Stock Car Science

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.

NASCAR had not one but two drivers on their roofs yesterday at Talladega. Ryan Newman’s wreck was by far the most spectacular, but I had to wince at Mark Martin looking a little shook up and disgusted during the post-infield-care-center interview. I don’t blame him – or Newman – for being angry. Yes, it’s really impressive to see three-wide around the turns ten rows back, but is that really great racing? Is it good enough racing to subject the drivers (and the fans) to the types of accidents that we keep seeing at Talladega and Daytona? Is it good racing when you can cruise around the back all afternoon and finish in the top ten with drivers that fought for the lead throughout the race?

We learned (or were reminded of) a couple important things at Talladega.

1. Most people do not understand the fundamental difference between the physics of drafting and the physics of bump drafting. There is a big difference between prohibiting bump drafting in the corners and prohibiting drafting in the corners
2. It appears to be much easier for the new car to become airborne than the old car
3. When cars are traveling in a pack, any accident is highly likely to involve more than one car

Drafting vs. Bump Drafting

Air rushing around the back of the race car creates a high pressure region at the front of the car and a low-pressure wake at the rear of the car. The high pressure in the front of the car (the car ‘punching a hole in the air’) creates a force opposite the direction the car is moving. The pressure behind the car is lower, which also acts in the direction opposite the car’s motion. So the car is fighting against a force pushing it backward in the front and a force pulling it backward in the back. If there are two cars running some distance from each other, each is experiencing two forces slowing it down: one in the front and one in the back (as indicated by the large red arrows in the top figure). Between the two cars, there are four big red arrows.

When a second car gets very close behind the first car, the air rushes over the two cars as if they were one, which removes the force at the rear of the first car and at the front of the second car. In the lower picture, there are only two big red arrows, so there is less total force working against the cars and – voila – they go 3- 5 mph faster. This is the important part: the two cars don’t need to touch to make this happen. This is plain ole drafting. You can get some more information on drafting and bump drafting in the Science of Speed segment "Drag and Drafting".

Bump drafting is totally different physics. The leading car is running at full throttle. The trailing car is being pulled along, which means that at full throttle, it can actually go faster than the leading car. The trailing car bumps into the leading car, transferring some momentum from the trailing car to the leading car. The leading car goes faster and pulls the trailing car along with it. ‘Bump’ is probably a misnomer. Brian Vickers says in the Drag and Drafting video that he’s come away from plate tracks with headaches because he got bumped so hard - but adds that he was happy to be hit that hard because that’s the way you go fast at a plate track.

In drafting, you’re essentially removing a force by driving within inches of each other. In bump drafting, you are applying a concentrated force from one car to another. Bump drafting takes significantly more skill. The black dot in the diagram below indicates the center of gravity of the leading car. Newton’s laws: If you apply a force, the car goes in the direction of the force. The two cars bump squarely in the top diagram. The force from the trailing car pushes the first car straight ahead.

The middle picture shows two cars in a turn. There’s no way to bump squarely because one car is rotated relative to the other. If you hit the car ahead of you, you create a torque, which is a force that makes things turn. Think of the leading car as a spinner, pinned by the dot in its center. If you hit it in a direction so that the force goes directly through the center, it won’t spin. If you hit it at an angle, the car will spin. This is why you don’t bump draft in the corners. It is very easy to hit someone on the side of the bumper, sending them into a spin and wiping out half the field.

You can cause a car to spin by bump drafting in a straightaway if the two cars are not fully aligned. The bottom diagram shows that hitting a car off-center – even when both cars are going straight – is pretty much equivalent to hitting a car in the corner.

The two techniques have one thing in common: the person in the trailing car is in control and the leading driver is really just along for the ride. The trailing driver decides when to push, where to push and how hard to push. A number of drivers’ have expressed discomfort with ‘being pushed’ (drafting) too hard in the corners because an overaggressive – or an inexperienced – driver can make your car unstable fairly easily. But there is a pretty significant difference between bumping and pushing. Requiring drivers to leave space going into the turns just caused an accordion effect with cars having to back up down the line.

Aerodynamics

One of the big reasons for the aerodynamic changes in the new car was to decrease the amount of wake behind the car. The large wake with the old car made it very difficult for one car to get up close behind another one because the swirling air from the wake didn’t provide enough downforce on the front wheels and the car got tight. The big difference between the old car and the new car in the rear is the spoiler on the old car vs. the inverted wing on the new. All of the air must go up and over a spoiler, creating a huge amount of turbulence behind the car. The wing allows air to flow on top of and below the wing, so there is less turbulence behind the car.

Let’s briefly review the aerodynamics of wings. As shown in the diagram below, air moving over the top of the wing moves faster than the air moving over the bottom. Faster-moving air exerts less pressure than slower-moving air, so a wing experiences more pressure and more force below than it does above. That’s what gives an airplane lift. The wing on the rear of the car is upside down, so there’s more force on the top than the bottom and the inverted wing on a NASCAR car provides downforce.

The key for the computational fluid dynamics simulations of the old vs. new cars (shown below, from a GM publication) is that red are areas of high pressure, meaning lots of downforce. Orange, yellow, green and blue show decreasing levels of pressure, so those colors mean less downforce. Note in the old car (on the left), there was a significant amount of downforce generated by the rear decklid, while in the new car (right), the vast majority of the downforce comes from the wing.

NASCAR cars have an aerodynamic instability problem when they get going too fast. They are fine as long as they are pointed forward, but when the car spins, the aerodynamics change significantly. The air moves very quickly along the rear window and the roof, and remember that fast-moving air doesn’t generate much pressure. The ’shark fin’ on the side of the rear window and the roof flaps were designed to slow down the airflow because slower-moving air creates more pressure. The roof flaps and the sharksfin were designed (the story is in my book, The Physics of NASCAR) for the old car and I don’t know how much work was done to compare the effectiveness of the roof flaps on the new car versus on the old car.

A couple things I noted this morning watching the video of Newman’s crash in slow motion.

1. Both roof flaps deployed at the same time. In theory, one is supposed to deploy first and then the other if it is needed. But things were happening very quickly.
2. The roof flaps deployed when the car was at about 135-150 degrees from heading in the correct direction. (180 degrees would be facing backward.)
3. The right rear wheel was already off the ground as the roof flaps were opening
4. One the car got fully backward, it flipped over without any twisting. You would expect a car that was spinning and then got airborne to continue the rotational motion, but the 39 did a really clean black flip.
5. Once the car made about a 60 degree angle with the track, the roof flaps went down again. I wouldn’t think that gravity would pull them down, so that suggests that the pressure had increased to a point where the roof flaps didn’t think they needed to be deployed.

A backward spoiler is still pretty much a spoiler. If you think about the inverted wing running backward, I wouldn’t be surprised if it were generating a lot of lift. Put yourself in the place of the people designing the car. Would you have thought to simulate the car going straight backward to see what happens? We may not even know enough about the aerodynamics of the cars to do an appropriate simulation. But between this incident and Carl Edwards’ takeoff, NASCAR needs put some serious resources into re-evaluating the aerodynamic behavior of the car at different speeds. Decreasing the restrictor plate holes, especially by fifteen thousandths of an inch (about four times the diameter of a human hair) is not going to affect safety much. The reduction took ten to fifteen mph off the cars, but it didn’t address the primary problem of plate racing, which is that the drivers are on the throttle wide open most of the time, which makes them run in a pack.

A number of drivers are suggesting that NASCAR needs to sit down with them and talk about how to make racing safer. It’s not the drivers NASCAR needs around the table. With all due respect to Ryan Newman’s engineering degree, it’s the top aerodynamics people at the race teams (some of whom actually have Ph.D.s), and people like Gary Nelson and Gary Eaker, who were largely responsible for designing the original roof flaps. NASCAR has many more times the number of people working marketing and licensing than they do on safety. The teams have some incredibly smart people in their aerodynamics departments who have spent the last three to four years trying to understand everything they can about the aerodynamics of this car. NASCAR needs to make use of those folks’ skill and talent because they simply don’t have the in-house resources necessary to do the job quickly and effectively.

NASCAR has a history of being a reactive organization - as Carl Edwards noted last spring, when he said "I guess we’ll do this until someone gets killed, and then we’ll change it". For a few scary moments Sunday, I was afraid we’d reached that situation. Take the initiative and solve this problem before someone gets killed. Don’t tells us that your rules weren’t the cause of the problems. Slowing the cars down is not enough: It is time for a major change, whether that be repaving Talladega to decrease the banking (as the late David Poole suggested), a major re-design of the aerodynamic safety equipment on the car, and/or introducing a significantly less powerful engine that could be run without plates.

Be very, very careful what you put into that head, because you will never, ever get it out.  Thomas Cardinal Wolsey (1471-1530)

Tolerance. In the immortal words of Matha Stewart, "It’s a good thing". In this case, we’re actually talking about engineering tolerance, although tolerance of other things – like people who don’t know what they’re talking about and persist in talking about it anyway – remains a virtue to strive for.

Engineering tolerance is "the permissible limit of variation in a physical dimension or other property". In English, that means how far you are allowed to miss the target number. The idea of an engineering tolerance is that you are trying to achieve the exact number, but you recognize that getting it that precise is unlikely because of random factors beyond your control. There is a difference between precision - how close you are to the target - and accuracy, which has more to do with how consistent you are.

Let’s use the analogy of building a car with hitting a bullseye with an arrow. The goal is to hit the bullseye and let’s say you are given five shots (i.e. five distinct cars). The top left picture below shows the ideal. All five arrows are in the bullseye. Now, in reality, you’re more likely to have something like figure B, where the average of the shots is in the bullseye, but the individual shots are scattered a little bit. NASCAR cars are made primarily by hand and you have to expect a little variation. NASCAR would write a rule saying that a shot would be legal if it fell anywhere within the third circle. The goal is the bullseye, but we’ll give you a little breathing room. That’s tolerance. (Actually, NASCAR would probably write a rule that they owned the target and the bullseye had to be a particular shade of red and you had to pay them royalties if you wanted to reproduce it.) Hold Figure C for a moment while I explain why this archery analogy is at all relevant to NASCAR.

As numerous people reported, the 48 and the 5 cars were taken to the NASCAR R & D Center after the Dover race for closer inspection. The 48 was chosen because it won the race and the 5 was the ‘random’ car. This week, the 14 and the 2 (winner and random) were taken back to the R&amp Center. And the 48 and the 5 were taken back just because NASCAR can. Hmmm… NASCAR must either be picking on Hendrick or showing Hendrick favoritism. I can point you to websites proving both.

After the Dover inspection, crew chiefs Chad Knauss and Alan Gustafson were told "not to bring the cars back". In NASCAR history, those words have a very particular meaning becuase they usually are the words that are used when a team figures out how to do something that violates the spirit, but not the letter, of the law. NASCAR can’t actually nail them for it, but they want to make sure they know that they won’t stand for it. In other words, OK, you got away with it once, but don’t try it again.

That isn’t what’s happening here. First, there was nothing illegal about either car either week. John Darby, Sprint Cup Series Director made that verey clear. This was a friendly reminder to the HMS teams that they were closer to the tolerance limits than NASCAR would like them to be.

How close is close? If I remember right (and I’m on the road away from my records), the tolerance on the body is one-eighth of an inch. If something is supposed to be one-inch long with a 1/8" tolerance, it could range from 7/8" to 1-1/8" long. An eighth of an inch is 0.125" or 125 thousands of an inch. The 5 car from Dover apparently was seven thousandths of an inch (0.007″) away from the target plus the tolerance. A ream of 20 lb paper is just about 2 inches thick. There are 500 sheets in the ream, so the thickness of one sheet is 2"/500 = 0.004", or four thousandths of an inch thick. The 5 car was about two sheets of paper away from being illegal - and note that the emphasis is on ‘away from being legal’. No rules were broken here. So why is NASCAR making such a big point about it?

Well, now go back to the drawing above and look at Figure C. In figure C, all of the shots are within the three-ring limit; howevver, they are all in exactly the same place. The point of tolerances is that you are supposed to be trying to achive the actual number, not getting as close as possible to the absolute limit.

Now, that’s really sort of a silly thing to expect teams to abide by. There is a 5 mph tolerance on the Pit Road speed limit. No one is trying to go the Pit Road speed limit. If the Pit Road speed limit is 45 mph, everyone is trying for 49 mph or 49.5 mph or 49.99 mph. When Juablo got nailed for speeding, he was 0.06 mph over the limit. Or was he 5.06 mph over the limit? Trust me, every single team in the garage is playing the same game. It’s how you win. There’s nothing illegal about doing it, but NASCAR wanted them to know that they were playing with fire.

We’re talking about thousands of inches here. Measuring things is not as straightforward as it seems it should be. If I give you the same piece and asks you to measure it fifty times, you will come up with a range of measurements. That’s the nature of measuring things. So it is entirely possible that a car leaves the shop being perfectly within the rules according to the measurements there, but when it’s measured at the R&amp Center, it’s illegal. Don’t forget that the car runs a race and (more often than not) the car hits things and that will affect the positions of parts as well.

So how do you get that detailed in your measurements?

At the track, NASCAR uses a template structure - similar to the one shown here (from the Super Chevy website) to check that each car conforms to the rulebook. The template grid is much more sensitive to variations than the old, two-dimensional templates that were used with the old car. But even the grid is not the most sensitive measuring device. I have seen someone tap the template to "make" it fit more than once. The template makes it easy to see gross violations, but racing these days comes down to thousandths of an inch and that is why the cars have to be brought back to the R&amp center for measurement..

Teams use templates at the shop; however, they rely much more on coordinate-measuring machines (CMM). CMMs consist of a probe (which may be mechanical, optical or other) and a reading device. Modern versions are attached to computers to collect the readings. Mechanical devices include the Romer and Faro arms, which are brand names of popular CMMs. These device looks like arms, with joints that mimic the elbow, wrist and fingers. Those joints allow motion along all three axes (up/down, left/right and back/forth), plus the ability to rotate about each of these axes. (Check out this interview with the inventor, Homer Eaton.) The arm is touched to the car in specific spots. The probe transmits its three-dimensional coordinates to the computer, thus forming a precise 3D map of the car inside the computer. The picture below shows a Faro arm (top) and a Romer arm in use at the NASCAR R&amp Center (bottom). The Romer arm is used to certify the chassis (with over 100 distinct points tested) as well as measure the body position and sheet metal thicknesses.

One of the challenges using mechanical CMMs is measuring over a very large volume. The NASCAR system measures over a 13′ x 20′ area defined by a set-up plate. To improve the measuring accuracy, 5/8" diameter seats are mounted in the plate every three feet. The placement of the seats is verified during installation using laser triangulation. Before measuring the car, the probe is touched to any three of the seats, which ensures that the probe uses the same origin every time. Triangulation is also the basis for the CMM. Remember all that geometry you learned in eighth grade? If you have an unknown length - the distance from the origin to the pointer - you make the unknown length one leg in a triangle. If you know the length of one side and two angles of your triangle, you automatically know the lengths of all sides and all angles.

On "This Week in NASCAR", Chad Knaus noted that HMS only had one Faro arm available and would usually measure the cars right after construction and not much after that. He also reported that they bought a second machine ($60,000) so that both the 24/48 and the 5/88 shops would have one. I"ve also heard that there were some differences in how HMS was measuring the cars and how the NASCAR R&amp center was measuring the cars. Differences in exactly how the measurement is done might make a measurement look fine at the shop, but be over tolerance at the R&amp Center. That wouldn’t be a big deal if you were aiming for one inch and your limit was 1-1/8′, but if you’re getting as close as possible to the 1-1/8" number, a couple of sheets of paper make a big difference.

The mechanical arms are really nifty pieces of technology, but laser scanning takes accuracy one step further because the only thing touching the car is light. A laser stripe is focused on the car, and a sensor analyzes the line on the surface of the car. A computer program uses triangulation to back-calculate what surface shape would cause the observed line to appear as it does. I’ve seen a couple of these systems in action at the major team shops. They are fast and accurate. They are also very expensive, but I know a couple of teams that laser scan every car before and after they hit the track, looking for subtle differences that might mean a few hundredths of a second per lap.

I’ve had a difficult time getting this confirmed, but what I’ve heard is that the area of the car being questioned by NASCAR is the rear end and its offset from the centerline. The old car was remarkably asymmetric, as shown below. See how it’s almost jelly bean shaped, curving to the left in the front and the rear? That assymetry is one reason the old car turned better than the new car. The new car (rear view below) is still a little asymmetric. Note the position of the wing with respect to the decklid to see the asymmetry; however, the asymmetry is much smaller in the new car. Apparently, a number of teams (HMS isn’t the first one to be warned) have been pushing the rear end as far as they can within the tolerances to help the car turn.

All NASCAR was doing was reminding the teams that that one reason for a tolerance is that there are bound to be discrepancies in measuring or fabricating and the tolerance is there to give you a safety cushion. The point is that you design for the specification and hope it is within tolerance. The teams, on the other hand, look at the tolerance as being "the grey area" and therefore legitimate for them to work with. Given everything that NASCAR prohibits, you can’t blame the teams for zeroing in on those things they can still innovate with.

So let’s just put the black helicopters away for the moment, remove the aluminum foil helmets and back slowly away from the digitizing arms. Remember that, in NASCAR, the best innovations are the ones that they we never hear about.