The Flap over Roof Flaps

Why Roof Flaps?

Roof flaps (the invention of which I detail in my book The Physics of NASCAR) help keep cars on the ground.  This is necessary because of Bernoulli’s law, which says basically that:

  • Faster-moving air exerts less pressure.
  • Slower-moving air exerts more pressure.

A wing develops lift because the air flowing under the wing moves slower than the air going over the wing. That creates more pressure under the wing than over the wing, which generates a net force upward.  That’s a good thing for an airplane.  Not so good for a race car.

A NASCAR race car is pretty stable when airflow goes from the nose to the tail.

The problems start when a car turns sideways because a sideways racecar looks a little like a wing.  Air flows easily over the roof of a sideways racecar. It stays attached to the car’s surface for a long time, and that creates a low pressure region on the top of the car. A little air gets under the car and all of a sudden, the car is flying.

If this happened all the time, you’d engineer the car to prevent it from happening – except whatever you engineered would slow down the car all the time.  Since this is only a problem when a car rotates(i.e. yaws),  you need a solution that only becomes active when the car is yawed.

The idea is to get the air to slow down when it goes over the roof, which increases the pressure on the top of the car and decreases the lift.   As shown at left, the roof flaps are flaps of metal that are normally flush with the roof. When the pressure on the roof gets low enough, the pressure differential between the underside of the flap and the top of the flap causes the roof flap to pop up.  The pop-ed up roof flap slows the air going over the top of the car, increasing the pressure and keeping the car on the ground.

Cars have two roof flaps (which are more appropriately called “hinged air deflectors”).  One runs along a line from left to right and one is angled at 45 degrees to the first (See: original patent #5374098)

The roof flap is a one-piece assembly made of carbon fiber composite.  The tethers, which are the strings running through the bottom of the tray and through the flaps, are made of a superstrong polymer called Vectran that stops the flaps from relying entirely on their hinges to prevent being ripped off the car when they stand up at 190 mph.  It’s a purely mechanical, deceptively simple design that works pretty reliably and doesn’t have a lot of moving parts.

Gen-6 Roof Flaps

Roof flaps were re-designed for the Gen-6 car.  The old roof flaps were 8″ x 12″.  The new ones are closer to 10″ x 18″.  Larger roof flaps slow down more air molecules than smaller roof flaps.  They are lighter, deploy faster and they have little fabric parachutes to improve their ability to slow down the air running over the top of the car.   The photo below is from one of my favorite magazines, Circle Track – check out their article to learn about other details of the Gen-6 car.  Compare the first diagram and the photo below.  See how much further over to the side of the roof the roof flaps run?


What are Roof Flap Spacers?

Roof flap spacers are metal disks that sit in the tray and give the roof flaps something to rest on so that they remain level with the roof surface.  There are two spacers for each flap, making a total of eight spacers in the car.


Teams buy roof flap assemblies from Roush-Yates.  The picture below is again the COT assembly, and the picture is from the Roush-Yates catalog.  The assembly costs $1130.00.   The key word here is assembly.  They come assembled and the idea is that you  drop them into their hole in the roof of the car.  COTRoofFlapPicture

And you don’t mess with them in any way before you drop them into the car.

The whole ‘roof flap spacer-gate’ thing (see Dustin Long’s MRN article for a picture of the confiscated parts) appears to be a simple case of teams shaving down components to save weight at the top of the car.

I know!  Four small cylinders – how is that going to change anything?  Remember that teams are scrapping for any advantage they can find.   Here’s the physics:

  • The grip on each tire is proportional to how hard the tire is being pushed into the track.
  • The force pushing each tire into the track is a combination of mechanical downforce (i.e. the weight of the car) and aerodynamic  downforce (the force of the air pushing down on the car)
  • The force on each tire changes as the body rolls during turns.  Accelerating out of a left turn transfers the weight of the car toward the right rear, so you lose grip on your left front tire.
  • The amount of weight that shifts during a turn depends on the height of the car’s center of gravity.  The higher the center of gravity, the more weight shifts and the more change in grip you have on your tires.

Teams have been using carbon fiber for things like dashboards in an attempt to keep the center of gravity as low as possible.

Note:  @DGodfatherMoody reports that each of the spacers was lightened by about 3 oz.  There are 8 spacers on the two roof flaps, so you’ve got 24 oz or about a pound and a half lighter.  That’s significant!

What are the Penalties Going to Be?

This is a hard one to predict.

  • You can’t say that it didn’t given anyone a performance advantage because of the above argument.  Maybe not a huge performance advantage, but come on – if it wasn’t going to make the car faster, why would you try it?  No one seems to think that the modified spacers had anything to do with aerodynamics.
  • The rule book says “The hinged air deflectors must be NASCAR-approved and obtained only through NASCAR-approved sources. The hinged air deflectors must be installed as specified in the instruction sheet supplied with the hinged air deflector kit.”  …and I’m guessing the instruction sheet doesn’t tell you to shave weight off the spacers.  Or do anything with the spacers.  It’s going to be hard to argue that you were working in the grey area with this.
  • It’s a safety device.  NASCAR doesn’t take kindly to monkeying with safety devices — even when the modification doesn’t impact the function of the safety device.
  • NASCAR has been increasingly grumpy about people trying to skirt the rules.  Penalties have been escalating and what might have been tolerated at the start of the season might have the hammer come down at the midpoint.
  • NASCAR has had an unprecedented number of penalties reduced by the appeals panel this year.  The last couple appeals outcomes may impact their thinking on the magnitude of the penalties.

If it were me, it would be a roll of the eyes and a shake of the head and telling the crew chiefs that if we catch you doing it again, you’re going to be watching the next couple of races from in front of the television.

Move along folks… nothing here to see, I think.

Parts of this blog were adapted from a blog previously published on the now defunct on 4/26/2009.


  1. Would it be possible to see on this site a picture of a roof flap, with a blow-up of how they go together with the spacers? Thanks, Hiandra.

  2. My issue with this is that 31 teams blatantly disregarded the rules on a piece of safety equipment and got away with it scott free. Not cool in my book, Nascar.

  3. Michael: I agree with you that teams shouldn’t mess AT ALL with safety equipment. I think the teams realized they were not compromising the safety of the device. I suspect NASCAR just sort of threw up their hands trying to figure out how to deal with this widespread a violation and the fact that it had been going on for quite some time before they caught it. But I bet there were some interesting conversations with crew chiefs and owners warning them that this doesn’t mean NASCAR is not going to come down really hard if they mess with safety equipment in the future. I’m glad I’m not Robin Pemberton – it was really a no-win situation for NASCAR.

  4. Personally, I feel every team involved should have had a minimum penalty of 12 points and a $50,000 fine. Because this was deliberate manipulation of a safety device, I think Nascar screwed up on this one.

    The teams that did no cheat deserve for those who did cheat to be penalized.

    And speaking of safety, how can running over a tire changer’s air-hose or leaving a wrench in the car as it leaves pit road result in a pit road penalty, but hitting a member of the pit crew doesn’t?!? Again, Nascar speaking safety but not putting its money where its mouth is (cough cough SAFER barriers everywhere!).

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