It happens with trailing-arm air suspensions, and is interesting to watch. Reyco Granning has an instructive video on its website,http://reycogranning.com, that clearly shows dock walk as it happens:
The forklift's weight compresses the air bags, which causes the suspension's trailing arms to rotate slightly, pushing against the axles, whose wheels don't roll because parking brakes are set (or should be), and/or the wheels are chocked.
So the reactive motion continues into the body, which is pushed slightly forward, says Bill Hicks, a product-planning leader at SAF Holland. Forward motion can drag the suspension, axles and unchocked wheels slightly. Repeated entry by the loaded forklift can push the trailer forward a little each time, until a bigger gap develops.
The Reyco Granning video shows that after three trips aboard a trailer by a forklift toting heavy paper rolls, the trailer has moved 4 inches.
The trailer doesn't move if it's hitched to a tractor, whose brakes keep it from moving and whose fifth wheel blocks the trailer's kingpin and body, and keeps the entire rig in place. But the trailer can move if the trailer has been dropped at the dock and its nose rests on its landing gear.
The forward motion strains the landing gear posts. Their braces can weaken and the posts can bend; in severe cases, gear has been known to collapse, Hicks says. Gear can be reinforced to deal with such motion.
Dock walk is minimized or prevented by certain suspensions, which use different designs to accomplish this goal:
- A parallelogram design in Meritor's Ride Sentry doesn't rotate, note Bill Wakefield, manager of strategy and product management, and Carl Ellis, a product manager.
- A trailing- and leading-arm de sign in Reyco Granning's Dockmaster "safety suspension" cancel each other's motion. The suspension actually tends to push the trailer closer to the dock. Engineers aren't sure why, says Wayne Powell, vice president, marketing, but don't worry about it, because rearward movement only enhances safety.
- Locking levers prevent trailing arms from moving, as employed by Hendrickson's SureLok and SAF Holland's PosiLok. The levers can be deployed in various ways, including by swinging open the trailer's rear doors. Usually cylinders powered by the trailer's compressed air system push them into place and later retract them. If they don't retract, the suspension becomes a stiff, unsprung mechanism, and trailing arms and other components can be damaged.
So drivers should check that levers have retracted before starting out.
These countermeasures solve the walking, but air bags can still compress.
"Squat" and "trailer drop" are other terms for it. That plus tire deflection can lower the trailer's floor by several inches.
Hendrickson calls its anti-dock walk and ride height lock device SureLok, notes Jim Rushe, trailer systems program manager for on-highway products. "We find that the ride height lock feature is desired in many instances in order to provide a smooth transition at the trailer floor, and it also maintains the height of the top trailer at the loading dock so that tall forklift loads can be moved onto the trailer."
Some operators prefer to dump air out of the bags so the suspension rests on its bumpers. This provides stability, but a more severe drop of the floor creates a steeper grade on the ramp that can cause the lift's tires to slip. Also, drivers can forget to dump and replace air (something remedied by automatic systems), and valves can freeze in cold weather, Rushe notes. Hendrickson offers an air dumping system along with its SureLok.
Most customers whose trailers regularly load and unload by forklift specify suspensions designed with dock walk in mind. They aren't needed by trailers that don't back up to docks, such as curtainsiders, tankers, flatbeds and dumps, where a regular trailing-arm suspension is fine.
Why not just stay with simple steel leaf-spring "mechanical" suspensions? Most suppliers offer both types. But air-ride is chosen by a majority of customers for vans, reefers and other trailer types because it cushions cargoes, reducing damage and freight claims, and protects trailers themselves against breakage of lights and structural parts, our supplier sources say. Here air-ride's advantages outweigh its drawbacks, including possible dock walk.
From the September 2012 issue of HDT