In the last laps of this year’s Indy 500, a wheel — a wheel tethered to its racecar — escaped the racetrack. The wheel/tire flew over the catch fence and — miraculously — through the narrow space separating two grandstands packed with of people.
In the end, the wheel damaged a white Chevy Cruze badly enough that it had to be towed. Track President Doug Boles did all the right things, which made for a good story in the end.
But those of us who remember wheels going into the grandstands and killing fans had a rough couple of minutes there waiting to find out the wheel’s fate. A wheel escaped a car at Indy in 1987 and killed a fan.
In 1998, a wheel came off a CART racecar at Michigan International Speedway, producing three fatalities and six injuries.
In 1999, an IndyCar race at Charlotte Motor Speedway was stopped when a loose wheel (and associated debris) killed three people and injured at least eight others.
These incidents led to requirements that teams tether the wheels to the car.
Whither Tethers?
Wheel tethers are polymer ropes that attach the wheel assembly to the car. You can see the wheel tether at work in the photo below (from For the Win). The tether holds the front wheel onto0 the car, even though every other attachment is gone.
IndyCar says that the tether didn’t fail. I interpret this to mean that they recovered the tether and it was in one piece. I would welcome an explicit statement to that effect, however.
Tethers are made from Zylon, a polymer very similar to Kevlar, but even stronger by weight. Most motorsports series that mandate tethers mandate that they be made from Zylon. A strip of Zylon is used across drivers’ helmet visors to protect them after Felipe Massa was struck by a car piece. Both F1 and IndyCar use Zylon sheathing to prevent objects from penetrating the cockpit.
While Zylon might sound like a wonder material, it has its issues. Like many polymers, it degrades in ultraviolet light and it doesn’t like being rubbed. IndyCar recently decreased rear wheel tether lifespans to two years from three years, and increased the diameter of the tether.
So What Happened?
A tether must loop around some part of the car. You can’t attach a tether directly to a wheel because the wheel rotates. Here’s how NASCAR tethers their wheels. (I don’t have access to the documents for IndyCar.)
The Zylong tethers connect the uprights to the chassis. As long as the wheel/lug connection remains solid, the wheel isn’t going anywhere.
Assuming the tether on the IndyCar retained its integrity, the impact between the two cars must have sheared off the structure the tether was wrapped around.
Investigation and Results
All of motorsports wants to know why the wheel came off because these types of tethers are so ubiquitous in the industry.
I hope IndyCar will share their investigation results with the other sanctioning bodies. Even one wheel leaving a racetrack is one wheel too many.
How much force is required to put the tether? Looking at the replays it looks like the rear wheel contacted the front wing of the other car. I wonder if the wing end plate might have cut the tether.
That’s a hard question to answer. The specs say 22,000 lbs, I think. But there’s a difference between pulling on something with 22,000 lbs and trying to slice though it. Jagged carbon fiber can be really sharp.