Understanding the Fundamentals of Drafting

Features, Racing I By Ross Bentley I September 23, 2015
A subscriber posed this query about drafting: "Obviously, drafting is super important in NASCAR, but what about in road racing at the amateur club level?”I asked John Block to answer the question and share his thoughts in this week’s feature article. John’s perspective is from his experience as a highly-successful race engineer, a driver, and coach.Before you close up on the bumper of the car in front of you, learn more about drafting from John’s article. -Ross
I would love to jump right in with long-winded explanations of fluid mechanics and all the obligatory equations. But Speed Secrets Weekly is more of a driver’s forum, so I will try to keep the engineering geek speak to a minimum. However, I will use this opportunity to remind readers that racing and obtaining performance is a process, not a recipe. If it were a recipe, the answers to the drafting questions would be simple, like "47.835 mph," and "184.35 ft." Alas, it’s just not that easy.
First, from a driving stand point, yes, drafting is important in most NASCAR races at intermediate speedways and super speedways, but not at places like Martinsville (1/2 mile oval). The main difference on these tracks is speed. 
Basically, for any racer, the faster you go, the more you can employ drafting.
Amateur club level racing on road courses is a bit different from NASCAR, however. Typically club racing "rules of the road" are to give racing room and avoid contact. I would caution getting too focused on drafting as it could very well lead to target fixation (staring at the back of the leading car) and end up in violating these rules or even worse. But I’m sure that’s not what people want to hear, and most of you want to use drafting to your advantage, so hang on – here comes some engineering info.
The whole reason we even care about drafting is that we do not have an infinite amount of power. This means we operate in what is known as "power limited acceleration." We could spend more money to make more power to go faster or we could do something to reduce the forces holding us back. Enter aerodynamic drag: one of the forces that limits our acceleration and the reason drafting works.
Look Ma, no algebra! Well, not exactly. If we examine the basic aero drag equation, what jumps out to me (as an engineer) is that the vehicle speed, in the math, is an exponential factor. Other factors like drag coefficient, frontal area, etc. are straight multipliers in that equation. This means for a given speed, a 10% increase or decrease in frontal area or drag coefficient results in a matching 10% increase or decrease in overall drag. However, the increment of drag increase from a 10% increase in speed can be way more than the reduction of drag by slowing down 10%. The point here is that when it comes to all things aero, speed is paramount.
For a club road racer, this translates to "the longer the straightaway, the more advantage there could be for drafting." Well, so much for Captain Obvious, but that brings us back to the question "At what speed does drafting start to work and how close do we need to be to get that tow?"
Drafting is a tricky business from a scientific standpoint. Most all cars have counter-rotating "Horse Shoe" vortexes shedding from the back of the car. Winged cars can throw in some horizontal "Von Karman" vortexes from the top of the wing and before you know it, the wake behind the car is a whirling dervish of fluid mechanics. This is one reason identifying the distance behind the next car to pick up a draft is a tricky one. So, the standard engineering weasel words answer is, "It depends."
At the professional level, I have done a lot of "wake management" work to make our car less advantageous to draft than other cars (both in open wheel and stock cars). This is the case when car A is a better follower than car B in a symbiotic tandem drafting situation, but when A is in front of B (the other way around), the trailing car cannot hang on to the lead car and it slowly pulls away, to a point.
From an engineering perspective, the multi-vehicle interaction is very complicated with all the vortexes in the wake. Add in changes to high pressure stagnation points and the conversation quickly turns into an engineering geek-speak-fest talking about coefficient profiles, the changes at various car lengths between vehicles, and the ensuing passing of the lead car. Bottom line is: different cars have different wakes impacting how the draft works.
A club racer should be aware of mixed classes on the track at the same time. Hopefully, you are on the track with cars of roughly the same size, shape, and power (just to keep everybody involved safe), but if by some odd chance you find yourself in a sedan and spot a Formula Vee up ahead, don’t even think about drafting!
Back to the question of "At what speed and how close do you need to be to get that tow?" Maybe now it makes a little more sense when the answer is "It depends." Bigger, draggy cars with less power will notice a draft at a lower speed than smaller slippery cars with gobs of HP. One car may start to pick up a draft at 60 to 70mph or less where another may not pick it up until 90, 100 or more. Likewise, how close to the leading car you need to be for it to work is also a function of the speed. The faster you go, the longer the wake holds its form like a "Separation Bubble" and the further back you can pick up the draft. At Daytona, I had one NASCAR driver tell me that if he could see the car up ahead he could feel the draft. I think he was kidding, but what happens at 200mph is amazing. On the other hand, I had an IRL driver tell me that he had to enter the draft at a certain angle or he thought it hurt him. Maybe in his case it didn’t hurt, it just did not help as much.
Let’s wrap this up by addressing the "What is the best way to draft?" question. When I’m not wearing the engineer’s hat, but wearing the coaching hat, I try to get drivers to recognize that there are several points ahead of the car while on the track. First is the control point. This is the distance ahead of the car at which if a control input (steering or whatever) is executed, the car will respond and/or react when you reach that point on the track. The control point also moves further out the faster you go.
Next is your mental focal point ahead of the car, not just visual, but mental. If at any time your focal point is between the control point and the front of car for more than a nanosecond, you are in danger of something unpleasant happening and that’s what I call the Danger Focal Zone. Important point: your focal point needs to be as far down the track as possible!
During drafting, as you come up to the leading car, it will move into that Danger Focal Zone. Here, the lead car is taking up a good portion of your field of vision and can become a distraction. You may have a tendency to focus on the car ahead of you (target fixation as mentioned earlier) rather than beyond your control point. In this case, drivers have been known to follow another car straight off the track or straight into a wreck.
This brings up the need for OODA loop strategy (the decision cycle of observe, orient, decide, and act, developed by military strategist and USAF Colonel John Boyd) when drafting and other driver coaching facets beyond the scope of the drafting question, but this may be a good subject for future story.

On a final note, when drafting, you should look through, around, or visualize through the lead car to stay on that all-important focal point.

– John Block
Twitter: @Autowareinc 

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