QMC Cranes Blog

The Long and Short of Boom Truck Axles

How pusher and tag axles can benefit drivers

A key advantage of the boom truck is the ability to take a powerful crane wherever you need to go. In the field some contractors prefer the ability of a rough terrain crane or a crawler lattice crane, but these are not capable of moving quickly from jobsite to jobsite. Crane rental outfits often make the case that the boom truck gives the best bang for your buck when you need a versatile crane, especially in urban areas.

A primary reason the boom truck excels is due to its short wheelbase, making it extremely maneuverable. A shorter distance between wheel axles ensures a tight turning radius and keeps drivers happy when needing to position a crane in close quarters. The downside comes when you need higher capacity cranes or are delivering heavy products. In these cases you typically need longer trucks, which can be both cumbersome and ineffective.

When determining your crane needs, it is important to understand axle capacities and federal regulations. Maximum axle weights are governed by two things: 1) axle capacity as rated by the manufacturer, and 2) federal bridge law. In nearly every case, the federal bridge law formula limits the amount of weight you can put on a single axle to tonnage substantially below the axle rating. To allow more weight to be carried, the wheelbase needs to be extended or more axles need to be added.

If you ever wonder why heavy hauling trucks are rolling down the road on five or six axles, it is most likely due to the regulations. On the plus side, the federal bridge formula allows more weight with additional axles. The downside is that as you add more axles to the truck you get diminishing returns, such as more weight, extra costs, less maneuverability, and less driver comfort. For those reasons, the key is to determine the appropriate number of axles for the type of work expected of the truck.

Three Axles and a Crane

A traditional boom truck is configured with three axles, usually on a 6 foot by 4 foot chassis with the rear tandem axle. Additional axles can be fixed, as in a tridem group, or liftable axles can be added as needed. The benefit of the lift axle is the ability to raise it up when it’s not needed to save on fuel consumption and tire wear.

When looking at the specifications of the crane it is important to identify which axle configuration will yield the highest loads, while still ensuring the maneuverability you need on the jobsite. The two primary configurations that our team sees are a lift axle in front of the rear tandem’s “pusher” axle or a lift axle behind the tandem’s “tag” axle. Both have their own benefits based on the crane.

A pusher axle can substantially increase the potential amount of payload on the deck of the crane because it typically sits directly below the middle of the deck. This position enables the pusher axle to take a high percentage of the weight of the payload, keeping it away from other axles where capacities may be limited.

In other units, adding the extra stability of a tag axle often makes sense because the boom truck can act like a large seesaw over the rear tandems. Specifically if most of the crane weight is behind the rear tandem axles, it reduces the weight on the front axle. This unloading effect puts more weight on the rear axles, leaving the front with capacity to spare. By adding the tag axle, the truck can comfortably lift the crane, while balancing the weight more evenly across the axles.

One of the best aspects of the tag axle is that when the additional payload is not on board the axle can be lifted, substantially decreasing the effective wheelbase and giving that extra maneuverability back to the driver.

Every boom truck needs to be evaluated for the best weight balance possible. There is no one truck configuration that fits all solutions. By ensuring a balance between truck and crane weight, owners can get the most out of every delivery without compromising driver satisfaction.

If you have questions about the layout of your boom truck, please let us know. We’ll be happy to talk with you about it.


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