Stock Car Science

I’ve had a number of requests to identify science people should look for at the track before the race weekend starts, so here are few things to watch for this weekend at Martinsville.

Martinsville was the second race I spent with the No. 19 team last year while I was researching my book, The Physics of NASCAR. Martinsville was a stark contrast to the first track I visited, Atlanta. Martinsville is a short track and, in addition to having a slight preference for short track racing, I really like the intimacy of the track. The garage is really more of a lean-to. There are no fancy windows for fans to watch the teams through. The National Anthem is usually performed by local people and they play it the way it was written. Even the drivers have to park their motorhomes outside the track. My favorite memory of Martinsville was the Sunday morning I spent sitting in my car waiting for the garage to open. There was a preacher singing hymns outside Jeff Burton’s hauler. The Sun hadn’t risen when the service started, but as it did, I watched the mist in the valleys lift. It was a wonderfully peaceful moment. Then the garage opened.

The most important thing on a car at Martinsville are brakes. Martinsville is 0.526 miles (2777 ft) in length. The straightaways are 800 feet. That means that about 57% of the track length is straight and 43% are turns. I’ve sketched out Bristol and Martinsville in the figure below. (And yes, Bristol is asymmetric. That’s not just my crummy drawing.) In addition to being asymmetric, Bristol is rounder. Drivers spend more time turning there. Martinsville is long and narrow. You’ll hear announcers call it ‘turns connected by dragstrips’, and that’s pretty much the way the drivers drive it. They get on the gas as soon as possible coming out the turn, get as much speed as they can down the straightaway, and then brake hard to enter the next turn. Keep an eye on the on-screen displays of brake and throttle during practices. The turns are tighter, so you need more grip to get around them, which means that the speeds at Martinsville tend to be slower. That doesn’t mean the racing is any less interesting or the cars are any easier to set up.

The huge demands Martinsville places on brakes means that teams use larger brakes. That means larger brake calipers (larger diameter calipers and often six calipers instead of four), plus the brake pads have a larger area and are thicker. It also means glowing rotors. A moving car has kinetic (motion) energy. When the car slows down, according to the law of conservation of energy, the kinetic energy has to be transformed into other kinds of energy. Kinetic energy is transformed mostly into heat at the brakes because brakes work using friction. If you rub your hands together for a few seconds and put them to your cheeks, you’ll find that your hands are warm. Where there is friction, there is also heat. When the brake pads drag against the brake rotor, the pads and the rotor get hot. At tracks where the driver doesn’t have to brake as hard, the rotors may heat up a little and for a short time. At Martinsville, you’ll see the rotors turn red for a much longer time. SPEED often has a camera on the brake rotors during practice and that gives you a great view of how hot the rotors get and how long they stay that way.

Just as friction between the tires and the track abrades the tire surface, friction between the brake rotor and the brake pad abrades the brake pad. During the race, look for close-ups of the pit stops. When the front tire changer removes the tire, look for black brake dust. That dust used to be brake pad. Sometimes there is so much dust that the tire changes have a hard time seeing until the dust settles.

The interaction between the brake pads and the rotor is very similar to the interaction between the tires and the track. Friction is one of those phenomena that scientists do not entirely understand, especially when you start dealing with materials like rubber that stretch and come apart. Friction originates from forces between molecules in the two things that are rubbing together–in this case, brake pad and rotor. There are two types of friction at work. Abrasive friction is the type of friction at work when a piece of sandpaper rubs on wood. The energy needed to remove the molecules in the brake pad comes from the kinetic energy of the car, so the kinetic energy of the car decreases and brake pad gets thinner.

The second type of friction is adhesive friction. When the molecules from the brake pad come off, some form the dust I mentioned earlier. Other molecules transfer to the brake rotor, forming a thin film of brake pad molecules. The friction between the brake pad and the film of brake pad material on the rotor is different than the friction between the brake pad and the bare rotor. The best analogy is to imagine that you have a piece of gum stuck to your shoe. If you step on the sidewalk, you’ll get one type of resistance. If you step on another piece of gum, the two pieces will stick together for awhile until you pull your foot away. The latter is adhesive friction. Brakes rely on both types of friction to decrease the motion energy of the car, and both types of friction produce heat.

The second thing to look for at Martinsville is load transfer. I’ll dive into that in more detail for the second Martinsville race, but here’s the brief summary. The grip each tire has is proportional to the force pushing down on the tire. When the car is sitting still, each tire has roughly the same fraction of the cars’ weight pushing down on it. When the driver brakes, the front tires have more force pushing down on them than the back. You’ll see the splitter go from a few inches off the track while the car is accelerating down the straightaway to almost riding right on the track when the driver brakes. That means that the front tires have more grip than the rear. The reverse happens on acceleration: More weight is on the rear wheels than on the front. When the car corners, the outside wheels support more load than the inside.

Coming out of the corners, you’ve got a combination of the shifts from accelerating and turning. The right rear gets the most load and the left front the least. If the driver is, for example, accelerating too much coming out of the corners, you will see the left front tire spin or actually leave the ground for a few seconds. The SPEED TV guys usually do a great job focusing on this issue when they cover practice. They’ll often highlight one or two cars that are having this problem, which is more of a problem in the new car because of the higher center of gravity. If your favorite car is one of the ones being highlighted, that’s generally not a good thing. I remember at the spring Martinsville race last year that Elliott was having a very difficult time getting on the throttle coming out of turn 2. They could actually superpose the qualifying laps of the cars and you could see that he was as good as the pole sitter into turn 1, but lost precious fractions of seconds coming out of turn 2.

For those of you in the Charlotte area, I’ll be explaining load transfer in greater depth, including some video from last year’s Martinsville race, during a talk I’ll be giving April 7th at the University of North Carolina–Charlotte. The talk (which is free to attend) will be at 7:00 p.m. in room 281 of the College of Health and Human Services Building on the UNC Charlotte Campus. Parking is available at the new Union Deck and you can find maps of the campus here.

Drew Donnelli asks: Can you offer any insight into the problems JGR seems to have with fuel supply?

Thanks for the question, Drew. As usual, with the help of a couple incredibly patient friends, I can provide a little information on the possible causes of the fuel intake problems that probably lost Denny Hamlin the Bristol race. Here’s a flowchart for the path the fuel takes from the fuel cell to the engine.

There has to be a constant supply of fuel for the engine to run properly. The sign of a fuel pickup problem is when a car that had been running just fine all of a sudden–often on a restart–sputters like it’s out of gas, but then recovers and continues.

Let’s start where the fuel starts, which is in the fuel cell. The picture below is from the ATL catalog. ATL is one company that supplies NASCAR-legal fuel cells and they have a very helpful catalog.

The fuel cell is a rectangular box filled with foam to prevent the fuel from sloshing around too much. There are one or two fuel pickups inside the box that are sometimes called "duck feet" because they are shaped like, well, duck feet. The pickups have doors at their entires so that fuel is trapped when it enters the fuel pickup.

The duck feet are located on the right-hand sides of the fuel cells on oval tracks. A car turns because the tires exert a force that prevents the car from going straight. That force (the centripetal force) points toward the center of the turn. The problem is that, because you are in the car, you feel like you’re being pushed towards the outside of the turn. Some people refer to this as a centrifugal force, but there is no force pushing you outward–it just seems that way. (I belabored this point in the book, so I won’t go into it in detail here. Suffice it to say that unexpected things happen in non-inertial reference frames.) The same phenomenon happens to the fuel: It tries to go straight while the car is turning, so the net effect is that the fuel is pushed toward the right-hand side of the fuel cell, hopefully, right into the pickups. The doors prevent the fuel from coming out again on the straightaways.

This strategy sometimes backfires on the more steeply banked tracks. Liquids always seek their own level. Gravity acts on them and doesn’t care what they are contained in. I’ve shown a drawing below of what the fuel would look like if the car were just sitting on a banked track. Amusingly enough, this is actually a picture I was asked to draw during my doctoral comps. The request was to show what happens to water in a glass that is tilted, but the picture is the same regardless of whether it is a glass of water or a fuel cell.

When there is a lot of fuel in the tank, this isn’t so much of a problem, as the top picture shows; however, when the fuel level gets low (as in the bottom picture), even the turning action of the car may not get enough fuel into the fuel pickup, causing the engine to sputter. This would be even more of a problem on a restart because the force with which the gas enters the pickup changes with the square of the car’s speed. Under caution, the cars are going more slowly, while the force of gravity is the same regardless of the car’s speed. It becomes a tug of war and if gravity wins, the driver whose car is experiencing this battle often loses.

A second problem is when there is something in the fuel line that isn’t liquid and/or isn’t fuel. Fuel vapor, air bubbles or water (remember second Atlanta last year?) all cause problems if they get in the fuel line. One of the more common culprits is vapor lock, which occurs when liquid fuel becomes vaporized before reaching the cylinder. Under caution, fuel is in the fuel lines for a longer time because it is not being used as quickly. The inlet lines that bring fuel to the engine are close enough to the exhaust that they get hot. If the fuel is heading to the engine quickly, there isn’t enough time for the heat to affect it significantly; however, some of the hydrocarbons in the fuel vaporize at relatively low temperatures. As one of my engine experts put it, “Vapor is difficult to pump!” (I need to check–I think the unleaded fuel, which has higher concentrations of toluene, is more likely to vaporize at lower temperatures than the old, leaded fuel did.)

Fuel vapor makes it difficult for the fuel pump to properly regulate fuel pressure, which again leads to sputtering on the restart. The problem is compounded when air gets into the fuel lines because, in addition to messing up the pressure regulation, combustion requires gasoline as well as air (in a pretty picky ratio).

UPDATE: In response to Lou’s comment (and I had to put this in the blog because the comments section wouldn’t let me add the link!):

I should have mentioned that the fuel pumps on NASCAR cars are mechanically driven for safety reasons. You don’t want the fuel pump on a crashed car to continue to pump fuel. The Waterman fuel pump, is a cable-driven pump that is mounted on the fuel cell (as opposed to on the engine). This type of fuel pump is run on the Chevy R07 engines, but I understand that the 2008 Toyota engines (which I assume Hamlin would have been running) also use this type of pump. I’m also told that Waterman originally was the only one offering such a pump, but that other manufacturers (like CV products) have started providing this type of fuel pump. One argument in favor of mounting the fuel pump on the fuel cell is that it lessens the chances of vapor lock; however, if you have any type of pressure regulating device near the engine that is expecting liquid and gets vapor, you’re still likely to have issues with fuel supply. Another positive of this type of fuel pump is that it should be more efficient in getting fuel out of the fuel cell, although I don’t have any testimonials as to whether that is actually the case or not. The web link to Waterman racing Components above has very useful pictures and information about the fuel pump.

The big question, of course, is why this would affect some teams and not others. How much fuel is in the fuel tank, the type of fuel pump (some are mounted back by the tank and others are mounted by the engine), and even the way components are mounted relative to the effective net force (i.e. the vector sum of centripetal force and gravity) can impact whether or not the driver suffers from a fuel intake problem on the restarts.

UPDATE: Jayski has a link with information from JGR through ESPN that mentions some of the same issues. One might ask why JGR seems more susceptible to the problem. It may be that we only take notice when it happens to a car that is in the front. Other teams may be experiencing similar issues, but we just haven’t noticed. Sorry to hear that this is making Denny age prematurely!

A slight errata:

This doesn’t change anything about the science surrouding the oil tank box cover and how it could provide an aerodynamic advantage, but it does clarify the situation with how the oil system is pressurized. Dan pointed out my error some time ago in the comments section to that blog, but it took me this long to get confirmation and an explanation from someone with enough patience to wade through it with me.

Here’s a flow chart of the engine oil system. I incorrectly stated that the entire oil system was “pressurized", but only part of it is pressurized. The top oil pump (which directs oil to the engine) puts the oil under pressure. The crankcase (bottom of the engine) is at a lower pressure than atmosphere thanks to the bottom oil pump. This makes perfect sense, because how would the oil get back to the oil tank if there wasn’t a pressure differential? The oil tank itself (and the overflow container, which is located in the trunk) are both at atmospheric pressure.

My oil tank expert wishes me to point out that I drew two oil pumps, but that it’s really just one oil pump with multiple stages. The top box represents the pressurizing stage and the bottom pump represents the scavaging stages. The scavenging stages are the ones that He also points out that there are multipe scavenge stages, which are the pump stages that bring the oil back to the oil tank. The problem was that it looked more confusing than helpful when I drew the diagram with a single oil pump.

Thanks to Dan for pointing this out, and for advancing my argument with my husband that the third garage at the new house would make a perfect home for an old stock car. Just so I don’t have to constantly bug people who are trying to get their cars ready for the next race.