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Getting maximum performance out of a car's rear spoiler

National Dragster, Apr 11, 2003 by Jones, Rick

The aerodynamics of a car are very sensitive to change, and an item that can be used to drastically alter the aerodynamic forces of a vehicle is a spoiler, which on an NHRA Pro Stock car consists of a lot more than meets the eye. A correctly designed spoiler will add the right amount of downforce to the rear without sacrificing much drag.

For Pro Stock cars, RJ Race Cars tries to develop a spoiler with superior aerodynamics for each particular body style that will meet NHRA rules, which allow nothing longer than a 14-inch spoiler (measured from the rear edge of the deck lid to the bottom of the wicker bill).

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NHRA rules require that the angle of a Pro Stock spoiler is not negative and that it remains level or positive after each run, and tech officials check this. In addition, rules require the use of a wicker bill, also known as a Gurney strip, that sticks up at least 1/4-inch above the rear part of the spoiler at a 90-degree angle. Using a wicker bill is a good way to add efficient downforce to a car without reducing the coefficient of drag (CD).

The angle at which spill plates are mounted to the spoiler must be carefully considered when designing a spoiler for a car. The spill plate is a vertical fin that attaches to the spoiler's side and transitions from the quarterpanel to the spoiler's back, which also is regulated. NHRA rules require that spill plates are not taller than four inches, that they stick half an inch to two inches above the spoiler, and that they are not longer than 24 inches. Also regulated is how far up the quarterpanel spill plates are attached. The angle of the spill plates,

viewed from the top, in relation to the center of the car is very important because it determines how much air gets under the spoiler. A Pro Stock spoiler can also be rolled down on each side no more than two inches off the flat part of the spoiler. This means that if a straightedge is laid across the spoiler, each side of the spoiler can be rolled down no more than two inches (measured at the transition/mounting point from the spoiler to the spill plate). Using a spoiler that is correctly installed and has a 1/4-inch, 90-degree wicker bill will provide a performance advantage. In wind-tunnel tests, we have found that, for most accounts, keeping the spoiler perfectly flat and using a wicker bill will reduce drag on the car and produce adequate downforce.

If a lane is bad and other racers are having handling problems, a racer will sometimes raise the spoiler. This is not necessarily the best thing to do because by raising the spoiler, the drag and downforce on the back of the car is increased. If too much downforce is added to the rear, the lift at the front of the car will increase, making it unstable. Once the spoiler is set, don't raise it when the track gets bad. Adjusting the spoiler in this type of situation can cause more problems than simply leaving it in its normal setting. To create more downforce, leave the wing flat and install a taller wicker bill. A 3/8-inch- or 1/2-- inch-tall wicker bill will add downforce with little CD loss.

Most door-car spoilers have four adjustable wing struts that keep the spoiler in place. The struts have a series of holes that allow the racer to adjust the height of the spoiler, usually in 1/4-inch increments. The cars that we build at RJ Race Cars have a 3/16-inch right-hand rod at the top end of the struts that allows a racer to fine-tune the spoiler angle. When setting the spoiler angle, I like to use a digital protractor in four places parallel to the centerline of the car (one on each side of the spoiler and one over each of the two center struts on each side of the centerline of the car). With the driver weight in the car, adjust the rod ends to fine-tune the spoiler so it has the same angle all the way across.

A correctly designed and setup spoiler can add performance to a car and make it a more balanced and safer entry.

Rick Jones owns RJ Race Cars Inc. and Quarter-Max Chassis & Racing Components in Galesburg, Ill. RJ Race Cars has been building doorslammers since 1987 and was named the Car Craft Chassis Builder of the Year in 1998 and 2002. Quarter-Max Chassis & Racing Components is Jones' mail-order division that offers high-quality components for Pro and Sportsman racers. Jones has published a chassis-tuning guide that is available through his mail-order company. Readers can send their questions or comments for Rick Jones via e-mail to ChassisChatter@rjracecars.com or via fax to (309) 343-0886.

Copyright National Hot Rod Association Apr 11, 2003
Provided by ProQuest Information and Learning Company. All rights Reserved

Getting maximum performance out of a car's rear spoiler

National Dragster, Apr 11, 2003 by Jones, Rick

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