BRISTOL, Tenn.—NASCAR's decision to turn back the clock for a Cup Series race on dirt—the first in more than 50 years—meant drivers in NASCAR's top-tier racing series were racing on bias-ply tires for the first time in more than 30 years.
As part of its strategy to boost interest in the 2021 NASCAR season, series officials and teams agreed to run one race this year on a packed dirt surface that's quite common among lower-tier, regional racing series.
Bristol Motor Speedway, site of the race, is a paved 1/2-mile banked oval; for this one-off event, the track hauled in 23,000 cubic yards of dirt over the course of several weeks in January and February and used Global Satellite Positioning (GPS) equipment on the bulldozers and graders to ensure it was laid down properly.
There is nine to 10 feet of dirt in the corners for a 19-degree banking and one to two feet on the front and backstretches. The racing surface on the track was about 50 feet wide, according to Steve Swift, senior vice president of operations and development for Speedway Motorsports, the track's operator.
The tires designated for this race feature a block-style tread pattern that was used for the NASCAR Truck Series as recently as 2019.
The leading edge of the blocks bite into the dirt and the grooves in the tread help evacuate the dirt, Goodyear said. The also has been widened to 11 inches, versus 10 inches on tires for modified, to give the NASCAR cars and trucks more grip.
A key reason for using bias-ply tires, according to Greg Stucker, Goodyear's director of racing, is that bias tires don't have belts under the tread area and thus are more "compliant" and able to conform to irregularities of the dirt surface. In addition, the tires designated for mounting on the cars' left, or driver, side are significantly shorter to build in more stagger between the left- and right-side tires.
Goodyear began transitioning from bias play to radial tires in NASCAR competition in 1989. None of the current Cup Series drivers has run a bias-ply tire in a Cup series race previously, Stucker noted.