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Hydroplaning happens when one or more tires is lifted from the road by a wedge of water that gets trapped in front of and under a tire as the vehicle drives through water. Hydroplaning most frequently occurs during heavy rainstorms when water creates puddles on the highway or expressway. In addition to the accompanying splash scaring the heck out of the driver, hydroplaning typically causes the steering wheel to jerk and the vehicle to abruptly pull towards the puddle.
The speed at which a tire hydroplanes is a function of water depth, vehicle speed, vehicle weight, tire width, tread depth and tread design. It depends on how much water has to be removed, how much weight is pressing down on the tires and how efficient the tread design is at evacuating water. While deeper water, lighter vehicles, wider tires, less tread depth and less efficient tread designs will cause tires to hydroplane at lower speeds; all tires will be forced to hydroplane at some speed.
As a rule, tread design affects hydroplaning resistance at high speeds and in deep water. Tread compound affects wet traction at lower speeds or in shallow water.
Directional tread designs (sometimes called unidirectional tread designs) are frequently used on tires intended to better resist hydroplaning. Their multiple tread grooves are aligned in a repeating “V” shape to increase the tire’s ability to channel water from between the tire’s footprint and the road. Somewhat like the vanes of a water pump continually pushing water in one direction through the engine, the grooves of a directional tire are designed to push water in one direction through the tire (forward on an angle to the sides). Directional tread designs are especially helpful in increasing hydroplaning resistance when relatively wide Plus Two, Plus Three or Plus Four tire and wheel applications result in fitting a much wider tire to a vehicle than its Original Equipment size.
In order to get a better understanding of the performance characteristics of directional tires we tested Goodyear’s Eagle F1 GS-D3 Max Performance tires on a 2003 SVT Cobra Mustang equipped with a six-speed transmission and a DOHC 4-valve V8 engine producing 390 horsepower and 390 foot-pounds of torque. The Eagle F1 GS-D3 tire was chosen because it features state-of-the-art directional tire technology and the SVT Cobra was chosen because its horsepower and torque could challenge any tire’s ability to provide traction in both dry and wet conditions. We also turned off the Cobra’s traction control to place all of the responsibility of managing the 390 horsepower solely on the drivers. The P275/40R17-sized Eagle F1 GS-D3 tires were mounted on 17" x 9" cast aluminum, alloy wheels.
Additionally, we also used this opportunity to confirm the performance of the directional tires in dry and wet conditions if they were mounted incorrectly (in the reverse direction). Therefore we ran a total of 4 separate track tests.
Mounted Correctly on a Dry Track
In this condition the Eagle F1 GS-D3 tires provided predictable handling, along with good cornering stability and grip. While they are not absolutely the fastest Max Performance tires in the dry, they complement the Cobra’s suspension and harnessed its horsepower while coming close to the best Max Performance dry tires’ lap times (within half a second). The Eagle F1 GS-D3 tires allowed the car to run 29.387 second average lap times, fast enough to confirm its position in the Max Performance tire category.
Mounted Backward on a Dry Track
Since we knew that directional tires are designed to emphasize wet performance, we didn’t expect to find much, if any, performance difference with the tires mounted “backwards” on a dry track. We were surprised at how close it was. In this condition the Eagle F1 GS-D3 tires continued to provide predictable handling and good grip. If anything, mounting the tires backwards helped free-up the Cobra a little, slightly reducing its steady state understeer. With no handling quirks or surprises, the car ran 29.465 second average lap times, within 8/100 of a second of the times turned in with the tires mounted correctly.
Mounted Correctly on a Wet Track
The Eagle F1 GS-D3 performs admirably in wet conditions. For that matter its confidence-inspiring control produced an average lap time of 30.373 seconds in the wet, just one second slower than the lap times it ran in the dry (and about one second faster than the lap times turned in by the best Max Performance dry tires). On the track in the wet, we have found that the Eagle F1 GS-D3 continues to provide predictable handling and enough grip to run the fastest average wet lap times of our Max Performance tires.
Mounted Backward on a Wet Track
This is where we expected to find some noticeable differences since the tires would now be pulling water toward the center of their footprint, reducing the available traction. However, the lap times and drivers’ comments indicated that the wet performance was still there. The Cobra ran average lap times of 30.387 seconds, just 1/100 second different than when the tires were mounted correctly.
Why wasn’t there more of a difference? There are two main things to consider.
Our test track was built with a 1-degree grade to the inside, very similar to how real roads are sloped to the shoulders or curbs. In addition to the absence of puddles, this constant slope is covered by a uniform layer of water as would be experienced on a real road during a moderate rainfall. It does not have the puddles associated with rutted roads or the standing water sometimes encountered during a heavy rainfall. Built to imitate real roads in good condition, our track challenges the tire tread compound’s ability to provide wet traction more than the tire tread design’s ability to provide hydroplaning resistance.
The track’s average lap speeds approach 40 mph and the top speed is around 55 mph. Evidently the uniform layer of water did not challenge the Eagle F1 GS-D3’s ability to provide wet traction at these relatively moderate speeds. Higher speeds, deeper water or puddles would eventually have increased the differences in wet performance related to how the tires were mounted.
So what is our conclusion? On dry roads and in moderate rain at city speeds, even though they always look cool, directional tires do not provide much of a wet traction advantage. However for drivers who spend a lot of their driving time on the highways, interstates or at higher speeds (during race track drivers schools and lapping days), directional tires will better resist hydroplaning.
Unfortunately directional tires do exhibit a drawback because their tread pattern is designed to only roll in one direction. It limits their tire rotation pattern possibilities. Without being remounted on their wheels, directional tires can only be rotated from the front to rear axles on the same side of the vehicle. This makes them more susceptible to irregular wear (sometimes called “saw tooth” or “heel & toe” wear).
To minimize this possibility, it is important that tires with directional tread designs be rotated more frequently than other tires, especially early in the tire’s life. While tire rotation can’t completely eliminate the possibility of irregular wear, it helps even out wear on most vehicles because the tires are experiencing different stresses as they are repeatedly reassigned from the “driven” to the “non-driven” axle, as well as to and from the vehicle’s steering axle.
NOTE: If directional tires are accidentally installed backwards during initial installation or following tire rotation; the driver should simply reduce speeds in the rain and have the tires installed correctly at the first opportunity. Running directional tires backwards for a brief period of time will not hurt their internal structure.
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