Aerodynamics »

Daytona Testing Notes: The Magic 200 MPH Mark — Not

Aerodynamics, Daytona, Engines No Responses »

Jan 132012

There is absolutely nothing magic about the 200-mph mark.

People have been treating the 200-mph number like it was handed down by a sacred oracle.

First off, a series of factors are required to make a car go airborne. ONE of them is high speed. Another is the car getting turned around at just the right angle. It’s not like the minute a car goes faster than 200 mph, it is in imminent danger of becoming airborne. The higher the speed, the higher the probability the car can leave the ground — IF other factors are also present.

Secondly, today’s car has very different aerodynamics than previous versions of the car. NASCAR apparently feels confident that the 202-205 mph range does not raise the probability of a car becoming airborne significantly. John Darby specifically said that NASCAR had wind tunnel testing data that led them to this conclusion. NASCAR believes that the slight increased risk is small relative to other benefits (the most significant of which appear to be saving engine builders/tuners from having heart palpitations due to the engines turning very high sustained rpms).

If you can’t let go of thinking about 200 mph holding some mystical power, remember that 200 mph is really just 321.9 kilometers per hour.

Doesn’t sound so magical that way, does it?

turning left, shifting right: why drivers move to the right to get air to the engine

Aerodynamics, Cooling, Daytona, Engines 5 Responses »

Feb 222011

Jack asks:

I’m curious as to why the rear cars are offsetting to the right, when offsetting to the left would let the rear driver see what is happening ahead of them and keep the radiator in cooler air, since the exhaust on these cars is on the right. I know that all those drivers and crew chiefs are smarter than I am, so I must be missing something.

Thanks for the question, Jack. Give yourself a little more credit: you bring up some really good points that I bet a lot of people didn’t see.

Drafting at Daytona has become more important than ever, with the two-car draft being the most effective means of getting speed. The problem is that this mode of drafting completely blocks the front grille, and that limits how much air gets in to cool the engine. The trailing car has to back off to let air into the grill when the engine gets warm.

Jack noticed that everyone was shifting to the right. I think it’s a matter of simple geometry and the fact that NASCAR is chiral. Chiral means simply that something twists one way. All of your DNA twists in one direction. NASCAR drivers turn (with two exceptions a year) ~~right~~ left. (Note: Thanks to the commenter. What WAS I thinking there?)

Below, I’ve drawn two cars trailing each other in line on the left, the trailing car shifted to the right (middle) and the trailing car shifted to the left (right).

When cars turn left, a natural gap opens up on the right-hand side between the cars. Moving to the right takes advantage of the gap and makes it slightly larger. If the trailing car moves to the left, I don’t think it’s going to get as much air. So despite the possibility of being able to see better, going to the left doesn’t look as effective to me as shifting to the right is if the goal is to get the most air into the engine.

Thanks for asking the question, Jack! I always read the comments, so if you have a question you’d like answered, please leave it in the comments for me.

Why Two and Not Three Cars in a Draft?

Aerodynamics, Daytona, Drag 13 Responses »

Feb 132011

The most talked-about feature of the racing at the ‘new’ Daytona is the two-car hookup. Just in time for Valentine’s Day, drivers are finding that the term ‘drafting partner’ is more accurate than ever before. Why two and not three-, four- and larger packs that used to be characteristic of Daytona?

Drafting 101

Anytime you move forward, you are working against something. To walk through a swimming pool, you have to push water molecules out of your way. To drive through air, you have to push the air molecules out of the way. The faster you go, the more air molecules you have to push out of the way in a given time.

Aerodynamic Forces on Cars

I’m going to focus on just the forces acting along the length of the car, ignoring sideforces. The key to my drawings is that the length of the arrows and their color indicates speed. Long green arrows indicate fast moving air, while red short arrows indicate slow moving, denser air. Some air gets under the car, while most of it goes up and around.

We are interested in two primary features: The front of the car acts like a wedge, pushing air out of the car’s way. The air molecules resist this motion, creating a force that pushes in the direction opposite the car’s motion. As the air passes over the car, it becomes turbulent at the back end, creating a partial vacuum at the rear of the car. The physical phenomena at the front and the rear of the car are different, but they have the same effect: they slow the car down. We can get rid of the little arrows and just represent the force of the air as arrows pushing against the front of the car and pulling backward on the rear of the car.

If the two cars are far apart, each car experiences forces on the front and the rear of the car. When they get close enough to each other, they appear as essentially a single object. The trailing car is traveling in the aerodynamic shadow of the first car, so it doesn’t get the huge blast of air on its hood. The trailing car prevents the air from getting as turbulent at the rear of the first car, so the force sucking back the first car is reduced. (To learn more, or at least see much better drawings than mine, see the Science of SPEED video.)

Drafting 102

Every television program explains the very basic aspects of drafting, but we need to go a little deeper to understand what’s different now.

The most important change in the car has been the improved match up between the rear and front bumpers with the new car. To get the maximum benefit from drafting, you really want the two objects to look like one, which means that they need to be as closely matched as possible. Compare the diagrams to the right. In the top diagram, two different shaped objects will have turbulence between them because of the height difference. (If the heights were reversed, there would be extra front drag.)

The second diagram shows two shapes that are the same size, but not very close to each other. They are so far apart that both experience the front drag and the rear turbulence.

The lowest diagram shows two objects of the same size fit right up against each other. The air travels over the two objects as if they were one. The better the back end of the first car and the front end of the rear car fit, the more of an advantage you will get from drafting. I apologize for my terribly drawing. Graphics has never been one of my strong points.

With the old car, cars could usually add 5-10 mph by drafting. We’re seeing much larger increases now – qualifying speeds are running 185 mph, while we’re seeing 205+ mph in the draft. This tells me that the cars fit together aerodynamically significantly better than the old cars did.

Why the Two-Car Draft Works Better than Before

The new car was introduced in 2007 and although the splitter has changed, that’s likely not the big effect. Drivers report that the repaving has really changed the character of the track. It’s got more grip, but the biggest effect is probably the smoothness. The key to good drafting is maintaining the relative positions of the two cars: they have to be close. Now the third dimension becomes important. The figure at left is meant to be a top view of two cars driving to the right, so we’re looking at the path of air around the sides of the cars.

On the top figure, the two cars are in perfect alignment, so the air can flow past them smoothly. If the trailing car stays the same distance behind the leading car, but slips slightly to the right, you’ve introduced edges. The top part of the trailing car (in red) now is having to push air aside. On the right sides of the cars, the misalignment of the trailing car means that there is a rear edge, and that means turbulence.

The old Daytona was bumpy. Those bumps made the cars move up and down relative to each other (which would decrease drafting effects). The bumpiness also made it harder for the drivers to control the cars, which made it more difficult to keep the cars aligned and close to each other. The new, smoother track seems to allow the drivers to keep the cars tucked up. The pull of the draft is so significant that we’re hearing drivers say that they have to ride the brakes. This is mostly unheard of -usually, crew chiefs have to remind the drivers to pump the brakes before hitting pit road because the brakes get very cold since the only place they were used was on pit road. Jaime McMurray had a brake rotor fail during practice and trashed his primary car when he blew a tire running over a piece of the broken rotor. That’s a surprising thing to happen at Daytona.

Hot Engines

One piece of evidence supporting the hypothesis that the cars are staying together better is rising engine temperatures. Air hitting the front of the car does produce drag; however, it also provides the air that goes into the car and cools the water in the radiator. If you draft too long, the trailing car’s engine starts to overheat. If you move to the side to try to expose the intake vents, you increase the drag and decrease the effectiveness of drafting.

One rumor is that NASCAR is going to require pop-off valves that would decrease the maximum temperatures the engines could reach. (How that works is a separate article.) This would decrease how long two cars could draft before they would have to separate or switch positions so that their engines didn’t overheat.

Why Two and Not Three

Go get three oranges from the kitchen. Try to juggle two of them. Not super easy, but not impossible. Now juggle three.

The reason we’re seeing two-car drafts and not three is that it is very hard just to keep two cars in position with each other without hitting each other or overheating. You’re asking the drivers to keep their minds focused on a lot of things, all while driving 200+ mph.

When two cars hook up, they take off. A third car would have to be right there in position, ready to latch on. All three drivers would have to focus on keeping the pack together. That’s far different than the old version of drafting, where becoming and staying part of the pack was easy… and fast. It’s additionally complicated because the old version drafting didn’t require the trailing driver to use his brakes. We’re seeing a lot more sudden drafting breakups as drivers realize they are overheating. Do you want to try to get precisely positioned behind someone at 200 mph who is dragging his brakes? The probability of getting two things to function together precisely is low. Add a third and it becomes very, very difficult to do.

The Fix?

Speeds reached 206 mph during the Shootout, which makes aerodynamicists nervous because a sideways racecar going 200+ mph has a strong proclivity to unexpectedly start doing an airplane impersonation. The usual head-first, tail-last position is just fine at high speeds – there is no reason that a stockcar can’t race at 230 mph or more; however, if the car gets turned sideways at that speed, it can become airborne, even when its roof flaps deploy. There’s not a magic “take-off” speed below which it is safe because it’s a combination of the speed and the angle the car makes with the direction it is traveling. We would be fine racing at 210 mph, provided that no one gets turned sideways. The consequences are uncomfortably large if a car does get airborne. Everyone remembers what happened when Carl Edwards got airborne at Talladega and no one wants to take a chance on that happening next Sunday. Most prognosticators are predicting that NASCAR will make a change after qualifying.

One quick fix (which has been used before) is to decrease the restrictor-plate size. This probably isn’t practical because the change in size to compensate for the higher speeds would have to be larger than NASCAR would prefer to make. The engines are tuned to work with a specific plate size, and changing the plate significantly could disproportionately affect one engine shop relative to others. This change would address the speeds, but it wouldn’t do anything about having only two-car drafts, which seems to be a problem if you believe twitter to be a representative sample.

The fairly simple fix is to limit the time two cars can be hooked up by making it easier for the radiator to overheat. If you force the radiator to start leaking steam at a lower temperature, the drivers can’t draft in pairs for as long as they can now. This is pretty simple to implement and the primary consequence will be sleepless nights for the engine tuners.

Mother Nature will help as well: the race will be during the day and temperatures will be higher, so there won’t be as much grip on the track. That should slow down the speeds as well, but it won’t change the preference for two-car drafting.

Etc.

Did you catch what Craig Ferguson said about NASCAR drivers and their understanding of science on the Late, Late Show? It’s in the first third of this clip.

Overpressured Shocks on the 5-Car: How Does that Create an Advantage?

Aerodynamics, Dover International Speedway, Downforce, Inspection, Rules, Shocks, Springs, Suspension 5 Responses »

Sep 272010

The 5 car got sent to the back for the start of the race last Sunday at Dover after qualifying third when their shocks didn’t clear post-qualifying inspection. Shocks and springs work together to control the rate at which the body of the car moves. The ideal attitude is the hound dog position: nose down, tail up, as demonstrated in the photo at right by my capable assistant Darwin. That position prevents air from getting underneath the car and it sticks the spoiler up in the air as much as possible, which means that more air hits it and creates more downforce.

A spring – one weapon in the setup arsenal - exerts a force proportional to the distance it is compressed. A 200-lb-per-inch spring will compress by one inch when a force of 200 pounds pushes down on it, 2 inches when a force of 400 pounds pushes down on it, etc. You can add spring rubbers to change the rate, but springs are pretty much straightforward tools.

Shocks (the exterior of on being shown at left) are a little more subtle: They exert force proportional to speed, not the distance. Shocks are great fun to play with: they have everything one likes in a mechanical device – lots of tiny little parts that can be put together in myriad ways, adjustments, oil and nitrogen gas. If you push them slowly, they present some resistance. If you try to push them quickly, they provide more resistance. Shocks make a good complement to springs in terms of damping out some of the energy from the bouncing, and for a crew chief, they also provide the ability to tune how the weight of the car shifts during acceleration, braking and turning.

Each team has a shock specialist who works with springs and shocks. A spring is a relatively simple device: it’s a metal rod that’s been twisted into a coil. A shock is much more complex, which means you can tailor the shock much more subtly than you can a spring. The crew chief will specify what type of behavior he or she wants and the shock specialist’s job is to figure out how to build the shock so that it has that specific behavior. Shock specialists are an interesting breed. Most shops have a display where they show which of their shocks were selected by NASCAR for disassembly. It’s something of an honor. The dissected shocks are displayed for everyone in the garage to see.

Although you are only allowed to use a specific set of discs and shims, the number of ways you can combine them is huge, so shocks represent a real degree of freedom for the teams to exploit to gain an advantage. Although the box that NASCAR allows the team to work within is small, this is an area where a knowledgeable person can make a huge difference for his driver.

From the outside, a shock has a body (shown mostly in brass on the drawing at left) and a shaft (the long silvery part extending downward. Many racing shocks are made to be rebuilt – the body unscrews. I’ve diagrammed the inside of the shock at right. The resistance of the shock to your trying to pull or push on it is because there’s a piston and series of shims that resist moving through the oil the shock is filled with. The piston is a disk with holes in it. The shims (the thinner disks that sit on either side of the piston) flex and bend depending of how fast you try to move the shock. The flexing allows the oil to move through the holes in the piston. You tune the shock by selecting a piston and shim stacks (one on either side) that determine how easily the shaft moves. (That’s the macroscopic view – there are a lot of subtleties that I’m glossing over here).

A shock has two motions: compression, which is pressing down on (or shortening) the shock and rebound, which is elongating it. You can tune the shock so that it behaves differently on rebound and compression by selecting the appropriate piston and shim stack – you’d like the car to resist compression and return to its fully extended position as fast as possible.

In addition to the piston attached to the shaft, there is a floating piston. That’s a solid disc with o-rings that seal it to the cylindrical sides of the shock. If you move the shaft through oil at atmospheric pressure, you incorporate air into the oil. Very tiny bubbles form, which change the oil’s viscosity. To ensure that you only have oil in the area where the shaft moves, the floating piston separates the oil. To keep it pushing on the oil, nitrogen gas fills the space above the floating piston. The gas is usually filled to a pressure above atmospheric pressure, which pressurizes the oil and creates a force pushing the shock shaft out. If you compress a shock and then let go of it, the shaft will extend all by itself. Not quickly, but over a few minutes. The higher the pressure, the harder it is to compress and the faster it extends.

NASCAR sets a maximum shock pressure. A closed container under high pressure is called a bomb. You’re just asking for a seal to blow, a piston to fail and you’ve got a catastrophic failure. Second, if you let the teams pressurize the shocks as much as they wanted, the rear shocks would essentially be non-moving. That’s not a great situation: it puts additional stress on the tires, makes for a very rough ride, and makes the car more challenge to keep under control when it hits a bump. An over-pressured shock will keep the spoiler up on the air, giving the car an aerodynamic and a mechanical advantage over the other cars.

At New Hampshire, a couple of cars didn’t pass the height sticks in post-race tech inspection. The teams were allowed to push the cars back around the garage and then the cars passed. Shocks respond differently when they get hot than they do when they are cold. If a team doesn’t pass something like the height sticks the first time, NASCAR lets them wait awhile for the car to cool down and settle. It’s much like how the recommended pressure of the tires on your car is specified when the car hasn’t been driven for awhile – the tires need to be cool. If you fill to the recommended pressure when the tires are how, the tires are going to be underinflated.

It’s very easy to overpressure a shock – gauges may differ, or the shock specialist might be rushing to respond to a last-minute request from the crew chief and miss the target pressure.

33 car penalties

Aerodynamics, Clint Bowyer, New Hampshire, Racing, Richmond, Rules, The Chase 7 Responses »

Sep 222010

After being a non-event (The 33 car from Richmond was “just barely legal” and NASCAR was checking with RCR to make sure they didn’t have a mistake on their build sheet) for a couple of days, the situation changed today when a 150-point, $150,000, 6-week crew chief/car chief suspension was announced based on violations from the New Hampshire car.

The primary part of the penalty (meaning the part besides actions detrimental to stock car racing) was

33 car body location specifications in reference to certified chassis did not meet #NASCAR-approved specs

The chassis is the tube frame that makes up the skeleton of the car. NASCAR specifies the chassis down to the exact size tube, wall thicknesses of the tube, and precise location. Prior to hanging the body, each chassis must be taken to the NASCAR R&D Center to be verified, which is done using a Romer arm. The chassis is then tagged with RFID tags that are scanned at the racktrack to ensure that no changes have been made.

After the chassis is certified, the team can hang the body. Instead of making measurements of the body directly, certain points are called out by their position relative to spots on the chassis. The violation was in the position of the bodywork, which suggests that any advantage that may have been gained was aerodynamic. A violation of the chassis might have been dealt with even more severely because the chassis is the primary protection mechanism for the driver. RCR confirmed in their statement that the body was too high relative to the chassis – something that one could argue might have provided an aerodynamic advantage by putting the spoiler up further in the air.

If this is the same issue that their Richmond car received such intense scrutiny for, it makes sense. Cars are built weeks ahead of time, as the hauler has to get them to the race track Thursday night, so it is likely that the body was already hung on the New Hampshire car by the time NASCAR told RCR that there was an issue with the Richmond car.

That moves Bowyer from first to last in the Chase, and leaves him without two very important members of his crew for the next six weeks.

UPDATE: RCR reveals that the out-of-tolerance measurement was 60 thousandths (0.06) of an inch. For reference, a sheet of paper is 0.004 inches, so 60 thousandths of an inch would be 15 sheets of paper. They also will argue in their appeal that the problem was created by the tow truck driver who pushed the #33 to Victory Lane after it run out of gas.

@bobpockrass reports that the general tolerance on the measurement in question is 70 thou, which means that the car was 0.13 inches off – more than an eighth of an inch. If you’re trying to be ‘just legal’, you don’t miss by 1/8″.

Check back: More as it develops.

Great post from Dustin Long on the inspection process.

An earlier post from me (on the old stockcarscience.com site) about the tolerances and Hendrick Motorsports’ situation last year.

Qualifying Trim vs. Race Trim

Drag, Engines, Glossary, Oil, Qualifying Trim, Race Trim, Racing 3 Responses »

Jul 092010

Thanks to Woogaroo for the suggestion of doing something on race trim vs. qualifying trim – two words you hear thrown around a lot, but often without a lot of explanation. I’ve embedded the video, but just in case, here’s the direct link to the YouTube version.

Something in racing you’re wondering about? Send questions to admin(at)buildingspeed.org.

The ABCs of Flying Cars: Aerodynamics, Brad and Carl

Atlanta Motor Speedway, Downforce, Drivers, Edwards Carl, Keselowski Brad, Roof Flaps, Talladega 2 Responses »

Mar 182010

I was frantically trying to finish this video blog this morning and still make my plane to Florida, where I’m looking forward to covering the 12 hours of Sebring American Le Mans Series (presented by Patron Tequila) race. You’ll have to excuse the glitches in the video editing while I am figuring out this new mode of communicating!

Eight Issues NASCAR Needs to Address in 2010

Aerodynamics, Greenness, journalism, News, Non-Dairy Coffee Creamer, The Chase, Tires No Responses »

Jan 042010

Because NASCAR likes nothing better than unsolicited suggestions, right?

If I could change just one thing about NASCAR during the off season, it would be banning people from calling into Sirius radio talk shows and suggesting versions of The Chase that rival the BCS and string theory for complexity. If you want to know what NASCAR might ever consider changing, check out the patent NASCAR holds on The Chase (patent number 7,207,568 entitled “Method of Conducting a Racing Series”).

I’m especially tired of whining about The Chase format when there are much more significant things to be addressed. Let’s talk about the state of motorsports journalism, for example. A number of excellent newspaper sports writers have been laid off in the last two years. Newspapers can’t afford to have dedicated motorsports coverage, you say? Apparently neither can NASCAR Scene, which laid off a significant fraction of their writing and editorial staff just today. My sympathies are with the folks who lost their jobs today. Some have been with the magazine literally their entire careers and some very recently moved from good situations to take what they thought was the ‘job of a lifetime’. I guess NASCAR fans are going to have to start getting the majority of their news from the NASCAR Citizen Journalists Media Corps.

All aspects of racing are facing the prospect of change, including the concept of racing itself. At the World Motorsport Symposium in England last November, people from all varieties of racing talked with great concern about the economic situation and how racing fits into the 21^st Century. People repeatedly mentioned one phrase: ‘the need for racing to be relevant‘.

Old-time fans can scoff that racing ought to be loud and smelly and it’s just a bunch of Prius-driving tree huggers that are causing all the problems, but the fact of the matter is that the world is changing. Race tracks in Europe are facing closure due to noise issues and emissions issues. Either racing changes or natural selection does the same number on racing it did on the dodo bird.

Between highly customizable entertainment coming at us from all directions, the glaccially slow economic recovery, people’s microsecond-long attention spans, animated gophers, and the fact that we must deal with increasing global tempertures, racing is a very obvious (although not justifiable) target. Racing series need to think about long-term planning. Not just what they’ll do next year, but what they’ll do in the next five years. Racing has an unfortunate history of being reactive. It’s time to get proactive. Now.

I normally struggle with my own New Year’s resolutions, so I thought maybe this year I’d just make resolutions for other people and see if they do any better. My suggestions, of course, focus on science. I do have a suggestion for changing The Chase, but it requires non-linear differential equations, non-dairy coffee creamer and quantum field theory, so I’m keeping it to myself. I’ve tried to order my suggestions, but take each of the heading numbers with about a plus or minus 2. Starting from least to most important (insert drumroll here):

8. Take Pit Road speeding penalties out of the race.

7. Get serious about diversity or stop talking about it.

6. Get serious about being ‘green’.

5. Rethink ‘parity’.

4. Beef up the ‘research and development’ part of the NASCAR Research & Development Center and establish formal mechanisms for involving the teams.

3. Stop being fuelish.

2. Give the New Car the tires it deserves

1. Fix the aero problems with the New Car.

I’ll be blogging about each one of these issues in the coming weeks.

Incidentally, I’m going to be double posting for a few weeks while I consolidate the buildingspeed.org and stockcarscience.com websites. Believe it or not, some of my sports car racing friends took umbrage at being talked aboout on a ‘stock car’ site! Plus, keeping up with the two different sites was stretching me just a little too thin, since I’m now also blogging about everything from Christmas tree lights to climate change at Cocktail Party Physics.