Stock Car Science

You all know Newton’s first law of motion (NASCAR version).

A race car going 130 mph down the frontstretch at Bristol is going to keep going 130 mph down the frontstretch at Bristol unless a force makes it do something different

In more science-y words, you can’t (thanks Josh) turn without a force that makes you turn. If you put a tennis ball on a string and whirl is around your head, the string keeps the ball turning in a circle. Race cars, obviously, don’t have strings. The thing that makes them turn are their tires.

Tires aren’t round, except when they’re stacked up horizontally in front of the hauler waiting to do their job. When a tire goes on a car, the part of the tire in contact with the ground is a flatish oblong shape called the contact patch.

When the car is sitting still, the contact patch is about the size of a man’s size 11 normal width shoe (or about 36 square inches). When the car is on the track, the contact patch size constantly changes, getting as small as 16 square inches per tire. Hold your hands up to ring a 4-inch by 4-inch area.

Think about how much force you need to change the motion of something like a 3600-lb car. To turn at Bristol going 130 mph takes a little more than two tons of force. What provides all that force are four surprisingly small patches of rubber in contact with the ground. The next time you get set to complain about Goodyear, think about what their tires are asked to do.

Goodyear provides basically different tires for different types of tracks. They’ll provide 25 different configurations of tires in 2009, and figuring out exactly what properties those tires need to have is why they have tire tests. Goodyear is constantly tweaking the tire properties - the type of rubber used for the tread, the strength of the sidewalls, the interior construction, etc. But there is now talk about making some more major changes in the tire to accomodate the heavier demands placed on the tires by the new car.

Regular readers of this blog know that the new car has a higher center of gravity, so more weight shifts to the right side of the car on left-hand turns, which places more load on the tires. The cars are set up to be more yawed. And still, the primary driver complaint you hear is that the cars won’t turn. So Goodyear is investigating make more major changes to the race tire.

Let’s start by looking at what we have now. The figure below shows (drawn to scale, even!) the current tire.

The tire has a bead diameter of 15 inches, which means that the wheel upon which the tire is mounted is a 15 inch diameter wheel. The outer diameter of the tire is 28.5 inches. Tire widths vary depending on the track, but most are between 10.8 inches and 11.8 inches.

Goodyear is talking about making a “taller” and “wider” tire, so let’s explore what options are open to them.

Dustin Longreports that the design of the tire is two inches taller (17 inches compared to 15 now) and 1.5 inches wider - and that it likely won’t be ready until 2011.

Wider Tires

Right now, it looks like Goodyear is considering a 1.5 inch wider tire, as shown above. Tires get their ability to make the car turn by friction between the tires and the track surface. The more friction, the more turning power. Friction, however, is a strange phenomenon. The frictional force between two surfaces doesn’t depend on the area in contact. So the assumption that a wider tire increases the frictional force is wrong.

What a wider tire does do, however, is change the fraction of the contact patch that is slipping relative to the fraction that is gripping. Consider two contact patches, both of which have the same area. The first one is 6 inches wide by 4 inches long. The second is 8 inches wide by 3 inches long. Both have a contact patch area of 28 24 square inches.

If you analyze the contact patch, the front edge grips while the area behind it is where slip happens. Slippage starts the same distance back from the leading edge of the contact patch. Let’s say for the sake of illustration that the first inch and a half of the contact patch is the gripping area. On the narrower contact patch, that’s 1.5 x 6 or 9 square inches, which makes that 9/24 or 37.5% of the contact patch area that is gripping. On our wider tire, we’ve got 1.5 x 8, which is 12 square inches of gripping area, which is 50% of the area of the tire. Since the contact patch areas are the same, you’re going to get more turning force from the wider tire since there’s a larger fraction of the contact patch gripping. For more details, I recommend Paul Haney’s excellent book “The Racing and High-Performance Tire”, which is available on his website. Haney is an excellent and very clear writer.

Increasing the tire width would definitely increase grip; however, that’s not the only consideration. The aspect ratio is the ratio of the sidewall height to the tire width. On commercial tires, that’s the number after the tire diameter. A 185/60R14 is a tire with a diameter width (thanks Ron!) of 185mm, an aspect ratio of 60%, and a wheel size of 14 inches.

The very first pneumatic tires had aspect ratios around 1, meaning that the width and the sidewall height were about equal. In 1984, 60% of commercially produced tires had aspect ratios of 80%.

Racing and other high-performance tires usually have lower aspect ratios, which means that the sidewall heights are closer to the tire width. If you think about turning, a lower aspect ratio means that there’re less flex in the sidewall, so you have better stability in turns. A larger aspect ratio gives you smooth handling, but a smaller aspect ratio gives you better responsiveness. Looking at a range from 10.8″ to 11.8″, the current aspect ratios would come out to be 57.2%-62.5%. Increasing the width by 1.5 inches without changing the height would change those values to 50.75-54.8%.

Decreasing aspect ratio increases the stresses in the belt cords, but decreases stresses in the carcass cords and bead wires, plus the stress distributions are more even across the tire. However, any irregularities in the road are transmitted more to the suspension system, which means the effect of bumps are magnified.

That’s important, as reported again by Dustin Long. He quotes Jimmie Johnson

From a feel standpoint, it seems like there’s less movement in the car where at Atlanta going through the bumps, not only is the car following the bumps and the interaction of the tire and the way it was compressing and cycling around, also had the car moving left to right, so as you’re going through those sensations it’s hard to tell if the car is loose or tight. Then you get to the center of the corner. At that point, usually your condition shows up but you’re left with the challenge of “Was I really loose getting in or is it just loose in the center.’ It just really confuses you and it takes away your confidence to carry speed in the cornera and affects your confidence in what changes you make to the car and really what is going on. It takes more time and is harder for the teams to find that magical setup to help that tire work right for you.

The tire plays a large part in the feel of the car. If the tire loses contact with the road because it’s following the bumps too well, that might throw even an expert driver like Johnson.

Lowering the aspect ratio reduces the average pressure of the contact area, as well as reduces the deflection of the tire, especially around turns. This could decrease tread wear. Lower aspect ratios also may allow the use of a softer tread compound because the stress distribution is more uniform.

Sounds good, huh? Unfortunately, there are a number of technical challenges in producing low-aspect-ratio tires. A problem called ‘reverse curvature’, which is the carcass cords curving inward instead of outward, can occur. This leads to pressure points and concentrated wear. The sidewall also must bear significantly more load in a low-aspect-ratio tire, which can lead to sidewall failure. A lot of strength has to be concentrated in a very short distance. The bead also must be constructed differently to provide greater support. The entire process of laying up the tire in the mold has to be done with greater precision, which means lower yields and higher requirements for quality control.

In the next post, we’ll look at what a ‘taller’ tire actually means and how the combination of taller and wider could mean changes for the new car.

The stock car science blog has been a little behind due to a really big project I’ve been working on that has taken up every spare moment of writing time the last few months. The good news is that there (finally) will be an announcement at Texas Motor Speedway the weekend of the April race. More as it develops.

The Las Vegas race, like last week’s California race, had more than its share of blown engines. The rise in the engine fatality rate is being attributed to a combination of factors. One is that the tires are a little grippier, which allows higher engine speeds. The second is that the temperatures have been cooler. Air density increases as the temperature drops. If you compare two equal volumes of air at different temperatures, the cooler volume will have more air molecules. The larger number of air molecules per volume increases power, but that also creates an increase in heat.

Four out of five Roush engines gave up at Las Vegas. Jack Roush noted that the team had chosen to use the higher ratio of the two allowed rear-end gears (3.89), which increased the engine speed by about 200 rpm. Qualifying runs were coming in around 9900 rpm. In addition to being grippier, the tires didn’t fall off as much as expected, so the speed stayed elevated for a longer time. Hendrick Motorsports’ problems at California were attributed to “valve train failures that were related to a specific batch of parts from a vendor".

This is the second week Toyota cars experienced engine problems. Last week at California, Brian Vickers won the pole, but was forced to start in the rear after discovering an engine problem post-qualifying. Michael Waltrip’s engine didn’t even let him make a qualifying attempt.

Vickers qualified 21st this week at California, but he, teammate Scott Speed and Michael Waltrip Racing’s David Reutimann (qualified 4th) and Marcos Ambrose (qualified 5th) all had to change engines.

Kyle Busch (who won the pole and ultimately the race) had to change his engine; however,Joe Gibbs Racing has their own engine program and Busch’s engine issues are unrelated to the other Toyota’s.

The engines for Red Bull Racing and Michael Waltrip Racing are provided by Toyota Racing Development. TRD thought they had resolved the issues experienced in California, but according to Lee White of Toyota, they actually went in the wrong direction.

“We’re going to use a heavier lubrication and not try to squeeze every ounce of horsepower out of them,” White said. He estimated the cost of using the new lubricants would be four to five horsepower.

The issue appears to be wear between the camshaft and the lifter. White said “It’s either a coating, lubrication, lack of lubrication, too much lubrication, not enough coating or a material situation or just the simple fact that we haven’t been testing.”

NASCAR engines use Flat tappet camshafts. The camshaft is a rotating shaft with lobes. The flat tappet lifters ride along the lobes and move the pushrods. The pushrod is attached to the rocker arm, the rocker arm is attached to the valve, so as the cam shaft rotates, the lobe profile determines how rapidly the valves open and shut.

Camshafts for racing usually have ‘radical’ lobe profiles. The cams on your car let the valves open and close at something of a leisurely pace. An engine going 9500 rpm opens and closes each valve 79 times each second. To get the most air/fuel mixture into the car and the most exhaust gas out as fast as possible, the valves must open and close quickly. The cam lobe profile is designed to achieve this.

As you might imagine, there is a lot of rubbing between the cam and the lifters. Roller lifters have a bearing that rotates, but flat lifters pretty much just rub. NASCAR mandates flat tappet lifters and that means a lot of friction. One way to decrease friction is with lubricants like oils. The oil gets between the two moving pieces and lubricates the interface.

But there are issues with oil. When the pressure between the moving parts gets high, it squishes the oil from between the parts. You have to use thicker oil that can stand up to the pressure; however, thicker oil increases friction in its own way because the parts have to move against the oil.

There are two issues: friction and wear, both of which can be addressed by coatings. Very thin (a tenth of a human hair’s width) coatings are deposited on the cams and/or lifters. Wear is two materials abrading each other. When the parts wear, they get smaller, they don’t contact they way they are supposed to, and they often get rougher. Friction, the resistance to motion of two things sliding past each other, costs horsepower in the engine.

Nitrides (chromium nitride and titanium nitride) are very hard ceramics that minimize wear. (Titanium nitride is the gold-colored coating you see on drill bits.) Coating a part is done by putting it in a vacuum chamber, removing most of the air molecules, then vaporizing your metal (titanium or chromium) and introducing some nitrogen. These types of coatings are often used on the parts of valves that contact the valve seats.

Friction is a second problem because engine power has to be used to overcome friction and that takes away from the power that goes to the wheels. Carbon comes in different forms. Two are graphite, which is one of the softest forms, and diamond, which is the hardest mineral known. Both are crystalline. If you make carbon amorphous-meaning that the atoms don’t have a regular arrangement-it’s called ‘diamond-like carbon’ or DLC. DLC has the best of both world-hardness and low friction. DLC has 10x less wear than titanium nitride and a lower coefficient of friction (which means its more slippery). DLC is actually a little too hard in some cases: it is fairly brittle, so it can crack if it gets hit too hard. Metal-doped DLC is not as hard, but also not as brittle. Since NASCAR engines often use ‘lofting’ (the lifter actually loses contact with the cam lobe), there’s a lot of impact when the pieces come back together.

Back at Daytona in 2007, a number of teams had issues with coatings that came off cams and/or lifters. I’ve been working on an article for the Materials Science Research Bulletin on materials used in NASCAR and talked to a number of people in the coating industry. They all stressed the following: Failure of coatings can happen in at least two ways: One is a processing failure of some type - a mistake was made by the company doing the coating. The second is when teams push the coatings past what they can take. Most coatings are two to four microns thick. A human hair is about 70 microns thick. It’s akin to teams running extreme camber on the front tires and then blaming Goodyear when they blow tires. (I should mention that none of the people I talked to supply cam or lifter coatings to TRD.) While coatings extend the properties of the parts, they don’t work magic. We haven’t yet invented a coating that requires absolutely no oil.

Both the engine people I spoke with said that, if they had tested at Las Vegas and California, the increased rpms would have been apparent and they would have stepped back a bit. It would be an interesting calculation to see how much the teams saved on testing compared to the costs of all those blown engines.

For more information

• Circle Track - The Science of Cams
• Circle Track - Flat Tappets and Lubrication

I’ve gotten a couple requests to talk about the story, but the Elliott Sadler YoYo saga is why I’m sticking to science. Gravity is gravity, doesn’t matter who is driving for whom and not even NASCAR can make rules about it. Regardless, however, of how much of the science you get right, racing always has a human element and there is no equation that can predict what an owner, driver or crew chief is going to do. That’s part of what makes it interesting.

I think Dave Moody, who is friends with both AJ Allmendinger (who was supposedly replacing Elliott) and with Elliott, had the most balanced and correct opinion. My feeling right now is that, although GEM has a right to do what they want in terms of who is in or out of the car, it’s low class to let a lot of other people know you’re planning on getting rid of him before telling him, not telling him face to face, and waiting until there are precious few other options open. Add in that Elliott had signed a contract extension earlier in the year and is getting married this weekend and that’s just a textbook case of questionable management decisions. I know people who sued universities after they were denied tenure. The ones that won promptly left for somewhere they felt they were really wanted.

Stock Car Science has been a little quiet over the off season, but we haven’t been taking any time off. I’ve got a lot of really neat things planned for the upcoming year, but for now, it’s time to look backward at the 2008 season. I was at the National Association of Science Writers conference and all of the editors said that ‘top ten lists’ are really big, so here is the list of Top Ten NASCAR Science Stories for 2008.

I didn’t include stuff like the Indy Tire scandal or the #83 sheet metal thinning - those seemed more stupid than clever.

10. The economy. Economics is called ‘the dismal science’ and nowhere is that more apt than now. This is actually likely to be the number one story of 2009, but it was just starting to hit us as 2008 went out the door. Letting people go is hard for the folks who are let go, and for the folks who had to let them go. I spent a day in the lobby at the 5/88 shop at Hendrick Motorsports (more about that next week) and was dismayed at the constant parade of young men turning in resumes. The woman at the reception desk spoke in glowing terms about the efforts Humpy Wheeler and others are making to help the folks who have lost jobs find new ones. They may fight tooth and nail at the track, but NASCAR really is a family.

9. The myth of the engineer crew chief. I was interviewing a technical director with an undergrad degree in physics and a grad degree in engineering. He pointed me to one of his favorite movies (Office Space), in which a young man explains that his job is to take information from the customers to the engineers. He is asked repeatedly by the dweebs who are trying to understand exactly what he does why the customers can’t just talk directly to the engineers. He progressively gets more and more irritated and finally blurts out something to the effect of, "Engineers can’t talk to normal people. Don’t you know that?" The trend that started a few years ago of assuming good engineers will automatically be good crew chiefs is waning. Just knowing the engineering doesn’t make you a good crew chief, but a good crew chief knows how to use engineering. Steve Letarte told me, "I’m not very smart, but I know how to talk to smart people." Knowing how to talk to smart people makes him pretty smart in my book. Bob Osborne, Carl Edwards’ crew chief, has just the right mix of skills. Now if we can just get the guys on Sirius XM Radio to stop using the words "geeky engineer" every time they talk about him…

8. Bobby Labonte’s Watkin’s Glen crash. Bobby crashed into the plastic barrels at the end of the pit road wall and exited the car in obvious pain. A couple tracks have finally put SAFER barriers on the inside walls and that’s a step in the right direction, but the pit road wall offers its own challenges. The end of pit road is one of the most dangerous remaining places. The pit road weall is a line defect. Hitting the wall head on is a really hard way to crash because there isn’t a lot of area over which to spread the impact. One of the most promising solutions for the end of pit road wall is Batelle’s Flexall, a honeycomb foam product that can deform up to seven times its original dimensions and they spring back again within a few minutes. (Remember how long it took to clean up that crash?) But the advances are going sort of slowly. One wonders what’s going to have to happen before research in this area really starts to get going.

7. Seven and Eight Post Rigs. I’m going to have a full post on the seven post (and now some have eight posts) rig. The guys at Red Bull Racing were kind enough to let me see theirs in action - it’s a really impressive machine, but not the panacea some suggest. Regardless, the teams that put effort into building and understanding the shakers are going to see big payoffs now that on-track testing has been banned for the year. Nick Hughes, the Technical Director at Michael Waltrip Racing, opined that people are sometimes don’t appreciate the high level of technology because the cars are "so simple". There is an awful lot of advanced engineering used to design and test these "simple" cars, even though you don’t see the machinery at the track. At Red Bull, they actually talk about the R and D building as "The Lab". Along with the engine dyno, these machines are going to continue to be very important in 2009.

6. The opening of the Windshear wind tunnel. One hundred and eighty miles an hour with a rolling road. I haven’t gotten a chance to see it myself yet, but the pictures from the Road and Track article look simply awesome. A $40 million + investment that one might question given the way the economy has turned (see #10); however, with the no-testing rule, wind tunnels are going to be just as important as engine dynos and seven-post rigs. I hear Windshear isn’t having a problem getting teams to sign up for time.

5. No changes to the new car. Everyone wants a lower center of gravity and more front-end travel. But the car is holistic. You can’t just lower the center of gravity without impacting other aspects of the vehicle dynamics. If NASCAR made big changes, much of the research the teams did this year would be trash. Give the engineers a little more time to get their heads around the car. It may be that some aspects of the car (side force and passing) are just never going to get much better. There are a lot of really bright folks working on the problem, but one year (plus a little) probably isn’t enough for them to figure it out entirely.

4. Yaw. I wrote about yaw – setting the rear end wheels so that they are toed, with the end effect being that the car looks like it’s already started to turn, even when it’s coming down the straightaway – at length. Typical NASCAR teams taking things to their logical level of absurdity: One team figures it out, another team does it even more and eventually it gets to such a ridiculous level that NASCAR has to make a rule about it. But it’s indicative of how creative people are in NASCAR. NASCAR took away the body asymmetry, so the engineers figured out how to get the asymmetry (critical for turning left) some other way. I look forward to seeing what they come up with this year.

3. Carl Edwards’ Loose Lid. No, not fighting with Kevin Harvick. The oil tank lid. OK, maybe I ranked this one higher than it deserved because my blog entry on it got more than 26,000 hits. It was such a great example of net force. Plus, Jack Roush’s explanation ("vibration harmonics" ) was a goldmine of science information. (Highly questionable, in my opinion, but when else do you get to talk about harmonics and NASCAR in the same article?) NASCAR takes the same attitude science takes: We can’t guess about people’s intentions. If NASCAR catches you (and they did), you get fined and penalized points. Carl lost not only his crew chief for six weeks and 100 points, but NASCAR took away the win, which cost Carl 10 points in the post-Chase competition.

2. The passing of Steve Peterson. Steve was the major NASCAR safety advocate. There are plenty of smart people making cars go fast, but people like Steve, Randy LaJoie, Dean Sicking, Tom Gideon, John Melvin (and a lot more people with less recognizable names) save lives. Steve had impact on everyone from karters to superstar NASCAR drivers. There are a lot of good folks at NASCAR R & D who will carry on, but just to make sure that there are more Steve Petersons in the pipeline, consider making a donation to the Steve Peterson Memorial Trust Fund for Motorsports Education and Safety: Contributions can be made through Wachovia bank account: 2000042225126 in the name of the Steve Peterson Memorial Trust Fund for Motorsports Education and Safety. Direct questions to Greg Peterson at 616-662-1612.

1. Michael McDowell walking away from his crash at Texas Motor Speedway. I was sitting in the infield, about 60 yards from turns 1 and 2 when McDowell got loose, hit the SAFER barrier, spun around, hit it again, and then tumbled down the track, flinging car parts in all directions. I was positive McDowell was going to be seriously injured if not killed. Amazingly, he walked away.

NASCAR had three goals with the new car: safety, improved competition and lowering costs. We can debate the merits of the second and third goal, and it’s true that NASCAR probably could have implemented many of their safety measures in the old car. But the fact that they were paying attention to safety saved at least one life in the 2008 NASCAR season . I got to spend a few hours examining what’s left of the 00 car for a project I’m working on. Randy LaJoie built the seat McDowell was in for Dale Jarrett in 2003 - that’s quite a testament to LaJoie that a five-year old seat did an amazingly effective job.

So there you have it, my bow to trendiness with a top ten list. There’s a lot in store for 2009: I’ve got some great experiences with a AVL engine dyno, the seven-post rig at RBR, some reports on the ‘green’ movement (which Marc at Full Throttle already started discussing). And a couple cool announcements to come. Welcome to 2009. Can’t wait to hit Daytona.