Tire technology has improved by leaps and bounds over the centuries, beginning with the use of iron bands that were heated in forges and quenched around wooden cart wheels and eventually evolving into the self-inflating, puncture-proof rubber compounds with which we are so familiar today. Many improvements in tire technology have been driven by the fierce competition on race tracks and drag strips, and by those in pursuit of that one one-hundredths of a second that can separate winner from loser. Only here are cars, drivers, and tires pushed to their absolute limits. Two types of tires have come to dominate drag racing – drag radials and bias-ply drag tires.
Makers of everything from street legal drag radials to Top Fuel super slicks, almost half a century’s worth of experience has helped make Mickey Thompson Performance Tires and Wheels not only a leader in the industry, but also one of its most recognizable names.
Mickey Thompson: The Racer, the Legend, and the Company
Marion Lee Thompson, or ‘Mickey’ as he came to be known, began his illustrious career working for the Los Angeles Times covering racing, and eventually became involved in the (then new) sport of drag racing. Thompson went on to become the first American to break the 400 mph land speed record, and ultimately set more speed and endurance records for cars than any other man in history. He was also an innovative designer who dabbled in everything from cylinder heads to whole competition engines.
Despite all of his achievements behind the wheel, perhaps the most important contribution made by Thompson to the racing community was the founding of a company oriented around manufacturing high performance tires for the weekend warrior and professional racer alike. Although Thompson and his wife were tragically murdered in 1988 by gunmen at their home in California, Thompson’s legacy continues to live on through the innovative and groundbreaking tires that bear his name.
Mickey Thompson Performance Tires and Wheels has always managed to stay on the cutting edge of racing technology. Today, they offer a wide line of street legal drag radials, racing slicks, and off-road tires, for a variety of vehicles.
A few of their important innovations include: the Wide/Low tires on 12” wheels that are in use at the Indy 500; the raised white letters visible on the sidewalls; development of the world’s fastest DOT approved street tire; and the creation of the widest and strongest forged aluminum wheel.
Back to Basics: A Brief History of Bias-Ply vs. Radial Tires
Developed by the Goodyear Tire and Rubber Company near the turn of the 20th century, bias-ply tires consist of layers of cord plies that are used to strengthen the casing. The plies are laid in alternating patterns diagonal to the bead cord, at angles ranging from 30 to 55 degrees depending on manufacturer. The tire bead itself is a small groove that keeps the tire locked into the wheel when it is inflated. Initially made from cotton, cord plies were later made from rayon, which was ultimately replaced by nylon in the mid-1940’s.
Bias-ply tires dominated the industry for over sixty years, until Michelin struck a deal with Sears in 1966 to mass-produce radial tires. Radial tires were developed a few years after Word War II, and first appeared in the ’50s on Packards. Radial tires utilize the same basic concept of cord-strengthened rubber, but with one important difference: the cords radiate at a 90 degree angle from the tire bead, going across the width of the tire from one lip to the other.
The radial tire design has several advantages over the bias-ply. The radial design allows for a greater contact patch with the ground, as well as greater
sidewall flexibility – thereby substantially improving steering (more ground contact = better handling). Radial tires also have less rolling resistance, allowing for increased gas mileage. By 1973, just as the first domestic gas crisis was hitting our shores, radial tires became standard equipment on almost every car, and the bias-ply tire faded to a supporting role.
Engineering the chemical compounds that go into these tires is an extremely involved process. We turned to Ken Warner and Carl Robinson at Mickey Thompson for some in-depth answers. While they wouldn’t divulge any specifics about the types of compounds Mickey Thompson uses, they did discuss how complicated the research and design is, requiring the efforts of a trained compounder. “The compounder is essentially a chemist,” states Warner. “He works with the various ingredients required to generate properties on the tread that are in alignment with the prospective goals for the tire.” The types of compounds used will determine such things as grip, stickiness, life of the tire, and safety.
Slicks vs Drag Radials
Bias-ply drag tires are ‘softer’ than drag radials and are generally used on lightweight, high horsepower vehicles such as Top Fuel dragsters, as well as by professional Heads-Up competitors. Radials are made from a harder compound and work better on heavier vehicles, which is part of the reason why certain radials can be used on the road.
Why Radials Replaced Bias-ply on the Street
Bias-ply road tires do not make for an enjoyable ride, as the softer compounds tend to result in a jello-like ride quality. They also follow grooves and cuts in the road, meaning your car may shift more than you’d want it to as compared to a radial tire. This is in large part the reason that radials replaced bias-ply tires on all passenger vehicles.
When it comes to safety, Mickey Thompson has built a strong reputation for manufacturing safe, reliable products by conducting arduous testing. “The most prolific test used in the market today is simply known as ‘The Wheel’,” says Warner. “This test takes a mounted and inflated tire and runs it against a rolling surface while applying a pre-determined load with a specific cycle count. Other common tests include an impact test and a radial load test. We use any and all lab testing available to ensure that Mickey Thompson produces safe reliable products.”
Bias-ply for Big Horsepower
Going through how tires are made and the chemically-enriched design process behind them brings us back to the tire that started it all. “Bias-play tires are going by the wayside,” according to Robinson. The technology in straight-up drag tires has not changed much, aside from the tire compounds and the cord strength. Eventually, technology will evolve beyond bias-ply and it is likely that we will even see radials on the highest performing hot rods.
So why do people still use bias-ply tires for heavy-duty drag racing? Because bias-ply tires are designed for some slippage off the line, which heats them up and results in better traction. A controlled spin makes it easier to fly through the first few gears, as well as to recover traction. They are also inherently stronger than radial tires, although they must be wider in order to maintain a large contact patch with the track. This is especially important for cars with a lot of horsepower, like Top Fuel dragsters. However, because bias-ply drag tires are designed to work as one unit, the sidewall strength is inevitably less than that of radials, resulting in “wrinkling.”
A key disadvantage of bias-ply tires comes from their tendency to “grow” and distort into an egg-shape as they get up to maximum speed. Mickey Thompson tires are generally considered “low growth” tires and will typically only distort between 1 to 1 ½” up to 150 mph. This is one of the few improvements that has been made to bias-ply drag tires over the years. Because of their construction, radial tires do not suffer from growth, and they generally last longer than bias-ply on both the street and the strip.
Because of the way in which the cords are positioned relative to each other, there is only so much that can be done to improve the 100-year-old technology of bias-ply tires. Mickey Thompson realizes this, and while they are always trying to advance their entire lineup of tires, make no mistake about it – radial tires are coming to dominate the drag strip for good reason.
Developing the Drag Radial
Developing a radial drag tire was a groundbreaking process for Mickey Thompson, and it was a tremendously difficult process from start to finish. They began back in 2001, starting with the 30×9 R15 – a popular tire choice among a variety of racers. However, because radial tires have unique properties on the drag strip, doing so successfully took time. “Radial drags have a dead hook,” explains Robinson. “So when you leave the line, it can be almost impossible to recover.” Drag radials give a whole different driving dynamic to the drag car and also require more internal air pressure, due to the interaction between the sidewall and tread. According to Robinson, “A radial tire should have around twenty pounds of [air] pressure, while a bias-play should be around twelve to fourteen pounds.”
After deciding on a tire size, Mickey Thompson cut the tread depth down and began a long series of experiments. “We started with a concept and went from there,” explains Robinson. That meant a lot of testing, on a lot of different cars, under a lot of different conditions. They researched things like temperature before, during, and after burnouts and straight-line runs. While there are a vast number of possible combinations, with so many variables involved, Mickey Thompson tested as many as they could to ensure that all of their tires equally balanced safety, reliability, and performance.
Drag Radials for the Future
Because of the way that radials hook right off the line (as opposed to allowing some controlled slip), you won’t typically see much (if any) difference in the 60ft times. However, the lower rolling resistance of radial tires can make a difference farther down the drag strip. If the tires can roll more easily, less horsepower is lost as a result of spinning. Those who run radials typically see lower ET’s, averaging in the .08-.12 range. That doesn’t sound like much until you consider the fact that the five fastest Top Fuel Dragster record holders are within .03 seconds of each other, and Top Fuel Dragsters still use ultra-wide bias-ply tires!
Radials also offer the distinct advantage of being much more applicable for street use. Mickey Thompson’s ET Street Radials are essentially DOT-approved racing tires for street use, and have proven to be faster than the bias-ply ET Streets that Mickey Thompson also offers. “These tires have exploded onto the market, offering incredible traction and performance, from cars on the street to the ‘Outlaw’ style racers seen in sanctions all over the world,” states Warner. “Drag radials share more in common with standard passenger car tires than they do with bias-ply drag tires.” As a result, they are more easily and safely converted to street use, and are little more than radial drags with rain grooves (called voids). This allows weekend warriors to run the same tire on the street as they do at the strip. The radial placement of the belts and the tread compound sets them apart from other standard and drag tires, and the water grooves allow them the slimmest margin of road worthiness.
However, there are also radial drag tires that have been designed solely for use on the strip. These are used in the ‘legal’ classes of races, such as Stock, Super Stock, and Comp Eliminator, and are used most often in cars equipped with automatic transmissions. Because drag radials dead-hook off the line and are constructed more stiffly than bias-ply tires, they are very unforgiving in regards to regaining traction. Consequently, they also often require much more work to the chassis in order to get the most out of them. So, while radials have several distinct advantages over bias-ply drag tires, they certainly won’t solve all of your on-track traction issues.
Mickey Thompson is constantly looking to improve both the carcass construction and the compounds they use, to improve safety, stickiness, and overall performance. According to Robinson, improving radial tires “is a natural progression for us.” Because of the advantages radials offer, they can be a cheap, easy way to improve your ET. This is especially true in Heads Up racing, where a few tenths of a second can mean the difference between victory and defeat. “This is huge for people who couldn’t really compete in Heads Up competition before,” states Robinson. Keep in mind, anyone who is running drag radials should also be using an automatic transmission. Doing so will prevent the tires from shaking or the engine from bogging down, since the torque converter provides a smooth launch off the line. Putting drag radials on a manually-equipped car in not unheard of, but doing so requires a more sensitive launch so the tires don’t spin.
Everyday Race Tire Advice
Whether you choose a drag radial or a drag tire, there are certain tips you should follow to ensure that you get the most from them. Ken Warner has this advice for all would-be drag racers.
• When storing drag tires or radials, keep them off the ground and out of the sunlight. Also, reduce air pressure to 3-6 PSI to retain the tire’s preferred shape.
• When inflating a tire, always understand where the boundaries are. Your tires will require different air pressure under cool conditions and maximum horsepower than on a hot day when the track is at 140 degrees with 70% humidity.
• Mickey Thompson does not recommend rim screws on radials. This is not to say that it hasn’t been done, but there is the potential for extreme bead seat damage from tires literally ripping along the bead when using screws. They recommend using bead lock wheels to secure radial tire applications.
• There is no set life span on any tire, radial or not. The only determining factor is your right foot, according to Carl Robinson. While Mickey Thompson is constantly looking to improve tire longevity, the length of burnouts, power level, and driving habits are all determining factors in tire life.
• In order to know when it’s time to replace your tires, it’s first extremely important to track the performance of your car and the set of tires you are running. When the performance begins to drop off, it will become evident. Warner suggests that racers – especially those competing in Heads Up drag racing – should consider having two sets of tires in service, so there is always a reliable back-up with a “known” performance window. This will keep you from beating yourself.