Semi Trailer Air Ride Suspension – TORONTO, Ont. — When was the last time you heard a driver say something positive about a trailer suspension? Most drivers would barely know that their trailers even had suspension, but a design innovation from Hendrickson actually prompted drivers to call the company asking what kind of suspension they were pulling. They wanted to encourage employers to specify the same suspension on future trailer orders.
This innovation, called Zero Maintenance Damping, started life about 10 years ago and was conceived as an idea to save maintenance. It first appeared on the Ultraa-K suspension in 2014, where it is now standard. It is also now available on select Vantraax and Intraax systems.
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“There are approximately 150,000 Ultraa-K suspensions in service now and nearly a million ZMD springs out there,” says Scott Fulton, director of product development at Hendrickson Trailer Commercial Vehicle Systems. “We really hope that it is just an improvement in the cost of the life cycle of an air suspension, but the traders of the positive feedback we received suggest it is much more than that.”
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Fulton says that they did not understand what was happening at first, so they took some trailers, the instruments up, and began to study what was happening. “We soon found that the ZMD suspension system reduced certain forces transmitted to the trailer and that it improved the ride in ways that we could document and understand.”
The Hendrickson zero maintenance damping system looks a lot like any other air suspension except it has no shock absorbers. A down-stop to prevent further extension now occupies this space.
The design intent of what became the ZMD was to reduce the total life cost of air suspensions. They looked at the various components and life expectancy and decided to cushion the shock they had to go. But shock absorbers provide the necessary damping in an air ride suspension.
All suspensions require some form of damping, which resists the movement of the suspension and the trailer. In a leaf spring package, for example, the friction between the spring leaves provides the damping force. Since a spring has no internal resistance, some damping mechanism is necessary. Hendrickson was able to eliminate traditional shock absorbers by designing a new spring and piston assembly.
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Essentially, the air bag and the piston form a single sealed air chamber separated by several small air passages between the bellows and the piston. As the air spring is compressed by the road inlets, the air is forced from the cavity inside the air bag into the cavity inside the piston. The resistance of the air moving in the passage between the two air chambers and the associated increase in pressure gives the former damping force delivered by shock absorbers. There are no more moving parts.
“Tests showed this design was absolutely effective as a hydraulic shock absorber, but it came with some unique and unforeseen side effects,” Fulton says. “It improves the ride quality of the overall truck/trailer combination for reasons that we only came to understand after we launched the product.”
The ZMD air spring is a two-chamber sealing system with several passages engineered so that the air passes from the air bag to the piston, providing the damping action.
The big difference between a spring and an air suspension is that the air suspension adapts to the load you are carrying. More weight requires more pressure in the system as controlled by the height control valve. What is not adjusted is the shock absorber. It is designed for the heaviest loads that the trailer will carry. With the ZMD, because the air is doing the damping, the damping rate also adjusts to the load.
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Where you see the real benefit is on low load trailers or lightly loaded trailers, because the damping rate adjusts down along with the load the trailer is carrying.
Fulton says shock absorbers are not uniform in how they cushion. It drops easily when it hits a bump and does all of its work as it extends back to its resting position, allowing the suspension to return to its ride height.
“That’s how it takes energy from the system,” he said. “The disadvantage is when hitting constant bumps. The shocks are compressed and the wheels can go off the road. With ZMD, we can wet in both directions.
There is yet another benefit: reducing the front and rear forces transmitted to the trailer. With the shock absorbers positioned as they usually are, approximately 45 degrees from the horizontal between the suspension arm and the trailer body, the shock load experienced by the arm is transmitted to the trailer as a force that causes that of the driver. felt like a punch in the kidneys.
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“We’ve changed the direction of the force going into the trailer from slightly forward to vertical, and we can adjust that force based on the load,” says Fulton. “Drivers get a more comfortable ride and there’s more protection for the cargo and trailer structure from road ingress, which helps extend the life of the trailer.”
Removing the shock absorber reduces the front and rear forces transmitted to the trailer from the suspension. With ZMD, all the force is transmitted vertically, providing a better ride for the driver and the cargo while reducing wear and tear on the trailer body.
I was initially a bit skeptical about the bold claims Hendrickson made regarding the ride improvements with ZMD. After a pre-ride briefing with Fulton, I understood the principle behind ZMD – forcing air through small openings to resist movement instead of mechanical dampers. I understood that removing a shock vector load source from the truck load would improve the driving experience, and I could well see how ditching the shocks would save a few maintenance dollars, but I honestly wasn’t prepared for the scale. improvement of the ride.
I drove two trailers on three loops around a test course in the Canton, Ohio area. The first was an empty van trailer equipped with a steel spring suspension. The second race was with an empty van equipped with an Ultraa-K suspension with ZMD technology. The third run was the same trailer with approximately 10,000 pounds of waste placed directly on the rear bogies. I used the same air-ride Volvo VNR for all three loops.
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The empty steel-spring trailer was predictably embarrassing. The road leading out of Hendrickson’s gate and access to the freeway was really nothing more than a loose group of pot holes that shook one of my video cameras right off its dashboard mount. Out on the highway, the concrete washboard continues to beat on the truck, which every trip over a seam causing a slap in the back. As I drove over a set of bridge joints at a modest speed, I could clearly feel three sets of axles passing over the joints, starting with the direction – thump, then thump-thump as the drives passed, and then a pause. , thump. -thump as the trailer axles went over the seams.
There were three sets of railroad tracks along the route. The first two weren’t too bad, the third was brutal, and with four sets of tracks, each grade crossing was severely deteriorated. This one was enough to bounce the trailer axles right on the ground at just 40 km/h.
The second loop around the track with the empty ZMD trailer felt about 500% better. I could still feel the potholes on the first part of the road, but they were considerably tamed by the trailer suspension. The lips were still visible, but the cues were gone. The washboard highway was still bumpy, but again, I just felt the ups and downs as the tractor axles crossed the slabs. That rear jolt that I felt with the steel-spring trailer was greatly reduced. The bridge seams were barely noticeable as the trailer axles passed over.
And this series of nasty trains still almost knocked me out of the seat as the tractor went on, but the trailer just a bit reminded me that we had just crossed over a rough set of trains.
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When we got back to Hendrickson, the crew put a 10,000-pound test load in the back of the trailer, right on the bogey, and we went for three rounds. To avoid the repetition, I will simply say that I could not have any input in the trailer yet. The bushings were just out of the axles to drag and drive.
Traveling over the seams of the bridges, I felt the thump and thump-thump as the tractor passed, but I could not feel anything in the trailer. Seriously. The nasty tracks half way around the route was also a non-event. I did not see the wheels from the ground and I could barely feel the trailer axles crossing the tracks. There was just the faintest soup from the back and absolutely no jolt in the waist to be felt at all.
Driving on the highway at highway speeds and hitting the usual variety of bumps was just like going over the seams of bridges. I felt the tractor wheels go over the bumps,
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