Hendrickson shows that suspension-related vibration is an engine torque effect
When truck technicians investigate driveline noise or vibration complaints, they tend to look in the usual places for the usual suspects: driveline imbalance and/or improper U-joint cancellation angles. If the problem persists, they check the clutch. And if that doesn't do the trick, they check the suspension.
With drivers demanding higher engine horsepower and torque ratings, the issue of maintaining proper driveline angularity is becoming more complex.
Recent studies conducted by Hendrickson Suspension Systems show that when drivers of heavy trucks with high-powered engines accelerate through the lower gears, driveline geometry changes, creating a torsional vibration effect that is more profound than commonly realized.
What happens is that the drive tandem's air-suspension ride height control drifts. Since the vehicle's frame rises as torque is released, a radical, if temporary, change in the U-joint working angles is produced.
In an effort to better understand the phenomenon and develop componentry to address it, Hendrickson collected data from a 75,000-lb.-GCW tractor-trailer. Accelerations from a series of standing starts were correlated with measurements of tractor frame lift. The engineers observed that each time the driver disengaged the clutch, the suspension's ride height returned to normal. But when the driver shifted gears and re-accelerated, wheel torque caused the frame to rise again.
It became clear that the 3-in. frame rise at the axle centerline wrenched driveline angles out of whack, inducing U-joint cancellation errors.
Hendrickson determined that the lift problem could be reduced by controlling the amount of suspension travel in the rebound direction when air springs are extended. However, this had to be accomplished without compromise to any of the crucial suspension functions, including vertical travel to absorb road inputs, comfortable ride, and enough axle travel to clear obstacles.
Several alternatives were evaluated before Hendrickson engineers hit on what they determined was the most effective solution -- a special spring design packaged inside a 13/4-in. bore shock absorber. This evolved into the "Hi-Torque" shock, which proved to be an effective means of minimizing frame rise.
Essentially, what they did was fit an internal spring over the shock piston rod. The spring is captured between the shock piston and rod guide bushing. As the shock is extended during rebound, the spring is compressed.
The Hi-Torque shock is part of Hendrickson's integrated EDGE system, which is designed to promote efficient driveline geometry for the HAS trailing-arm air ride. It is now required on tandem axle vehicles with engines developing 1,550 lb.-ft. or more of peak torque, or with final drive ratios of 4.11 to 4.6.
Hendrickson engineers point out that all current air suspensions have some form of height control that can be evaluated against a trio of performance characteristics: dead band (the range of vertical movement that the suspension does not respond to); dynamic height drift (the ability to maintain height in normal operation); and response time (the time needed to respond to sudden ride-height changes).
Many of today's height-control systems have rather slow response times and aren't designed to react to the dynamic short-duration events that impact suspensions. However, Hendrickson claims that EDGE incorporates a reliable combination of dead band, height drift, and response-time controls.
Yet, the suspension maker is quick to note that proper setup is crucial to the success of the system. EDGE is effective in limiting torsional driveline vibration only if parameters are maintained. The driveline must be set up correctly. Ride height must be accurate. And a Hi-Torque shock is needed with a height control valve that has minimum dead band. Otherwise, U-joints will be unduly strained.