Our drag race and street-strip wheel and tire series rolls on. This time we’ll look at the rubber part of the equation. Years ago, slicks ruled the roost in terms of drag race tires. You wanted to race? You ran slicks. But today, many popular racing categories mandate radial tires. DOT drag radials were born. Fair enough, but the DOT requirement also places a heavy burden on the tire manufacturer: The company has to come up with 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 the DOT.
Given the strange mix of necessary ingredients, the “street” tires used in street car drag racing might at first glance look 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 things have an influence on the overall mass of a fastest street car tire, and they’re both important. The first thing one has 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” drag radial car to tip the Toledos at a figure that’s more than 3,000 pounds. That same car can run six-second quarter miles.
It should come as no surprise to racers that drag slicks for many applications are often constructed as light as possible (particularly in competitive classes where ET counts). 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 something like a M&H Racemaster P325-50R15 as an example, the overall tire weight is 25 pounds + or – (at least on our bathroom scale). By the way, we weighed a similarly sized drag radial from another manufacturer and it tipped the scales at 29 pounds, while another came in at a fat 37 pounds. On the other hand, a similarly sized conventional drag slick weighed in at 23 pounds. Where does the weight, or perhaps more important, lack of it come from? M&H was careful in the design of their tire. That’s why the weight of their drag radial isn’t over the top, but they do point out that a bit more belt material is required in the drag radial tire. This makes for a more rigid sidewall and of course, a heavier tire. Meeting the DOT requirements is the big reason for the added heft.
When it comes to compounds every manufacturer is different. Using the M&H Racemaster tire as an example, an entire new line of compounds was developed just for their newest “HB” series of street car drag race tires. Here’s how they work:
|Compound:||#8, HB11, #703, #704, #20|
Note that the drag radial “HB” compound is on the scale’s soft side.
So far so good. What about actually using the tire? Let’s look at the burnout first (the type of burnout you perform correlates directly to the tire compound). For their drag radial, M&H offers this advice: “A hard burnout is not necessary. For the first pass of the day, make a light to moderate burnout. After that, a light burnout should suffice. Continue the burnout until the engine starts to pull down. A dry hop after the burnout isn’t recommended. For a stick shift car, perform a light burnout, haze the tires and stage immediately. Generally speaking, drag radials work better with a light burnout rather than a hard burnout. Drag radials tires may require a fairly hard burnout on the first and second pass to break them in.”
How long does a set of drag radials last? Modern drag radials tires are similar to slicks, and as most of you know, slick life tends to vary from combination to combination. It also depends upon how hard you are on the tires in the burnout box. The real measure of tire life is inconsistency in the 60-foot and 330-foot times. They’re most often caused by tread wear or carcass breakdown. Keep in mind that while slicks have wear holes on the tread face, street car tires don’t. When the grooves in a drag radial are gone, then it’s time to buy new rubber (the various sanctioning body rules usually stipulate that some tread must remain). Another consideration is this: Cars that hook hard can breakdown the tire carcass. Every thirty or so laps, it’s a good idea to go over your tires carefully (if you have a seriously fast, heavy car, it’s not a bad idea to go over the tires before every race).
What about tubes? Believe it or not, some street car “drag tires” are designed as tube-type tires. There are a number of reasons for this, but primarily, a tube helps to maintain air pressure in specific tires (tires designed for tubes). If you run bias ply tires without tubes, it’s not that uncommon for them to deflate rapidly (that even includes several of the more streetable types with a full compliment of tread). Not so with a drag radial – they should be run tubeless.
What’s the deal with air pressure on a drag radial? And where do you start? M&H Racemaster points out that proper air pressure is critical when it comes to DOT drag radials. Recommending a specific air pressure isn’t easy, since there are so many variables involved. Like what? Factors such as the weight distribution of the car, chassis setup, transmission type, wheel size and so on add up. Those things can have an effect upon operating pressures. When you poll drag racers you’ll often find that “less is better” when it comes to tire pressure. That’s not always true. Although there are exceptions to the rule, higher pressures (and maybe higher than you might expect) generally work best with drag radial tires.
M&H Racemaster offers the following base line pressures for drag strip use:
|Tire Size||Air Pressure|
|Under 30″ diameter||12-16 PSI & up|
|Over 30″ diameter||11-14 PSI & up|
If you’re using the tires on the street, check the sidewall for maximum pressure. Figure 75% of that figure. For example a P325-50R15 has a maximum pressure of 35 pounds. 75% of that pressure works out to 26.25-pounds.
When all is said and done, you can see drag radials aren’t slicks with grooves cut in them. They’re specialized race tires – Plain and simple. Next issue we’ll take a look at front tires. There’s more there than meets the eye.