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".
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
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
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 takeoff 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 30inch 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
