While leaf spring suspensions are designed to provide comfort while loaded, the ride can sometimes be harsh when operating without a load. "Depending upon the type of leaf spring, it can offer poor ride quality, especially when you look at an unloaded situation," says Stevenson. "They are optimized for comfort under load. People sometimes forget the trucks are running pretty much the same amount of time loaded as they are unloaded."
For service and lube trucks, unladen ride quality is not as much of an issue. These trucks are usually laden.
Steel leaf springs also have a weight disadvantage. "The rubber and the air springs can be built lighter than the steel leaf springs," explains Lindgren. "It impacts how much you can haul. If you can haul a little bit more, you can usually get a little bit better revenue. So weight becomes very important."
Good articulation does not usually go hand in hand with good roll stability, but leaf springs can provide good roll control if they are properly outfitted. "The leaf suspension is pretty good in roll stiffness as long as you add certain things, such as a stabilizer bar," says Ferro.
The Sterling TufTrac is an example of a leaf spring suspension with a torque rod. This type of setup maintains a uniform load over each wheel as the suspension articulates, and its design is much lighter than some leaf spring and even some rubber block designs.
Mack Trucks has found its Camelback leaf spring suspension is a popular choice for the construction market. "Primarily, it has a parallelogram design, meaning the spring centers can adjust from end to end to aid in equalizing ground pressure and, most importantly, allowing the bogie to steer around a corner and not drag the rear-most axle, scrubbing off very expensive rubber," says McKenna. "We offer an Anti-Sway spring pack that has increasing spring leaf thickness as the spring leaf gets progressively shorter —in effect, giving the entire spring pack stability through the entire load and unload cycle, resulting in good ride quality, loaded or empty." It weighs more than a rubber block suspension, but offers good ride, stability and tire wear at an affordable price.
Rubber blocks offer compromise
"One of the advantages of a rubber block [suspension] is it is pretty consistent in terms of its roll stability," says Stevenson.
Ferro agrees, noting, "The rubber block suspension has inherent lateral stiffness and roll stiffness."
Rubber springs can also be tuned for increased ride quality. "It is easier to get a variable spring rate with a rubber suspension than it is with a steel leaf spring," says Lindgren. "Steel leaf springs typically have a linear spring rate."
"At General Motors, a lot more of our customers are requesting the rubber blocks just because of the better ride and roll control," says Ferro.
Increased ride quality and roll stiffness make rubber block suspensions especially popular with dump truck operators. "For Kenworth, we typically recommend using the Chalmers or Hendrickson HAULMAAX suspensions," says Lindgren.
A disadvantage of a rubber block suspension is its higher initial cost compared to leaf spring suspensions, notes Ferro. In addition, the ride quality of the rubber block typically is not as good as air ride suspensions.
Air ride gains popularity
The demand for improved ride quality is driving the development of air suspensions. "There is probably more development money spent on air suspension systems than any other kind," says Lindgren. "That appears to be the direction of the future. We are seeing air suspensions being specifically designed for vocational applications."
In the past, some on-highway air suspensions were pressed into off-road service. This created articulation, stability and durability issues with the air bags. But times have changed. "The Sterling AirLiner is specifically designed for vocational applications, as opposed to in the past where you had highway air suspensions being put in vocational applications," says Stevenson.
Lindgren reports that Kenworth also has a series of air suspensions designed specifically to tackle vocational applications.
Air suspensions typically offer the best ride quality. "The air ride provides a good ride, whether it is laden or unladen," says Ferro.
This translates into greater productivity. "You reduce driver fatigue," Ferro explains. "You can stay in the vehicle longer without getting tired."