Question about "drag".

Factual Questions
1

Now that plate racing at Daytona is in the air, could you explain something to me about “drag”.

One thing they always say is that “two cars travelling together can go faster than one”.

For one, they sometimes employ a bump draft, where the second car sitting in the draft can accelerate, bump the guy in front of him, and remain in the draft, going a little faster than before.

However, they can also go faster than one car just by staying together.

That indicates that it’s not just the amount of air the front car plowing through, bit also something happening with the upsetting of his drag by the car behind him.

Now, the second car alss has drag, but is it the same amount of drag as the first car would have? How does that interact with the decrease in resistance he’s feeling at the front of his car?

What inspired this question was a discussion at a biking forum (I’m “mth”) about whether adding a rider on the back of a paceline reduces the drag on the whole paceline.

defn: a paceline is basically a bunch of riders riding in a line to stay out of the draft. Clearly, longer pacelines are more efficient since you need to spend less time in the front, but are they also more efficient for this other reason, the “removal” of draft from the guy in front of you? It would clearly have a diminishing effect as more and more riders get on, but is that a valid statement?

2

I am not an aerodynamic engineer, but an explanation of airflow I once received really opened my eyes regarding how it can cause things like lift and drag.

Like any physical object, particles in a flow of air change direction only when a force is applied. But Newton’s third law tells us that for every action there is an equal and opposite reaction. Thus, a force that causes an airflow to deflect, say, upward will produce a reactive force that pushes the deflecting object downward.

In the case of an automobile moving thru the air, drag comes from two sources. First, obviously, the car pushes the airflow up over the windshield; the car pushes the airflow up and forward to bend it, so the reactive force pushes on the car in a down and backward direction.

The second source of drag is more subtle; after the air passes over the roof of the car, it bends back downward over the trunk and taillights. Note that the air curves over the rear of the car, so it must be pulled both down and forward (a drawing would help here; it’s bent forward by the usual tendency for the airflow to cling to a surface rather than leave an open area of lower pressure behind the car). The reactive force, in the opposite direction, pulls the car up (lift on the rear wheels) and backward, slowing the car down.

With two cars driving in a line, the airflow never curls down the tail of the first car before reaching the second; it more or less passes from one car roof the the next. The first car then does not suffer the drag caused by the airflow slipping down the rear of the car, while the second car benefits from avoiding the first source of drag from plowing “clean” air up and out of the way.

Theoretically, then, the two cars suffer as much drag as, say, a limo of similar length, but remember there are two separate engines in the racecar case, while the limo must make do with only one.

3

If there was ever a thread tailor-made for an Aerospace Engineer racing fanatic! Anyway, when two cars are drafting, the front car blocks the free airstream from impinging on the front of the drafting vehicle, so the drag is less. Also, the trailing car fills in the area behind the lead car, which reduces the vortex behind the lead car and the drag is less for the leader as well, but not as much as for the trailing car.

Drafting also effects downforce and handling. Sometimes you’ll hear drivers comment about how their car handles a certain way in the draft, but a different way by itself. Since the air is no longer pushing on the front of the trailing car, front downforce is reduced and the car will start to understeer (or “push”). That is, the car will not want to turn as well. For the lead car, the effect is to lift the airstream off of the spoiler and cause the car to oversteer (also called “loose”). The back of the car will want to snap around and spin the car in a turn. (Drivers sometimes take advantage of this by getting the lead car a little loose, forcing the driver to check up a bit.)

The net result is that two cars running together on a high-speed oval (such as Daytona, for example) will lap faster than a single car running alone. If there is a group of cars drafting in line and a car steps out of the draft, it will generally fall back until it can get back in line. Interestingly, three cars will be a little bit faster than two, and four a bit faster than three, beyond which you get diminishing returns (the reasons are a bit complicated for this post, but it has to do with the ratio of overall length to cross sectional area getting longer). This is why you’ll often see a lead pack of three or four cars breaking away from the rest of the group.

Sometimes a car can also use the draft to slingshot around another car. Basically, it is taking advantage of the reduced drag to run up on the lead car and build up enough momentum to pull out and pass the lead car before the increased drag of running in the full airstream slows it back down to it’s normal speed. This requires careful timing to pull off successfully.

Then there is bump drafting, where the trailing car will actually run up and bump the lead car. This will give the leader an additional 1-2 mph boost and pull the trailing car along with it. (The danger is all this bumping might close up your air inlets and cause the car to overheat).

All of this assumes you’re talking about NASCAR-style cars. For high-downforce cars, such as a Formula 1 or Indy car, the turbulence and reduced downforce upset the trailing car so much, they generally don’t form the drafting packs you see in stock car racing. Usually they use drafting (which is generally called slipstreaming in European racing) just for slingshot passes.

Also, you don’t have to be at race-car speeds to experience drafting. Bicycles can draft as you’ve noted (which is why these races tend to form a large peloton). I race in triathlons and during the swim (which is a mass start) you can even draft off of other swimmers.

(On preview, I notice CJJ* beat me to it, but since my post has some information his doesn’t, I’ll post anyway.)

4

Thanks guys. That’s what I was looking for (been away from the computer for a few days, including a bicycle race myself.)

I only got to watch half of Daytona, but the Busch race was great.