The NextGen Chassis

The NextGen chassis evolves the NASCAR-designed Car of Tomorrow (COT) chassis. Let’s examine what’s different and what’s the same.

The COT chassis marked the first time NASCAR dictated the position of every last piece of tubing in a chassis — not to mention how to weld it together. They put everything known about motorsports safety at the time into it, and improved it when warranted. We’ve had fourteen years with very few serious injuries, and zero deaths.

How do you improve on that?

We know even more now, plus there’s strong imperative to cut costs. Add that to prior experience, and you’ve got the Next Gen Chassis.

What Is a Chassis?

We rarely see bare chassis, but they are the most important part of the race car from a safety perspective. A chassis is the structural basis of a car: The engine, suspension, fuel cell, etc. all mount to the chassis. The car’s body — the part with fancy wraps and decals — covers the chassis.

Think of the chassis as a car’s skeleton and the body as its skin. The graphic below (courtesy of NASCAR) shows the Next Gen car on the left and the NextGen chassis (along with other features) on the right.

A graphic from NASCAR showing the Ford Mustang on the left and a blueprint on the right showing the NextGen chassis and other features

The NextGen Chassis

I’ve removed some of the distracting stuff from the chassis in the illustration below. Sorry I don’t have access to a better diagram, but hopefully, you can discern the five chassis components. From left to right:

  • The Rear Bumper Assembly
  • The Rear Subframe
  • The Center Assembly Section (where the driver sits)
  • The Front Subframe
  • The Front Bumper Assembly
A clean-up schematic from the blueprint to highlight the Next Gen chassis
Only two joining spots are visible in the diagram, but there are four for each connection.The inset, from a video put out by Stewart Haas, shows a close-up of one of the connections. Note that they’ve run the lower bolts the opposite direction from the upper bolts.

For comparison, here’s a drawing of the original COT chassis

A schematic drawing of the COT NASCAR chassis
The original COT chassis

Similarities and Differences

  • The COT instructions listed criteria for the front and rear bumper supports, but the NextGen bumper assemblies will be uniform for all cars.
  • Unlike the COT chassis, NextGen chassis components bolt together instead of being joined with welds. Bolts should make it easier, faster and cheaper to fix a wrecked race car. Only the damaged part needs to be switched out.
  • The NextGen roll cage offers even more protection for the driver.
    • While still leaving room for a roof hatch, the top of the roll cage offers more protection.
    • Additional cross members (some of which were added to the COT after notable wrecks) stabilize the roll cage.
    • There looks to be more protection in the front.
  • The rear subframe fully encloses and protects the fuel cell in the NextGen chassis compared to the COT chassis.

Why Stick With Steel for the NextGen Chassis?

In 2008, I asked then-NASCAR VP of Competition Robin Pemberton what made a metal exotic, and thus prohibited by NASCAR rules. His answer:

If you have to ask… it’s exotic.

-Robin Pemberton

NASCAR’s relaxed its attitude toward materials since then. For example, we’ve seen increasing amount of carbon fiber composite in places like dashboards, mirror mounts and seats. The NextGen splitter introduced an innovative new composite material.

The engineers designing the NextGen had choices for constructing the chassis.

Carbon fiber and related (i.e. hemp and flax) composites are very strong and very light. They’re also difficult to recycle, create sharp shards when they break, require specialized manufacturing techniques, and are expensive.

NASCAR did consider a hybrid, sports-car-like chassis, but ultimately stuck with steel.

Metals are versatile, easily sourced, and more easily manipulated without a lot of fancy tooling. Before moving to composite bodies, fabricators used to form everything on the car’s body with basic metalworking tools such as the English wheel, which is about as simple a tool possible.

The COT chassis mandated ‘magnetic steel’, which NASCAR tested by sticking a magnet to it. But that still leaves a plethora of possibilities.

All About Steel

Steel is an alloy — a combination of one or metals that produces properties different than either metal by itself. The simplest variety — the kind that’s been found dating back to 1800 BC — is simply iron mixed with a little carbon. Steel was probably discovered by accident, when carbon from the fires used to work iron got mixed in.

A “little” carbon means between a few tenths of a percent to around two percent. For every one-thousand iron atoms, there are a few to a dozen carbon atoms. Although small in number, those carbon atoms have a HUGE effect on the steel’s properties. Depending on how much carbon you add, you can make the steel easier to shape, stronger and/or less likely to crack.

And, of course, if a little carbon makes iron better, why not add other things? Like chromium. Or silicon. Or molybdenum, nickel, niobium… Chromium added to iron makes stainless steel, which doesn’t rust, but is heavier than regular steel.

Each combination of elements yields different properties. Start with six trace additions, add options for heat treating, cryogenic cooling, cold working, hot working, quenching, and any other number of processing techniques and you have an infinite number of steel types with almost any property you could want. As you might expect, the more complicated the process and the costlier the ingredients, the more expensive the steel.

NASCAR actually defines steel as “composed of a majority of iron while including carbon and other elements, which has a density between 0.265 and 0.300 lbs/in3 (i.e. 7.33 – 8.30 g/cm3).

Stock Car Steel

Stock car chassis generally use steel tubing with round or rectangular cross sections. Steels like 1018, 1020 or 1026 include (in addition to iron and carbon) very tiny amounts of phosphorous, sulfur and manganese. That makes them ideal for this purpose because they can be cold worked and fabricated to form essentially seamless tubes, with precise inner and outer diameters.

Chrome-molybdenum steel alloy, known as 4130 or chromalloy, includes has tiny, tiny amounts of chromium, manganese, molybdenum, phosphorus, sulfur, and silicon. 4130 is stronger than the one-thousand series mild steels, with about the same density.

If you make two identical parts, the chromalloy part will be much stronger than the mild steel part. Or, as as race would look at it: I can get the same strength as mild steel with a lighter chromalloy part.

There are even dual-phase steels that combine the best of two different types of steel, like Docol. Docolo is stronger then chromalloy by 10-15%, but you can’t heat treat it, or it loses it strength.

Choosing the right steel is a balance between the properties you want, how hard you want to work to make the chassis and how much you’re willing to pay.

Why Not Exotic-Metal NextGen Chassis?

Given the importance of safety, why not move to newer, ‘space age’ metals found in rockets and planes? Note that when people refer to ‘aluminum’ or titanium’ in these contexts, they rarely mean the metal. They’re talking about one of the many possible alloys.

Aluminum alloys are about half as strong as steel, but a quarter of the density. You may need a larger (or thicker) piece to get the same strength, but the aluminum piece will still be lighter.

Titanium alloys are 30% stronger than steel and 50% lighter. And although they’re 60% heavier than aluminum, they’re twice as strong.

But… and you knew there was going to be a but… these metal alloys are from ten to a hundred times more expensive. In addition, some require much more specialized welding techniques.

But there’s another reason why NASCAR didn’t build the strongest possible NextGen chassis: The strongest chassis isn’t the safest.

The Goldilocks Chassis: Strong, but Not Too Strong

The faster a car goes, the more motion energy it has. A race car going 200 mph has motion energy equivalent to a couple pounds of TNT. All of that motion energy must change into other types of energy when the car stops, whether that’s a pit stop or a hard hit against the wall.

When a car pits, motion energy slowly converts into sound (squealing brakes), heat (brake rotors and tires heat up) and light (sparks). The conversion takes tens of seconds as the driver slows from race speed to pit road speed and finally to a stop.

A graphic explanation of conversation of energy in a pit stop.

When a car crashes, all that motion energy must transform much faster — sometimes in milliseconds. Heat, light and sound still play a part (the sound of crunching metal and squealing brakes), but these mechanisms can’t transform energy fast enough.

So other types of energy become involved, like crunching energy (deformation), spinning energy (rotational), and the energy it takes to move a SAFER barrier or another car.

A graphic explanation of conservation of energy in a crash.

Here’s the key: You want to transform energy any way except those that involve your driver

If the chassis doesn’t give, then all the energy that could be dissipated by the car transfers to the driver. We once thought that a stiff chassis and unstretchable belts were ideal, but research showed that a little give is better. So there’s no need to resort to expensive titanium alloys when good ol’ steel actually does a better job.

So Why Not A SAFER-Type NextGen Chassis?

So if a somewhat floppier chassis is better from a safety standpoint, but not make a SAFER-style chassis that could absorb a lot of energy. Here’s the problem: All of the suspension components — shocks, springs, roll bars, etc. — are also attached to the chassis. If a spring doesn’t have something to push against, it can’t do its job. In addition, chassis are subject to tremendous forces as they go around the track. You don’t want wheels moving out of position every time you turn.

Designing a safe and fast chassis is just one more Goldilocks task in NASCAR: You have to find parameters that are just right.

For more about energetics and crashes, check out this video I did awhile back for the National Science Foundation. It’s a little dated but while NASCAR may change, basic physics doesn’t.

Useful Videos

NASCAR shows some features of the modular chassis.

Stewart Hass takes delivery of their first NextGen chassis.

If you want to watch the giant robot welders at Technique Chassis, who make the NextGen chassis, here’s a video. (Note that all of the robot welds are inspected by expert welders to ensure uniformity.)

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