The Evolution of the Stock Car

ADDITION: If you are more comfortable with Portuguese, check out this translation by Artur Weber and Adelina Domingos – and thanks to them for being interested enough in my work to do the tough job of translation.

While many fans decried the look of the Car of Tomorrow, the most significant thing about the COT was a part you couldn’t see: The Chassis.

The first time I was around a stock car, I was surprised at how flimsy the sheet metal on it actually is. The body is for aerodynamics and looks. The real strength of a stock car is in its chassis.

This marked the first time that NASCAR sent  computerized drawing to teams and told them that they not only had to build exactly that chassis, but that the chassis would be inspected using computerized coordinate arms and lasers, marked with RFID tags, and re-checked at the track and after accidents.

Nine years later, we’re back to a more familiar-looking race car, but there have been minimal changes to the chassis underlying it. That’s a testament to the research that went into the original COT. Criticize the COT all you want, but the COT chassis was (is) a huge step forward in terms of safety.

The general chassis philosophy is built around a central roll cage that acts as a steel cocoon around the driver. The rear clip must protect the fuel cell and the front clip must keep the engine from being shoved back into the cockpit; beyond that, the primary goal of these sections is to crush in a predictable manner and absorb energy in a crash.

Although NASCAR has not had a fatal accident on track in any of its top three racing series since 2001, there are still serious accidents. The most concerning are the hard hits that lead to concussions because of the long-term, insidious nature of the injury.

But if you look beyond concussions, the most serious injuries that spring to mind are:

There’s a clear trend: The current car isn’t protecting the drivers’ feet and legs in the worst crashes. This is especially a problem for the larger (meaning taller) drivers. Danica Patrick was involved in a similar crash at Talladega last year. Even hitting a SAFER barrier, the crash bent all three pedals and left her with some pretty bad bruises.  Greg Zipadelli of Stewart-Hass racing, noted that, given her size (or lack of same), she was able to pull her feet out of the way and likely avoided any more serious injuries that way. Your Michael Waltrip or Elliott Sadler (both over 6′ tall) don’t have that as an option.

Even thought SAFER barriers have been extended at many tracks, the NASCAR R&D Center started their own studies to see how they might be able to improve the chassis in this area.

Crashes: It’s All About Conservation of Energy

Energy comes in many different forms: heat, light, sound, motion, chemical and more. The Law of Conservation of Energy tells us that energy cannot be destroyed or created, only changed into other forms of energy.

Race cars have a lot of motion (a.k.a. kinetic) energy. The faster a car goes, the more kinetic energy it has. A race car going 180 mph has nine times the motion energy of a typical passenger car going 60 mph.

A race car going 200 mph has motion energy equivalent to a couple pounds of TNT. When that car comes to a stop, the law of Conservation of Energy dictates that all motion energy the car carries must be transformed into other kinds of energy.

When a car pulls into the pits, motion energy is gradually converted into sound (squealing brakes), heat (brake rotors and tires heat up) and light (sparks). This is a controlled process that takes place over tens of seconds as the driver slows from race speed to pit road speed and finally to a stop.

When a car crashes, all that motion energy has to be transformed in a matter of seconds. Heat, light and sound energies are still involved — the sound of crunching metal, squealing brakes, etc. are all there, but there are new types of energy introduced. A crash introduces other types of energy, such as the energy of deformation (a.k.a. crunching energy) and rotational (or spinning) energy. (Yes, rotational energy is a type of motion energy, but it’s very different from the translational kinetic energy of cars which they’re racing on the track.)

Here’s the key: You want to design the car to give motion energy ways to transform that don’t involve your driver. You want all of the crunching to happen in the car so that the energy to crunch is gone by the time it would otherwise reach the driver.

For more about crashes, check out this video I did awhile back for the National Science Foundation.

The 2017 Chassis Changes

All of their previous experience, plus extensive computer simulations and crash testing have led the NASCAR R&D team to make changes to the chassis construction they hope will better protect the drivers’ legs during crashes.

• The firewall (the metal sheet that separates the cockpit from the engine compartment) will be thicker.
• The floorboard and toeboard area will be made out of one piece and beefed up by construction methods and thickness of materials used.
• There will be about an inch of energy-absorbing foam added to the toeboard area, which will help dissipate energy without crushing around the driver’s legs.
• All of the above changes affect existing parts, but there will be one new piece added: In addition to making the anti-intrusion panel on the driver’s side door thicker, a new piece of anti-intrusion plating will be added behind the driver. This piece will run from the existing anti-intrusion plating on the door bars into the rear sub-frame to further protect the driver from objects entering the cockpit.
• Finally, NASCAR is introducing a new method of fabricating these pieces that will allow for stronger, more extensive welds. (There are literally more than a hundred welds on a chassis and welds are one of the primary places that fail in a crash.)

Here’s the kicker, though: Anytime a crew chief sees the words thicker, “beefed up” or stronger, what he (or she) reads is “heavier”. Accompanying these changes is a new minimum weight: 3275 lb with a 200-lb driver, which is 20 lbs more than before.

But Only on Plate Tracks in 2017

These enhancements were announced last July. They were optional for 2016 and are mandatory in 2017, but only on restrictor-plate tracks. They will become mandatory at all tracks in 2018. (And these rules apply to both XFINITY and Monster Cup teams.)

The extended roll out is in part because the modifications aren’t trivial. One crew chief called them “a massive rebuild of the car“. NASCAR tried to make changes within the existing chassis so that teams didn’t all of a sudden find themselves with a dozen useless chassis. A gradual phase-in gives teams some time to adapt to the changes.

The changes are significant enough that all chassis will have to be re-certified. NASCAR doesn’t have a huge staff at the R&D Center, so they have to give themselves some breathing room to ensure that they have time to do the certifications.

Some of the teams can comply with directives like the additional foam in the toeboard area without major problems only because they have shorter drivers and room for the foam. Meeting this requirement for taller drivers requires a significant re-working of the chassis.

The Handling Penalty

When the changes were announced in July 2016, NASCAR suggested that they were not expected to adversely affect the weight of the vehicle or change the balance, If you mention this to any crew chief in the garage, you’ll likely hear a derisive snort.

One of the big changes with the COT was that the car’s Center of Gravity got a couple inches higher. Over the last few years, teams have pulled out all the stops trying to lower their cars’ centers of gravity by doing things like making instrument panels out of very expensive carbon fiber to remove a half-pound of weight they can then put low on the left side of the car.

These safety changes add weight above the current CG, which means the overall effect is raising the CG – which decreases handling.

What’s a Center of Gravity?

A Center of Gravity (CG) is the point on an object where it will balance perfectly in all directions. For a uniform, symmetrical object, the CG is at the center — even if you’re talking about something like a donut, which puts the CG in the middle of the hole. That’s okay. It’s an imaginary point, so it doesn’t have to be on the object.

If you have a non-uniform, squirrely-shaped object like a racecar, you have to either do an experiment or a bunch of calculations (or both) to figure out where it is. In most circle track stock-type cars, the CG is located to the left of the centerline, somewhere around the driver’s butt. (This varies greatly with specific rules packages and requirements, but in general, it’s a good first approximation.) The weight is weighted toward the bottom of the car, which is why the CG is not at the geometrical center of the car.

What’s that Got to Do with Going Fast?

Here’s why that’s important:

How fast you go depends on how much grip your tires have. How much grip each tire has depends on how much force (mechanical and aero) pushes it into the track.

BUT THERE’S A CATCH (Of course there is): You can only go as fast as your least grippy tire.

Why Does Grip Change as a Car Circles the Track?

We’ve talked elsewhere about the change in aerodynamic force with speed, but the modifications we’re talking about don’t impact that at all. What we’re worried about here is called load transfer.

Engineers break a car into two pieces to understand how the cars behavior changes when accelerating, braking or turning: Sprung mass and unsprung mass. (You can think ‘weight’ when you read ‘mass’ for the purposes of this discussion.)

 

The tires, wheels and axles are connected to the rest of the car via suspension parts like springs and shocks. We can break the car into the all the mass (weight) supported by the suspension (which we call the sprung weight because it’s on springs) and the rest of the car’s mass, which we call the unsprung weight. (And yes, part of the suspension goes with the unsprung weight and part with the sprung weight, which is why I drew the line the way I did.)

Think about what happens to this car when you speed up or slow down. The unsprung weight is in contact with the ground, but the sprung weight isn’t rigidly connected. If you slam on the brakes, the sprung weight is going to keep going forward until it’s pulled back by the springs.

That means when you brake, the load transfers from the rear wheels to the front where. You may have had perfect balance before, but now that you hit the brakes, you have more grip in your front wheels than your rear wheels: You’re loose.

The converse also applies: If you accelerate, the sprung weight shifts from front to rear and you are tight.

Guess what happens when you turn? Yep. When you turn left, the load transfers right.

Now let’s combine these. When you turn left and accelerate (as you would when coming out of a corner), you get load transfer from left to right and front to back. You end up with lots of grip on your right rear wheel and very little grip on your left front.

But remember: You can only go as fast as your least grippy tire. SO even though you’ve got tons of grip on your right rear, you’ve got almost nothing on the left front and you will be slow.

And this has what to do with the new changes?

How much load transfers when you accelerate, brake or turn depends on the CG height. The higher the CG, the more change in weight on your tires. The new safety enhancements are going to raise the CG, which will make the cars transfer more weight and thus harder to handle.

Everyone has to follow the same rules at the plate tracks, so everyone has the same penalty in terms of worse handling.

But when we go to Atlanta, where these safety improvements are optional, how many teams do you think are going to implement them?

None. The competition is so tight that adding another 20 lbs is enough to make you a non-factor.

It’ll be 2018 before we see these improvements at all races in the top two series.

Addendum

Josh Hamilton (one of the good guys in NASCAR) tweeted me with the following:

The total overall minimum weight has been increased (which I pointed out in the bullet list); however, the issue here is not the total weight, it’s the distribution of the weight.

If I go with the new footbox, my car’s CG will be raised. Another team has to have the same weight, but if they’re not using the footbox, they can put that weight in the frame rails, which will lower (or at worse, not change) their car’s CG. So there’s still a handling penalty; but again, the changes will be uniform in 2018 and no longer an issue then.