5 axle 48' tractor trailer
|Sway Resistance/Rearward Amplification Ratio ¹
¹ The Sway Resistance to Rearward Amplification ratio approximates the lateral instability contributed by tandem axles and dual wheels. The lower the ratio, the less stable and maneuverable the vehicle is and the more likely it is to cause highway damage due to sway. Whenever a truck turns, the inner tire of a dual wheel set must travel a different distance than the outer tire forcing one of the tires to skid. If there are multiple axles, one of the axles must also skid whenever the truck turns. Because the flexible sidewalls of radial tires store energy during turns until enough force builds up to cause a skid, the sudden release of energy changes the truck's direction abruptly, resulting in side to side oscillations that shift the vehicle's weight from one side to the other, overloading the tires of each side of the vehicle alternately. The resulting lateral scrubbing forces on the overloaded pavement are thought to be the cause of most pavement wear. Lateral stability can be improved by reducing the number of axles or the axle spacing, increasing the wheel base length, switching from dual wheels to single tires, and reducing the axle weight on towed units. To be scientifically accurate, both the Sway Resistance and Rearward Amplification forces should be multiplied by the vehicle's deceleration divided by the gravitational constant. But, as used here only in a ratio, these terms cancel each other out, allowing the above equations to be simplified in legislation.
² Some car haulers and maxi-cubes have 5th wheel turntables well behind the drive wheels of the truck called "stingers" which contribute to instability. These should be repositioned closer to the drive wheels or equipped with single tires to improve stability.
³ Replacing dual wheels with single tires typically reduces Rearward Amplification by more than half. As an alternative, cargo weight on the second trailer can also be cut in half to reduce Rearward Amplification to within legal limits. The Safe Truk computer program makes it easy to determine compliance.
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