Tri-axle & Quad-axle Trailers
Trailers with multiple axles are used to tow heavy loads. This is due to both the safe carrying capacity of the trailer itself, as well as legal limits set by individual jurisdictions.
In Washington State, the number of axles a semi trailer has, along with the amount of space between the axles, determines the weight of the load the trailer is legally permitted to haul. RCW 46.44.041 defines these regulations using the following formula:
W = 500((LN/N-1)+12N+36)
W is the maximum weight in pounds (to the nearest 500 pounds) carried on any group of two (2) or more consecutive axles.
L is the distance in feet between the extremes of any group of two (2) or more consecutive axles.
N is the number of axles under consideration.
More information regarding RCW 46.44.041 can be found here: link here
Multi-axle Semi Trailer Haul Weight Example
Considering a standard hauling scenario, a tri-axle semi trailer, towed by a semi truck, would have a total of six axles. Three of these axles would belong to the semi truck with the remainder belonging to the semi trailer. According to the RCW mentioned above, the “maximum load in pounds carried on any group of 2 or more consecutive axles,” assuming a “distance in feet between the extremes of any group of 2 or more consecutive axles” of 18, would be 64,500 pounds. At the maximum, a load of up to 105,000 pounds could be legally towed in Washington provided the setup included an axle group distance of 85 feet or greater.
Assuming a quad-axle semi truck and trailer combination where the axle extreme is no less or greater than 21 feet apart, the maximum legal gross weight would be 72,000 pounds. With an axle extreme of 72 feet or greater, a maximum of 105,000 pounds would be legal.
The above information is for example purposes only and should not be considered official. To maintain legal compliance it is recommended that you confirm the operating restrictions for all locations in which you intend on hauling.
Multi-Axle Trailer Information:
Source: Link here