The Science of …Missing Oil Tank Covers

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. I can–with a little help from Josh Browne, Chief Race Engineer at Red Bull Racing, who helped me disentangle a couple of things I heard on television.

First, the location. Unlike your car, the oil in a NASCAR car isn’t stored in the engine (which would be a wet sump system). NASCAR uses a dry sump system, in which oil is stored in an oil tank reservoir. The oil tank reservoir is located behind the driver’s seat and is surrounded on the sides and top by sheet metal, which forms the oil tank. You can see a picture of the oil reservoir in the Stock Car Science Building A Car section. The sheet metal plays an important role in minimizing 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. The teams usually duct tape the lid on. The picture below 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. (added) 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. We give different names to those forces depending on the direction in which they act. 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 we give the forces that push straight down or up on the car. Downforce pushes the tires harder into the track, while lift pulls up on the car. These two forces are in direct opposition to each other. Whichever one is bigger 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. Josh told me that there may also be a drag reduction–there was with the old car, but he wasn’t sure about the new one.

On NASCAR Now Monday, Ray Evernham commented that having the lid off “allowed the rear wing to make more downforce”. The total downforce is the difference between all the forces pushing down and all the forces pushing up. From my discussions with Josh, I assume Ray means that the open oil tank lid decreases the force of air pushing up on the car, which means that there is less lift. If the rear wing (or spoiler in the case of the Nationwide cars) produces the same amount of downforce, but 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. I can’t see how having the oil tank cover missing would lead the wing to generate any additional downforce unless the decreased lift changed the car’s attitude, in which case all bets are off.

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.

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. Carl Edwards said on NASCAR This Week that a “bolt backed out”, but it’s a little hard to see how losing one bolt would lead to the disappearance of the entire plate. Boris Said asked the important question on NASCAR Now Monday: “Was the lid in the car?”

As of Wednesday morning, still no ruling from NASCAR. We’ll have to wait to see how serious they think this violation was.

UPDATE: $100,000, 100 points and six weeks suspension for crew chief Bob Osborne (who, incidentally, has a B.S. in Mechanical Engineering from Penn State).

ALSO SEE: There is additional information on this topic on my follow-up blog on vibrational harmonics.

Sorry to keep extending this post, but the questions keep coming in. The oil tank cover was indeed in the car, although it wasn’t on top of the oil tank enclosure where it was supposed to have been. ESPN has a nice article summarizing this.

To the numerous people who have emailed to ask if I think the No 99 “cheated”: Science, like NASCAR, doesn’t utilize mind reading as one of its methods. If you decide to use a single bolt to hold down an irregularly shaped piece of sheet metal a little less than a square foot in area, you are accepting a certain amount of risk because you know you’re going to be penalized if that single bolt fails. If nothing else, they deserve the penalty for bad engineering design. Somewhere in State College, PA, there is a mechanical engineering professor shaking his head sadly, wondering if Mr. Osborne was asleep in class the day they discussed safety factors. Or, Osborne may just be following the rule Josh Browne taught me while I was researching my book, The Physics of NASCAR:

“Sure we have have a safety factor. It just happens to be equal to one.”


  1. Diandra-
    This is good stuff! Can’t wait to get your book and read more of your blogs!

  2. Great site. I am glad to see you are contributing to our knowledge of all things NASCAR in this way. Now I know where to come when I have technical questions. No more dependence on the networks and Jeff Hammond for me!

  3. What a great explanation. I miss most of those shows, so was not aware of Ray Evernham’s remarks. Many of us learned something today.

    And a special thanks for the time Josh Browne took to help us understand how it effects the total downforce of the car.

  4. Just the other morning, I was pondering these oil tank cover issues that have seemed to have sprung up everywhere and what was the advantage. Maybe Diandra will cover this. Then, “BANG” – here it is!

    Great job, and fascinating explanations on all subjects covered!

  5. Ditto what Grump said, above!

    Terrific concept here, and we’ll be adding you to our blogroll so readers can jump to your site for NASCAR physics! (A course I most definitely would’ve failed, since all of that influx-trajectory stuff is way over my head, grin …)

    Good luck with book sales!

  6. Thanks for the clarification. But I’m still a little in the dark on how the oil tank lid is held in place. Some reports have indicated it is held by one bolt and others have indicated duct tape is used. And just how does Rousch secure theirs? Lastly, was the lid in the car after the race?

    Even with these questions open, yours has been the BEST reporting on this issue i have seen to date — GREAT JOB!!

  7. thought that diagram looked familiar: i am mid-way thru your book & it’s amazing! i started watching nascar at the insistence of my engineer friends who were determined to make me lose my “it’s just turning left in circles” bias and so they explained racing in terms of the sciences involved. since then, i’ve been working on understanding the hard sciences involved in nascar — as well as the history of the sport and the current drivers! your work is adding to my understanding and i will be suggesting that others buy their own copy of your book b/c i am NOT giving mine up! when the pit tank issue came up, i immediately went to your book and so had a clear understanding of why it was such “a big deal.” many thanks!

  8. I’m still confused. If this was a oil
    tank, wasn’t there oil in the tank? If
    so, then how could air come up through
    the tank and get in the car? And if the
    tank was empty then wouldn’t the engine
    burn up due to lack of oil???? Please,
    someone straighten me up on this!

  9. Thanks for the question, Loren. I see where I wasn’t clear.
    The oil reservior itself (the dry sump system) is closed and pressurized. You are right: If oil were leaking, Carl would have had major engine problems.
    The oil tank cover is the sheet metal surrounding the oil tank on the top and sides. The top oil tank cover is the piece that apparently wasn’t where it was supposed to be in the post-race inspection. So there wasn’t a problem with the oil tank itself, but with the metal surrounding the oil tank.

  10. Okay, I got it. I found the answer to my own question. In case there are others wondering the same thing here is what I found out. The oil reservoir is incased inside a metal box. This box is sometimes referred to as the “oil tank”. There is no bottom on the “oil tank” due to the oil lines and drain plug access to the “oil reservoir” inside the “oil tank”. If the lid was partially or completely off then air from under the vehicle would be allowed to pass up through the box and then would escape through the windows and/or other openings in the car. This would result in less air under the vehicle and therefore more downforce giving the car an advantage due to better traction.

  11. I think much of the confusion is about terminology. A better name for the “oil tank” in this article would be “oil reservoir enclosure”. It’s nothing more that sheet metal formed to contain and shield the oil reservoir from the driver and the car’s interior. Leaving the cover off the enclosure opens a pathway for air pressure under the car to enter the car’s interior.

    Yes, I’d also like to know if the cover was found lying inside the car somewhere.

  12. If you look at any of the pictures of Carl doing his trademark back somersault that were taken from the right-hand side of the car, you can see the lid inside the car. It isn’t just a little off the the oil reservoir enclosure (I like Clark’s terminology).

  13. If the oil tank was inside the sheet metal your talking about it would block the air coming from under the car. You would have to remove the oil reservoir from the enclosure to get air to move from under the car to inside the car. Also you stated that air inside the causes drag and as it builds up there is no were for it go. You got air presure inside the car and under the car working agianst each other. I think its the old battle Nascar fighting Roush because his cars are to strong coming out of the hole this year.

  14. James: The oil tank enclosure is much larger than the oil tank reservoir. There is plenty of space around the oil tank reservior for air to flow. There are a number of articles (see my harmonic vibrations blog) where other teams claim to have done wind tunnel tests with the oil tank cover lid off and saw significant increases in downforce. As for whether NASCAR is picking on Roush, my perception from the time I’ve spent at the track is that NASCAR is busy enough that they simply don’t have time to wage coordinated campaigns against any one owner or manufacturer. This was such an obvious problem that if they hadn’t come down on the team, they would have spent the next two weeks answering complaints about why they were favoring Roush. Plus, this week, there would be 43 cars exhibiting extreme creativity in their choice of oil tank cover hardware. Mr. Osborne took a calculated risk and, vibrational harmonics or not, he lost.

  15. I guess its okay to cheat nationwide series because you only get 1/4 of the penalites. Good learning area before you get to cup series.???????

  16. This is just a Game with nascar they didnt do anything to there CHEVy Driver they can cheat and get away with a slap on the wrist.. ROUSH RACING DOES NOT CHEAT!!!! ONLY NASCAR DOES.. No respect!!!

  17. Thanks,

    Your site is great. If I can, I’d like to help. I’m a 3D designer using SolidWorks, AutoCad, Intellicad, and Truespace. If you need graphics, 3D models or exploded views of a subject, I might be able to help. I would need specs or samples of a given object but can give you accurate 3D sims, vids, or pics of it.

    Let me know if I can help.


  18. I understand that air comes up thru the reservoir box and allows air to escape from under the car, but after it enters the greenhouse area of the car, where does it go? The cars are built so aero-consious, wouldn’t the air would be trapped inside the car, creating a parachute effect against the rear window, thusly slowing the car down. And a solution to this would be to completely seal the box, with the box caulked where the lines come out.

  19. Actually the oil reservoir is NOT pressurized, it is vented to the atmosphere.
    Your comments referring to one hold down bolt being a “bad engineering design” and reference to “safety factors” are wrong. The bolt did not “fail”, it simply loosened to the point where it fell out, most likely due to some crew guy who failed to tighten it sufficiently. The design is such that only one fastener is required to retain the cover, maybe you don’t understand the concept of efficient design. Removing a single bolt to add oil during a pit stop is much more efficient than having to remove four.
    Assuming that Roush engineers are ignorant only proves that you are the ignorant party.

  20. Unfortunately we do not have access to the data from the impounded car, so we will not likely know the truth to the supposed advantage. The simplified discussion of physics involved is interesting from an academic perspective, but inconclusive as far as “cheating by design,” given all of the forces acting on the car. Bottom line is NASCAR is too complex a sport for consistant rules and regulations and all we can do is sit back and hope it all balances out and we get to root for more than 7 or 8 drivers that have a real chance of winning when the green flag drops.

  21. I have trouble believing that Roush would cheat knowing the severity of the penalties that NASCAR is handing out for messing with the COT. Messing with it is what kept Jr. out last year, wrong bracket holding the wing.

    This seems to be getting more attention than Cheatin’ Chad and Jimmy got when they won Dayton with their adjustable rear window.

  22. I think this will relate the same way but I would like to get your opinion of my theory to see if we are all understanding this the same way.
    As a dirt late model racer we design the bodies of the cars in a certain way because as the air flows over the car it creates a vacuum under the body making tremendous down force. Therefore as big as you can make the underneath of the car the more down force. In the case of the oil tank cover being open, that makes the underneath of the car physically bigger because it makes that vacuum under the car also a vacuum inside the car. Resulting in more down force over the total car because the total volume of vacuum is so much larger.

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