IndyCar continuing analysis of Kirkwood’s Indy 500 crash

IndyCar continuing analysis of Kirkwood's Indy 500 crash
IndyCar continuing analysis of Kirkwood's Indy 500 crash

The NTT IndyCar Series has completed its investigation into the root cause of the failure that saw the left-rear wheel assembly on Kyle Kirkwood’s Andretti Autosport car break free and sail over the Turn 2 grandstands during the Indianapolis 500 on May 28.

From the ongoing analysis conducted by IndyCar and its official chassis supplier Dallara, a mandatory, series-wide componentry update to address the matter will be implemented starting this weekend at the Hy-Vee doubleheader at Iowa Speedway.

Kirkwood was hit by the crashing Arrow McLaren car driven by Felix Rosenqvist, who impacted the Turn 1 wall, slid down through the infield, and rolled back onto the racing line at Turn 2 where the No. 6 Chevy clipped the side of Kirkwood’s No. 27 Honda as it streaked by.

After having a significant portion of the left-rear corner torn away, Kirkwood’s three-wheeled car rotated, hit the SAFER barrier, and inverted. Momentum carried the car hundreds of feet down the backstraight before it came to a stop. Both drivers emerged unhurt from the incident.

To the great relief of the series and the Indianapolis Motor Speedway, the No. 27’s intact wheel assembly – comprised of a tire, wheel, wheel hub, wheel bearings, and wheel nut – did not come into contact with fans after clearing the fence, instead landing in a parking lot where it struck a parked car and came to rest.

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With decades of experience using steel tethers as safety devices to hold wings, suspensions, and other items to an IndyCar’s central spine in the event of an accident, the departure of the No. 27’s left-rear wheel and related attachments came as a surprise.

Kirkwood’s detached wheel initially raised questions about whether the tethers (in blue) had failed or been severed.

It led to questions of whether the tether had been severed or somehow failed in the hard hit from the No. 6, but as the investigation revealed, the tether was not the source of the corner’s disconnection from the No. 27. The investigation’s target quickly moved to the rear suspension and what’s believed to be a first-time failure.

The analysis centered on a suspension component called the upright, which carries the brake calipers and discs, wheels and tires, and connects to the top and bottom suspension A-arms and pushrods that actuate the dampers. Specific to the rear suspension, the upright also joins the rear wheels to the transmission through driveshafts that are affixed to the wheel hubs positioned in the center of the uprights.

Drawing from the crashed items Dallara had to inspect and the virtual accident recreation done by the company, the left-rear’s separation has been attributed to a freakish failure of the large retaining nut that holds the wheel hub and wheel bearings into the upright.

The severe hit from the No. 6, which applied an immense pulling force on the entire wheel and hub assembly, ripped the retaining nut off the threads on the back of the hub. And with nothing left to keep the hub, bearings, wheel, tire, and more inside the upright, the package exited the car in an instant and fired over the Turn 2 fence.

The left-rear suspension tether system, like the rest of the tethers that were strengthened for 2023, did its job by keeping the upper and lower a-arms and the upright attached to Kirkwood’s car.

In response to the findings, Dallara has designed a new nut that is wider and machined from a different grade of steel, which has been done to decrease the likelihood of it being stripped off of the hub’s threads that protrude through the back of the upright. The revision, said to be 60-percent more robust than the outgoing nut, should greatly reduce the chances of a repeat failure in the future.

Dallara has designed a wider nut (shown on the right) with the aim of reducing the likelihood of it being stripped from the wheel hub’s threads compared to the original version (left).

IndyCar teams were informed through an official technical bulletin last week that the new wheel hub nuts manufactured by Dallara are required for use on the rear hubs from Iowa onwards.

“The parts and pieces from the 27 car were shipped to Dallara in Italy immediately after the race and they got right on this,” IndyCar president Jay Frye told RACER. “A few weeks ago, they finished a synopsis and reenactment of what happened, and it was extremely well done. One of the conclusions they came up with was to update the rear wheel retaining nut, so that’s the first update we’re making.

“And there’s more that happened in other parts of the crash that we’re continuing to evaluate and it’s possible for other updates to come from the analysis. Safety is very important to IndyCar and this new retaining nut is a relatively simple and impactful change that we can make immediately based off what they learned.”

The upgraded assembly (shown with the nut attached to the hub) will be introduced at Iowa this weekend.

Dallara put the new retaining nuts through load testing in Italy before signing off on the pieces and recommending their adoption by IndyCar.

“It was an unforeseen event, but if it happened once, it could happen again if changes aren’t made,” Frye continued. “The moment it was recommended and approved, Dallara did a great job of manufacturing them for the whole field so they could go on the cars as quickly as possible because we’re doing everything we can to make sure it doesn’t happen again.”


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