DOT Drag Radials 101: What You Need to Know About Drag Radials
There are two kinds of street/strip cars—street cars that occasionally see the dragstrip, and drag cars that that occasionally see the street. Both cars need tires, but can that really be the same tire? We set out to break them down and give you the skinny on fat tires. We talk with some of the leaders in the industry, Mickey Thompson, Hoosier, Toyo, BFGoodrich, and Nitto, to see what their tricks are.
Breaking down each tire by its construction is a critical step in deciding what you need. Tires are not created equally; in fact, they are all different, depending on the intended usage. Bolting a set of wrinkle-wall slicks on a road car would mean disaster by the first hard turn. You have to know what you are going to do with them before you click the “buy it now” button.
For street cars, we are looking at DOT-rated tires. These are tires that have passed the strict Department Of Transportation regulations for street use. Some wrinkle-wall tires, like the Mickey Thompson ET Street slicks, meet the regulations just enough to get the D.O.T. stamp on the sidewall, but just because a tire is DOT rated does not mean that it is a good idea to go to the grocery store while wearing them. You can do it, but it’s kind of like running in flip flops—it may look cool (running in flip flops does not look cool), but you may end up broken and scarred. wrinkle-wall tires are designed for one reason, and that’s the hook up under big power and make the car launch and go straight down the dragstrip. But that flexible sidewall that provides traction in a straight line does not work in a corner-turning situation.
To be approved for street use, a tire must meet specific standards like hydroplane resistance, tread depth, and sidewall stiffness. The bare minimum tread depth is 2/32nds for street use; most drag radials are in the 5/32nds range whereas a true street tire is 10/32nds. Tire compound is a factor, but most DOT track tires use a special compound that is sticky enough for the track and safe enough for driving you home.
Speaking of the Street, Radial and Bias Ply DOT Tires
A clutch car will work much better with bias-ply slicks. I’m not saying radials won’t work, but it takes a sophisticated clutch and suspension. – Carl Robinson
The sticking point for choosing your tire design is the transmission, or more over, whether you have a clutch or a torque convertor. Carl Robinson of Mickey Thompson Tires told us, “A clutch car will work much better with bias-ply ET Street tires. I’m not saying radials won’t work, but it takes a sophisticated clutch and suspension to take advantage of the reduced rolling resistance of a radial early in the run.” Robinson also said that power adders with stock suspension cars have really pushed the use of tubes in bias ply tires. The tubes provide additional support for the bias construction, particularly in the sidewall, improving the grip of the contact patch.
Most manufacturers have a disclaimer about not mixing bias-ply tires and radials on the same car. While this is true for street driving, drag racing is different. You don’t have to deal with corners and bumps in drag racing, which is where the two designs are drastically different. If you drive the car to and from the track and run bias-ply DOT drag tires, you need a set of street radials to drive it home.
Every manufacturer has proprietary rubber compounds. The tread compound not only determines the life of the tire, but also the length of the burnout. Regular street tires typically do not like burnouts; the heat makes them greasy and slick, though some high-performance street tires (such as the BFGoodrich Radial TA KDW) like a short little burnout. Drag tires, however, usually require some level of burnout to heat them up. Again, every brand is different. Mickey Thompson suggests the tires be heated to within 15 degrees of the track temperature.
If you have a new high-performance car like a Viper or Corvette, you need a large diameter tire to fit the stock wheels. For that, Mickey Thompson has the ET Street Radial II (below), which covers wheels up to 20-inches. Nitto specializes in large diameter drag radials too with their NT555R (pictured above) as well as their NT05R.
BFGoodrich suggests a light spin to clean up the rubber and put a little haze on it as well. If you put too much heat in them they start to ball up, kind of like when you rub an adhesive and get little balls of goo. That balling up can actually work against you, slowing down the car in the first 60 feet. In the end, you need to take the manufacturer’s suggestions and experiment with your car.
How Long Do They Last?
Another aspect of the rubber compound is the longevity. For a fully sponsored racer, tires are not an issue, but for the average Joe, a set of tires needs to last as long as possible. When we asked about the life expectancy of drag radials, Robinson’s reply was, “Would that be the first or the third set?” On average, a set of drag radials with a mix of track time and highway miles is between 3,000 and 5,000 miles.
In order to provide the grip, the compound is considerably softer than a stock or even an ultra-high performance street tire. That grip comes at a cost of lifespan. It all depends on how often you are going to run the drag tires. Keeping them properly stored for the majority of the time will certainly add to the life of the tire. Trying to impress the blonde in the Mini Cooper next you at the stoplight with a 60-second third gear burnout is not doing your tires any favors. Besides, most girls who drive Minis care more about the power in your checking account than under your car’s hood.
Reading The Sidewall
Every tire has a tire wear rating as part of the UTQG (Universal Tire Quality Grade) as required by the DOT. This rating is based on the tests performed by the National Highway Traffic Safety Administration (NHTSA) over a specific 400-mile loop, driven for 7,200 miles in west Texas. These numbers range from 600+ down to 0. For comparison sake, a typical street tire has a treadwear rating in the 400-600 range. These tires last a long time, providing a comfortable ride in all weather conditions.
The axiom is that every time you double up on the treadwear rating, the tire will last twice as long: a 400 lasts twice as long as a 200, for instance. A BFGoodrich Radial TA (the classic muscle car tire) has a treadwear rating of 400. The ultra-high performance street version of the TA, the G-Force TA KDW, is rated at 300, and the ultra-sticky G-Force TA Drag Radial features a 00 treadwear rating. While these standards have left some room for interpretation, a 00 rating means that the tire did not last the entire course, and that is performed under street conditions, not racing.
If you look at the rest of the UTQG scores, there will be some puzzling grades, such as the traction grade. The G-Force KDW is listed with AA traction, which means on asphalt the tire will pull over .54 G, and over .41 G on concrete. The Drag Radial on the other hand, with its sticky compound, is rated with a B grade. This is because drag radials are not designed for wet traction, and that is what the DOT traction test is based on.
The other main grade is temperature resistance. This is actually a fairly important detail for a drag tire. The temperature of a tire increases as speed and time at that speed increases. These are portrayed in A, B, and C. This is different from the speed ratings, which measures speed. All tires in the US must be rated at a C or better, which means the tire is capable of 85-100 MPH without damage. B rating is 100 to 115, and A is over 115 MPH. Of course, while a drag radial is capable of handling well over 100 MPH, it is not capable of these speeds for long durations. It would not be advisable to drive a car on drag radials at elevated speeds as the temperature build-up could cause them to fail in a spectacular fashion, and that would not be good for anyone.
It is important to note that some manufacturers expressly label their DOT drag tires as drag race use only. Not all DOT drag tires are capable of safely getting you home; they are built for drag racing only, and just barely meet the DOT specs. This is basically a slick (pardon the pun) tactic to get around the rules—sure, it meets DOT standards, but don’t actually use it that way or you will die.
At the same time, some brands market their DOT drag tires specifically for the “drive to the track and race on the same tires” crowd, which is how it should be. BFGoodrich and Mickey Thompson drag radials have the most tread of them all. The G-Force radial was designed for straight-line performance along with high-speed stability and cornering performance for streetability. The point is that you have to decide what you need. If you trailer the car and need a DOT tire for class restrictions, then choosing a non-streetable tire is acceptable, but if you want to drive the car home after a day of racing, you need a tire that has some realistic street capabilities.
The days of the 15-inch wheel are almost completely gone, save for the sportsman drag racer. Because of the proliferation of 17- to 20-inch wheels in motorsports, there is a need for drag tires that fit those wheels. Mickey Thompson, Nitto, BFGoodrich, and Hoosier all offer large-diameter DOT drag radials. In fact, Nitto’s NT05R is only available in 17- to 20-inch sizes.
The main reason for stepping into a larger wheel is because of the large rear wheel disc brakes that require a larger wheel, particularly in the late-model cars like Vipers and Corvettes. There is a tradeoff as you go to the larger wheels, and that is something you need to consider for your street machine. A steep-geared drag car can handle 30-inch tires, but throw those on a car with 3.73 gears, and you will know what slow is.
The Nitto NT05R 20-inch tire is a 315/35R20, with a 28.74-inch overall diameter and a 12.48-inch width, far from the 10.5 tire that many street car classes require. That 28.74-inch diameter means you have about four inches of sidewall (28.74-20=8.74/2= 4.37” of sidewall), compared to the same diameter tire in a 15-inch wheel, which would yield 6.5 inches of sidewall. That extra two inches of sidewall is a lot of room for extra traction.
Radial tire sidewalls may be stiff, but they will still flex a little, providing the wider contact patch that you want. When at all possible, run the smallest wheel that your brakes will allow. This maximizes the sidewall height, putting more power to the ground. It may not “look” as cool, but the quicker timeslip looks even better.
Bias Ply Roll Out Measurement
Bias ply tires are made with nylon cords that criss-cross each other. Over time, the cords shrink, reducing the rolling height of the tire. Bias plies must be matched to within a half-inch of roll-out of each other. Mickey Thompson Tires has a specific procedure spelled out on their website to check the roll-out.
Tire pressure in a drag tire is crucial for launch and traction control. While 32 psi is optimum for street driving, running that much pressure on the track is going to leave you spinning off the line. Every car is different and every tire is different, so experimentation is required, but there are some starting guidelines. Mickey Thompson suggests keeping a log book of air pressures and suspension settings with each run (you should be doing this anyway) to determine what your best pressure is.
M/T goes so far as to break down each tire with their minimum suggested tire pressure by vehicle weight. For example, a 28-inch-tall Pro Drag Radial on a 3,200-lb car should run a minimum of 12 psi; the same tire on a 2,800-pound car can go as low as 10 psi. They are even more specific for the ET Street radials with a specific range of air pressure for 275 and smaller (14-18 psi) and 295 and larger tires (12-16 psi). These numbers can be used as a good starting point for other brands as well, but Mickey Thompson claims that their tires tend to perform better with a little more pressure than other brands.
The Quick Guide to Picking the Right Radial
Track Only Use
- Hoosier D.O.T Drag Radial – Hoosier’s tire is the only tire in the lineup that is not designed for any type of street use. The tire utilizes two very simple rain grooves designed to maximize the amount of tire on the contact patch. While a relatively new tire, the Hoosiers have seen success in 275 and outlaw drag radial races. While this is designed as a track-only tire, Hoosier does make these tires in 15 to 18-inch applications
Track and Light Street Use
- Mickey Thompson ET Street (I and II) – The ET Street is designed for limited street use and will last less than 7,000 miles. M/T’s ET Street Radial is world-renowned for its hardcore drag racing capabilities and have clocked 6-second quarter mile times. The ET Street Radial II is made from the same compound as the Street Radial I, though it is designed for better wet weather traction, due to the increased number or rain grooves. The Street Radial II is mainly designed for 17-inch+ wheel combinations.
- Toyo Proxes TQ – Toyo uses a unique compound that is designed for both IRS and solid axle rear ends. Much like the M/T ET Street, it can be driven on the street and strip but its 00 treadwear will put life expectancy at around 7,000 miles or less (depending on how much you go to the track).
- Nitto NT05R – This is Nitto’s newest drag radial that is based on the tread design of their NT05 Ultra High Performance road handling tire. This is a single 0 treadwear tire and will last slightly longer then the other tires in this class. While this is intended to be a track tire, it can also be driven on the street.
- BFGoodrich g-Force T/A – Also sporting a 0 treadwear like the Nitto NT05R, the BFG will last closer to 10,000 miles while doing a decent job during wet weather situations with its integrated rain channels. BFG’s Comp T/A drag radial was the first ever drag radial to meet DOT street tire standards. The main differences between the two tires is that the new g-Force offers a broader range of performance. The g-Force T/A was the spec tire in the NMRA Drag Radial category for many years and is a proven 7-second tire.
Street and Track
- Nitto NT555R – Nitto’s NT555R has been a very successful tire, especially in the late model muscle crowd, for its dual purpose street/strip use. With its 100 treadwear and improved water channels, The NT555R can live for around 10,000 miles and combat wet weather situation; it is the longest lasting drag radial in this article.
The last step in dealing with tires is how to store them. Some of this depends on the design of the tire. Bias ply tires are not stable enough to sit inflated with the car on them for long periods. Flat spots develop quickly in a bias ply. Radial tires do not have this problem, but you still need to take care of them while they are put away for the off-season. All drag radials are considered summer dry-weather tires, so there is no reason to keep them on your car when it is wet and cold outside.
Bias ply tires should be stored either off the car or with the car on jack stands with the load off the tires. Drop the tire pressure to 5 psi. To protect the tires from UV damage, keep them covered and out of direct sunlight. Fluorescent light can damage them as well. Another tip is to keep them away from electric motors. Electric motors generate ozone, which eats rubber. There is also no need to use tire dressings; just clean the sidewalls with mild detergent and rinse, leave the tread surface alone.
Radials are not as temperamental as bias ply tires, but many of the same principles hold true. You can leave a radial under pressure, but if you take them off the car and air them down, they will last longer. The environment for the rubber is the same as a bias ply: no light, no electric motors in close proximity and clean them with soap and rinse them well.
There are significant differences in a DOT tire, just like anything else. The key to making the right selection for your car is to know what your car needs to go fast. Keep in mind that street tires allow the tires to slip, drag tires don’t. If you have a weak link, slicks and drag radials will find it and point it out in spectacular fashion. There are also other rules that you must adhere to in order to pass tech when upgrading to a drag tire, such as axle and lug nut specs. As long as you adhere to the rules and practice safe racing, you will go faster.