Driving Link’s Smooth Riding Semi-Active Cab Suspension

Link Manufacturing knows a thing or two about cab air suspensions. The company pioneered cab suspensions back in the early 1980s, when Link’s founder, former cab-over-engine driver Bill Nibbelink, decided he’d had enough of the bone-jarring ride in his COE. He invented the first cab suspension to use air bags and shock absorbers, the likes of which is now fitted to nearly every truck cab in North America. All the OEMs have a standard cab suspension, but Link’s Cabmate maintains a 40% OEM marketshare. And it’s about to get even better. 

Link is now in the final stages of development on a semi-active cab suspension it plans to release to OEMs in 2019 called Road Optimized Intelligence, or ROI Cabmate.

Semi-active means variable-rate damping in the suspension’s shock absorbers. The damping rate is electronically controlled using input from two accelerometers built into the rear of the cab and two position sensors that monitor the movement in the truck’s main suspension. The suspension’s electronic controller uses information from these sources to set the damping rate in the suspension’s shock absorbers, which it adjusts hundreds of times every second to maximize ride comfort.

Link is currently exploring two methods of managing the dampening in the hydraulic shock absorber: an orifice that rapidly adjusts its size to either allow or limit fluid migration within the body of the shock; or with magnetorheological fluid, which thickens or thins with the application of a specific voltage and thus impedes or permits the movement of the piston within the body of the shock.

The dampening rate in an off-the-shelf shock absorber isn’t variable, so it can only do so much to respond to movement. In this semi-active system the dampening rate is infinitely variable, so it can respond to large or small bumps accurately to keep the ride smoother than was previously possible. 

The truck I drove for the test was using the latter method, but either approach works well, explains Tye Davis, Link senior engineer. “We are still evaluating the two methods, but if we get the same performance either way, we will choose the option that minimizes the overall cost of the system.”

During my visit to Link’s manufacturing facility in Sioux Center, Iowa, I toured the testing rigs where all manner of suspension inputs were simulated, from “cliff event” bumps to the washboard-type bumps found in concrete highways. It was easy to see the suspension responding to the various inputs while the system was on or off. Link has charts and graphs that clearly demonstrate the technical effectiveness of such a suspension design, but during the test drive the differences were equally obvious to my back and my backside.

The test course was decidedly less scientific than the test rigs, but it represented real-world conditions. The first portion of the test was a section of concrete highway with uneven slabs that give the washboard feel. It’s located a few minutes from the plant on Route 18 near Hull, Iowa. We made a run over that stretch of road with the factory suspension installed, then returned to the plant to switch that suspension out for the new ROI Cabmate semi-active suspension.

The test truck was a 2016 Peterbilt Model 579 with an 80-inch sleeper, a truck already known to have pretty good ride characteristics.

The result was palpable — and visible, too. We recorded the two runs on video, and you could see there was less movement of the driver’s seat relative to the cab. But more importantly, the movement was modulated so the ride actually felt smoother.

We also did a before-and-after test over a couple of railroad crossings. Again, the difference could be both felt and seen. On one of the crossings, my seat bottomed out with the standard suspension, but smoothly moved up and down with the semi-active suspension. In this case, the suspension limited the travel of the shock, preventing the movement of the cab from causing the seat suspension to travel fully downward and hit the stops. There certainly was movement in the seat suspension, but it was much smoother than with the standard cab suspension.

The implications for team drivers can hardly be overstated. Lying as they usually do in the rear of the sleeper at the farthest point from the fulcrum (the front cab mounts), the range of motion would be at its worst. The semi-active suspension limits the up and down movement of the cab dramatically and most importantly, smoothly. I can think of a few options that actually help with driver retention, and I wouldn’t hesitate to add this one to the list.

The ROI Cabmate also features electronic height control. The system minimizes air consumption compared to traditional-height control valves, because it does not fill or exhaust air in response to dynamic suspension motion. And as might be expected, Link says the semi-active suspension can reduce wear and tear on the cab structure, too.

Watch our video how the range of movement changes with the use of the standard and the semi-active suspension. You’ll also notice a pretty big smile on my face; the ROI Cabmate really does make a difference.

Video: Focus On... Link's Semi-Active Cab Suspension