Many years ago, the Willis Brothers sang about a trucker who ended up in Boston going the wrong way on a one-way street. When he was finally flagged down by the police and told to go back, the driver replied, “Give me 40 acres and I'll turn this rig around.” If you've ever felt a kinship with that trucker when driving one of your department's apparatus, you may want to consider several ways to improve your next rig's maneuverability.

Start with the basics

For most people, maneuverability simply equates to turning radius — the tighter a vehicle's turning radius, the more maneuverable it is. Although that view overlooks such factors as overall vehicle length, steering ratio, visibility and others, turning radius still is the most common measure of a vehicle's ability to negotiate around tight corners and position itself in crowded spaces.

Curb-to-curb turning radius is the smallest radius the vehicle can turn without having the front wheels go over the curbs on either side of a street. Remember that it's the radius, not the diameter. Thus, a vehicle with a 25-foot curb-to-curb turning radius can turn around in a 50-foot-wide street without the front wheels going over the curbs.

Wall-to-wall turning radius is the smallest radius the vehicle can turn without the forward corners of the cab or the ends of the front bumper touching the walls on either side of an alley. The wall-to-wall turning radius is always greater than the curb-to-curb turning radius.

Of the two, the curb-to-curb turning radius is the most commonly used in sales literature because vehicles usually have to maneuver in open streets more often than in confined alleys. It's also the smaller, and therefore more impressive, figure to present to prospective buyers.

The basic formula to determine the curb-to-curb turning radius of a vehicle with front wheel steering is:

CCR (feet) = WB (inches) × CF/OWC (°)

Where WB equals the wheelbase, OWC equals outside wheel cut and CF equals conversion factor. The wheelbase is measured in inches from the center line of the front axle to the center line of the rear axle for vehicles with a single rear axle, or to the center line of the tandem spacing for vehicles with tandem rear axles.

The wheelcut is measured in degrees on the outer front tire, not the inner front tire. When a manufacturer states that their vehicle has a 45° wheelcut, they are talking about the inner tire wheelcut. The outer tire wheelcut is always less, and that's what is used to calculate the turning radius.

The conversion factor depends on the front axle, tires, wheels and wheelbase. It is different for every combination of these components and is something that has to be determined by the vehicle manufacturer.

Looking at the formula, you can see that if you either decrease the wheelbase or increase the wheelcut, you will reduce the turning radius. Although that may look simple, there are many ways to do it. Each one has its own advantages and disadvantages.

Shorter wheelbases

Decreasing the wheelbase is a common way to improve maneuverability. The most obvious way to do this is to reduce the length of the body. Unfortunately, that also reduces the space for the pump, hosebed, ladders and compartments. One way to make it work is to move the pump module to the rear of the vehicle. In this location, the pump can be installed in a shorter length, and the pump panel can be placed across the width of the vehicle instead of taking up room along the length. With the pump out of the way, the body and the rear axle can be moved forward to dramatically reduce the wheelbase. One disadvantage of this approach is that the longer rear body overhang may reduce the angle of departure.

Other pump configurations that can reduce the wheelbase include PTO-driven pumps mounted low in the frame, front-mounted pumps and hydraulically driven pumps. Some manufacturers build compact midship pumps that take up as little as 24 inches of vehicle length for customers who prefer a midship design.

Another common way to reduce the wheelbase is to use a custom chassis with the front axle mounted under the cab rather than a commercial chassis with the front axle mounted ahead of the cab. For the same size body, this can significantly reduce the wheelbase. One disadvantage of this approach is that it shifts more weight onto the front axle. The extra weight may require larger front tires and wheels, which may reduce the wheelcut. The extra front overhang also may increase the wall-to-wall turning radius and decrease the angle of approach. Using a commercial chassis with a set-back front axle has the same advantages and disadvantages to a lesser degree.

Finally, you can reduce the wheelbase by 26 inches or more by using a single rear axle with a self-steering tag axle instead of a tandem rear axle. Some of the disadvantages of this approach include either using a more expensive forward-reverse castering tag axle or having to raise the tag axle to operate the vehicle in reverse. A single rear drive axle may also have a lower traction rating than a tandem rear axle, and the tires may be more prone to spin on slippery surfaces.

Tighter wheelcuts

Increasing the wheelcut is another common way to improve maneuverability. Some manufacturers offer front axles with a wider distance, or track, between the tires. With these wide-track axles, the tires are located farther outboard from the frame rail and have room to make a sharper wheelcut.

Other manufacturers offer sharper wheelcuts simply because the weight on the front axle allows them to use smaller tires and wheels. This is especially true of vehicles built on commercial chassis with set-forward front axles, where the front axle weight rating is 12,000 pounds or less and the wheelcut may be as high as 55°. Compare this to vehicles built on custom chassis with set-back front axles, where the front axle weight rating is 18,000 pounds or more and the wheelcut may be as low as 40°. In most cases, less weight on the front axle means smaller tires and greater wheelcuts. This advantage is offset by a longer wheelbase, and a comparison of the resulting turning radius would have to be calculated.

One final thought. When the salesperson tells you a vehicle has “up to” a certain wheelcut, ask for details. In many cases, the figure you are being quoted is only good for vehicles with the smallest tires and wheels, not necessarily the ones you need to specify on your apparatus. This figure also may be good only in left-hand turns; in right-hand turns, the left front tire may hit the steering arm and the wheelcut may be restricted. Ask for both. And when you get the vehicle, make sure the factory has adjusted the axle stops to allow the greatest wheelcut in both directions. You won't get the tightest turning radius unless you check. If you adjust the stops, be careful that you allow sufficient clearance between the tires and any hoses or other components in the vicinity.

Other ways

Besides decreasing the wheelbase and increasing the wheelcut, manufacturers have come up with several other ways to improve vehicle maneuverability.

Tillered trailers with steerable rear wheels have been used for ladder trucks since the late-1800s. The concept now is being applied to many other applications, such as rescues and hazmat units, where the combination of bulky equipment and a variety of support components make single vehicles very difficult to maneuver. Tractor-trailer combinations solve that problem and have the added advantage of allowing a department to replace the tractor without having to replace the trailer, thus effectively doubling the life of the vehicle. This concept does require a trained tiller operator, however, and the turning capability does have certain limitations. For example, if the tractor makes a very tight U-turn, the trailer will actually back up for a portion of the turn rather than follow the tractor around the circle.

Perhaps the most unique approach to improving maneuverability is the Pierce All-Steer system that offers the driver the choice of three steering modes to best guide the apparatus under varying conditions. On the road, the rear wheels are locked in the straight-ahead position, and the vehicle maneuvers like any other front-steer vehicle. At speeds under 35mph, the driver may select the coordinated-steer mode, in which the rear wheels turn in the opposite direction of the front wheels to decrease the turning radius by as much as 30%, depending on speed. Finally, at speeds under 10mph, the driver may select the crab mode, in which the front and rear wheels turn in the same direction to allow the vehicle to move sideways as it moves in forward or reverse. The steering system is electronically controlled and includes safety interlocks and self-diagnostics to ensure reliable operation. It is especially valuable for longer vehicles, such as heavy rescues and aerials, that may have to maneuver past other apparatus and obstacles at a crowded incident scene.

Look before you leap

There are lots of ways to improve a vehicle's maneuverability. Each has its own advantages and disadvantages. Remember that in most cases, a tighter turning radius puts a greater side load on the tires and can result in accelerated tire wear if it's used frequently. Don't spec it if you don't need it.

To ensure you get the maneuverability you need, work with the apparatus manufacturer to specify the best configuration for your application. If the turning radius or some other aspect of maneuverability is important to you, write it into a performance spec in terms of curb-to-curb or wall-to-wall turning radius in both left-hand and right-hand directions. Remember that there are many factors involved in determining maneuverability — tire size, wheel offset, axle track, wheelbase, wheel cut, steering geometry and others. You should investigate them all to specify the best vehicle for you.