Which configuration do you need for your business: set-back or set-forward? That’s likely the first question you’ll be asked when you get ready to spec your new vocational truck. And your answer will be determined largely by bridge laws. We asked our Caterpillar Truck Experts to provide a quick refresher on these requirements and explain some key differences between trucks with set-back and set-forward axles.
Bridge Laws 101
Bridge laws were first developed in the 1950s to protect bridges and road surfaces of the interstate highway system from early wear and catastrophic failure. These laws specify how much weight any single axle or group of axles can carry based on the number of axles and overall wheel base of the truck. The maximum allowable weight is determined by a mathematical formula, along with specific exceptions and restrictions. Bridge laws can vary from country to country, state to state, and even locally.
Typically, the farther apart you can spread a truck’s axles, the more payload you can carry and still meet bridge law requirements. By moving the front axle forward, the overall wheel base is increased, and in many areas that allows for more payload. That’s the #1 benefit of set-forward-axle trucks. On the other hand, set-back-axle models let you get more of the payload on the front axle, which can be useful in areas with bridge laws that don’t favor lift axles. Here are some other key differences between the two:
Set-Back Axles Offer:
• A tighter turning radius. With the front axle set further back, you can make tighter turns. That kind of maneuverability is especially important in confined spaces and on busy jobsites. (FYI, the axle on the Cat CT660 is set back further than any other manufacturer’s truck, resulting in the industry’s best turning radius.)
• Better visibility. With the axle set back, truck manufacturers can lower the radiator and slope the hood. These design elements result in dramatically improved visibility, allowing your drivers to see more of the road or jobsite—a big safety benefit. A sloped hood is also more aerodynamic, which should help you improve fuel economy.
Set-Forward Axles Offer:
• The highest payload capacity. Bridge laws favor longer wheel bases, which in some cases make the set-forward axle the clear choice.
• More mounting/attachment possibilities. With the axle set forward, you have more flexibility in the front for attachments. On the Cat CT681, we took advantage of that space by offering optional Front Frame Extensions (FFE) and a Front Engine Power Take-Off (FEPTO)—which make it easy to mount and power snow plows, hose reels, winches, hydraulic pumps and other attachments.
Ultimately, the set-back/set-forward axle decision comes down to what’s most important for your particular application—and what will enable you to achieve the maximum allowable payload. Your Cat dealer can help you weigh all the benefits and trade-offs and make the right choice for your business.