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Dr. Diandra: How teams race the same NASCAR Cup car at different tracks

After winning last week, Ross Chastain’s crew chief Phil Surgen noted that the car sitting in Talladega’s Victory Lane was the same car they’d won with at COTA.

“Really comes down to just the whole Next Gen platform,” Surgen said. “All the cars are the same, have the same range of adjustments, same group of parts. All the cars right now are essentially universal. We took that car after it was done at COTA, cleaned it up, set it up a little bit differently to come here.”

It turns out “cleaning up” a Next Gen car and setting it up “a little bit differently” is a major undertaking. So much of one that you may question whether it’s even fair to call it “the same car.”

Next Gen anatomy

Think of a car’s chassis as its skeleton. Before the Next Gen car, teams made their own chassis by welding steel tubing to NASCAR’s specifications. Now teams buy already welded chassis pieces and bolt them together. The change is similar to making a bespoke bookcase versus buying one from IKEA. While your sturdy, made-from-scratch furniture will never change, you can disassemble and re-assemble the IKEA bookcase. Of course NASCAR has tech inspections and IKEA doesn’t.

A graphic showing the three main parts of the Next Gen chassis.

The main portion of the chassis comes in three pieces, as shown in the diagram above. The center section protects the driver from impacts, heat, flames and fumes. The front sub-frame assembly supports the engine and front suspension (among other things), and the rear sub-frame assembly does the same for the fuel cell and rear suspension.

Everything on the car, from suspension to body panels, attaches to the chassis. That means every aspect of the assembled car depends on precise chassis assembly.

A tolerance for units

NASCAR fabricators (and inspectors) talk in thousandths of inches. Sometimes, instead of saying ‘thousandths’, they say ‘thou’. This confuses people because most of us think ‘thou’ is thousands instead of thousandths. A thousand is 1,000 and a thousandth is 0.001. When I learned machining in graduate school, we called one one-thousandth of an inch a mil. You’ve likely seen that unit before on trash-bag boxes. A typical kitchen trash bag is a little less than a mil thick. A good outdoor trash bag is about three mils.

But since NASCAR sticks with thousandths, so will I.

To give you an idea of the sizes we’re discussing, here’s a photo of a ruler onto which I’ve superposed some values.

A photo of a ruler, with units shown in mils

As the graphic shows, 250 thousandths is 1/4 inch, 125 thousandths is 1/8 inch, and 0.160 inches is a little over 5/32 of an inch.

Why did I pick 0.160 thousandths?

Because that’s the maximum value of a little-talked-about part used in connecting chassis pieces. A part that gives teams the ability to introduce small amounts of asymmetry into the car.


Shims are pieces of material typically used as fillers and spacers. You’ve probably seen packs of wedge-shaped wood shims in the home improvement store. They taper from just about nothing to a quarter inch. You break off the size you need. Those shims are good for things like leveling cabinets against an uneven floor or wall.

Precision shims, such as those used in manufacturing, can be made of any material and often come in increments of one one-thousandth of an inch (0.001").

NASCAR allows teams to put shims of up to 160 thousandths of an inch between the main section and each of the sub-assemblies. In the diagram below, the shims are in red, the bolts used to attach them to the front sub-assembly are in dark blue, and the nuts are orange.

A graphic showing the shim placement between the center and front sub-assembly sections

Graphic Credit: NASCAR Rule Book

Teams aren’t required to make all four shims the same thickness — or even to use shims on all four attachment points. Using different combinations of shims, they can shift the front and rear of the car slightly up, down, right or left. Because everything attaches to the chassis, those shifts ultimately affect everything from the suspension to the aerodynamics. The shim itself isn’t very big, but making that little of a shift at the join with the center section produces a larger shift by the time you get to the front or rear end of the car.

Cleaning up

When a car returns from the track, most teams remove everything from the door tops down. They remove the wrap and unbolt each composite body panel. They remove the engine, fuel cell, suspension, transaxle, hoses... even the windows.

Teams keep detailed notes on each car part. They know where, when and how long each piece was used. The team retires parts at the end of their reliable lifetimes. They clean and examine parts that can be re-used.

If the car will run at a track similar to the track it came from, the front and rear sub-assemblies might be left in place — especially if the car performed well. More often than not, though, the front and rear sub-assemblies come off, too.

Setting up the car “a little differently”

Teams reattach the same or different front and rear sub-assemblies with shimming specific to the track at which that car will race next. Chassis positioning determines suspension geometry, which affects everything from skew to travel. Different tracks call for different springs and shocks, different gear ratios, and sometimes, even different side windows. Teams replace body panels, then re-wrap the car in the sponsor’s colors.

This is not a fast process.

“It would be really difficult to turn (a car) around every week,” Surgen said. “Every third or fourth week, with the cleanup and the prep time that goes into it, you could use it every third or fourth week pretty easily.”

What about tech?

NASCAR’s Next Gen chassis design gives teams a fair amount of adjustment space. But how much does that matter given that each car must pass the Optical Screening System (OSS) in tech inspection?

According to the NASCAR rule book, the OSS allows +/-150 thousandths tolerance on body panels. If a measurement is specified at 3.000 inches, the car passes if the measurement is between 2.850 inches and 3.150 inches. That gives the team just short of a third of an inch from minimum to maximum value.

The OSS allows tolerances of +/-200 thousandths on the windshields. Teams get 0.4 inches of leeway there. As we saw last week, they attempt to use every last thousandth they can.

Is it the same car?

When a team talks about bringing ‘the same car’, that might mean only that they’re using the same center section, or even the same three main chassis sections. But most everything else on the car could be entirely different from what was on it at the last race.

Some people have suggested that the interchangeability of car parts means that anyone can put together a Next Gen car. That’s a huge oversimplification. There is a still a box within teams must work. Putting together a car requires high precision tools, experience and lots of knowledge to ensure the car provides the biggest advantage possible, but still passes inspection.