Aerodynamics is an extremely complicated subject, even for engineers, so it is not a surprise that many "rules of thumb" are adopted by some, and then defended with almost religious intensity against anyone who challenges the over-simplifications.
The goal of racing is to finish ahead of your competitors. This is accomplished by any combination of "factors": a) A faster car; b) A more skilled driver; c) A driver with very fast reflexes; and d) A car/driver able to maintain control in the event of unexpected or unpredictable on-track challenges.
Stockholm Seminar Slides
Katz Book @ Amazon: Race-Car-Aerodynamics
Also, I suggest just to "Google" the term "Katz Race Car Aerodynamics" for some papers he has written.
In iRacing, you will quickly find there are spirited debates on the "best" setup. Each car has it's own forum and "community" and each community seems to have one (or more) self appointed and usually well intentioned "guru" who promotes a certain "rules of thumb". For the most part, I find most of these rules of thumbs to be over-simplifications and often based on faulty engineering and racing principles.
For the most part, this discussion will be about cars running in the iRacing sim, but most of the principles apply as well to real life. There are some exceptions. iRacing is a racing simulation--a very sophisticated video game. Being successful in iRacing is somewhat related to being successful in video games. In real life--mistakes are very costly in terms of money and personal injury. In iRacing mistakes are pretty much remedied with a simple reset button.
A brief history:
As you can see, the widespread use of downforce began in the 1970's. Above is a "wingless" Lotus with essentially no downforce. (Those who promote the concept of "low downforce is always better" should be drive the Lotus 49--it is a real handfull and much slower than a comparable modern car with downforce.)
The first use of downforce was to use inverted wings--take an airplane wing and turn it upside down. From the photo below, it can be seen that even that did not keep the car on the pavement.
The development over the years has advanced to include spoilers to reduce lift, drag reducing bodywork and "undercar" effects as seen above.
In essence, these graphs shows the benefit from downforce:
Grip, or the amount of lateral force that can be transmitted to the car in the corner increases dramatically with the vertical force on the tire/s AND the slip angle--to a point. So anything that decreases lift or increases downforce increases grip in the corner.
In addition, anything that decreases lift or increases downforce also increases braking and acceleration forces that can be input through the tire.
So..more downforce gives shorting braking distances and faster deceleration, along with increases in lateral cornering forces and less wheel spin on acceleration. Faster corner approach, faster corner speed, and faster corner exit.
Of course, these enormous benefits come with a cost: DRAG. For the most part, increases in downforce produce increased drag--sort of like having a parachute attached.
The balancing act is this:
If there is a shortage of grip in the corners (too much sliding or a loss of control) then downforce will increase grip and increase speed POTENTIAL in mid corner. (The driver must utilize some of all of this potential speed.)
If the track requires hard braking, then downforce will allow the car to approach the corner with more speed and will allow it to accelerate out of the corner faster.
But...Increased drag will REDUCE the rate of acceleration and top speed.
For example, lets examine just part of the race track---the long straight along with the corner leading to the straight and the one immediately following the straight:
A) The car with high downforce will enter the straight with more speed.
B) The car with high downforce will accelerate more slowly and will achieve a lower top speed.
C) The car with high downforce will be able to brake later and harder at the end of the straight.
D) The car with high downforce will travel through the ending corner with more speed.
You will often hear, "With high downforce, you are a sitting duck--you will be passed on the straights". Well, that depends on how much more downforce you are running than your competitor.
The higher downforce car will be slower for a part, BUT NOT ALL, of the straight. Typically, the lower downforce car will gain significantly in the second half of the straight, up until the braking zone.
But, here is a comeback.
If the downforce produces sufficient benefits, the car with higher downforce may have built a sufficient gap that the speed advantage of the lower downforce car is insufficient to close the gap. (So often, it depends on whether there is a sufficiently long series of turns before the long straight to build a gap--think Atlanta vs Road America or Spa.)
In many cases, the passing process will include the braking zone, and here the high downforce car has the advantage. It is not a certainty that the lower downforce car can complete the pass.
And, a car with higher downforce, can mitigate the speed loss from drag, by using the draft of the car ahead, and then may be able to outbrake the car ahead and complete a pass in the braking zone.
When the last corner of the race is a high speed one, where downforce produces an advantage--like at Indy, I have seen the 2nd place car with higher downforce pop out of the draft, passing for the lead going into T3 and using the superior grip thru T4 to win the race.
But...a "logical" argument for low downforce, even for inexperienced drivers is that it is easier to go fast on the straights than it is to use the maximum corner grip that provides the major speed advantage from higher downforce. This argument assumes that inexperienced driver will brake earlier, drive slower in mid-corner, and get on throttle later than the more experienced drivers.
What I have noticed is that once the less experienced driver witnesses other cars pulling away in the corners, he/she will often push the car harder-invariably too hard---and will lose control.
Let''s review:
The goal of racing is to finish ahead of your competitors. This is accomplished by any combination of "factors": a) A faster car; b) A more skilled driver; c) A driver with very fast reflexes; and d) A car/driver able to maintain control in the event of unexpected or unpredictable on-track challenges.
a) There is always an "optimum" downforce that will produce the theoretical "fastest" car.
b) A skilled driver will be able to fully optimize the car's speed potential--however it is setup.
c) A driver with very fast reflexes will be able to drive a car with lower downforce.
d) Often, situations occur that require higher levels of grip than normal and anticipated.
The claim that a driver with very fast reflexes will be able to drive a car with lower downforce has to do with the fact that the road surface is uneven and the corner radius is not constant. Grip requirements change thru the braking zone and through the corner--requiring lots of changing driver inputs. A higher downforce will produce some "reserve" grip---not needed by the driver with lightning fast reflexes, but quite beneficial for the driver with slower reaction times. This holds true also in those unanticipated track events that invariably occur in racing traffic.
In conclusion: 1) Downforce settings are driver dependent--each driver may require or desire a different level; 2) Having a certain level of "reserve" grip from a higher downforce can be beneficial; 3) The lowest downforce nor even the theoretical "fastest" car will NOT win races with more frequency than a car with a reasonable level of "reserve" grip in the hands of a skilled driver who can "fully optimize the car's speed potential"--however it is setup.
A marginal excess of downforce, if not excessive, is not like "training wheels" for beginners. In fact, a marginal excess of downforce, while admittedly helping some less experienced drivers, will often provide the greatest benefit to the most experienced driver who has the knowledge and skill to optimize the car's speed potential.
My personal opinion is that the best or "optimal" setup is not the one that produces the fastest hot lap, but rather the setup that provides the fastest average speed over about ten laps, with the least variation or standard deviation from the average.
Note: Keep in mind that increasing downforce only helps if there is a shortage of grip. Here is a test:
Take iRacing's HPD Prototype, set it for LOW downforce and run it flat out at Indy. You never lift nor slide. (There is no shortage of grip.) Using MEDIUM downforce will result in lap times 4% slower. Then do the same test at The Milwaukee Mile track. Even though the MEDIUM downforce car will be slower on the straights, the lap times will be around 3% faster than the LOW downforce setup.
Even a modern race car with lots of downforce will fly 😎:
