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 five parts of the Next Gen chassis.
Graphic credit: NASCAR Rule Book

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.

NASCAR Clash heat race lineups


LOS ANGELES — Justin Haley, Kyle Busch, Christopher Bell and William Byron will start on the pole for their heat races Sunday at the Los Angeles Memorial Coliseum. 

There will be nine cars in each of the four heat races. Here’s a look at each of the those heat races.

Clash heat race starting lineups

Heat 1

This heat has four drivers who did not make last year’s Clash: Alex Bowman, Aric Almirola, Chris Buescher and Ty Dillon. Almirola starts second, Bowman third, Buescher eighth and Dillon ninth. This heat also has defending Clash winner and reigning Cup champion Joey Logano, who starts fifth.

Heat 2

Richard Childress Racing teammates Busch and Austin Dillon start 1-2. This race has five former champions: Busch, Kyle Larson (starting third), Kevin Harvick (fourth), Martin Truex Jr. (fifth) and Chase Elliott (eighth).

Heat 3

Toyota drivers will start first (Bell), second (Denny Hamlin) and fifth (Tyler Reddick). Ryan Blaney starts last in this heat after his fastest qualifying lap was disallowed Saturday.

Heat 4 

Byron will be joined on the front row by AJ Allmendinger in this heat. The second row will have Ross Chastain and Bubba Wallace.

The top five in each heat advances to Sunday night’s Clash. Those not advancing go to one of two last chance qualifying races. The top three in each of those races advances to the Clash. The 27 and final spot in the Clash is reserved for the driver highest in points who has yet to make the field.

Justin Haley tops field in Clash qualifying


LOS ANGELES — Justin Haley posted the fastest lap in Saturday’s qualifying for the Busch Light Clash at the Los Angeles Memorial Coliseum.

Haley will start the first of four heats on the pole after a lap of 67.099 mph (13.413 seconds). The four heat races will be held Sunday afternoon, followed by two last chance qualifying races and then the Busch Clash on Sunday night.

Clash qualifying results

“I feel pretty confident about where we are,” Haley said. “I’m not sure why we’re so good here.”

The top four qualifiers will start on the pole for their heat race.

Kyle Busch, who was second on the speed chart with a lap of 66.406 mph, will start on the pole for the second heat. That comes in his first race with Richard Childress Racing after having spent the past 15 seasons at Joe Gibbs Racing.

Christopher Bell, third on the speed chart with a lap of 66.328 mph, will start on the pole for the third heat. William Byron, fourth in qualifying with a lap of 66.196 mph, will start on the pole in the fourth heat race.

The pole-sitters for each of the four heat races last year all won their heat. That included Haley, who was third fastest in qualifying last year and won the third heat from the pole.

Ty Gibbs was not allowed to qualify because of unapproved adjustments his team made while making repairs to his car after the door foam caught fire during practice. NASCAR deemed that the Joe Gibbs Racing team made adjustments to the car not directly related to the damage.

Ryan Blaney‘s fastest qualifying lap was disallowed after he stopped the car in Turn 4 and turned it around and to go back to the backstretch and build speed for his final lap. NASCAR disallowed the time from that final lap for the maneuver.

Section 7.8.F of the Cup Rule Book states: “Unless otherwise determined by the Series Managing Director, drivers who encounter a problem during Qualifying will not be permitted to travel counter Race direction.”

The top five finishers in each of the four 25-lap heat races advance to the Clash. The top three in the two 50-lap last chance races move on to the Clash. The final spot in the 27-car field is reserved for the driver highest in points not yet in the field.

Chase Briscoe, AJ Allmendinger in first on-track conflict of the season.


LOS ANGELES — The first on-track conflict of the 2023 NASCAR Cup season?

Did you have Chase Briscoe and AJ Allmendinger?

They made contact during Saturday night’s practice session at the Los Angeles Memorial Coliseum for the Busch Light Clash.

Busch Clash practice results

Briscoe explained what happened from his point of view.

“(Allmendinger) was slowing down so much on the straightaway to get a gap (away from other cars),” Briscoe told Motor Racing Network. “I felt like I was beside him pretty far down the straightaway. I got in there a little hot for sure, but, honestly, I thought he was going to give it to me since we were in practice. Went into (Turn) 3 and he just drove me straight into the fence. Definitely frustrating. … Just unfortunate. We don’t have a single back-up car out there between the four of us at SHR. 

“Definitely will set us behind quite a bit. Just chalk it up in the memory blank.”

Asked what happened with Briscoe, Allmendinger told MRN: “He ran inside of me, so I made sure I paid him back and sent him into the fence.

“It’s practice. I get it, I’m struggling and in the way, but come barreling in there. I just showed my displeasure for it. That’s not the issue. We’re just not very good right now.”

Earlier in practice, Ty Gibbs had to climb out of his car after it caught on fire. Gibbs exiting the car safely. The Joe Gibbs Racing team worked on making repairs to his No. 54 car. NASCAR stated that the car would not be allowed to qualify because of unapproved adjustments, modifications not directly related to the damage.

NASCAR will not race at Auto Club Speedway in 2024


LOS ANGELES — Auto Club Speedway will not host a NASCAR race next year because of plans to convert the 2-mile speedway into a short track.

It will mark only the second time the Cup Series has not raced at the Southern California track since first competing there in 1997. Cup did not race at the track in 2021 because of the pandemic.

Dave Allen, Auto Club Speedway president, also said Saturday that “it’s possible” that the track might not host a NASCAR race in 2025 because of how long it could take to make the conversion. 

MORE: Details for Sunday’s Clash at the Coliseum 

NASCAR came to the Fontana, California, track during the sport’s expansion in the late 1990s that also saw Cup debut at Texas (1997), Las Vegas (1998) and Homestead (1999).

Auto Club Speedway begins the West Coast swing this season, hosting the Cup Series on Feb. 26, a week after the Daytona 500. The series then goes to Las Vegas and Phoenix the following two weeks.

Auto Club Speedway has been among a favorite of drivers because of its aging pavement that put more of the car’s control in the hands of competitors. 

Allen said that officials continue to work on the track’s design. It is expected to be a half-mile track. With NASCAR already having a half-mile high-banked track (Bristol) and half-mile low-banked track (Martinsville), Allen said that a goal is to make Auto Club Speedway stand out.

“It has to make a statement, and making sure that we have a racetrack that is unique to itself here and different than any of the tracks they go to is very important,” Allen said. “Having said that, it’s equally important … to make sure that the fan experience part is unique.”

Kyle Larson, who won last year’s Cup race at Auto Club Speedway, said that he talked to Allen on Saturday was told the track project likely will take about 18 months. 

“I don’t know exactly the extent of what they’re doing with the track, how big it’s going to be, the shape or banking and all that, and I love the 2-mile track, but I think the more short tracks we can have, the better off our sport is going to be,” Larson said.

With Auto Club Speedway off the schedule in 2024, it would mean the only time Cup raced in the Los Angeles area would be at the Los Angeles Memorial Coliseum. NASCAR has a three-year contract with the Coliseum to race there and holds the option to return.

Sunday’s Busch Light Clash at the Coliseum marks the second year of that agreement. Last year’s inaugural event at the Coliseum drew about 50,000 fans. NASCAR has not publicly stated if it will return to the Coliseum next year.