Recently a company called RCD Classics figured out a way to install a conventional cover on a Bilstein shock. It's actually a noteworthy achievement. Though a Bilstein looks like most other shocks, its construction doesn't lend itself to a metal cover. Instead, they usually feature a plastic shroud, which most people in the hot rod world forego altogether. The bare look doesn't exactly turn everybody on though-the vintage/traditional crowd in particular loves that OEM look, and OEM shocks almost always have that metal cover.
True, metal-shrouded shocks are pretty much standard issue. And in fact, the garden-variety covered shock costs considerably less than even a bare Bilstein, much less one of RCD's chromed-and-covered ones. So why spend more for something that looks just like something you can get for less money?
One word: Technology. Though ordinary looking, a Bilstein (stein as in beer stein) is an incredibly sophisticated piece of machinery. A valve network built into its piston head automatically reacts to speed and road conditions. Forget about adjusting knobs or even electronics; this passive mechanism, tuned specifically for each application, responds instantaneously without any user input. In fact, a Bilstein damper can cycle through its entire adjustment range within 2 mm if need be. To the metrically disinclined, that's less than 0.080 inch.
Curious as to how it does it? Well RCD's Steve Duck pointed out a few interesting features. There's more than meets the eye.
Let it Bleed
To understand what makes a Bilstein different, we have to understand how shock absorbers work. But before we do that, let's exchange the term shock absorber with the more correct term damper. Those oil-filled tubular things at each corner of your car do not absorb shock. Much like the slab on the snout of a crankshaft or that flat coil inside a valvespring, they suppress or damp unwanted movements, in this case bounciness and body roll.
They do it by converting kinetic energy (motion) to thermal energy (heat). In fact, all dampers employ bleed circuitry to do it.
A bleed is a controlled leak in either the piston itself or the damper's cylinder head. It permits a specific amount of fluid to move or bleed through the piston or cylinder head. The friction the oil encounters as it's forced through the bleed circuit generates heat. "The bleed circuit in an ordinary damper is a group of holes," Duck revealed. Various things, including vehicle weight, suspension geometry, spring rate, and intended vehicle use govern the number and size of these holes. They're simple and, at least for most applications, sufficient. But bleed circuits have a catch. Two, in fact: excessive heat and hydrostatic lock.
Heat is an intentional by-product, but excessive heat isn't. Bleed holes have a comfortable flow range. They will flow a bit more if asked to but will generate excessive heat trying. "Excessive heat does all sorts of nasty things, like cause the damper to lose its effectiveness or, in extreme cases, melt its own seals," Duck noted.
That's not the worst that can happen either. At the upper end of the flow rate is the bleed holes' maximum flow capacity, and there is no working around that. "If the piston can't move fast enough, the damper hydrostatically locks up and acts as if it's as solid as a bar," he added.
Ever encounter a bone-jarring pothole? More than likely it was the damper transmitting the brunt of the impact directly to the chassis because it couldn't keep up with the suspension movement, Duck maintained. "If the impact is great enough and the vehicle is heavy enough, things can break," he said.
In layman's terms, the damper gets dramatically stiffer as it increases in speed until it literally won't move any faster. We refer to this as a progressive rate. Dampers that rely exclusively on bleeds have very progressive rates.