Broken Brake Rotors show Next Gen’s Limits

Four of the eight ‘accidents’ listed on the official caution for yesterday’s WWTR at Gateway Cup Series race were not cars running into each other, but rather broken brake rotors.

Rotors are a spec part. Four rotors out of 144 total isn’t a large enough fraction to suggest there was something wrong with the rotors.

So why did these brake rotors break?

What’s a Rotor?

The rotor is a disc that rotates with the wheel, between the brake calipers. Pressure from the brake pads slows the rotor (and thus the car.)

I took the photo below at Las Vegas last March. The best thing about sole-source parts is that very few teams tell you not to take pictures of their cars anymore.

A photo of a rotor from the No. 41 car at Las Vegas in March 2023
This photo shows a rear wheel with the rotor setup for Las Vegas.

The temperature sticker changes colors according to temperature. After a practice run, it tells the crew chief the hottest temperature reached.

The comma-like grooves on the rotors help with cooling by directing airflow.

On the view below, you can see a splotch of red temperature-sensitive paint on the rotor edge. You can just see the vents along the sides of the rotors. That’s the primary cooling mechanism for the rotor.

The paint turns colors according to how hot it gets. It provides only the maximum temperature reached. It’s old technology, but it works.

A photo from March 2023 during inspection at Las Vegas showing the temperature-sensitive paint on the edge of the brake rotor
This photo shows the front wheel, with the Las Vegas brake rotor.

AP Racing provides all rotors for all teams. NASCAR allows only two rotor models for the Cup Series: a light-duty and a heavy-duty rotor. The sanctioning body specifies which rotors may be used for which track. The same set of calipers are used for the front wheels at all tracks. The same for the rear wheels.

Why Did They Break?

While some series (like Formula 1) use carbon-fiber brake rotors, NASCAR sticks with cast iron because they are durable and much less expensive. Cast iron is a great material, but it does not like thermal shock.

Most materials expand when heated and shrink when cooled. When the expansion and cooling happens quickly, it tends not to be uniform. That causes different parts of the material to change size at different rates and by different amounts. The material becomes strained. If it collects enough strain, it cracks.

Clint Bowyer used the example on the television broadcast that if you take a glass out of the dishwasher it put something cold in it, it will shatter. (It sounded like the voice of experience. You tend to remember physics when the lesson includes explosions.)

At short tracks, the brake rotor stay pretty hot. At superspeedways, the rotor is employed mostly for pitting.

Gateway has tight corners and long straightaways. The driver brakes coming into each set of turns and is usually off the brake by the middle of the corner. That means the brake has from when the driver releases it and all the way down the straightaway — to cool down.

The driver hits the brake again, and the rotor heats up. Depending on how hard and how long a driver brakes, the rotor will experience a different degree of strain.

When a rotor does crack, the rotation shoots the broken pieces out of the wheel well — as Chase Briscoe found out when a piece of rotor went through his radiator.

What Does That Have to Do with the One-Car Model?

Before the Next Gen, teams had a much wider range of latitude in terms of selecting rotors and calipers. The well-off teams could fine-tune their parts to each track, and to their driver preferences.

The rotor NASCAR has chosen for Gateway isn’t ideal. It’s not ideal for anyone, so it’s perfectly fair.

But the broken rotors this weekend are part of the large problem, which is that is may be impossible to narrow down car parts to such a small set and still offer great racing at every track. That’s not to say NASCAR shouldn’t try. But it may just be impossible to make the same car run great at intermediate tracks and at short tracks.

Or to give teams a choice between only two different rotors as the number of track types included in a season continues to expand

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1 Comment

  1. Dr D….perhaps the larger wheel/tire & brake package has created a situation where the car can be driven deeper before braking harder, spiking temps, then the cycle repeats with less recovery time before the next braking event, eventually stressing the rotor in the current package.

    Oval tracks over a mile weren’t typically the problem, and road courses often had their own package, but in the NextGen those “norms” may no longer exist. Could adequacy of brake cooling (or possibly over-cooling?) be part of the situation?

    I’m just reminded that before 1973, drum brakes were used, and 500 miles at Riverside was a REAL test, though nothing like COTA today…then Donohue & Penske put discs on a Matador, and it was ON. Road course races today are far shorter distance events than Riverside and I’m not certain whether the NextGen rotor failures have all come on ovals, or if any happened at a road course.

    Bigger brakes than those fit behind 15″ wheels of the former Cup & current Xfinity/Truck chassis are common in production vehicles today. Of course the packages used are quite well-developed & specialized….perhaps AP (NextGen brake source) and NASCAR are working on revisions that will put an end to the problems that have been surfacing.

    Just some things to ponder, if you might have data analytics that could bring any focus beyond what seems to be known now.

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