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A tandem axle truck is equipped with two drive axles, such as those found on the rear of the typical 18-wheeled semi-rig's tractor. Utilizing eight tires and wheels on the drive axles, this type of truck is able to support a tremendous amount of weight as well as provide improved traction despite poor road conditions. While the engine sends power to the lead axle, the trailing axle receives its power via a short drive shaft extending from the rear of the lead axle housing. This typically allows the driver of the truck to select drive power from the front axle only, or by activating a switch, the power is divided between the two drive axles. This provides increased traction in inclement weather conditions.
The incredible strength of the tandem axle truck makes it a good choice for use as a dump truck, cement mixing truck and tanker truck. It is equipped with very large spring packs that are designed to withstand the rigors of heavy load transportation. By applying torque to both drive axles, the heavy truck is able to make a smoother start from a stopped position. This is possible by spreading the weight and torque over a larger and broader area. With both axles pulling, both sets of axle-springs are flexed as the truck pulls the load into motion.
Especially important on a tandem axle truck used in a dump truck application, the heavy-duty rear axle springs are required to not only support the truck's heavy weight when loaded — they must also stabilize the truck when the dump-box is raised high into the air. This is also true in tanker-type tandem axle designs. The sloshing of the liquid in the tank is sometimes able to rock one side of the truck's tires off of the road. Weaker suspension systems would likely fail under this type of stress.
In very heavy-weight capacity trucks that operate on softer ground and secondary roads, the tandem axle truck is occasionally fitted with an air lift, third axle to disperse the weight across a wider area. This not only supports the weight of the load on the soft ground and prevents the truck from becoming stuck, it also aids the vehicle in adhering to seasonal weight restrictions. Year-round restrictions such as bridge weight limits are met by dividing the amount of weight carried by each axle.
Tandem trucks are necessary, but they beat local roads to death by carrying those tremendous weights over them. We need them, but they are a problem -- much like a lot of other things.