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Racing Technology
Street-legal slicks
by Wayne Scraba
Once upon a time, recapped Atlas Bucrons were the hot ticket for drag race cars. Eventually, tire technology evolved to where dedicated drag slicks were readily available. With that came classes of competition in which "street" tires with a tread pattern were required, and the cheater slick was born.
The cycle continued, and good old-fashioned slicks became the norm for almost all serious drag race applications. But then, fastest street car drag racing blossomed. Sticky street-tire compounds were released. The tire wars heated up, and before long, tire technology made an almost complete circle. No, we're not back to recaps, but there are a number of extremely capable cheater slicks available in today's marketplace.
What's the point? It's simple. The rules for many categories of street car drag racing mandate a tire with Department of Transportation (DOT) approval. In these categories, if you show up with slicks, you can't play. But the DOT requirement also places a heavy burden on the tire manufacturer: The company has to develop a tire that can effectively cope with an almost obscene amount of horsepower, but at the same time, it must pass a rigid set of requirements laid out by DOT.
Matters of mass and compound
Because of the mix of requirements, the "street" tires used in street car drag racing might look exactly like slicks with a couple of grooves cut in them, but they have a number of subtle and not-so-subtle differences.
One issue is weight. Two important factors have an influence on the overall mass of a fastest street car tire. The first thing to keep in mind is the fact a typical fastest street car is quite often heavier than it's true drag race counterpart, even if the dimensions of the tires are identical. It's not uncommon for an Outlaw Pro Street racer to tip the scales at a figure that's very close to 3,000 pounds or more. That car can have an ultimate performance on par with a current NHRA Pro Stock car, but the Pro car weighs 2,350 pounds.
Of course, it's no secret that drag slicks for unlimited applications are often constructed as light as possible. That just isn't the case with a tire that is forced to pass a DOT requirement. A DOT tire must have a load range, and it must have that load range cast into the side of the tire.
Using the Mickey Thompson 33x18.5-15LT shown in the accompanying photos as an example, the overall tire weight is 43 pounds. A similarly sized conventional M/T E.T. Drag slick tips the scales at 37 pounds. Where does the weight come from? The company points out that more belt material was required in the E.T. Street race tire. This made for a more rigid sidewall and, of course, a heavier tire.
In simple terms, the manufacturer is forced to build a more robust tire if it has to conform to heavier cars and DOT requirements. M&H Tire perhaps sums it up best: "The manufacturer must offer the right product for the weight of the vehicle. That's the manufacturer's job."
Compounding is another issue to be addressed by the manufacturer. A couple of avenues are taken by companies that have entered the street car drag racing arena. For example, Hoosier compounds its Quick Time Pro street tires differently than it does its drag tires. M&H Racemaster has developed an entire new line of compounds just for its newest HB series of street car drag race tires. On the other hand, Mickey Thompson uses compounds identical to the company's most popular drag slick compound, M5. Below is a compound chart supplied by Mickey Thompson Tires comparing its E.T. Drag slicks and E.T. Street tires.
Burnouts and compounding
The type of burnout you perform is related to the tire compound. Typically, for a car equipped with M5 compound tires, M/T offers this advice: "For the first pass of the day, make a hard burnout. Once that is accomplished, for an automatic transmission car, make a fairly hard burnout on the first pass or two. After that, a light burnout should suffice. For a stick-shift car similar to a Super Stocker, perform a light burnout, haze the tires, and stage immediately. Generally, tires work better with a light burnout rather than a hard burnout. E.T. Street tires may require a fairly hard burnout on the first and second pass to break them in."
In terms of life, these new-generation fastest street car tires are much the same as slicks, and slick life can vary from car to car. According to the experts, inconsistent 60-foot and 330-foot times are caused by tread wear or carcass breakdown. These should be signals that it's time to change tires.
Though slicks have wear indicators on the tread face, street car tires don't. Typically, when the grooves in a street car drag race tire are gone, then it's time to buy new rubber (the various sanctioning body rules might also stipulate that some tread must remain). Remember, cars that launch hard can cause the tire carcass material to break down. A good rule of thumb is to inspect your tires carefully after 30 passes. And if the car is very quick, the tires should be inspected more frequently.
Totally tubular
Street car slicks are typically designed as tube-type tires. There are a number of reasons for this, primarily safety. In addition, a tube helps maintain air pressure. It's not uncommon for a tubeless fastest street car tire to deflate quickly — that even includes several of the more streetable types with a full compliment of tread.
M/T states that tubes will enhance the reaction time, increase the stiffness of the tire, and reduce sidewall shock and deflection when launching the car. Finally, the manufacturers polled claim that a tube helps with consistency because the tube actually absorbs some of the heat from the tire.
Which tubes are correct? In drag racing, a natural rubber tube is the most common. It's not a good idea to use tubes designed for large trucks because they are not sized correctly. For the most part, each manufacturer of drag race street tires will have an appropriate tube size for a given tire. What if you don't use the correct tube? Figure on replacing tubes often. One of the biggest causes of tube failure is the use of an incorrectly sized tube.
According to Mickey Thompson Tech Bulletin No. 2, you may notice a low spot or wrinkle in a tube-type tire. This is often caused by the wrong tube size. It can also be caused by a faulty installation. To confirm this, break down the tire and rotate it on the rim. You'll note that the low spot (if it is still there) has not moved. If you break down the tire and replace the tube with a standard tubeless-style valve stem, then the low spot or wrinkle is eliminated. This should tell you that the tube is the root of the low spot. To eliminate wrinkles, the tube must be inflated and deflated during installation. If the wrinkle persists, you may have to lubricate the tube with talcum powder and/or break down the tire and work the wrinkles out by hand.
Size matters
What range of tires are available? You might be surprised at the choices. Using Hoosier and Mickey Thompson as examples, sizes range from eight inches wide by 26 inches tall all the way to 22 1/2 inches wide by 35 inches tall, which have true tread widths of 17 inches! Mickey Thompson has 14 tires in its E.T. Street series, and Hoosier offers 17 tires in its Quick Time Pro series. Mickey Thompson sizes are in the 15- and 16-inch rim-diameter range, and Hoosier's lineup covers 14-, 15-, and 16-inch rim-diameter sizes.
In terms of rim width, each manufacturer publishes a range of wheel rim widths that are applicable to the tire you select. This is most often referred to as the "design rim" in the application charts. Typically, street drag tires are available in sizes to fit rim widths ranging from six inches to 16-plus inches.
On another front, M&H Racemaster is in the process of releasing an entire new line of DOT race tires. In the past, the M&H tires were of primarily a full-tread design. The new lineup, called the HB-10 series, is based on the in-vogue cheater-slick configuration; in other words, shallow grooves and maximum dry weather, racetrack traction.
What's interesting about the M&H lineup is its focused approach to sizing. Many of the rules currently in style for fastest street car drag racing limit certain classes to a 10-inch-wide tire. For practical purposes, these tires are most often 11.5 inches wide (the maximum most sanctioning bodies will accept). None of the rules limit diameter, and as a result, M&H will offer at least three tires to fit this rule: an 11.5x30 tire, an 11.5x31 tire, and a monstrous 11.5x32 tire (shades of vintage Top Fuel tires). Other sizes will be offered as well. Expect to see these tires available for sale within 30 days.
Air apparent
How much air pressure is required for these tires? According to Mickey Thompson Tires, proper air pressure is critical in its E.T. Street drag tire, especially from a performance perspective. M/T notes that recommending air pressure isn't easy because there are so many variables involved. For example, the weight distribution of the car, the transmission type, the chassis setup, wheel size, and other factors add up, and they can have an effect on operating pressures.
M/T offers this advice: "In drag racing, many racers feel that less is better with regard to air pressure. This is not always the case. While there are exceptions to every rule, we have found higher pressures generally work best with Mickey Thompson tires. Not only do the higher pressures lead to quicker times, but they also contribute to a safer, more stable ride at the finish line."
Mickey Thompson has six categories as recommended starting points for tire pressure. According to M/T, the actual optimum air pressure may vary significantly, depending, of course, on the above variables. The recommended baseline pressures are as follows:
In the end, you're going to find that tires designed especially for street car drag racing aren't slicks with a couple of grooves cut in them. Instead, they're dedicated race tires designed from the ground up for the application. Due to the reduced tread depth and the compounds used in these tires, they should be used on dry pavement only and aren't suitable for highway use. Remember, these are specialized race tires.