Drive Toward Optimum Performance

There are basically four types of drive axle suspension systems for medium- and heavy-duty trucks: rigid mount, steel leaf springs, rubber blocks and air ride. The choice between systems directly impacts the vehicle's ride quality, roll stability, durability, mobility and handling. To maximize productivity and safety, you must understand the trade-offs.

No system works well in every application. "As with almost every truck spec'ing question, the answer is that it depends," says Brian Lindgren, vocational sales director, Kenworth. You must decide which attributes are most important in your application.

On construction sites, one of the most difficult trade-offs is between the feel of the ride and handling. "You are always making a choice between ride quality and roll stability," says Steven Ferro, lead design engineer, medium-duty suspension systems, General Motors.

Suspension designers must pick and choose which conflicting attributes a given suspension system will emphasize. "Roll stability and articulation often work against one another," says Lindgren. "Durability tends to be a trade-off with weight —the heavier ones can be built with more metal so they last longer. Handling, in most cases, comes as a function of roll stability."

Operation in rough, uneven terrain requires different suspension solutions than those running on well-graded jobsites. "If you need a suspension that has extreme articulation because you are going into a site that large earthmovers have just been through, then you might have to trade off more of a lightweight design to get one that has the utmost in articulation," says Matt Stevenson, product marketing manager, Sterling and Western Star Trucks. Cost is also an issue that results in trade-offs.

Even regional regulations affect your choice. "In the eastern United States, customers demand a more robust specification for duty that certainly will weigh more than a similar application in, say California, where weight is the primary driver of the spec," explains David McKenna, product manager, Mack Engines, Transmissions & Axles.

But there has been a shift in the types of suspension systems preferred by vocational customers. "If we go back in history, virtually all trucks were on some kind of a steel spring," recalls Lindgren. "Then rubber started making inroads. About five years ago, it was about one-third steel spring, one-third rubber and one-third air for vocational trucks." And the market continues to shift. "It seems to be settling out to either rubber or air."

Rigid suspensions offer stability
Rigid suspensions have advantages in terms of simplicity, durability and stability, but they offer poor ride quality.

Optimally, a rigid beam suspension will be the lightest and most stable," says McKenna. "It will also be the roughest ride laden and un-laden."

Lindgren adds, "If you went to the old Hendrickson R suspension systems that didn't have any springs, that is the best in roll stability." But you probably would not enjoy the ride quality.

Leaf springs prove cost effective
The leaf spring suspension is a good fit for anyone who places a premium on the initial purchase price. "It is the lowest cost," says Ferro.

In addition, leaf spring suspensions typically offer good articulation for negotiating uneven terrain. "The leaf spring with torque rod design or some of the lighter weight rubber block suspensions with torque rods usually offer the best articulation," says Stevenson.

This prevents the suspension from bottoming out. "When a suspension bottoms out, that invariably means that one axle is no longer in contact with the ground, causing wheel spin," says McKenna. "Depending upon wheel speed, when that end regains traction, this could be very costly." This favors systems with good articulation, such as the leaf spring rrangements.

Ground clearance is also important in these environments. "That is the single most significant reason that Mack has top-loading rear axle carriers that greatly increase ground clearance and keep the driveline angles straight," says McKenna.