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Ford OEM Parts - Shock Factor - Tech

Black Jeep
Posted February 1, 2006

An Inside Look With Bilstein

Several years ago, while conducting the vehicle evaluations for another publication, we were shocked (no pun intended) to discover that we had blown out the OEM shocks on a brand-new Ford we were testing.

This wasn't an uncommon occurrence for the off-road guys, who typically auditioned trucks and vehicles like they were racing Baja 1000. But for a truck driven solely on the street for our test, this was highly uncommon - or so we thought. No doubt we drove the truck hard, with cornering tests and several passes up and down a very steep and windy stretch of highway well known for motor vehicle testing.

Actually, with just over three full days of driving, we wore out the tires and brakes on all the vehicles, and had shock failure on two. Generally speaking, OEM shocks are built to last longer than most of the aftermarket shocks out there. This may sound surprising, however, consider that shocks are covered under standard manufacturer warranty and you'll understand why they're made to last.

Like tires, the aftermarket of shocks is literally flooded with junk. Parts with a "Lifetime Warranty" that will last between 10k and 20k miles (how's that for a shock?) are unfortunately common enough. Eighty-nine dollars for a solid, high-performance shock doesn't sound too bad until you times that by four and tack on an additional $50 per corner (on average) for installation if you don't bolt them on yourself. So a good majority of enthusiasts trim their budget and buy low-quality shocks. What they don't realize is that even with a lifetime warranty, it will cost them a couple hundred bucks to replace them sooner than expected. Sadly, most people only spend extra money on parts they can see - note the abundance of pristine, never-see-dirt, pre-runner and monster-style trucks now commuting on the freeways.

The bottom line is that shocks are arguably the most important safety feature on your vehicle - they keep the rubber on the road. This may not seem as important to the rigs that are taken to their destination on a flatbed and only see trail time, but the majority of four-wheelers today actually drive to their destination. This means that traction on the highway is just as important as it is off the highway.

In general, shocks are broken down into two basic categories: twin and mono-tube. And of course there is the adaptation of the mono-tube in the McPherson strut. Realistically, we don't use twin-tube shocks on off-road vehicles, so for the purpose of this feature we focus on the mono-tube and the various types of mono-tubes, hydraulic, gas-charged, and air piston.

Mono-tube shocks are higher-performance shocks as they are made to tighter tolerances. They have less variance and are best for off-road and racing applications as they dissipate heat faster. On the down side, they cannot be used in all applications such as struts. They allow a larger bore size to be used in the same space which gives you more damping control.

Generally speaking, hydraulic-type shocks are not practical for the rigors of today's off-highway driving. Hydraulic shocks, like gas-charged shocks, use a piston and special valving to dampen energy. Because the fluid cannot be compressed, it dissipates this energy by passing through the valves. Excessive movement actually results in the fluid starting to bubble which further hinders performance. To rebound, it must pass back through the valves. This recovery time relates to poor performance, thus making them impractical.

Gas-charged shocks, struts, and air-piston designs have much more advanced valving and result in faster reaction time - keeping your tires where they were meant to be. Simply stated, shock absorbers convert the kinetic energy of the spring movements into heat. This heat is then dissipated through the shock tube. Hydraulic fluid or oil actually holds heat longer versus gas or air which helps to dissipate heat, thus the faster the response time.

We sat down with Shane Casad who works in product development for ThyssenKrupp Bilstein of America to gain some perspective on what goes into the research and development of a new product.

4WD&SU: What are some of the key factors you consider when developing a shock absorber?

Shane: The major factor is building the shock to accommodate the application to improve the overall performance, ride, and handling.

4WD&SU: Explain the process in developing a new shock absorber.

Shane: Answering this question depends on what type of shock we are trying to build, as well as the desired performance increase. In this case, I can only assume you are referring to an application for a street legal off-road vehicle. Here is the process: When we build an application, we go through a process we refer to as "ride work." Basically, a vehicle is brought into our shop in San Diego, California. We determine the goals of the application and decide on which damper will be required. We measure for dimensions, in many cases optimizing travel. Once the prototype shocks are mounted, the vehicle is then driven on a set test track consisting of surface streets, windy roads, freeways, and rough dirt roads. During this time, the shocks will be taken off the vehicle and revalved until the desired goals are realized (during the revalve or "shock-tuning" phase, the damping rates are adjusted on the piston head for rebound and compression). On a Jeep or truck application, this process usually takes two to three days.

4WD&SU: Bilstein is well known in both on-road and off-road racing markets. Are there two separate engineering teams that address each market?

Shane: For Bilstein of America, there is one engineering team that addresses both motorsports and street applications. However, we have different Tech Groups that actually apply the products to the various markets. Our main specialties here are off-road racing, circle track, OEM, and aftermarket street performance. In Germany, there is a specialty motorsports group that works autonomously on the aftermarket and OEM groups; however, the only off-road vehicles they are involved with are World Rally cars


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