The purpose of a shock absorber is to dampen the movement of the vehicle body as it rides over irregular surfaces or negotiates a turn. That is why they are technically called “dampeners.” The level to which a shock dampens body movement is purely a function of the oil inside and the valves through which the oil has to pass. The “right” damping is somewhat subjective. Some people prefer a stiffer ride, while others feel that a vehicle that rides too stiff is just plain wrong. In the engineering world, damping is discussed relative to “critical damping.” More than critical is “overdamped"; less than critical is described in terms of fractions of critical:
Typically, what most people agree is “the right” damping is about ½ of critical damping. That is, if you push down on the back of your vehicle and let go, it should bounce back above the point where it started and then settle back to its original height. The graph above shows what ½ critical damping looks like—on paper. Flip the graph upside down, and you can see more clearly how it relates to my pushing-down-on-your bumper analogy. That is about what we shoot for in our GoWesty HD Bilstein and Fox shocks for both Eurovan and Vanagon. So, which one should you buy?
Any shock absorber of any brand or quality can do the job. Both FOX and Bilstien are top-quality, world-renowned brands. The differences between these two brands and other shocks available on the market for the Eurovan and Vanagon are simple: quality and design. You might find a shock that feels the same when you first put it on, but it's not likely. The engineering and quality control that goes into a top brand like Bilstein or Fox is not easily matched. But even if you did find one that you felt was dampened just as well, it would likely not be up to snuff for nearly as long as a Bilstein or Fox shock. So, the question remains: Fox or Bilstein? The answer depends on what kind of roads you plan on negotiating.
Both of these shocks will feel about the same on a smooth road and around corners—and even over the same bumps. The difference between them starts to materialize when you go over lots of bumps for an extended period—like miles and miles of washboard, for example. What happens to any shock when it is dampening movement is that it heats up. And, as stated above, the oil inside is a key component as to how well a dampener can do its job. As the oil heats up, its viscosity decreases, the dampening action decreases, and you get further away from the ½ critical damping we all know and love. Shocks can get so hot they will literally puke out all their oil and stop working entirely (been there, done that). The Fox shocks we sell will handle severe service for between about 5-10 times longer (depending on application) than a Bilstein shock before giving up the ghost. This is not due to any quality issue; it is purely a design issue. The Fox shocks are designed to carry more oil and have more surface area—so they can dissipate heat better than the Bilstein shocks we offer. It should be noted that Bilstein also makes super high performance, off-road shocks—just not for Vanagons or Eurovans.
Thus, buying a Fox shock will ensure worry-free travel over the occasional washboard road. If you are absolutely, positively sure that you will not ever subject your Eurovan or Vanagon to that sort of (ab)use, then you might as well save a few bucks and settle for a fantastic, world-class shock like the Bilstein. For those of you who seek out the road less traveled (we're talking washboard roads here), the Fox shock is your best option!