the hood. But, thankfully, that usually isn't necessary. As we learned, adjusting the wedge in one wheel can affect the weight distribution on all of them. As it turns out, most of the time pit crews need to adjust only one wheel.

If the driver makes another pit stop because the car's too tight, the tire changer must subtract wedge. Instead of the left side, he mi­ght go to the opening on the right side of the rear window with his ratchet. He screws down half a round, putting slightly more pressure on the right-rear spring -- but this time he subtracts wedge and loosens the car for left turns.

­So now that that you've gotten down and dirty to learn the nuts and bolts of NASCAR wedge adjustment, learn more about NASCAR and other related subjects from the links on the next page.

Related HowStuffWorks Articles

More Great Links

Sources

  • Bloomfield, Louis A. How Things Work: The Physics of Everyday Life. 2nd Ed.John Wiley & Sons, Inc, New York: 2001.
  • Burt, William. "Stock Car Race Fan's Reference Guide." MotorBooks/MBI Publishing Company, 1999. (Nov. 5, 2008) http://books.google.com/books?id=pWHAA4dDf34C
  • Demere, Mac. "NASCAR Mid-Race Adjustments." Valvoline. (Nov. 5, 2008) http://www.valvoline.com/carcare/articleviewer.asp?pg=pht20070501mr&cccid=4&scccid=4
  • Grassroots Motorsports. "Understanding corner weights."Grassroots Motorsports. (Nov. 5, 2008) http://grassrootsmotorsports.com/articles/understanding-corner-weights/
  • Guerrero, Michael. "What is Wedge?" Stock Car Racing. (Nov. 5, 2008) http://www.stockcarracing.com/techarticles/39742_what_is_wedge/index.html
  • Ovcharik, Miroslav. "Front Bias, Left Bias, and Cross Weight: Suspension Turning with Weight Adjustments." ArticleCity.com. (Nov. 5, 2008) http://www.articlecity.com/articles/auto_and_trucks/article_1601.shtml

­