NASCAR started transitioning away from bias-ply tires in 1989. So why did they start making a comeback in 2013?
Dirt Tires are Different
One of my favorite Nebraska memories was coming home from the I-80 Speedway covered in a dusting of red clay. (Planting in that clay was another question.) NASCAR is bringing a little of that dirt spirit to the Truck Series.
Wednesday’s Mudsummer Classic race at Eldora Speedway was a great show. A dirt track challenges drivers (and their crew chiefs), who are mostly used to asphalt and an occasional foray into concrete. A lot changes when you trade surfaces: Set ups, driving approaches, pit strategy and, perhaps most significantly, tires.
Goodyear has a long history of equipping cars for dirt racing. The tire they developed for NASCAR’s first foray into dirt in 40 years (in 2013) was based on their standard 10-inch wide dirt modified tire. They tested that tire with Tony Stewart and the Dillon brothers (that sounds like a Wild West gang, doesn’t it?) at Eldora back in October 2012. After that test, they made the tire wider (11 inches) to help with grip. Trucks, being on the order of a thousand pounds heavier than a typical modified, require greater force to turn.
Goodyear provided much more stagger than they do for asphalt or concrete to help with turning. Put a red Solo cup on its side and give it a push. It automatically rolls in a circle because the drinking end has a larger circumference (distance around) than the base. The same strategy helps race vehicles that only turn one direction.
Belts and Treads
Dirt tires have two primary differences compared to the standard truck tire used at the other races.
The first is that dirt-track tires have treads. The Goodyear Wranglers employed at Eldora use the G23 tread pattern (shown at left). The treads serve the same purpose they serve on rain tires. The front edges of the blocks provide bite into the dirt. The grooves give loose dirt a path to move out of the way so that the rubber can grip the track. The tread compound is softer, which again improves grip on the dirt surface.
The second difference is that dirt tires are bias-ply instead of radial. The first tires appeared in the mid-1800s. They were pieces of solid rubber that provided a bumpy, uncomfortable ride. The pneumatic tire was introduced in the mid 1800’s, but they wore quickly and punctured easily.
A tire ply is a fabric made of layers of rigid reinforcing cords embedded in rubber. The idea of using cords in tire plies to increase strength was introduced in the early 1900s. The reinforcing fibers (cords) can be anything from cotton, rayon, polyester and steel to fiberglass or Kevlar.
The ‘bias’ part of the bias-ply comes from the use of the word bias to mean ‘at an angle’. That comes from sewing, where cloth is often cut ‘on the bias’: at a 45-degree angle to the grain of the fabric. It makes the fabric drape better.
The plies alternate directions, with the cords running between 35-60 degrees with respect to the bead. The diagram below, right, labels the plies as ‘cord body’.
The cords in a radial tire go straight across the tire from bead to bead (left, above). Because the radial cords go in one direction, the tire isn’t as strong. Belts made of steel, polyester or Kevlar-type polymers are laid over the plies and under the tread.
Radial vs. Bias Ply Tires
The radial tire was patented in 1915. Michelin led the way in popularizing radial tires after WWII. But radial tires were at a disadvantage. Belts make radial tires more rigid. A radial tire can’t deform easily to match the shape of the surface. That means they offer a rougher ride than bias-ply tires. However, the gasoline crisis of the 1970’s made mileage more important than comfort.
Bias-ply tires have advantages and disadvantages. Bias-ply tires have friction and thus generate more heat and more wear. Bias ply tires are inherently round, which means that the contact patch is smaller. Extra rubber has to be built up at the shoulders to provide a flat tread surface.
But — importantly for racing — the sidewall and the tread on a bias-ply tire are one piece. They move in concert with each other. As a result, a bias-ply tire will give more in a turn. The separate belt package in a radial tire lets the sidewall flex.
Bias ply tires give more and allow better grip on an irregular surface like dirt. Radials would make the racing a lot harder — and not just for the drivers who don’t have much prior experience on dirt!
Many lower level series, as well as weekend racers, use bias-ply tires for another very important reason: they’re (in general) cheaper than radial race tires. This doesn’t mean they’re cheap, but it does help cut costs a little, which means more money to put toward going faster.
NOTE: This post was refreshed on 2021-03-24 to make the explanations clearer and clean up the formatting.