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While much of the talk about truck suspensions these days centers on the incredible rise in popularity of air ride, there are other advancements making waves, even a couple of fascinating design concepts that have yet to leap off the drawing board, that deserve discussion. According to suspension engineers at independent suppliers and truck OEMs alike, whether or not the most futuristic advances ever

While much of the talk about truck suspensions these days centers on the incredible rise in popularity of air ride, there are other advancements making waves, even a couple of fascinating design concepts that have yet to leap off the drawing board, that deserve discussion. According to suspension engineers at independent suppliers and truck OEMs alike, whether or not the most futuristic advances ever reach the pavement, there are innovations aplenty being built into today's heavy-duty truck and tractor suspensions that will benefit fleet owners by protecting their equipment investment and boosting their appeal to truck drivers.

Typical of many OEMs, Peterbilt Motors Co. allows buyers to order trucks either with their proprietary suspensions or those provided by outside manufacturers.

“Suspensions are a core competency here,” says Todd Acker, Peterbilt's on-highway marketing manager. “We offer our proprietary suspensions for capacity ratings of 46,000 lbs. or below, as well as Hendrickson models for ratings above 46,000 lbs.”

Acker says air ride is king nowadays across the Peterbilt product line. “Our top two drive-axle suspensions are our FlexAir and Low Air Leaf models,” he notes. “And on steer axles, buyers can order a Hendrickson air-ride suspension if they choose.”

Peterbilt chief engineer Landon Sproull expects the popularity of air ride up front will grow and points out that there is also interest in the potential development of independent front suspensions (IFS) to smooth the ride for Class 8 highway tractors. IFS is currently available on such vehicles as fire trucks, motor homes and motor coaches but not on highway tractors.

According to Sproull, on today's trucks, just adding air all over is not the answer. “Front and rear air-ride suspensions perform differently,” he advises.


Plus with drive axle and cab sleeper air suspensions in place, putting air on the front axle could be too much of a good thing. “You have to be careful that having all that smoothness from air everywhere could sacrifice lateral stability, what drivers call road feel,” Sproull points out.

To avoid such issues, Sproull says to approach suspensions application by application. “For example, weight is critical for a dump truck so first you'd want to narrow your choices down and then choose a suspension based on purchase price vs. unit weight. There's about an 80-lb. difference between our Flex Air and Lower Air models but the payoff will vary by application and business situation. And ROI over time is also critical to the spec'ing decision.”

Looking a bit further out, Sproull says active control of suspensions via electronics is certainly feasible but the concept is still very much under development. “There is work going on toward active suspensions, which may mean putting in electronic controls to handle various features. Active suspensions may become a way for OEMs to differentiate their products.”

According to Dan Fuchs, chief engineer-chassis for the Freightliner Group, air ride now accounts for “a good 90% of on-highway rear suspensions. The most obvious trend now is the pick-up in air on steers in vehicles with front axle capacities above 12,000 lbs.

“Air ride on the front axle is often perceived as providing further refinement to ride,” he continues. “The margin of improvement is slight because the load on the front axle is so constant compared to what the drive axle encounters. We offer the Hendrickson Airtek for steer axles and from 10 to 20% of linehaul tractors now have it.”


Down the road, Fuchs says to look for “full air on the front as well as the rear; the idea being making that commitment all the way around will gain the most benefit.”

Fuchs says IFS could be regarded as the next step beyond front-axle air ride to further improve ride and stability but does not see it arriving “until 2012 or later in this market. It has not been embraced in Europe and the re-packaging OEMs would have to do to accommodate it would bring a substantial weight increase.”

In the meantime, Fuchs expects air ride will make further inroads on vocational trucks, which he also expects will offer more higher-capacity rear axles, rated for 46,000 and even 52,000 lbs., that may use air suspensions to help maintain traction off road.

In Fuchs' view, bringing electronic controls to suspensions is another matter altogether. “It's a significant cost step to active control of the suspension because today we have reliable pneumatic controls in place. Also, to have an active suspension would also require active shock absorbers, further driving up the costs.”

According to Tom Davis, on-highway marketing manager for Mack Trucks, “99.9% of the drive axles of highway tractors are now on air ride due to the ride improvement provided and the fact these suspensions have become lighter over the years. We also offer air on the steer with the optional Hendrickson Airtek suspension.

“Active suspension for trucks is something we are evaluating,” he continues, “but not something to be expected soon. Likewise with IFS. That's being evaluated with our engine group but is still at the conceptual stage.”

“We've studied IFS,” adds David McKenna, Mack's engine, axle and transmission marketing manager, “but it would add a couple hundred pounds of weight. As it is, the ‘07 engines and the expected new brake regulation will add weight. IFS may be a technology that is ahead of its time for heavy trucks.”


Steve Ginter, Mack's vocational marketing manager, says the two Granite truck models are still mostly spec'd with mechanical suspensions, either proprietary Camel Back units or ones supplied by either Hendrickson and Chalmers.

“We will be updating our Camel Back mechanical suspension over the next year to year and a half,” relates Ginter, “to improve its ride characteristics and make a slight reduction in its weight that will keep moving this technology forward.”

Roger Elkins, product manager for truck, bus & motor home suspensions for the Holland Group, points out the reason there is such an array of trucks suspensions available from both OEMs and independent makers is “there's a vast number of applications that must be satisfied — there's a greater number of jobs trucks are doing.”

He says that air ride on highway trucks is just about a given and now it is gravitating toward the vocational side due to “concerns about driver comfort over long work shifts and the desire to lower lifecycle cost by protecting the vehicle from bumps and vibration.”

Elkins explains the appeal of a parallelogram-design suspension for drive axles is that by being “non-torque reactive,” it prevents the pinion angle changes that a torquey high-horsepower drive-train can cause, which in turn saves wear and tear on pinion bearings, u-joints, and other parts.

“There are only a few non-reactive air-ride suspensions for heavy-duty trucks available,” says Elkins, “including our Holland Neway AD Series for single, tandem and tridem severe-duty applications. Its parallelogram design reduces driveline wear and vibration by maintaining a constant pinion angle.”

Turning to future electronics on suspensions, Elkins suggests “you'll see the active concept develop in terms of the suspension being tied in with the engine, brakes, steering etc. so that the suspension is ‘talking’ to the rest of the truck via a black box. There are many ways to accomplish this, but certainly cost and complexity will be involved. The real question will be whether this will become part of a ‘smarter’ truck.”


Holland already produces IFS for motor homes and Elkins says that “back before the last downturn some truck OEMs were showing it on their concept vehicles but it is more difficult to package than a rigid axle.

“IFS for highway trucks may make sense from a marketing viewpoint — simply in terms of something new to promote. But fleets would have to try it out to see if it adds value for their particular operations,” he says.

“An IFS vs. a rigid axle air suspension is not something all fleets would go for based on the upcharge for a minor performance enhancement,” Elkins adds.

Steve Keifer, marketing manager for Hendrickson Truck Suspension Systems, says suspension development boils down to four key issues — reducing lifecycle cost; trimming unit weight; smoothing truck ride; and improving performance, “be that traction, torque management, handling or stability.”


Keifer confirms the non-torque-reactive trend is very strong. “All our drive-axle suspensions now under development are non-reactive to eliminate suspension-induced driveline vibration resulting from high torque output from engines.

“For example,” he continues, “this spring we rolled out our HTB tandem-drive axle suspension Its parallelogram design offers significant enhancements over the traditional trailing arm. With no steel springs, it is non torque reactive and weighs 240 lbs. less than a trailing arm suspension. We feel it will improve ride quality by 10 to 15%.”

Moving up front, Keifer points out that Hendrickson's Airtek is currently the “only front air ride suspension in data books for most on-highway brands.” While that suspension was designed expressly for highway trucks, he reports that a version of Airtek specifically engineered for vocational trucks will be released by late next year. “This model will give vocational customers at least a 25% ride improvement and up to a 190-lb. weight savings.”


While he concedes things could change over time, Keifer figures the “marginal improvement in ride over an Airtek suspension” that IFS would deliver would not offset the increased cost enough to attract fleet buyers.

As for the potential of electronics being applied to suspensions, Keifer points out that Hendrickson has been providing “semi-active” suspensions for military applications. “However, we have no immediate plans to bring this technology to Class 8 commercial vehicles,” he states. “As with IFS, it's a benefits-vs.-cost issue. The semi-active design does perform beautifully in the severe off-road conditions military vehicles encounter.”

Wayne Powell, director of marketing for Tuthill Transport Technologies, maker of Reyco Granning suspensions, says there's a “need now in over-the-road trucking for non-torque-reactive drive axle suspensions on trucks spec'd with higher-horsepower engines.”

He says the traditional drive axle suspension design, combining steel spring trailing arms and air bags at or behind the axle, can react to torque input to the point that u-joints and drivelines incur damage.

According to Powell, Tuthill has addressed this with its recently introduced Reyco Granning AlignDrive RD 2300NR suspension. He says this unit boasts a parallelogram design that helps offset the impact of higher torque loads on the drive axle from engines producing over 1,000 lb-ft of torque‥

“As a non-torque reactive suspension, AlignDrive has fewer wear points, which means greater durability and reduced service intervals,” he states. “Pinion angle ranges of as little as 1.5 to 7 degrees can be achieved. Shrinking the driveline-operating angle reduces noise and vibration.”

While he regards the non-torque-reactive suspensions as sorely needed by some truck fleets, Powell is not so sure IFS will find a market on Class 8 trucks.

“We already make IFS for motor homes and even some fire trucks,” Powell relates, “but I'm not sure truck fleets would want to pay the premium this would require to get the ride and handling benefits. At this point, to pay a little more on a $500,0000 motor home is one thing; a price premium on a working truck is another. We are working on ways to reduce the cost of IFS for heavy trucks and think one day an OEM will offer it as a ride-and-handling option.


“On a heavy truck,” he adds, “the area at the front end gives you the option to use long leaf springs for a good ride at a reasonably low cost. Lighter-duty vehicles went to IFS a long time ago so we know there are handling advantages.”

Powell regards active “or reactive” suspensions as off on the horizon as well. “We are working with a customer to develop an electronically controlled suspension,” he advises, “but there are cost and reliability issues to resolve. And that is complicated by the fact truck ride is very subjective, making it hard to measure and quantify.”

Steve Slesinski, Dana Corp.'s director of product planning for commercial vehicle systems, contends that IFS may arrive in Class 8 trucks in “maybe five years, depending on how several factors come together.

“IFS is designed into our technology demonstration truck,” he continues. “In fact, that's our third-generation IFS design for a Class 8 tractor.”

According to Slesinski, IFS offers some key improvements over other suspension designs. ‘There's a substantial improvement in handling and control because each wheel is independent as the truck travels over bumps etc.,” he advises. “We have our IFS concept mounted in a conventional tractor to demonstrate the handling characteristics. And the maneuverability is equal to or surpasses current offerings. We think weight will be equivalent on a production truck.

“But,” Slesinski continues, “before IFS can be viable, engine compartments will have to be redesigned by OEMs. Packaging is a big issue. But the needs of the 2010 engines could be a driving factor in terms of managing heat rejection under the hood.”

Slesinski says that, for Dana, bringing air ride to the front axle is the next step on the road to IFS. “We're now in the process of commercializing our air ride concept for the steer axle,” he reports.

“It may be one to two years before this product arrives. We're now at work establishing its reliability. “With any advanced suspension concept,” he adds, “arrival is really a matter of when customers want it.”

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