While a vehicle’s springs do the heavy work when it comes to maintaining driving stability, it’s the shock absorbers that determine how smooth the ride will be.
By Jim Brightly, F358406
The term “shock absorber” serves as the name of an important piece of automotive equipment as well as a description of its function within a vehicle’s suspension system. Commonly referred to simply as a “shock,” this device doesn’t support any portion of a vehicle’s weight “” that’s what springs are designed to do. The sole function of the shock absorber is to control axle movement.
How does a shock absorber work? Without getting into too much detail, a shock works by passing hydraulic fluid back and forth between two reservoirs through a series of valves. This dampens the vertical movement of a vehicle’s axle without transferring the movement to the coach’s frame and body. A shock absorber transfers the energy of movement to heat, and then dissipates the heat to the air passing over its surface. Shocks can be designed with differing valves for controlling incoming (upward) or outgoing (downward) movement. For motorhome applications, the shock absorber design is most likely to be 50 percent upward and 50 percent downward.
It’s hard to talk about shocks without also mentioning springs, since the job of shock absorbers is to regulate the action of springs. So to begin any discussion about shocks, it’s important to understand what a spring is and how it operates.
For motorhome chassis, springs come in two major types: coil and multi-leaf (there are other designs that are called “springs,” but they are not specifically springs). Coil springs are made from steel rods that are heated and then wrapped around a central core. The rod diameter and the tensile strength of the steel used in the production of the coil determine the weight rating of the coil.
Leaf springs are made from strips “” or leaves “” of what is called spring steel (so named because it’s flexible, not brittle, and tends to return to its original arc), with the main leaf having rolled ends for attachment points. There is also a center pin in each “stack” of leaves (which slips through a hole in each leaf to keep the leaves centered over the axle) and two or more end clamps on each stack (these clamps hold the leaves in alignment; otherwise, they could spread out from the center pin like a pair of fans). In many cases, the spring manufacturer will slip Teflon or plastic shims between the leaves to keep them from squeaking or binding. Leaf springs also are used to align the axles to the chassis frame, which is why they are used more extensively on solid axles than coil springs are. Although coil springs are said to provide a better ride than leaf springs, leaf springs perform more jobs and are somewhat less expensive overall.
Springs “” regardless of their design “” do not control rebound in the same sense as shocks do. The springs are there to support the weight of the coach and attempt to minimize or eliminate the transfer of movement from the axles to the chassis and, thus, the coach. Without shocks to absorb and control spring rebound, the springs could theoretically oscillate until next Tuesday after each roadway obstacle. If you’ve ever seen a car continue to bounce up and down a half block after going over a bump, you have witnessed a vehicle with shock absorbers that are worn out and need to be replaced.
Speaking of bouncing cars, that’s how we used to tell when shocks needed to be replaced. We’d jump up and down on our car’s bumper; if it stopped oscillating after one or two vertical movements, the shocks were still good. If not, we’d run down to the nearest auto parts store for a new set.
Unfortunately, we can’t determine shock wear on a motorhome chassis by bouncing up and down on the vehicle’s bumper. Its suspension is way too strong “” even with worn shocks “” and will not move. We have to use other methods to determine the shock’s condition. One is to look for leaks. Some leaks may not be readily apparent, so you may need to check for areas where dirt has collected on seepage. Also, make sure to examine each shock for broken or bent shock mounts or damaged portions of the shock, such as a bent or broken shaft, weld, etc.
You might want to take the time and effort to remove each shock and attempt to compress and extend it by hand. Does the shaft slide in and out effortlessly, or is it extremely difficult to operate? If the shock compresses easily, it probably should be replaced. If you find one shock that’s bad, even if it shows no obvious signs of leakage, then more than likely all the other shocks that were installed at the same time are bad. If they’ve been on your motorhome for more than 100,000 miles, they probably should be replaced.
After some research, I’ve determined there are three major players in the motorhome shock absorber market, all commercial members of FMCA: Bilstein (C5209), Koni (C6019), and RoadKing, distributed by Henderson’s Line-Up (C5009). Here’s some information about each of these companies and products that they offer.
Bilstein’s Comfitrac piston design, now found in many Bilstein monotube shock absorbers for motorhomes, is said to deliver overall improved ride comfort and enhanced handling characteristics. The body of Bilstein gas-pressure shock absorbers is fabricated using a special extrusion method for added strength. The seamless design, with a large “” 46 millimeter “” piston, also helps maximize heat dissipation and shock life. The Comfitrac piston head design allows independent tuning of the compression and rebound damping forces to provide optimum ride comfort and performance. According to Bilstein literature, the self-adjusting piston instantly adapts to changing road conditions and provides maximum vehicle body motion control while maintaining superior comfort.
The shock also uses Bilstein’s High Frequency Bypass valving technology, which reportedly reduces road harshness input. And since it’s a monotube shock, there are fewer moving parts to fail or wear out, offering increased durability and reliable performance.
Introduced a few years ago, Koni’s Frequency Selective Damper (FSD) shock design makes it possible for the shock to adjust itself according to road conditions. The FSD includes a special valve that reportedly filters out annoying vibrations before they enter the motorhome. As described in company literature, here’s how it works.
Each FSD shock has two parallel oil flow openings: one that travels through the piston assembly, and another that travels through an FSD valve. The FSD valve is pressure-sensitive and remains open during high-frequency inputs, such as expansion joints and rough roads, allowing the shock oil to continue to flow easily through the FSD valve and provide a smooth ride. However, if the motorhome starts to bounce, roll, or sway, the FSD valve closes and forces the shock oil to travel through the piston assembly, damping the forces until the motorhome settles down.
The new RoadKing shock absorber from King Shocks is designed specifically for the heavy loads and high mileage typically encountered by large motorhomes. Built using proven designs from the heavy-duty trucking industry, RoadKing shocks feature an extra-large 2 5/16-inch bore, a 3/4-inch stainless-steel hard-chromed piston rod, and a spherical rod bearing mounting system. The result is a shock absorber that delivers up to 4,500 pounds of damping force per wheel and reportedly will last up to 350,000 miles. Yet, according to product literature, it is said to dramatically reduce ride harshness, provide excellent stability over varying road surfaces, and increase tire mileage by up to 30 percent. It also can be fully rebuilt.
RoadKing Shocks LLC
RoadKing from Henderson’s Line-Up