After getting its start about 25 years ago in large off-highway, 200-ton capacity mining trucks, Liquid Spring introduced its multiple-patented Compressible Liquid Adaptive Suspension System (CLASS®) to the bus and ambulance industries in 2012. During that time, the company’s market share in the ambulance industry, where an upgraded air spring technology system was already being used, grew from 0% in 2012 to about 95% today.
On the bus side, Liquid Spring initially made inroads in a sector of the industry where a comfortable ride was of the utmost importance.
“On the commercial side, we initially saw some success with operators that were working with retirement homes, who had passengers using the bus every day and could notice the smoother ride on buses that had the system,” says Richard Meyer, president for Liquid Spring Technologies Inc., the parent company of Liquid Spring.
Now, with the positive feedback from end-users and transit looking to provide a better customer experience, the CLASS system is starting to make an impact in the public transit industry, particularly in paratransit.
“Back in 2012, we didn’t believe our product was going to be readily accepted in the transit bus market because of the associated cost,” Meyer explains. “That may have been a good assumption in 2012, but it is not a good assumption today. We believe the change is due to passengers requesting a smoother ride and transit agencies wanting to increase ridership by enhancing the customer experience.”
Algorithms Control the Flow
LiquidSpring’s CLASS system changes spring stiffness instantaneously over a wide range without driver intervention. To accomplish this, a compressible liquid is “properly” configured and algorithms are created to electronically control the flow of liquid. In doing so, low- and high-spring rates are instantaneously and seamlessly available to achieve desired ride and handling characteristics.
The patented suspension system is designed to replace conventional springs (steel or air), shock absorbers, and stabilizer bars by utilizing a compressible liquid. The compressible liquid provides the spring and damping. CLASS connects a first liquid volume (strut/damper) and a second volume (liquid only) with a hydraulic line and valve. The volumes are filled with a compressible liquid and pressurized, which serves as the spring. When the valve is open, all of the liquid is available to compress, providing the lowest spring stiffness; when closed, only the liquid in the first volume (strut/damper) is available to compress, thereby providing the highest spring stiffness. When the valve is Pulse Width Modulated (PWM), there is a seamless spring stiffness change. This configuration enables CLASS to change the spring rate and damping at each wheel independently, reducing harshness while increasing roll and pitch control.
The system, which has been outfitted on chassis from suppliers including Ford, GM, Ram, and Freightliner, offers comfort and sports modes. In the comfort mode, high stiffness is automatically activated for aggressive driving or evasive maneuvers, otherwise it is minimally active to maximize comfort. In sport mode, high stiffness is moderately active for more aggressive driving.
Monitoring Via Microprocessor
Operationally, the system works by monitoring the vehicle’s steered direction, speed, brake application, and the motion at each wheel. This information is processed by an on-board microprocessor using its preprogrammed eight-degree-of-freedom vehicle and non-linear fluid models. The microprocessor instructs the corner valve for each strut/damper module to adjust the effective liquid volume, thus changing the stiffness and damping as required to minimize transmitted power and resonant vibrations from the suspension to the chassis, which cause a harsh ride.
CLASS includes a leveling system that maintains a pre-set “proper” ride height, regardless of load, or as vehicle load changes. The leveling system also has the ability for leveling to “true” earth.
Ride Quality and Handling
Features and benefits of the system include a safer ride through electronic stability control system; a significant improvement in both ride quality and handling; a reduction in shock and road vibrations; reduced driver fatigue; increased vehicle life due to fewer vibrations into the vehicle; and a reduction in passenger complaints.
Coupeville, Wash.’s Island Transit has the CLASS system on 13 of its 29-foot Glaval Legacy buses, built on a Freightliner chassis, and has just ordered another seven to be delivered by the end of the year. Ken Riley, maintenance & facilities manager at Island Transit, said customer feedback about the rough ride was a catalyst for adding the system to its buses.
“On some of our routes, we don’t pack the buses completely full, and when you don’t have a certain amount of weight on the bus, the air suspension tends to ride much rougher,” he says. “We learned that unless we have a full bus, the ride is going to be rough. We actually had to move our security camera system from one location to a different location because the rough ride was actually jarring the hard drives — that’s how rough the ride was.”
Riley adds that because the agency is on an island there are a lot of corners and his operators also noticed handling issues on the back-end when negotiating those corners.
After meeting Liquid Spring at a conference and test riding a vehicle, as well as garnering feedback from those in the ambulance industry, Island Transit eventually decided to go with the system, using some grant funding that was available to them.
“By garnering feedback, we hear from our operators that the buses with the system ride really nice, especially going around corners, and the bumps don’t seem so bad,” says Riley. “Also one of our operators noticed on a windy day that they didn’t rock like some of the older buses.”
Although it’s early, Riley also adds that there have been zero maintenance issues, with the agency only having to calibrate the system.
Meyer explains that the CLASS system is virtually maintenance-free, with a lifecycle that is expected to not only last as long as the life of the bus and then some.
“Our system is designed for no maintenance other than normal wear-and-tear items like bushings and hydraulic seals for our struts. The fluid doesn’t have to be replaced because it doesn’t break down,” he says. “Also, the system should be good as long as the bus itself and should, in fact, extend the life of the chassis because there is less vibration coming from the road into the chassis itself.”
Meyer says the company is making a growing impact in transit and expects that to continue as agencies look to continue providing a comfortable, high-end experience for the customer. He also says the company will move up to larger transit buses as its footprint in the industry grows.
“Our company hopes to have more penetration in the cutaway bus market, with the goal of getting into larger buses going forward,” he says. “That wouldn’t be in the next year, but going out two or three years from now we can see ourselves in the larger bus market as well.”