Dana Frontrunner
New suspension for over-the-road heavy trucks.
Steve Sturgess
Executive Editor
Responding to market demand for a better-riding and sharper-handling front suspension, Dana has developed the FrontRunner for over-the-road heavy trucks.
Featuring the familiar E-1202W forged I-beam axle, the new suspension features a soft leaf spring and variable-rate air suspension in a combination designed to address upcoming braking regulations that will demand suspensions capable of reacting to higher brake torques.
The FrontRunner's most obvious feature is the reaction beam to the rear of the axle that feeds brake torque directly into the three-bearing rear spring hanger. The inside links of this beam are coupled by a torque tube that allows Dana to tailor the roll stiffness of the front suspension for improved transient handling or for a softer, more compliant ride.
According to Dana engineers, the design intent was to provide the improved ride quality of an air suspension without the "getting used to" factor that drivers identify with the air sprung steer axle options as offered by Volvo and Mack.
To demonstrate the benefits of the new suspension – due for limited release in the spring of '06 and full production by the end of the year – Dana staged a recent demonstration at its Marshall, Mich., proving ground and on a mixed-road on-highway
Features
At the introduction, Dana product manager Mark Davis identified the characteristics that define a front suspension that handles well. It must offer minimum deflection in steering input and in bounce and rebound, he said. Also, it should have a low rate and frequency for good ride and to maintain tire contact on the road surface, and offer significant roll stability.
In fact, a standard steel leaf spring does most of this. But fleets are looking for a little more ride comfort for drivers, so the answer is to combine the features of both.
So the FrontRunner has a low-rate conventional leaf spring that – even without the air – will support a 12,000-pound front end just off the bump stops. Combined with this leaf is an air spring on top of a specially profiled spring seat/air piston on each side. The profile of this piston gives a softening rate on spring deflection, but an extremely well controlled anti-dive for the front end under panic-stop braking.
The second unique feature is the brake torque reaction "link," which effectively parallels the rear half of the spring. Approximating a parallelogram with the spring, this beam mounts to the spring seat and the rear shackle about two to three inches away from the spring. The geometry eliminates any tendency for the spring to wind up under braking, improving the robustness of the spring combination with today's brakes. It also readies the suspension for anticipated changes to FMVSS121 that will likely call for a 30 percent reduction in stopping distances for heavy trucks sometime in the 2007-2009 timeframe.
As such, the FrontRunner suspension is designed to be effectively a bolt-in change for truck manufacturers at a time when they are wrestling to accommodate engineering changes dictated by 2007 emissions regulations.
In addition to the brake-torque reaction, these beams are connected side-to-side by a torque tube. As the suspension deflects in a curve – one arm up, the other down – the torque tube twists. This provides a roll stiffness that allows Dana to tune the suspension to the requirements of each truck OEM, by varying the wall thickness and diameter of the tube. On the International 9400 demonstration vehicle – and likely on International's production trucks and especially on the upcoming 4000-cab highway heavy due next year – this anti-roll feature is tuned for comfort rather than roll reduction.
Other tuning potential is through the shock absorbers and the spring length adopted by the truck manufacturer.
With its bolt-in design, the FrontRunner puts the steering arm location where the standard steel suspension locates it, making installation easy and also allowing it to be an option.
At the press event, there was even some conversation about making the FrontRunner available in the aftermarket, where a second owner could specify a ride-enhanced or performance-oriented front suspension replacement.
Spring Subtleties
The suspension offers a 30 percent lower spring rate than a standard steel suspension, made possible by the self-leveling characteristics of an air suspension. This is achieved with a single leveling valve – important in the event of an air spring failure, said engineering supervisor Gregg Richardson, when the suspension would settle to its unassisted just-off-the-bumpstops level, allowing a truck to limp in for repairs.
One of the more clever design features of the FrontRunner is the air spring piston. It features a negative taper so that over the bounce travel (four inches for International's design requirement versus Peterbilt's three inches as an example), the spring offers its maximum rate at the normal ride height. As the spring deflects, the effective spring area reduces as the rubber spring rolling lobe descends over the piston, lowering the spring rate (spring force equals air pressure in the spring times the effective area at the lobe) for excellent ride quality.
Even more impressive is the anti-dive under braking also conferred by the piston profile. At the last part of the lobe travel, it contacts the wide aluminum spring base and the lobe mushrooms out, significantly increasing the cross sectional area and increasing the spring rate to provide excellent control of front suspension dive under heavy braking.
Cool Runnin's
Demonstrating the ride and handling of the new suspension, Dana had lined up two virtually identical International 9400 tractors from its on-highway transportation fleet. One was a standard taper-leaf sprung unit; the other featured the FrontRunner setup. Both had Sheppard M100 steering gears, and they pulled identical wide spread air-ride 40-foot flatbeds loaded to 80,000 pounds GVW.
My first drive was in the conventionally sprung tractor-trailer and I found it a surprisingly nice truck to drive. The steering was light and precise – as are all Sheppard installations I have experienced – making the International easy to maintain in position with only minimal correction and a light touch on the wheel. And in its fully loaded condition, I had absolutely no complaint about the comfortable ride, either.
Next up, I sat in the passenger seat of the FrontRunner truck while my colleague Tom Berg drove. This was a fixed seat, so I was more conscious of the road surface, and the Interstate had deteriorated over this leg so there was a lot more to feel. Nevertheless, the air-ride soaked it up well, and the major suspension input was from the tandem suspension, felt mainly as back slap through the fixed seat. The quality of the ride was a little different too, with a lower frequency rolling with the bumps rather than the jolt you get from a conventional leaf-spring suspension over major bumps like bridge-deck joints.
When it was my turn to drive this unit, I immediately felt at ease behind the wheel. The truck tracked equally as well as the leaf-spring truck with no additional steering wheel inputs. It was just as easy to consistently run the trailer tires a couple of inches off the white line as before – a factor that proved very valuable in the last stage of the route on rural two-lane approaching the proving ground. Here, lane widths were only just wider than the width of the vehicle and oncoming traffic included other trucks. At no point did the steering or handling produce sweaty palms.
The same can't be said of the test track itself, where demo drivers put the two outfits through a slalom course. In the leaf-spring truck, entering the course at 35 mph – a speed I considered foolhardy in the extreme – the truck rolled wildly from one side to the other through the alternate swings of the steering wheel, made to negotiate the successive cones. Fellow trucking writer Jim Park (from our Canadian sister magazine HighwayStar) and I, unanchored in the sleeper, finished up in a heap, having slid and crashed into one side of the sleeper and then the other several times. It was most undignified.
To be truthful, I thought we came off surprisingly well, because I fully expected we'd finish up in a heap to one side as the thing rolled over, but the low center of gravity of the concrete block test load likely saved our skins.
After this, the sudden-avoidance lane change was fairly tame, but did show significant roll to the outside front with each direction change.
In the panic stop, the nose dived down as the ABS fired off, and while we were not staring at the road surface, a significant amount of the sky disappeared from view.
By contrast, the same heavy braking brought the front air spring down to its high-rate position and neatly controlled the dive. The lane change, too, was much more controlled as the roll stiffness built into the FrontRunner kept the International much more satisfactorily under control.
Having learned our lesson, Park and I braced ourselves for the slalom, but again, there was significantly less drama this time.
In Conclusion
The antics at the test track were absolutely convincing: The higher roll stiffness imparts a significant handling improvement to the suspension. And that was a good thing, because on the road, it is almost impossible to tell which vehicle is the air-ride-suspended unit. They both steered very well, maintaining line and never reacting to road ridges or ruts. There was a moderate improvement in the ride and a pleasing elimination of any bump-thump through the steering or even up through the floor on the FrontRunner tractor.
However, echoing engineer Richardson's comments, the drive suspension needs to be carefully matched to the steer axle's, because this became the predominant influence on ride quality.
Thirty minutes at the wheel was not really sufficient to get used to one truck before switching to the other. However, I think it very likely a driver who switches from a leaf-spring setup to a FrontRunner would spot the ride difference immediately, but would detect no degradation in the handling and steering response.
Would that make this suspension worth the upcharge it will likely command from the OEMs? Well, it's maintenance-free with rubber bushings and it has the proven low-maintenance E-1202 axle. As an air-ride, it undoubtedly bestows an easier life on the cab, which should result in fewer electrical and gauge problems and help maintain a tighter cab over the life of the truck.
And if it eases driver turnover because the fleet can be seen to be spec'ing trucks with the driver in mind, then it has some hard-to-quantify but hidden paybacks.
Oh yes, and it'll save about 75 pounds over today's specs and maybe 125 to 150 pounds over tomorrow's trucks, which will need heavier conventional suspensions to handle the bigger brakes we'll be seeing on trucks on a few years.
And if that improved handling results in just one fewer accident, it'll pay for itself many times over.
