From the Lab Notebook: Las Vegas and the Mysteriously Missing Oil Tank Cover

As we head for Las Vegas this weekend, I thought I’d repost on of my most popular posts from stockcarscience.com on 3/5/2008  since the redirects for the old stockcarscience.com site don’t work reliably.  The post is about Carl Edwards’ 2008 win at Las Vegas when the team was subsequently fined for having their oil tank cover lid askew at the end of the race.  I have edited the post extensively, adding some new information and better graphics.

Danny LaDue asks: Can you explain the location of a NASCAR oil tank reservoir and how the lack of one could improve aerodynamics?

Thanks for the question, Danny.

NPR got this one wrong.  Frank DeFord in his usual Wednesday commentary made a comment that was essentially — look, the lid was still in the car, it didn’t give him a weight advantage, so NASCAR was wrong to penalize the team.  Don’t these folks known anything?

That’s the problem with aerodynamics — you can’t see it happening.

Unlike your car, the oil in a NASCAR car isn’t stored in the engine (called a wet sump system).  NASCAR uses a dry sump system, in which oil is

stored in an oil reservoir. The oil reservoir is located behind the driver’s seat and is surrounded on the sides and top by sheet metal, which forms the oil tank.   The sheet metal minimizes heat radiating into the car, traps fumes from the hot oil, and serves as an additional firewall.  This function is so important that NASCAR doesn’t allow the top of the tank to be attached using quick connect fasteners. Some teams duct tape the lid on. The picture to the right shows the location of the oil tank with respect to the chassis. It doesn’t show the cover, which would sit on top of the tank.  The oil reservoir itself is closed and pressurized.

So if the oil tank cover plays such an important role, why would you leave it loose, much less leave it off?   The answer is aerodynamics.  The air exerts forces on the car in different directions.   Drag is the force air creates along the length of the car.  Air creates drag when it hits the front of the car,  but it also creates drag when it gets inside the car because there is no way for it to get out.   Drag always acts opposite the direction the car is trying to move, so you want to eliminate as much drag as possible.

Downforce and lift are the names for the forces pushing straight down or up on the car. Downforce pushes the tires harder into the track and provide grip, while lift pulls up on the car.  These two forces are in direct opposition to each other.  The bigger force wins.  You want to maximize downforce and minimize lift.

The oil tank is open to the bottom of the car. Air under the car creates lift.  Even though you try to keep the splitter close to the ground, there is always some air that gets under the car.  If the oil tank lid isn’t firmly tightened down, it creates a path for air to get out of the car, which reduces lift.

When the amount of lift decreases because of the loose oil tank cover, then the net downforce is larger because there is less air pushing upward. More downforce translates directly into more speed, as shown in the figure below.  Remember learning about ‘net force’ in physics?  Yep – it is actually useful.  The loose oil tank cover likely provided a little extra downforce — in a sport where races are won by thousandths of seconds, even “a little” advantage is important.

One of Rusty Wallace’s cars originally penalized in the Nationwide series won its appeal on the basis that all of the bolts on the oil tank cover were engaged fully and the design of the oil reservoir was such that it led to the apparent opening. I can imagine (especially having seen graduate students overtighten bolts) that if you screwed down really hard on the bolts and the oil tank lid were on the thin side, you might be able to warp the cover on the oil tank lid a little and get some air escape.  The problem with this argument is that you can only use a ‘bad design’ argument once because NASCAR will make you redesign it.

The case of the No. 99 car’s oil reservoir lid is a little different, though, because the reports have been that the lid was entirely missing. In fords, the oil tank cover is held on by a single bolt.  Carl Edwards said on NASCAR This Week that a “bolt backed out”.  Jack Roush made the argument that the vibrations in the car caused vibration harmonics that caused the bolt to unscrew itself.  Even if that’s true (and I have to admit I’m a little skeptical about it), should you really have a safety feature held in place by a single bolt?

NASCAR fined the driver and the owner 100 points (old points scheme!), fined crew chief Bob Osborne (B.S. in Mechanical Engineering from Penn State) $100,000, 100 points and suspended him for six weeks.