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From the July, 2011 issue of Modified Mustangs & Fords / By Mark Houlahan
Rare is it that we're talking about 14-inch rims and skinny tires here at MM&F. More often than not, the feature cars you see on these pages are running some serious performance rubber on at least a 16-inch rim, if not a 17- or even 18-inch rim, and why not? If you're building a classic Mustang or Ford with better handling and braking that puts out more power, you truly need a better performing tire that has more grip for better traction, shorter braking, and improved handling. A larger contact patch and shorter sidewall all work together to enhance these attributes in a classic Mustang or Ford. Of course, these modern radials try to do everything better, but if you're more of a track junkie and all you want to do is go fast in a straight line, then usually a specialty tire is more in line with your needs.
When drag racers hit the tracks back in the '50s and '60s, there weren't any radial tires to begin with, but there were slick tires. Slicks were tires completely devoid of any tread pattern for the optimum in straight line traction and contact patch. Their sidewalls were soft, allowing the tires to bite into the pavement. Eventually, we saw the development of the DOT (Department of Transportation)-approved bias-ply drag/street tire, which had just enough tread on it to be street legal in the DOT's eyes, but was made from a soft compound rubber and did its best to act like a slick in the traction department. Both of these tire technologies have had a long and fruitful life. So much so that they're still available and used widely today on a full range of carsùfrom classic Mustangs to street rods and hot rods.
The latest drag tire technology to reach the track surface is the drag radial. Found as either a full slick or with a DOT tread, the drag radial offers a high traction rubber compound wrapped around a standard radial tire carcass. The radial allows for heavier loads, and can be more stable at speed, something that makes many racers feel more "comfortable" at the big end.
So what's the right tire for your ride? Only you can ultimately decide what tire best suits your purpose, vehicle, horsepower, and other factors, but we've reached out to the best in the tire business to help you answer those questions and fit your ride with the right rubber. Check out the following captions and see what bias-ply versus radial drag tires can do for your e.t. the next time you hit the quarter-mile.
Drag Tires: Bias-ply versus Radial
We spoke with several tire company engineers and when we asked them about bias-ply and radial construction drag tires they all agreed that it comes down to tire construction and compound.
Tom Kundrik of Mickey Thompson tires tells us radial tires, as compared to bias-ply tires, absorb more energy. "Why is that?" we asked. Well, it starts with the way the components of the tire are assembled. In a bias tire, the tread and sidewalls share the same plies, causing the sidewall to flex. This flex can be transmitted to the tread, causing deformation of the contact patch. This construction creates more friction with the ground, which is a good characteristic for a "drag tire," but in some cases can be parasitic. With radial construction, the sidewall plies run across the tire and the tread plies, also known as "breakers," run in a longitudinal manner. This construction technique produces a flat, stable and wider footprint on the ground. The radial has less distortion of the contact patch surface and better ground pressure distribution.
We gained further construction details when we spoke with Eric Gullett of Coker tire, who manufactures M&H tires and Phoenix drag tires.
"Bias-ply tires are constructed with ply cord that runs in a criss-cross pattern at a 45-degree angle to the bead. A radial tire has cord that runs at a 90-degree angle to the bead, allowing the tire to be more stable and to better conform to the surface," Gullett stated. The folks from Nitto tire explained further that there is a difference in the ply angle (internal construction) between the two types. In a bias-ply tire, cords are angled away from the direction of the tire to criss-cross each other. In a radial tire, the ply angle runs parallel to the radius of the tire (from the center spoke out to the rolling surface of the tire).
Track-Tested Tires
Mickey Thompson's Kundrick notes the most important "characteristic" of radial race tires is called "dead hook." If the radial tire is over-powered or starts to lose traction it will not recover as will a bias tire, because the radial construction doesn't absorb power. This is why radial-tire class drag racers with lots of power can really be a handful just trying to get the car to launch right. It takes a fine symphony of rpm, clutch, and throttle to get these cars to hook and leave and not bog or blow the tires off on the launch. Another distinct trait is tire growth. Due to the fact that there are steel belts running the circumference of the tire, it can't grow. Finally and in a general sense, radial drag tires are best suited for cars with automatic transmissions.
"Many Stock, Super Stock, and Comp Racers have tried to utilize the radials and in very few cases (where the clutch was adjustable enough) they have achieved a net performance improvement. Typically they have to 'give-up' elapsed time early in the run (to ensure good traction) and make up that time, plus some in the back half of the racetrack," Kundrick states. This points to the key trait of radials--reduced rolling resistance. As we all know, many years ago when radial technology first hit the highway on production cars, the critical difference between the two construction types was a better ride and increased mileage.
Gullett from Coker Tire tells us that if you buy a generic drag-racing slick, it's more than likely a bias-ply tire. The difference between a bias-ply and a radial is the tire's inner construction, as we just discussed, but it affects the way the car launches and how it handles at speed. Generally, you can run a little more air pressure in a radial tire, which reduces rolling resistance and increases stability on the top end. If you've ever raced on a bias-ply, you've felt that tire "wiggle" at the top end of the track. There's also a difference in a "drag radial" and a "radial slick."
A drag radial is a DOT-approved tire with actual tread that many Outlaw street car racers use, while the radial slick looks just like a regular slick, the only difference is the internal construction. A radial tire will, in theory, be quicker than a bias-ply tire, if you use it to its full potential. The increased air pressure capability means a quicker elapsed time due to the reduced rolling resistance. However, you must adjust your suspension accordingly to eliminate the chance of tire spin.
Radials are generally used on bracket racing cars or index racing cars, because of their consistency, but there isn't a downfall to using them on a heads-up race car or even a street/strip car. Drag radials, such as the M&H Racemaster, are generally used on street cars, as well as many Outlaw-style race teams. The main difference is the suspension setup, as it differs between the two tires.
By design, the radial slicks are more stable, but you should never have any stability issues out of a bias-ply slick if the pressures are right and you have the appropriate suspension setup. The only reason people have issues with wobble or sway on the top end is related to tire pressure, or mixing bias-ply and radial tires on the same vehicle.
Finally, Nitto Tires added that the sidewall and overall construction of the bias-ply slick is less rigid than a drag radial tire. Therefore, upon launch at a dragstrip, the footprint left by a bias-ply slick can be bigger than its drag radial equivalent. Also, the bias-ply slick being less rigid can be more "forgiving" if a driver spins the tires. However, if the track condition is optimal and well groomed, the rigidity of a drag radial can result in faster performance than the bias-ply slick.
A drag radial is a DOT-approved tire with actual tread that many outlaw street car racers use, while the radial slick looks just like a regular slick, the only difference is the internal construction
Drag Tires 101
We asked our tire manufacturers what they get the most questions about concerning drag tires and they were all fairly unanimous that it came down to tire pressure questions, type of tire for the car's horsepower or race class, and vehicle weight and its effects on tires. We also asked our tire companies if mixing construction types front-to-back is hazardous or not, as we've seen (and I'm sure you have too) Mustangs with bias-ply slicks on the back and their street radials up front.
Mickey Thompson's Kundrick agreed with our statement and told us that a trailered drag car is not a major issue because the car is only going straight down the dragstrip, but Mickey Thompson discourages the idea of mixing tire compounds for street use. "Yes, it is most definitely hazardous," Gullett from Coker added. Whether you choose to run bias-ply tires or radials, make sure the other end of the car has matching tires. The difference in construction makes the tires react differently, even when on the same car, so it can sometimes create a dangerous situation at speed.
Tire pressure questions are certainly at the top of the list, if not the number one question. As racers move forward and adapt to new technology, the radial-constructed drag tires take a different tire pressure than their old-school bias-ply tires they've used in the past.
Kundrick told us that in a racing scenario, when moving from a bias to a radial, the radial will almost always require more pressure to maximize its performance. "In general, when in discussion regarding tire pressure, we would ask two questions that we combine to yield a 'power-to-weight' ratio. Along that train of thought, the heavier the car, the higher the pressure. Also, when asked what air pressure is recommended, we would be likely to suggest an 'operating range' versus a specific number. The range being variable based on the ambient temperature, track conditions, and corrected altitude."
Gullett explains, "Tire pressure depends greatly on the weight of the vehicle. The heavier the vehicle, the more pressure it generally needs to perform at its potential. For vehicles over 3,200 pounds (most average street/strip cars), we suggest starting at 10 psi and adding pressure slowly until you reach the breaking point, in terms of traction. Running the most pressure possible reduces rolling resistance, thus making for quicker times. Radial slicks generally require 3-4 more psi than a bias-ply slick.
"For drag radial tires, such as our M&H Racemaster, you should never run below 24 psi on the street, and keep pressures above 15 psi at the track. The normal range of pressures, depending on the vehicle's weight and suspension setup is 15-20 psi." Weight is the biggest factor. Track conditions also affect how that car hooks, so consider that when you're tuning the air pressure or suspension. Horsepower, obviously, has an effect on overall traction, but the baseline tire pressures are geared mainly toward vehicle weight.
Nitto Tires stated that for on-road pressure, always refer to the factory recommended setting for the vehicle. This ensures a safe and reliable ride on the street. On the track, Nitto recommends starting at 20 psi and decrease/adjust from there. Monitor the tires and wheels to see if slippage occurs. If so, that means that the pressure can be too low. However, there are too many variables to consider having a standard one-size-fits all solution, Nitto tells us. Factors such as temperature, horsepower, weight, chassis, track condition, and more are all contributing factors to finding the optimal tire pressure settings.
"Bottom line is to consider that the tire has the potential of generating a certain level of 'spring rate.' The higher you can run the pressure, the faster the car will go. Go too high and the tire will slip; too low and the tire will change shape absorbing power and potentially deforming the contact patch creating more rolling resistance," Kundrick tells us.
Time For New Shoes
So, you're ready to hit the track with your classic Mustang or Ford and your baby needs a new set of track shoes. Don't worry, we've got you covered. Now that you've read all about bias and radial compounds and how they work, we had our tire companies show us what they offer in both bias and radial slicks, and DOT tires for your classic Ford. We know you want to hit the 'strip and see what your Mustang or Ford can do, and now you'll know what tire will work best for your ride and where to start with tire pressures to optimize your combination's traction. Lower e.t.'s here we come!
By design, the radial slicks are more stable, but you should never have any stability issues out of a bias-ply slick if the pressures are right and you have the appropriate suspension setup. The only reason people have issues with wobble or sway on the top end is related to tire pressure, or mixing bias-ply and radial tires on the same vehicle. The footprint left by a bias-ply slick can be bigger than its drag radial equivalent.
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