Leaf Relief

DETROIT — If there's one thing this industry reveres, it's a cheap and effective design.

How else to describe the rear suspension of every pickup truck and most truck-derived sport/utility vehicles (SUVs) currently sold? Solid rear axles and leaf-spring suspensions have been around since Daimler met Benz, and that fact is what helps trucks and SUVs to be such huge profit centers: They use century-old technology!

But even the remorseless penny pinching of the auto industry may have wrung the last squeak out of the leaf spring. Automakers are working overtime to update pickups and SUVs with — hold onto your hat — honest-to-goodness independent rear suspensions (IRS).

“There are numerous very active (truck IRS) programs,” under way at the OEs, says Rich Kowalczyk, manager-mechanical integration and chassis module groups at Delphi Automotive Systems' Energy & Chassis Systems division and one of the industry's authorities on truck suspension design. “There's a lot of marketing pressure to go in that direction; everybody's working on it, looking at where the market's going.”

The “why” beyond this fundamental technology shift is simple: the inevitable customer demand for more refinement. Although leaf springs and solid axles comprise pretty effective suspension for hauling heavy loads or towing a boat, the setup isn't the end-all in ride comfort or handling finesse.

“Multi-leaf springs and a solid axle work very well for both light loads and heavy payloads,” Mr. Kowalczyk counters. “It's completely passive and very cheap.”

But the plain truth is that a startlingly scant percentage of light-duty pickup/SUV buyers engage in heavy hauling or extreme off-roading. The pickup truck (and the SUVs derived from them) has emerged in the last decade as a “lifestyle” vehicle — largely removed from the severe duty cycles for which all pickups traditionally have been designed.

The “how” is a bit more complicated.

Bill Gillespie, director of chassis systems engineering at Delphi Chassis and Energy Systems, says that there's a complex market dynamic under way — one that ultimately will determine how IRS for future trucks and SUVs is adopted.

For example, both Mr. Gillespie and Mr. Kowalczyk agree that the market is likely to further differentiate between light-duty trucks and heavy-duty versions. The light-duty product development is much more likely to be headed for IRS because that's where the “lifestyle” buyers are; heavy-duty pickups will continue to be bought by those using the trucks in genuine “work”-type duty cycles, they say, and as such will be better served to retain the excellent load-bearing qualities of solid-axle/leaf rear spring suspensions.

“The truck and SUV buyer now wants the same kind of ride and handling that's available with a car,” says one OE suspension engineer who asks anonymity. “They want the SUV package but don't want to deal with the fact that they're really driving a truck.”

The obvious choice is to go with one of the growing league of so-called “crossover” vehicles — that is, those vehicles built on car-type platforms, typically already using more sophisticated IRS.

But that ignores that enormous existing manufacturing investment in body-on-frame vehicles. All of the domestic automakers, in particular, have a preponderant portion of their manufacturing base gradually adapted to take advantage of the nearly decade-long U.S. fascination with trucks and SUVs.

The answer, says Mr. Kowalczyk, is either to adapt an existing chassis or to design a next-generation light truck to employ IRS. Although he believes most automakers are leaning toward the latter solution — as usual, they're three years behind the customer-preference “curve” — some may consider adapting existing chassis.

“It's relatively easy to adapt a solid axle vehicle to IRS,” says Mr. Kowalczyk, although it's handy to have an intensive redesign in the works. Case in point: Ford Motor Co.'s new 2002 Explorer/Mountaineer, which uses a full-frame chassis, but one cleverly modified to accept an IRS by running the rear wheels' halfshafts through “portholes” in the frame itself.

Mr. Kowalczyk says there are advantages to Ford's strategy — apart from the ride/handling improvement, IRS lowers the load floor, which is particularly useful in staking out territory for a third-row seating area.

But he says the Ford answer is not without caveat. For one, the frame rails are heavier than would be necessary if the vehicle were solid-axled. The design also makes the Explorer/Mountaineer harder to assemble.

What about a “modular” approach — one that would use a single chassis able to be fitted with either a rear IRS or solid-axle module, depending on its use?

“You can have a truck that tries to serve both roles,” claims Mr. Kowalcyzk, “but you'll still have a ‘bias’ that makes it feel like a truck. It's difficult to have one platform for both users. For trucks, you shape the vehicle around the capability. There's a lot of ride height, tall aspect-ratio tires, high spring rates.” In essence, the sort of designed-in engineering that wouldn't necessarily lend itself to ride-improvement shortcuts.

The engineers agree, then, that the best solution lies in next-generation platforms — ones designed to use IRS.

Perhaps the best example is one that's already out there: the ahead-of-its-time Mercedes-Benz M-Class.

The M-Class, launched for the '98 model year, was designed from the start as a body-on-frame chassis using IRS. Although in the years subsequent to its launch the M-Class now is judged as not as refined as some newer competitors, its design is the archetype for what domestic automakers are likely to do with next-generation pickup/SUV platforms to leverage the refinement of IRS.

“Mercedes is very good at designing suspensions,” says Delphi's Mr. Gillespie. “Most other companies probably will follow in that (the M-Class) direction.”

Mr. Kowalczyk reminds that although there is no overwhelming technical hurdle preventing pickups and SUVs that employ IRS from hauling even the heaviest of loads, the design would require some type of load-leveling setup, such as the “air-spring” system available on some current SUVs, to handle the large tire camber changes heavy loads can impart. Then, too, come packing issues for those new components.

All that, not to mention the numerous requisite suspension links, brings about the inevitable cost penalty. How much, though, is anybody's guess. The Delphi engineers reckon “a couple of hundred dollars,” at least.

Nonetheless, the onward march of progress seems clear. One supplier engineer believes that, “Ten years from now, it (truck IRS) will be pretty much standard. And as other electronic systems become cost-reduced, it will make even more sense to combine attributes.”

The two Delphi engineers, intimate with OE development programs, also have no doubt. Will we be seeing, say, General Motors Corp. Yukon Denali-class vehicles with IRS?

“Yes,” asserts Mr. Gillespie. “Although it's clear that the tradeoffs are very complicated, including the vehicle assembly process. Currently, there's no right answer.”