Both heavy loads and high speeds contribute to the strong centrifugal forces acting on an aircraft tire. The relationship of speed versus centrifugal force is obvious. The effect of coupling speed with a heavy load is shown in the drawing below.

This drawing depicts a tire rotating counterclockwise. The heavy solid horizontal line represents the ground or runway. The distance "CX" is half the footprint length. Because the tire is pneumatic, it deflects when coming into contact with the ground. This deflection is represented by the distance "BC" or "XZ". In the same length of time that a point on the surface of the tire travels the last half of the footprint "CX", it must also move radially outward the distance "ZX".

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As the tire leaves the deflected area, it attempts to return to its normal shape. Due to centrifugal force and inertia, the tread surface doesn't stop at its normal periphery but overshoots, thus distorting the tire from its natural shape. This sets up a traction wave in the tread surface, which takes several cycles to fully dissipate.

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This photograph shows just how severe a traction wave can become under certain operating conditions.

The following parameters help explain the magnitude of forces acting on the tire carcass and tread as it runs on a test dynamometer:

Speed 250 MPH
Revolutions per Minute 4,200
Deflection 1.9 inches

At this speed, it takes only 1/800 of a second to travel 1/2 the length of the footprint (CX). In that same time, the tread surface must move radially outward 1.9 inches. This means an average radial acceleration of 200,000 ft./sec./sec. That's over 6,000 G's!

The picture doesn't show it, but the traction wave took three to four cycles before dampening out. This means the tread is going through 12,000 to 16,000 oscillations per minute.

Obviously, a tire cannot withstand this type of punishment. How can a traction wave be reduced or eliminated? In other words, what factors affect the traction wave? The following page shows effects of SPEED and UNDERINFLATION.

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Traction Wave vs. Speed

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The above photographs show the tread of a tire as it leaves the footprint moving toward the reader. The only test variable is speed, showing from left to right 190, 210, 225 mph. The higher the speed, the more pronounced the traction wave.

One of the major tasks of the tire design engineer is to minimize this traction wave at the required speeds and loads.

Traction Wave vs. Underinflation

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All tires in the above photographs are traveling at 225 mph. In the picture to the far left there is an appreciable traction wave because the tire is properly inflated. The only test variable is pressure, showing from left to right rated pressure, -10 psi, -15 psi, -20 psi. Obviously, the greater the underinflation, the more pronounced the traction wave.

Note how the grooves open and close as the tread passes through the traction wave.

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The centrifugal forces that generate a traction wave, combined with the thousands of revolution cycles, can cause tread problems such as Groove Cracking and Rib Undercutting, which could result in tread loss.

GROOVE CRACKING is a circumferential crack that can develop in the base of the groove caused by the repeated flexing of the groove when a traction wave is present. Tires should be inspected frequently and removed if any fabric is visible.

RIB UNDERCUTTING is normally a continuation of the groove cracking that continues under the tread rib between the rubber and the tread reinforcing fabric.

Rib undercutting can progress to a point where pieces of the rib or the whole rib can become separated from the carcass. In severe cases the complete tread can come off the carcass. Progression from deep groove cracks to undercutting and ultimate tread loss can occur rather quickly. Therefore, careful examination of the tires before each take-off is extremely important. The tire should be removed if the fabric is exposed.

Before leaving the subject of centrifugal force, it is interesting to look at the magnitude of these forces due to speed only, disregarding other radial accelerations caused by loads and deflections. This chart shows the centrifugal forces acting on one ounce of tread rubber on a 30-inch diameter tire.

Centrifugal Forces
30 Inch Diameter Tire
MPH G'S FORCE ON 1 OZ
OF TREAD
FORCE ON TOTAL
TREAD (8 LBS)
100 500 33 LBS 4,000 LBS
200 2000 130 LBS 16,600 LBS
300 4500 300 LBS 38,500 LBS
400 8000 528 LBS 67,500 LBS

The force increases as the square of the speed from 500 g's, or 33 lbs. per ounce, at 100 mph, to an extreme of 8000 g's, or 528 lbs. per ounce, at 400 mph.

An average tread for this size tire would weigh approximately 8 lbs. This means that the effective weight of the total tread at 200 mph would be 16,600 lbs. and at 400 mph would be 67,500 lbs.

With forces like these, it is amazing that a tread can stay on a tire at all.

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Courtesy of The Goodyear Tire & Rubber Company