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The Science of Fast

The Dirt on Radials vs. Bias-Ply Tires

ne of my favorite memories from Nebraska was coming home from the I-80 Speedway covered in a dusting of red clay. (Planting in that clay was another question entirely.) But there's something about dirt tracks that you just don't get at the asphalt ones.

One of my favorite memories from Nebraska was coming home from the I-80 Speedway  covered in a dusting of red clay. (Planting in that clay was another question entirely.) But there’s something about dirt tracks you just don’t get at the asphalt ones.

It’s great that NASCAR is bringing a little of that 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.

Some people made a lot of noise about Goodyear and their ability to produce a tire that would stand up to dirt-track racing, but Goodyear has a long history of equipping cars for the dirt. The tire they developed for NASCAR’s first foray into dirt in 40 years (in 2013) was based on rhwie standard 10-inch wide dirt modified tire. They started testing with that tire with Tony Stewart and the Dillon brothers (that sounds like a gang from the Wild West, doesn’t it?)  at Eldora back in October 2012.   They modified the tire based on that test, making it a 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.

In addition to the individual tire construction, Goodyear also provided some built-in stagger.  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 bottom. The same strategy is used for race vehicles that only turn one direction.  At the 2013 race (I’ve been unable to find the specs for this year), the left-side tires had a circumference of 85.8 inches compared to 88.5 inches on the right.  Teams got four sets of tires for the event.

Goodyear_Wrangler_G23DirtTireRegardless of whether they were left- or right-side tires, the tires used at Eldora 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 for the Eldora race use the G23 tread pattern (shown at left). The treads serve the same purpose they serve on rain tires – they provide a path for loose dirt to move out of the way so that the rubber can grip the track. The edges of the blocks provide bite. The tread compound used is a softer compound, which again improves grip on the dirt surface.

The second difference is that the tires used at Eldora are bias-ply tires, not the radial tires that are standard in NASCAR.  Let’s back up a little and remember a little tire anatomy 101.  The diagram at right is from the Michelin website (the people who led the way in popularizing radial tires way back, just a little after WWII).

Tire_Anatomy_new

The part we’re interested in for the sake of this discussion is the plies.  Plies are a type of fabric made up of layers of rigid cords embedded in rubber. The body plies run from the outer bead to the inner bead. The reinforcing fibers (cords) have been made from materials like cotton, rayon, polyester, steel, fiberglass and aramids like Kevlar.

If you ask Goodyear which materials they use in their race tires, they will neither confirm or deny any particular material or combination of materials they use — which should give you an idea how important the cords are in terms of giving the tire desirable properties..

tirecords_wikipediaIf everything works the way it should, you will never see tire cords. When a driver hears during a race that the tires that were just removed had cord showing, they know there’s either a problem with the setup or with their driving. The cords are there for strength and stability. A tire’s grip comes from the tread – not the cords.

The plies have directionality, depending on how cords are arranged.  They are usually parallel to each other – it you look carefully at the picture to the left, you can see the rows of cording.  Whether a tire is radial or bias-ply depends on the way the cords point.

The first tires appeared in the mid-1800s and were simple pieces of solid rubber. They provided a bumpy, uncomfortable ride. The pneumatic tire was introduced in the mid 1800’s, but a tire made soley of rubber didn’t last very long, nor allow you to go very fast. Tires needed to be stronger and more resistant to road hazards. The idea of using cords in tire plies to increase strength was introduced in the early 1900s. A number of plies was used – two to four was common – to reinforce the rubber in the tread.w

The first tires were bias ply tires, which literally means that the cords in different plies were oriented in alternating directions running between 35-60 degrees with respect to the bead, which increases the strength of the tire, as shown in the diagram below, at right.

radialvsbiasply

In a radial tire, the cords go straight across the tire, as shown in the left picture. The cords are 90 degrees with respect to the direction you’re going. The radial tire was patented in 1915 and, as I mentioned earlier, Michelin really pushed the development of the radial tire starting in 1946.  Radial tires didn’t offer as smooth a ride as bias-ply tires; however, the gasoline crisis of the 1970’s made people value their improved gas mileage (which happens because they have less friction.) Almost all passenger car tires at this point are radials.  Because the radial cords go in one direction, the tire isn’t as strong, which is why radial tires have an additional belt package. Belts made of steel, polyester or Kevlar-type polymers are inserted over the plies and under the tread. Those belts greatly increase the strength of the tire..

Bias-ply tires create more friction and thus more heat and more wear.  A bias ply tire is inherently round, which means that the contact patch is smaller. Extra rubber has to be built up at the shoulders to provide a flat surface. Importantly, the sidewall and the tread on a bias-ply tire are one piece, which means that they move in concert with each other.  As a result, a bias-ply tire will give more in a turn because of the lateral force. The separate belt package in the radial separates the sidewall (the plies) from the tread (the belts) so the sidewall can flex, leaving the tread to hug the ground. 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.

 

1 thought on “The Dirt on Radials vs. Bias-Ply Tires

  1. Radial tires have ruined competitive NASCAR racing. With bias-ply tires, stagger was more important in a car’s set up and drivers had to know their set up and how to take care of their tires. Radial tires make drivers out of everyone and their consistence makes for boring races. It has created follow the leader racing and mega dollars spent on making each car aerodynamically the same. It is no wonder one third of the seats are empty at most races.

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