Oct 172014
 

Every year at this time, we hear that Talladega is a wild card because “Anyone can win”.  Which, of course, made me wonder — can anyone win?

Who Wins Races?

Let’s start by looking at who wins races in general. I analyzed the last three years and everything we have so far for this year and put it in a table. Why a table? Because tables help you see your way through all the numbers.  What I was interested in was trying to find a correlation between who wins and how “good” a driver they are, as determined by how high they finish in the standings at the end of the year.

The number in each box is the percent of all wins run by drivers in the top 5, top 10, top 15, top 20 and the Chase.  Note that I discarded some situations, like Brian Vickers, who won a race in 2013, but sat out much of the season due to illness and finished 78th in points. Same thing for Hamlin and Stewart, neither of whom ran all the races that year, but won a race.

Note that the new rule – that anyone who wins is automatically in the top 16 is going to invalidate this type of an analysis in the future because someone who would’ve finished lower in points gets boosted up by the win.

Year T5 T10 T15 T20 Chase
2011 44.4 63.9 83.3 91.7 77.8
2012 48.6 88.6 94.3 100 88.6
2013 61.8 70.6 91.2 94.1 85.3
2014 35.5 71.0 96.8 100 100

Here’s a gratuitously colorful graph of the same data, just for Moody:

BSPEED_PercentWinsByFinishingStatus

The take-away message:  It is very unusual for a driver who ends the season outside the top 15 to win a race. In fact, for the last three years, more than 70% of the races are won by the top ten drivers. (And I don’t know about the goofy perspective Excel uses in those graphs.  It makes it look like the numbers for 2013 and 2014 are less than 70% – but they’re not. I promise.)

But What About Talladega?

If Talladega really is an ‘equal opportunity racetrack’ in terms of winning, then the stats ought to look very different over the years. I analyzed Talladega races all the way back to 1990, which is almost 50 races. You know what? It’s not that different from the average.

Year T5 T10 T15 T20 Chase
2011 44.4 63.9 83.3 91.7 77.8
2012 48.6 88.6 94.3 100 88.6
2013 61.8 70.6 91.2 94.1 85.3
2014 35.5 71.0 96.8 100.0 100.0
Talladega 44.7 72.3 91.5 95.8

The stats are almost identical relative to every other race track out there. Out of the 47 races I included, only two were won by drivers outside the top twenty.

Jamie McMurray – 2009 Fall (22)

David Ragan – 2013 – Spring (28)

I omitted the Spring race in 2009 because the driver (some guy named Brad Keselowski (?)) only finished in 38th place – but only ran 15 out of the 36 races. So if you’re currently running below 20th place, you’ve got less than a 5% chance of winning.

Even the year Michael Waltrip – the patron saint of teams hoping for an upset at a plate track – won, he finished 15th.

Wait a Minute… That Can’t Be Right

We all remember David Ragan winning Talladega and Daytona and Trevor Bayne winning the Daytona 500.  Is it true that if you’re not in the top 15 and you’re going to win, it’s likely going to happen at a plate track?  Let’s look at the exceptions.

Year Driver Finishing Rank Track
2013 Martin Truex, Jr. 16 Sonoma
2013 David Ragan 28 Talladega
2012 Joey Logano 17 Pocono
2012 Marco Ambrose 18 Watkins Glen
2011 Trevor Bayne 53 Daytona
2010 Regan Smith 26 Darlington
2010 Paul Menard 17 Indy
2010 Marcos Ambrose 19 Watkins Glen

This year Aric Almirola won Daytona, and I’ve left that out because we don’t know where anyone is finishing yet. He could be 15th or better still.

But even if you counted him, not even half of the “upsets” take place at restrictor plate tracks.

But I swear I remember all these times…

I gotta tell you. I sweated this one out. I have looked at Dega Data for two straight days because I knew there had to be something interesting in there.

And I finally found it – but it runs counter to all my intuition. This is one of those things scientists have to be very, very careful about – not letting our expectations get in the way of reality. If you expect to see something, you’re more likely to see it.

So why does everyone think anyone can win Talladega?

It’s not at all surprising – it’s called the von Restorff or isolation effect. It’s named after a woman named Hedwig von Restorff (1906-1962), a psychiatrist and children’s doctor who conducted a set of memory experiments and found that an isolated dissimilar item surrounded by otherwise similar items would be better remembered.  In other words, it basically says that when something stands out as being very unusual, we tend to remember it.  For example, consider two lists

A B
9 TCE
21 GTS
3 JWN
16 PDY
QXK QXK
5 RWV
13 MTX
8 LEB
54 DVQ
3 PYN

The same three-letter sequence is in both lists. If I showed you the lists, then took them away and asked you what you remembered, you’d remember the letters better if I’d given you the A list than if I’d given you the B-list.  We tend to remember the unusual. And there’s a reverse effect, in that you may actually remember less about the things that don’t stand out.

Now if only I could wipe Michael Waltrip’s last dance (and 70’s mustache) out of my memory.

Oct 032014
 

@NASCARRealTime, @TheOrangeCone and @CircleTrackNerd had an interesting dialog when the 2015 rules were announced. They were debating whether the track records that are now standing are going to be essentially locked into history. The debate ended with an appeal to me and Goody’s Headache Powder. TwitterConvo_TrackRecords

When the Gen-6 car was introduced in 2013, new track speed records were established at 19 of 32 qualifying sessions. Yes, that’s more tracks than we run, but the record at Martinsville, for example, was broken in the spring and again in the fall. Another way to look at it is that out of 20 tracks where there was an opportunity to break a track record (meaning we exclude Dega and Daytona because their records are pre-restrictor plate, plus rainouts) – it happened at 16 places.

Why? The primary change was the much lighter car – they took 150 lbs off relative to the Gen-5 car while maintaining the same engine power and increasing downforce.

That changes in 2015, as one of the new rules NASCAR announced is a 1.170″ tapered spacer that will reduce power by about 125 hp. Gene Stefanyshyn (senior vice president of innovation and racing development for NASCAR) expects this is only going to decrease speeds by no more than 3-4 mph in most instances.

That seems like a weird trade off, right? 125 hp = 3-4 mph? Well, that’s because the engine isn’t the only place they’re making changes. They’re going to decrease the spoiler size to six inches, which will take away about 300 lbs of downforce, but will also reduce the drag on the car.

Here’s the theory: racing on ovals is won and lost in the corners. The primary impact of horsepower (all other things held equal) is determining maximum straightaway speed. In the corners, you’re not (except for plate tracks) using all the horsepower you have – you’re more limited by your lateral grip, which is determined by downforce.

Any driver can mash the gas coming down the frontstretch. What makes a difference is how soon they get off the gas/onto the brakes coming into the corner and how soon they get onto the throttle coming out of the corner. Let’s say you have to slow to 180mph to make a corner. It makes a difference when you start braking if you’re going 210 mph vs. going 200 mph.

You may actually be able to take the corner faster if you aren’t slowing the car down quite so much. A number of the drivers and NASCAR officials have stated that slowing down the cars a little (and remember, we’re talking 3-4 mph) should give drivers more options in the corners and thus make for more exciting racing.

But What About the Records?

Yes. A lot of records were broken in 2013. But a number of those records have been broken this year. The overall trend of pole qualifying times is up. Even when a rules change or a track change decreases the qualifying time, the next year, it starts creeping back up. I plotted qualifying times for a couple tracks to show this. Everyone’s been talking about these records being broken as if the speeds were stuck and then suddenly they jumped up. Not at all.

TrackPoleSpeeds_Charlotte2

So here’s Charlotte.  There are year-to-year oscillations, but the overall qualifying times have ben nothing but increasing.  On average, over the last twenty years, they’ve increased by about 0.7 mph each year.  So let’s assume that speeds are down across the board by 3 mph. In four or five years, they will likely be right back where they were before. You see a big jump in the slope of the curve (how fast it’s getting larger) from Gen 1 to Gen 2, but after Gen 2, it’s been pretty consistent.

I put each of the car generations on the graph to see how much difference changing car models actually made, but the track condition also makes a huge difference. Let’s blow up the last twenty years.

TrackPoleSpeeds_CharlotteRecent2

So there was a big jump after the 1994 repave. Then remember 2005 when we all learned a new word: levigation? They diamond ground the track, which made it very rough. Pole speeds jumped and the fall race that year was an unmitigated disaster, with tires blowing left and right. They did a formal repave in 2006.

And if you really want to see what a different track surfaces make, take a look at Kansas.

TrackPoleSpeeds_Kansas

After the re-pave, the pole speed jumped from 176 mph to 191 mph. There’s almost no history to rely on, but the following year, the fall speed was 4.3 mph slower than the spring speed.

In addition to major changes in the track, you get year-to-year oscillations due to things like weather and the tires Goodyear provides. One of the goals for the new set up is to allow Goodyear to make grippier tires that wear out faster, which could have a big impact on qualifying and (more importantly) racing.

So are the track records safe?  Probably for a couple of years.  But I’m not betting for much beyond that. The guys designing the race cars are just too clever to let little things like rules keep them down. The impressive thing is going to be if they figure out how to make the cars faster while also making the engines more reliable and longer lasting.

A final note. In the end, we judge drivers on race wins and championships. Poles may help you win a race, but I guarantee you if you give a driver a choice between a win and a pole, they’re going to choose the win.

Sep 292014
 

Last time, I explained what the center of gravity (CG) is. This time, let’s look at why we care.

A fast reminder – the grip you have on each tire depends on the force pressing down on that tire. The force pressing down depends on the weight on that wheel, plus the aerodynamic downforce. Today, I’m ignoring aerodynamic downforce for the sake of argument.

Let’s start by trying to get the car so that the weight is distributed equally on all four tires.  (Yes, I know that’s actually not what you want for ovals, but I’m trying to make things a little simpler here, okay?)  Let’s assume a 3600 lb car+driver, so that would be 900 lbs on each wheel.

Here’s the problem. That weight changes when you brake, accelerate or turn.  You can divide a car into two pieces:  the sprung mass is that part of the car that is supported above the suspension.  (You often include part of the suspension mass in there as well). The unsprung mass is the wheels, tires, and lower half of the suspension.

The unsprung mass is more or less tied to the ground. It responds directly to any bumps or wiggles. The sprung mass, however, is attached via mushy, springy things like springs and shocks, which means that it doesn’t respond directly to changes at the wheels. This is good in the sense that a suspension isolates the passengers from a rough road; however, we know that in racing, the goal is speed, not comfort. The moving around of the sprung mass complicates things because it changes how much weight pushes down on each tire. In other words, the sprung mass changes how much grip you have.

TippyTruckSignThe tippy truck sign (at left) is a good general breakdown of sprung and unsprung. The top half of the truck (the rectangle) is the sprung weight and the unsprung is the axle and wheels.

It’s not a perfect analogy, though. I hate to break it to you, but the sign is wrong. The truck doesn’t just rigidly rotate. The wheels stay on the ground (unless you’re taking that curve really fast), but the sprung mass does shift.  When you break, weight shifts from the rear wheels to the front. When you accelerate, it’s the reverse. Weight shifts from the front to the rear. When you turn left (as the truck in the picture is doing), the sprung weight shifts to the right.

That means that the grip on each tire also changes every time you speed up, slow down or turn. That’s why a car can be loose coming out of a corner and tight going in. The set up is the same – but the top part of the car is shifting constantly.

And here’s why the center of gravity is important. The amount of the shift – is proportional to how far off the ground the CG is .

CG_Height_2

The car in the left picture has a lower CG than the car in the right. That means that when the right car goes around a corner, it will experience a bigger shift in grip from the inner wheels to the outer compared to the car with the lower CG.

This is why SUVs and semis are much more likely to tip over than sports cars. When the CG shifts outside the box formed by the wheels, the vehicle will tip over. Stability means that your CG is firmly over your base.  Football players use the three-point stance (crouched low) because it does two things: it forms a wide base (the triangle formed by the two feet and the one hand) and it lower your CG. This is one you can try at home. Especially if you have little brothers because, face it, that’s what they’re there for, right?  Try to knock one over when they’re standing up with their feet together. Then have them move their feet apart. Finally, let them crouch down like a football player on the line and try to knock them over now.

In addition to tuckering yourself out, you’ve managed to show that a lower CG is more stable. For a race car, that translates into less weight shifting on acceleration, braking or cornering. This was a big issue with the COT (Gen-5) car because the CG was much higher (a few inches if I remember right) than in the previous versions of the car. Just raising the CG messed up everything the engineers had figured out in the old car, never mind all the other changes.

This is why the ballast for the race car is placed in the (left) frame rails. You don’t want to raise the CG. More weight lower in the car brings the CG closer to the ground and helps improve the grip on all four wheels around the corner.  Teams are even using carbon fiber composite (a very expensive material) in dashboards and seats to try to save weight and keep that CG as low as possible.

So that’s why keeping the CG low in the car is so important. The weight of the car doesn’t change – but the distribution over each wheel does and weight translates directly to grip. You can only go as fast as your least grippy tire, so the less change in weight transfer as you turn, the faster you’re going to be able to go.

 

Sep 192014
 

You hear engineers and crew chiefs talking a lot about the racecar’s “center of gravity”. There’s a reason for all the talk. The center of gravity really is the point around which everything else on the car rotates.

Terminology: CG vs CM

You will hear people in racing use the terms “center of gravity” and “center of mass” interchangeably.  It drives physics professors and people who work at NASA crazy. No one else really minds.

The reason for the two terms is that things weigh differently depending on the gravitational field they are subject to. If you are on the Moon, for example, your weight would be one sixth what it is on the Earth. Your mass – the amounts of ‘stuff’ that makes you you – doesn’t change, but the pull of the planet (or satellite) on which you’re standing makes you feel heavier or lighter.

The location of an object’s Center of Gravity is the same whether it’s sitting on Earth, the Moon, or Pluto. Each has a pretty constant gravitational field at their surface. If you work for NASA, you have to make calculations of things like galaxies, where the gravitational field changes throughout the object. That matters, and it means that the center of the mass and the center of gravity aren’t always the same thing.

 

CGIconHowever, if you’re driving a race car — unless something goes terribly, terribly, wrong — you will always be in a uniform gravitational field and the center of gravity will be the exact same thing as the center of mass. So we tend to use the two terms interchangeably.

We abbreviate them CM for Center of Mass and CG for Center of Gravity.  The abbreviations are much short and it makes us look like we really know what we’re doing.  Also, we use the cool icon at right to denote the CG.  You may have seen this icon on crash test dummies (the real thing, not the band) before.

What is it?

The simplest explanation is that the CG/CM is the balance point of an object. Finding the CG/CM ob a uniform object is simple. Take a ruler, for example. If I asked you to balance the ruler on one finger, where would you put your finger?

CG_Ruler_FingerYep. Right in the exact middle. If I gave you a yardstick, the CG would be at the 1-inch mark. The CG of a meter stick is at the 0.50 meter mark. For anything uniform, it’s pretty easy – it’s at the geometrical center.

CGofObjects

And you’ll notice from the donut in the lower right-hand corner that the CG/CM doesn’t have to be on the object. It could be out there in space. It’s an imaginary point.

Finding the CG for a racecar is a little more challenging because racecars are not uniform objects. There are heavy parts, light parts, and they’re all weirdly shaped. For most race cars that turn left, the CG/CM is located somwhere near the driver’s seat.   (I mean literally the driver’s seat.  His or her butt.) The diagram below is from an excellent book by Bob Emmons called the Racer’s Math Handbook.

Emmons_RaceCarCG

You want the CG as low and as left as possible in the car (for oval tracks) because that’s the position that’s going to help you turn faster and circle track racing is won and lost in the turns. The location of the CG is often limited by your sanctioning body’s rules, either explicitly (specifying front/rear and left/right weight distributions) or implicitly (limiting how much ballast can go in the left frame rail).

Finding the CG height of a racecar is usually done experimentally by measuring how the weight is distributed between the front and the rear of the car, then raising the car and re-measuring. Longacre has a nice online calculator that not only tells you how to do the measurement, but explains why it works. Note that the CG of the car will be different with and without the driver in the car, especially if you have a heavier driver.

In Part II, I’m going to examine the relationship between CG and stability.

Related Posts:

Sep 152014
 

In the last blog entry, I explained what brake bias was and how it could be used to improve the car’s handling during green-flag runs. This time, let’s look under the hood (or I guess, more accurately, under the dash) and see how this is accomplished.

Let’s start with the schematic from last time:

BrakeBiasinaRaceCar

 

 

WilwodBrakeBiasKnobNow we’re going to look at what happens at that little dial icon in the graphic. We start with a knob in the cockpit. It’s a simple knob, with two directions: front and rear. Turn to the right and more brake force goes toward your rear brakes. Turn to the left and the brakes are biased in favor of the front. The cable runs down to the brake bias bar, which I show below in a schematic drawing first, then a picture of the real thing.

On the left side of the drawing is a threaded rod – the bias bar (running up and down) with a center pivot point and two rods (one at either end) that are used to apply force to the two master cylinders.

 

BrakeBalanceBar_Alcon_Schematic

The next picture shows the entire pedal box assembly for a Rally Car.

BrakeBiasPedalBoxRallyCar

 

When you press on the brake pedal, the bias bar moves to the right, which increases the pressure on the master cylinders. When the bias bar is perfectly centered, meaning that the distance from the center to the left rod and the distance from the center to the right rod are equal, then the force is applied equally to the two cylinders.

Now consider moving the center pivot to the left or the right. This changes the distances from the center to the left and right rods and tilts the bias bar (like is shown in the diagram at top).  Now when you press on the brakes, the force is split between the two master cylinders differently. The side that the pivot is closer to will get a larger fraction of the force.  Instead of a 50:50 split, you can get a 60:40 split, for example.

You can see in the photo immediately above  that the center pivot is already adjusted slightly to the right of the picture, so the right master cylinder is going to get more of the braking force than the left one. Often, because of the inherent weight distribution of the car, the ‘neutral position’ of the brakes is biased a little toward the front or the rear.

And that’s pretty much how it’s done.

Sep 052014
 

With all the talk about giving the drivers the ability to change aspects of the setup from within the car, I thought some comments on what types of changes they can make would be appropriate.  Since the only control they have right now is brake bias, let’s start there.

The first thing you need to know is that the braking system on a race car is different from the one on most passenger cars.  On a  standard passenger car, the brake pedal is connected to a master cylinder – a hydraulic-filled container that transfers force from your foot on the brake pedal to the brakes.

BrakeSystem_NormalCarWhen you press on the brake pedal, the master cylinder exerts force on the fluid that fills your brake lines. That fluid pushes on the brake calipers and slows down the car. In everyday driving, most people don’t push the boundaries of traction or of their braking system, so this works pretty well.

In a race car, however, you’re always on the hairy edge of losing traction. Your goal is to go as fast as possible, so you have to run on the edge. That means the balance of the car is absolutely critical.

I’ll remind you of what I’ve decided to call the zeroth law of racing (we do that in science. When something is so fundamental, we make not the first rule, but the zeroth rule.) That rule would be: You can only go as fast as your least grippy tire. The corollary to this is that you can also only slow down as good as your least grippy tire.  For that reason, racers like to have a little more control over how the brakes are applied.  A NASCAR race car has separate master cylinders for the front and the rear tires.

BrakeBiasinaRaceCar

 

Note the little dial where the brake lines split into front and rear. That’s the brake bias, a device that allows the driver to change the proportion of the force that’s going to the front versus the rear brakes.

You’ve heard drivers talk about cars being loose and tight.  Loose is when the front wheels have more grip than the rear wheels. When you’re going around a corner on an icy road and the rear wheels swing out, that’s loose.  Tight is when the rear wheels have more grip than the front wheels. That’s when you have the steering wheel all the way to the left and the car is still going straight.

Look at the extreme case of doing donuts.  You do donuts  (in a rear-wheel drive car, at least) by holding the front tires still, turning the wheel and powering the rear tires. The forces on the front tires essentially pin the front of the car to the ground, while the rear is free to move. This would be the extreme case of an unbalanced car. Because the front wheels are pinned and the rear wheels aren’t, the car goes in circles.

Back that off a little. Anytime you have unbalanced forces, the car is going to want to rotate. On a dirt track, drivers talk about steering with the throttle – you can turn the car by giving it a burst of power to the rear wheels. The need to steer with the throttle is primarily because of the lack of grip on dirt – the front wheels have less effect on steering. It’s a much smaller effect on asphalt. But the principle is the same. When you have better braking in the front wheels than the real wheels – or vice-versa – the car is going to want to turn. You can use that to your benefit to some extent, but remember the goal is always balance.

Hence the need for brake bias.  Let’s say, for example, that we have a front-heavy car. If we apply the same braking force to the front and the rear, we’re going to lock up the rear brakes before we’ve gotten everything out of the front brakes we need.  We need to bias the brakes so that the front gets more braking force.

Remember that the distribution of the car’s weight on the four tires is always changing, depending on whether the car is speeding up, slowing down or turning a corner. So no one brake bias setting is perfect through an entire corner. You’re generally optimizing for corner entry, so you an be the last car braking into the turn.  You want to build up as much speed as possible down the straightaway. If you can wait a fraction of a second longer than your competitors to put on the brakes, you’ve got an advantage.

Traditionally, set-up changes can only be made during pit stops. The brake bias is the one thing the driver can change from inside the car. (At the moment, at least.) The driver can’t change brake bias through a corner, but think about what changes over the course of a green flag run. There are two biggies.  Tires change in two ways. For the first few laps, they’re underpressured and heating up. After that, they’re wearing. Both things change their grip levels.

Second, you’re going from a full to an empty fuel tank. The fuel tank holds somewhere around 18 gallons.  Gasoline is roughly 7 lb per gallon, so that’s a rear weight change of about 120 lbs. Brake bias allows the driver to compensate for those rather significant changes in weight distribution – and thus need for braking – inbetween pit stops.  There’s a knob in the car that’s allows the driver to dial in how much brake goes to the front and how much goes to the rear.  I’ll get into the details of how that works in the next post.

 

 

 

Aug 222014
 

TurbulentSmokeA persistent motorsports issue (and not only with stock cars) is the aerodynamic passing problem. You can’t pass without grip. Grip is a direct result of downforce. Downforce comes from two places: the weight of the car (mechanical grip) and the billions and billions of air molecules hitting the car (a.k.a aerogrip).

Racecars are designed to take advantage of aerodynamic downforce. Everything from their shape to the aerodynamic appendages added to the car are all optimized to produce downforce. You can play around with mechanical grip some by adjusting the weight on each corner of the car and trying to control how the weight changes as the car turns, brakes and accelerates. Aerodynamic grip is even more subtle.

And complicated.

Aerodynamicists think about fluid flow (fluid meaning liquid or gas) in terms of two extremes. Laminar flow is when the air (or water) moves predictably over a surface in nice, uniform sheets with relatively little variation from sheet to sheet.  In the diagram below explaining how a wing works (an airplane wing; turn your computer upside down if you want to see how a car wing works) the air is represented by nice, neat lines that very politely crowd each other as they work their way around the wing. Changes in pressure and velocity happen gradually.LaminarFlowOveraWing

The other extreme is turbulence – when the air (or water) flows in swirls that are not at all well behaved.  Turbulence is chaotic – large differences in pressure and velocity that change quickly. Turbulence is very difficult to describe mathematically because it’s just so darn complicated.

It’s easier to see experimentally. Pour some cream into your morning coffee and stir it with the back of your spoon. The spoon moves the cream out of the way, creating a gap. The cream swirls around the back of the spoon and fills the gap, forming a lovely swirling pattern.  You can see the same thing in the wake of boat – the water flows back in to fill the gap the front of the boat made. Smoke rising from a cigarette is turbulent as it mixes with the air.  Breaking waves are turbulent.

In turbulent flow, the air molecules end up going in all different directions.  If you’ve ever driven very close to the back of a semi on the expressway, you’ll feel your car buffeted from different directions – that’s the turbulence.

Laminar and turbulent flow are both evident in the aerodynamics of racecars. The front of the car is smooth and sloped.  The cross section of the car (what you’d get if you took a slide of the car perpendicular to the direction the car’s traveling) gets larger and larger as you get further from the front fascia. The car keeps pushing a bigger and bigger hole in the air.

Things change when you reach the B-post.  Now the car needs to push away less air because it’s sloping down. Its cross section is getting smaller. The air starts swirling in around the rear window, becoming turbulent. The wake of a racecar is similar to the wake of a boat. The water’s going in all directions, trying to fill the hole made by the front part of the boat.

A technique called computational fluid dynamics lets engineers visualize the airflow. The diagram here is from Ford Racing and shows the turbulence on the 2013 Ford Fusion.  This visualization shows you where there are big changes in the airflow.  You can see the giant wake behind the car. It’s strongest the closest to the rear of the car, but note that the wake extends almost two car lengths behind the car.

2013_FordFusion_CFDTopandSide

If you want to learn more about Ford’s CFD calculations and the role they play in designing racecars, check out their YouTube video – it’s worth a gander.

The wake creates drag on your car and slows it down just a little, but as the driver of said car, it’s not really a big concern. For the guy running behind me, however, my wake is a really big problem. Laminar air makes downforce. Turbulent air doesn’t.

And that’s the origin of the passing problem. A fast car catches up with the car ahead of it. As the trailing car research the leading car’s back end, the turbulence from the wake of the first car makes the flow over the front of the trailing car turbulent, which means the trailing car loses downforce or becomes ‘aeroloose’. And you can see from the CFD calculation that you don’t have to get so close for aeropush to become a problem.

Right after the Michigan race last week, NASCAR ran a big test (10 teams) to try out some options for possible rules changes for the 2015 season.  In case you think this is a simple problem to solve, they had two approaches: more downforce and less downforce.

On the increase downforce side, the first change was to a bigger splitter – nine inches tall.  The problem with increasing the splitter is that it unbalances the car. One of they key principles in racing is that you can only go as fast as your least grippy tire. Grip is proportional to downforce.  If you increase the rear downforce without making a commensurate change in the front downforce, you get a really tight car.  Lots of grip in the back, but the front tire – the ones that turn the car – don’t have enough grip.

Increasing the front splitter has its own challenges, so NASCAR turned to dive planes. Dive planes have been used for a long time on sports cars. They’re simply small, curved pieces of metal or carbon fiber composite. NASCAR used two dive planes – one above the other – and put one set on each side of the car.  The dive planes started at the front fascia and swoop upward, ending at the front fender. The pictures below are from the twitter feeds of @nateryan (top) and @2spotter (Joey Meier, bottom).
divePlane_NateRyan

DivePlane_Joey

The principle behind the dive plane is that it takes the turbulent air coming onto the front of the car and funnels it to make the flow more laminar.  More laminar flow should translate to more downforce.

NASCAR made the point that the dive planes may not be part of the final rules package; however, having the dive planes allowed them another little benefit – they could put pressure sensors on the dive planes and measure how the downforce changed for the different configurations.

The ‘prime rules package’ that was tested consisted of the larger spoiler, a lower rear differential gear, and decreased horsepower.  They tested 850hp, 800 hp and 750hp.  The second test package was actually a lower downforce package, in which they went with a smaller spoiler and they removed an underbody piece that had been new this year.  The estimate is that these changes decreased the overall downforce by 28-30%.

And (of course) the drivers were not very enthusiastic about the prime rules package. They liked the lower downforce better. Reporting from the track suggested that the prime rules package gave rise to in-line racing, while the lower downforce package got drivers really excited about possibilities for passing.

Unfortunately, there’s no time for another test because NASCAR really needs to have the 2015 rules finalized pretty darn quick so the teams have time to prepare for next season.  Right now, a bunch of NASCAR engineers are sitting back at the R&D Center, trying to make sense of the gigabytes of data they collected during the test. I’m sort of glad I’m not the one who has to make this decision!!

I didn’t mention one of the big changes in this blog post – the ability for the driver to modify the trackbar position from inside the car, but I will comment on that in the near future.

Aug 082014
 

TirePressureIconRunning on underinflated tires can be dangerous.  Underinflated tires they create more friction and more heat, which leads to not only bad handling, but also can produce structural problems.

You may remember the late 1990’s, when Firestone tires had problems with tires blowing out or treads coming off.  The flat tires caused vehicles to roll over and there were more than numerous (Wikipedia says 250, but there’s no source for the stat)  fatalities and many more injuries. The majority (but not all) of the vehicles on which this happened were Ford Explorers. So there’s a rule now that any car made after 2007 has to have tire pressure sensors that warn the driver when a tire is significantly under-inflated. (Significantly means around 25% under pressure.)

Underinflated tires produce high stresses and temperatures. In a correctly inflated tire, the gas inside the tire supports most of the car’s weight. If the tire is underinflated, then there’s not enough gas pressure and the sidewall of the tire has to support the weight.  An underinflated tire flexes a lot as it rolls, which causes two major problems.  One is that it put more stress on the tire, and the second is that it produces more heat. (Graphic from http://www.tirebuyer.com/education/tire-pressure-and-performance.)TireUnderOverInflation

Underinflated tires heat up faster. You need some heat in the tire for it to work right; but there’s a Goldilocks situation here. Too cold and they don’t have any grip. Too hot and they fail. The pressure has to be just right.

Which makes you wonder, how hard can it be to just put the right pressure in the tires in the first place?

When it comes to racing, it’s because street tires are rarely called upon to sustain the extreme conditions race tires endure.  The issue is what we call tire pressure build – the increase in the tire pressure due to the heating of the tire and the gas inside the tire. At it’s most basic, heat is simply the motion of molecules. The faster the molecules are moving, the higher the temperature.

In a tire, the faster the molecules move (i.e. the hotter they get), the harder they hit the walls of the tire and the higher the tire pressure.  This is why your car’s owner manual tells you to measure the air pressure in your tires when the tires are cold. The pressure changes a lot with temperature. The video below, which I did a few years ago with the National Science Foundation, delves a little deeper into the specifics of tires and gas pressure. (And if the video doesn’t embed correctly, then try here.)

For the temperatures passenger car tires reach, a good rule of thumb is that every 10 degrees Fahrenheit corresponds to 1 psi (pounds per square inch) increase in pressure. NASCAR tires routinely change pressure by 20-40 psi from the tire sitting on the pit wall to the time it’s run a couple of laps. So the pressure you put into the tire is no where near the pressure you have three to five laps into the race. For a tire to have the right pressure during a long run, it has to have a much lower pressure when it first goes on the car. It takes a few laps for the tire to heat up, so the tire is really being stressed in the initial part of a run.

It’s a calculated risk teams take in how low they will start their tires.  Goodyear specifies a recommended pressure for each tire. NASCAR officials often check right front tire pressures in the pits.  The idea is that because the tires work in concert, if you the left front is way off, the car won’t be set up well. But there’s plenty of evidence that teams are getting around the recommendations and starting off with very low tire pressures.

Lee Spencer had an article this week suggesting that NASCAR might finally give in and require tire sensors as a way to get around the “shenanigans” (Her word, but one I love using). She uses the example of the left rear tire failures of the 48 car at New Hampshire a couple weeks ago. Johnson swore it wasn’t because of low tire pressure. Goodyear begged to differ.

TirePressureSensor_McLarenSo why not just put tire sensors on all four tires and let NASCAR officials monitor the tire pressure before the tires go on the car? Teams already use sensors during testing. They’re basically replacement caps for the tires that measure and transmit the tire pressure through an RF (radio frequency) link to a data logger on the car.  Places like McLaren (where the picture at right comes from) already have sensors for both inner and outer liners. The technology is pretty much already there to require sensors on the cars.

So why not just go ahead and put sensors on the cars?

Because NASCAR is very careful when it comes to opening cans of worms. Who will you allow to have access to the data?  The pit official? The team? Just before the tires go on the car, or will you allow access to that data during (or even after) the race? A lot of the same issues that arose when NASCAR made the transition to Electronic Fuel Injection will come into play here. How much technical data do we want the teams to have?

Sure enough, the first thing the husband said when I mentioned the idea to him was “That’d be great because then the teams would know when they had a tire going down.”

Do we want to go there? Are you encouraging crew chiefs to try edgier setups because they expect they’ll have a bit of a warning before they lose a tire? Do you want to take the driver’s ability to sense what’s happening with the car out of the picture?  NASCAR historically has had a de facto ‘no real-time telemetry’ rule for a very long time. The science and engineering in NASCAR go into the developing and preparing the car.  At race time, you turn over the majority of the control to the driver.

The other issue that always comes up is how teams will attempt to game the system. When NASCAR allowed for skew in the rear end of the cars, teams kept pushing the skew until NASCAR had to say “enough”. That we’re even talking about tires and tire sensors is because of teams ignoring the recommended pressures. Pushing the envelope is their job.

The discussions, according to Lee Spencer’s interview of Goodyear’s Greg Stucker, are ongoing, but NASCAR has a tendency to spend quite a bit of time thinking through issues like this before taking action.  The problem needs to be addressed, both to protect Goodyear’s reputation and for driver safety.

 

 

 

Jul 292014
 

My friend at the Milwaukee Journal Sentinal, Dave Kallmann (whose online column should be a regular read for race fans) asked about the confiscated firewalls from the Number 11 car at Indy.  That reminded me of the  first NASCAR race I was supposed to attend as research for my book The Physics of NASCAR. That was California in 2007. I was to follow around the number 19 car, at that time driven by Elliott Sadler and crew chiefed by Josh Browne.

VentedScrewsThen Josh and three other crew chiefs got themselves suspended at Daytona for using bolts in the spoilers that had tiny holes drilled all the way through the shanks. I know what these are (and where to find a picture) because we used to use them in the lab in our vacuum systems.  If you put a bolt into a hole, you trap air in the hole. We’re trying to leave on about 1 in every 1,000,000,000,000 molecules in the vacuum chamber, so it’s absolute critical that we can pull them all out.

The reason the team used… excuse me, I mean allegedly used… the bolts is because it gave the air trapped in the trunk of the car a way out and that should decrease drag and thus increase speed. So my first research race turned out to be Atlanta.

During Indy post-race inspection, some rear firewalls from the number 11 car were confiscated. A firewall is a piece of sheet metal that puts a barrier between the driver and anything you don’t want the driver exposed to, which may include fire, hot oil and other fluids, carbon monoxide, smoke, etc.  I’ve indicated the front firewalls on the picture because I couldn’t find a good picture of rear firewalls.

Firewallpic

Apparently the firewalls, or their positioning, was suspicious to the NASCAR inspectors. It’s possible there was just an error in the way the parts were installed (the teams manufacture a lot of cars and sometimes there are mistakes). But there is also the possibility that moving the rear firewalls around does pretty much the same thing that the vented bolts did: they provide a path for air to get out of the car and thus reduce drag and increase speed.

Seems like a pretty minor thing to move a few pieces of sheet metal around just a little; however, the positions of the sheet metal are specified pretty precisely. Modifying the arrangement can compromise drive safety in terms of an opening allowing smoke or fire into the cockpit.

We’ll see this afternoon whether NACSAR comes down hard on the team because this is a safety-associated issue. It may just have been a mistake; however, NASCAR doesn’t consider intention in determining the severity of a penalty.

Incidentally, Dave does an online race chat on Wednesdays at 1 pm Central. It’s not limited to NASCAR, so if you have a chance, join in the chat.

And can you believe it? I’m finally getting back to Milwaukee and it ends up being a week when the Brewers are out of town the entire week.  I’m going to have to just break down and go see them when they come to town and play the Nats.  At least Karl Ratzsch’s is still there.  And the Zoo.

 

 

 

 

Jul 252014
 

One of my favorite memories from Nebraska was coming home from the I-80 Speedway  covered in a dusting of red clay. (Planting in that clay was another question entirely.) But there’s something about dirt tracks you just don’t get at the asphalt ones.

It’s great that NASCAR is bringing a little of that spirit to the Truck Series.  Wednesday’s Mudsummer Classic race at Eldora Speedway was a great show. A dirt track challenges drivers and their crew chiefs who are mostly used to asphalt and an occasional foray into concrete.  A lot changes when you trade surfaces: Set ups, driving approaches, pit strategy and, perhaps most significantly, tires.

Some people made a lot of noise about Goodyear and their ability to produce a tire that would stand up to dirt-track racing, but Goodyear has a long history of equipping cars for the dirt. The tire they developed for NASCAR’s first foray into dirt in 40 years (in 2013) was based on rhwie standard 10-inch wide dirt modified tire. They started testing with that tire with Tony Stewart and the Dillon brothers (that sounds like a gang from the Wild West, doesn’t it?)  at Eldora back in October 2012.   They modified the tire based on that test, making it a wider (11 inches) to help with grip. Trucks, being on the order of a thousand pounds heavier than a typical modified, require greater force to turn.

In addition to the individual tire construction, Goodyear also provided some built-in stagger.  Put a red Solo cup on its side and give it a push. It automatically rolls in a circle because the drinking end has a larger circumference (distance around) than the bottom. The same strategy is used for race vehicles that only turn one direction.  At the 2013 race (I’ve been unable to find the specs for this year), the left-side tires had a circumference of 85.8 inches compared to 88.5 inches on the right.  Teams got four sets of tires for the event.

Goodyear_Wrangler_G23DirtTireRegardless of whether they were left- or right-side tires, the tires used at Eldora have two primary differences compared to the standard truck tire used at the other races.

The first is that dirt-track tires have treads. The Goodyear Wranglers employed for the Eldora race use the G23 tread pattern (shown at left). The treads serve the same purpose they serve on rain tires – they provide a path for loose dirt to move out of the way so that the rubber can grip the track. The edges of the blocks provide bite. The tread compound used is a softer compound, which again improves grip on the dirt surface.

The second difference is that the tires used at Eldora are bias-ply tires, not the radial tires that are standard in NASCAR.  Let’s back up a little and remember a little tire anatomy 101.  The diagram at right is from the Michelin website (the people who led the way in popularizing radial tires way back, just a little after WWII).

Tire_Anatomy_new

The part we’re interested in for the sake of this discussion is the plies.  Plies are a type of fabric made up of layers of rigid cords embedded in rubber. The body plies run from the outer bead to the inner bead. The reinforcing fibers (cords) have been made from materials like cotton, rayon, polyester, steel, fiberglass and aramids like Kevlar.

If you ask Goodyear which materials they use in their race tires, they will neither confirm or deny any particular material or combination of materials they use — which should give you an idea how important the cords are in terms of giving the tire desirable properties..

tirecords_wikipediaIf everything works the way it should, you will never see tire cords. When a driver hears during a race that the tires that were just removed had cord showing, they know there’s either a problem with the setup or with their driving. The cords are there for strength and stability. A tire’s grip comes from the tread – not the cords.

The plies have directionality, depending on how cords are arranged.  They are usually parallel to each other – it you look carefully at the picture to the left, you can see the rows of cording.  Whether a tire is radial or bias-ply depends on the way the cords point.

The first tires appeared in the mid-1800s and were simple pieces of solid rubber. They provided a bumpy, uncomfortable ride. The pneumatic tire was introduced in the mid 1800’s, but a tire made soley of rubber didn’t last very long, nor allow you to go very fast. Tires needed to be stronger and more resistant to road hazards. The idea of using cords in tire plies to increase strength was introduced in the early 1900s. A number of plies was used – two to four was common – to reinforce the rubber in the tread.w

The first tires were bias ply tires, which literally means that the cords in different plies were oriented in alternating directions running between 35-60 degrees with respect to the bead, which increases the strength of the tire, as shown in the diagram below, at right.

radialvsbiasply

In a radial tire, the cords go straight across the tire, as shown in the left picture. The cords are 90 degrees with respect to the direction you’re going. The radial tire was patented in 1915 and, as I mentioned earlier, Michelin really pushed the development of the radial tire starting in 1946.  Radial tires didn’t offer as smooth a ride as bias-ply tires; however, the gasoline crisis of the 1970’s made people value their improved gas mileage (which happens because they have less friction.) Almost all passenger car tires at this point are radials.  Because the radial cords go in one direction, the tire isn’t as strong, which is why radial tires have an additional belt package. Belts made of steel, polyester or Kevlar-type polymers are inserted over the plies and under the tread. Those belts greatly increase the strength of the tire..

Bias-ply tires create more friction and thus more heat and more wear.  A bias ply tire is inherently round, which means that the contact patch is smaller. Extra rubber has to be built up at the shoulders to provide a flat surface. Importantly, the sidewall and the tread on a bias-ply tire are one piece, which means that they move in concert with each other.  As a result, a bias-ply tire will give more in a turn because of the lateral force. The separate belt package in the radial separates the sidewall (the plies) from the tread (the belts) so the sidewall can flex, leaving the tread to hug the ground. Bias ply tires give more and allow better grip on an irregular surface like dirt. Radials would make the racing a lot harder — and not just for the drivers who don’t have much prior experience on dirt!

Many lower level series, as well as weekend racers, use bias-ply tires for another very important reason: they’re (in general) cheaper than radial race tires. This doesn’t mean they’re cheap, but it does help cut costs a little, which means more money to put toward going faster.