Aerodynamic Downforce: A Passing Fad?

The question of why it is so difficult for cars to pass each other at 1.5 mile and 2 mile tracks is getting more and more attention.  Carl Edwards put it succinctly:

“I firmly believe, and NASCAR hates it when I say this, that we should not be racing with downforce, sideforce and all these aerodynamic devices.  We do not need splitters on the race cars and giant spoilers.  I have not been around long enough to say something definitely, but it is pretty common sense: if all the cars are very similar and all the drivers are within a tenth of a second of each other but are relying on clean air and downforce, then by definition if the guy in front of you is disturbing the air then your car is not going to be able to go as fast as it could in clean air.”

Other drivers disagree — Jimmie Johnson, for example, believes that changes must be made to the track to enable passing.  But let’s look for a moment at the aero/mechanical force question.

Grip (aka static friction) is determined by two factors:  how well the tire grabs the track and how hard the tires are pushed into the track.  Goodyear is entirely responsible for the first factor given that everyone runs the same tires and teams are not allowed to modify those tires in any way.

There are two components to the second factor: mechanical grip and aerogrip.  Mechanical grip is easy:  that’s the weight of the car.  Given the minimum weight of 3450 lbs and a Jeff Gorden-sized driver, you’ve got a total of 3600 lbs of weight pushing the four tires into the ground.  How that weight is distributed between the tires is a topic all to itself, plus the weight distribution changes as the car accelerates, brakes and/or turns.  Regardless of where the weight pushes, there is never any more force that the total weight of the car.  (You can only go as fast as your least grippy tire, so getting that weight evenly distributed is something engineers spend a huge amount of time trying to do.)

The other factor is aerodynamic force.  When air molecules hit a surface, they push down on the surface.  Although each molecule is very tiny, the billions and billions of molecules hitting a car’s surface (especially at high speed) can create thousands of pounds of additional downforce on the car.

Aerodynamic downforce varies with speed squared.  If you double your speed, you quadruple the aerodynamic downforce.  A car that has 1000 lbs of aerodynamics downforce at 90 mph will have 4000 lbs of aerodynamic downforce at 180 mph.  This is why aerodynamics are less important at tracks like Martinsville where the speeds are lower, and incredibly important at tracks like Texas, Michigan, Atlanta, etc.

Mechanical grip doesn’t change much when you drive around other cars – but aerogrip does.  Aerodynamic downforce changes depending on how the air flows over the car.  A nice smooth laminar flow is like sheets of air tracing the car’s contour as they flow.  That’s “clean air”.  The opposite is “dirty air”, which means that the air is turbulent or being diverted in some undesirable way.

So imagine that you are running a couple car lengths behind the leader.  You both have comparable cars, with (say) half of your total grip due to aerodynamics and half being mechanical.  As you get closer to the car you’re trying to overtake, the air coming off the rear of that car disturbs the air flowing over your car.  You feel the front end grip decrease and you know the television commentators are telling the audience that you’re “aeroloose”.  You’ve got no choice but to back off and get the air flowing back over your car.  Or you could just keep going and spin yourself out.

Let’s look at an alternative:  say that aerodynamic force is only a very small fraction of the total force.  Getting close to another car decreases your aero downforce, but if you’ve got a better setup on your car, the improved mechanical grip might just be enough to compensate and you could then be able to pass.

The spoiler and the splitter create a pressure differential.  The splitter “splits” air by forcing it to go around the car.  The pressure on top of the splitter is higher than the pressure on the bottom and thus the net force is downward.  The spoiler provides a surface for the air coming over the top of the car to hit and thus provides rear downforce.  A shorter spoiler provides less area and thus less overall force.

So can we follow Carl Edwards’ advice and just get rid of the aerodynamic devices?  Sure – if you’re willing to have the cars be slower.  The fewer aerodynamic devices, the slower the cars are going to be.  It’s a question of scale.  You have to decrease the aerodynamics enough so that they don’t dominate, but not so much that we’re falling asleep during the six hours it takes to run 400 miles at a 1.5 mile track.

NASCAR’s recent change to skirt heights is an example of what I believe is an attempt to make minor changes to the aerodynamics of the car without requiring teams to make a major reboot on a car that is in its last year of  existence.  Robin Pemberton said that the recent changes were informed by the research being done on the 2013 car in terms of both competition and safety.

Keeping the skirts higher off the ground has two potential impacts:  one is that it can help air get out from under the car in case of a spin.  Air underneath a car pushes upward, which decreases downforce and – in extreme cases – can cause the car to leave the ground.  The higher skirts will give the air a plan to escape when the car spins.  Prior to the change, skirts had to be 3 – 4.5 inches off the ground (measured when the car is sitting still during tech).  The new rules require the right side skirts to be 4 to 4.5 inches off the ground and the left-side skirts to be 4.5 to 5 inches rm the ground.

The other consequence of this change is that it will be harder for teams to keep the car sealed to the ground.  Air flowing under the car will push upward, which decreases grip.  Even though the spoiler and splitter will not change, the net force pushing down will decrease.