What Is a Tandem Axle Truck?

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.

Sliding your truck's tandem axles is a relatively easy task. However, you must know the proper procedure to do it correctly and achieve desired results. There are three parts to this process: unlocking your trailer's locking pins, sliding the truck's tandems to the proper positions and then locking the trailer's locking pins.

Before you begin, check to ensure that your truck is correctly coupled to the trailer. Leave your engine running, but put your semi in neutral gear. Engage the brakes for only the truck, get out of the cab, and find the locking lever. It's typically in front of your trailer's wheels on the driver's side of the trailer. Once you've found the lever, your first steps are to undo the trailer's locking pins:

The trailer's wheels will remain in position thanks to its brakes. Meanwhile, you can move your truck to slide the axles into the proper position. Start by selecting your truck's lowest gear. To slide the tandems backward, slowly move your truck forward. To shift the tandems forward, gradually move your truck backward. Stop when you've reached the appropriate position.

Once you've gotten the axles where you want them, engage your truck's brakes and get out of the cab. You'll follow some easy steps to reengage your trailer's locking pins:

Federal laws regulate gross vehicle weights and axle loads on interstate highways. You'll want to be aware of these maximums. Heaviest permitted gross vehicle weight is 80,000. The maximum on a single axle is 20,000 pounds, with 34,000 pounds as the upper limit on a tandem axle group.

Most tandem axle dump trucks can carry up to 13 tons per load. You may also see capacity measured in cubic yards. For perspective, a cubic yard is a bundle of material about three feet in length, width and height. That's equivalent to 27 cubic feet. A typical dump truck can carry 10-14 cubic yards.

Keep in mind that federal and state bridge laws also govern payload weights. If you're not certain, check your truck's weight rating for how much payload it can carry. Look at its gross axle, vehicle and combined weight ratings for additional details. Remember that maximum loads can vary between city streets and freeways.

As you may recall, tandem axle trucks have axles with differential assemblies: one with a standard differential and the other paired with a power divider. Understandably, there may be some confusion in determining which differential is active at any given moment. But in truth, both axles are driving the truck. How? Let's find out with a quick overview.

When there's no loss of traction, both axles drive the truck. The differential or power divider lock can be engaged, which permits each axle to drive the truck whether or not the other has sufficient traction. Each of these power dividers also incorporates a nest assembly to offset small differences in tire wear. When you lock your axles, you deactivate this offsetting function. You risk damage to one or both axles in these cases. For best results, avoid locking your axles unless your truck is completely stopped.

Depending on the truck's design, it may also be equipped with wheel differential locks that independently allow wheels to turn in spite of existing traction conditions. However, remember that driving in proper traction conditions with a differential lock on can cause serious damage to that differential.