I couldn't find a link to this article from National Dragster, so I posted it here in its entirety. Its drag racing related, but its also true for autocross and road racing:
National Dragster
Issue 15: April 30, 1999
Racing Technology
The hook factor: the ups and downs of shock absorbers
by Wayne Scraba
In the April 16 issue, I took a close look at how to make the front suspension in a Stocker work. This time around, I'll look at shock absorbers. What follows might surprise you. A simple truth in making a Stocker work is that shock absorbers are key. The reason for this is simple: If you can control the wheel motion, then you can control the dynamics of the race car. The better the control of the wheel motion, the better the control of the dynamics of the entire car. Interpretation? In the world of the drag racer, this boils down to refined hook. It also means your tuning capabilities are amplified many fold.
Certainly, there are cases where one can get a car down the track with a simple set of worn-out stock front shocks or a set of outdated non-adjustable 90-10s, but how good is that particular car going to work? What happens when the conditions change? What do you do when the car "works" at one track and doesn't at another? What happens if you run out of adjustment on your adjustable shocks? There's no question this is a major dilemma (and it has been addressed to some degree by single- and double-adjustable shocks), but in the not-so-distant future you'll see plenty of Sportsman drag cars ditching their old-tech hardware and replacing it with high-end shock-absorber packages. We're already seeing it in the Pro Stock ranks. More than a few teams have switched from common double-adjustable shocks to hand-assembled, nitrogen-gas-charged, infinitely adjustable dampers. Some Sportsman teams aren't far behind; I know of several Super Stock racers who have switched to Penske shocks and similar high-tech shocks from other manufacturers.
What makes today's infinitely adjustable shocks so different from other dampers available today? Plenty of subtle and not-so-subtle differences can be seen between them and more pedestrian shocks. Penske shocks (which are used for the basis of this column because these are the multiple-adjustable dampers with which I'm most familiar), achieve compression and rebound damping by fluid being forced through a series of high-quality valve washer stacks located on either side of the internal main piston. The damping forces of the shock absorber can be revised by simply changing the valve shims. Double- and triple-adjustable shocks permit you to fine-tune the valving requirements externally. The compression adjustments are controlled by the knobs on the remote reservoirs. The rebound adjustment is located at the eyelet; or, in the case of the late-model F car shocks shown in the photos, by way of a hex head adjuster located inside the shock shaft. These adjustments are easily accessible without removing the shock from the car. The following is a close look at how shocks work, how they're revalved (in the pits or in your shop), and how they're adjusted:
Valving
Different shock companies use different lingo. Quite often the words "bump," "rebound," "compression," and "extension" are used interchangeably. A shock absorber travels in two directions: It gets shorter (compresses), and it gets longer (extends). Some shock-absorber manufacturers call this "bump" and "rebound," but that can get confusing. To grasp what this is all about, pretend that you drive your car over a good old-fashioned speed bump. The speed bump "bumps" the shock, which in turn compresses it. After you drive over the speed bump, the shock rebounds and extends. Penske prefers to call these actions "compression" and "rebound."
The damping characteristics of the Penske 8100 Series drag race shock are determined by a series of compression and rebound valve stacks located on either side of the main piston, inside the shock body. Each of these valve stacks consists of a series of high-quality stainless-steel shims. The design of the shims allows them to flex under the forces of oil that flows through the shock absorber's piston ports. Once the force is removed, these shims return to their original, unflexed state.
Here's the neat part: The actual thickness of each shim determines the amount of damping force the shock absorber will produce. Penske services groups of shims or valve stacks in 13 different combinations. Each of the combinations includes constants (which are always the same size) and shims or valve stacks with different thickness. The respective stacks are identified with letters (AA, AA+, A, A+, B, B+, C, C+, D, D+, E, E+, and F, which happen to correspond to the valving letter designation). As an example, if your particular shock absorber uses "A" series compression valving, where all of the shims in the stack are .006-inch thick, and you replace them with "B" series compression valving, where all of the shims in the stack are .008-inch thick, the compression damping of the shock will increase. Reducing the thickness of the shims in the stack obviously decreases the damping force.
There's more here than meets the eye. With each set of shocks produced, the owner receives a technical manual. In this manual is a complete set of shock absorber dyno graphs. Each graph depicts the characteristics of a specific type of valving. This means if you have a set of shocks with "A+" valving on the rebound and "B+" valving on the compression, you can check the dyno chart, evaluate the way your shock works, then make changes in the valving. And you don't need to return the shocks to the dyno to determine the results. It's all done for you; each set of valving combinations is depicted in the manual.
So how are these stacks replaced? While there isn't room in this column to get into a step-by-step disassembly and assembly process, it all boils down to depressurizing the remote reservoir (this sounds fancy, but the reservoir uses a common Schraeder-style valve on one end), clamping the shock body in the lower-mount eyelet area in a vise, then unscrewing the shaft bearing assembly, which has an external wrenching hex. You then have access to the shaft and, of course, the shock piston and the valve stack. It's that simple.
Compression adjustment
The remote compression adjuster in the reservoir is a fine-tuning device for the main valving inside the actual shock body. As mentioned previously, the Penske 8100 Series drag race shock compression adjuster is located in the remote reservoir. This reservoir serves as an extension of the shock absorber, and it's designed to hold the vital elements of a gas-charged shock ? oil and nitrogen. Why use a remote reservoir? By having the reservoir mounted away from the shock, capacity can be added. This means a larger volume of oil and nitrogen is available to the shock, but at the same time, it allows for much smaller shock packaging.
Why increase the volume of nitrogen? At first glance, drag racing might seem like a venue where shocks aren't heavily stressed. That's completely wrong. The speed of the shock absorber piston is extremely fast in a drag race application; in simple terms, the shock "works" very hard in a short time frame. By increasing the volume of the nitrogen, the shock will become much more consistent, especially when faced with the piston speeds seen in a drag race application.
How does the compression adjuster work? According to Penske, "fluid is forced into the remote reservoir in direct proportion to the area of the shaft entering the shock body." As fluid enters the reservoir, it is forced to pass through the compression adjuster. Inside the adjuster is a CD drum. This drum has six different settings, numbered (appropriately) one through six. Number one is full soft, and number six is full firm. As fluid is forced through the CD drum, it is metered, though one of the holes in the drum. These holes correspond to the external settings. Once it passes through this opening in the drum, it enters the reservoir body, displacing the existing fluid, which reacts on the floating piston. The design of the internal floating piston inside the reservoir is such that shock oil and nitrogen are separated. This eliminates aeration inside the shock.
A problem with many shocks is fluid packing up when shock shaft velocities increase rapidly. When the fluid packs up, it cannot enter the hole in the adjuster (in the case of the Penske shock, this is the CD drum). The result is usually an increase in shock damping force. In other words, the compression damping is thrown out of whack, and what you have is a probable link to tire shake in a Pro Stock car. Penske has come up with a rather innovative solution: They use an internal blow-off valve that opens immediately, relieving the fluid packup. In turn, this allows the shock valving to operate correctly and provides a much more linear damping curve.
Rebound adjustment
The rebound adjuster on the Penske drag shock is typically located in the eyelet at the base of the main shaft. And like the compression adjuster, it only acts as a fine-tuning device for the shock valving. In the shocks shown in the photos, the adjuster is contained inside the main shaft. Aside from being a slick piece of machining, the adjuster at the top of the front shock is tuned by simply inserting a 1/8-inch hex Allen-wrench inside the shaft and turning it in either one direction. Here's how the rebound adjuster works:
During the rebound (extension) phase of the shock movement, fluid is forced through two ports contained in the main shaft. Just behind these two ports (inside the shaft) is a needle-and-jet assembly much like a carburetor needle-and-seat. The needle-and-jet are adjustable by way of the external rebound adjuster. If you turn the rebound adjuster clockwise, the needle is forced upward into the jet. This restricts the flow of the shock fluid, which creates firmer rebound damping forces. If you turn the adjuster counterclockwise, more oil is allowed to pass through the jet. This creates lighter, or softer, rebound damping force. The rebound adjuster is really a timed control for the rebound shims on the main piston to work.
Two needles are available from Penske for the drag race series of shocks. The standard needle uses a 10-degree taper on the end. Optionally available is a needle with a 5-degree taper. The decrease in the taper increases the fine-tuning capability.
In terms of adjustment, the rebound adjuster is designed with two full turns of adjustment. Typically, these adjustments are called flats ? for each side of the hex on the adjuster. A hex has six sides. Two full turns of the adjuster equals 12 flats. This means there are 12 adjustments from full hard to full soft on the rebound adjuster.
Slow-speed bleed bypass
This is a neat feature that will be important to any racer who has to deal with small bumps at the big end of the track. Penske incorporates a slow-speed bleed bypass on its 8100 Series drag shocks (slow-speed action of a shock is considered to be where the shaft velocities are between zero and two inches per second). In practice, this system allows oil in the shock to flow through small orifices that bypass the valve shim stacks.
Why is this so important? Simple. On the top end of the track, your race car isn't seeing a bunch of shock motion. The suspension has settled down, and for all intents and purposes, it's there for the ride. Without the bleed system, a car can tend to dart over small surface irregularities; remember, we're dealing with drag shocks, which, given their unique valving, can be easily upset. The slow-speed bypass action tends to help the driver with the feel of the race car. What it boils down to is a sense of control at high speed. This feature may not help your car run any quicker, but it certainly will make driving it more agreeable.
Baseline settings
The initial setting on shock absorbers should not be at the center point of adjustment. The first thing you need to do is set a baseline for the shock absorbers. For cars that run eight-second elapsed times and slower, the best bet is to set the front and rear shocks on the softest rebound setting. These soft settings allow the front end to lift as quickly as possible, transferring as much weight as possible to the back of the car. It will also allow the car to plant the tires immediately. Next, set the compression adjuster to full soft (the lowest value). This means that the nose will settle down quickly. What you'll have is a car set to initially react violently. On a very quick car, it's just prudent to begin with a conservative tune-up, then slowly adjust it to hook.
The only caveat to the above is in a stick-shift car, a violent automatic-transmission car, a short-wheelbase car with lots of power, or any car with a bunch of horsepower. It's best to initially set the rear shocks with firmer compression and rebound damping at the rear.
Once you've established a baseline for the shock setting, make a short pass with the car; 1st gear and part of the way through 2nd gear only. At this point, you have to treat the car as a system. The primary goal is to leave the rebound control alone for the time being and work on the compression setting. Try dialing in more compression (bump control) until the car loses traction. At this point, you know you've gone too far with the adjustment, and you'll have to back off by one click. Only at this point should you begin work on the rebound settings.
The following, courtesy of Hal Lees of Hal Shocks, are the steps you should take when tuning the front and rear shock combination.
If the car wheelstands excessively or bounces in the gear change (more likely with a lower-horsepower application), adjust the front shocks first. If the car rattles the rear tires, wheel hops, or has way too much body separation, adjust the rear shock absorbers first.
The idea is to get a smooth transition in the front-end movement as the car launches through the first gear change. Bouncing and jerking motions do not help the launch. If the car is violent on the launch and physically jerks the front wheels off the ground, the rebound shock setting is too soft, or loose. If the car bounces on the gear change, the shock needs a stiffer compression setting. When the car bounces on the gear change, it's coming down on the front-suspension travel limiter, then bouncing back up. Obviously, if the shock is set too tight (stiff), then the front won't move sufficiently to transfer weight. On a similar note, a too stiff setting on the front shock will tend to bounce the car on the tire after the launch. Don't get this confused with bouncing off the limiter.
When it comes to the back shock absorber, the idea is to shock the tire as hard as possible (track conditions permitting). Keep in mind that it's the shock that actually controls how much force, or hit, you're applying to the slick. If the shock is too loose on the extension (rebound), you might get way too much rear body separation. If the shock is too tight, the car will flatten the tire excessively or simply cause the car to spin. As mentioned previously, start soft on the rear and keep tightening up the valving until the car slows.
Once the initial fear factor of working with sophisticated shock absorbers passes, you're going to find out one thing very quickly: Shock absorbers can account for more improvements in performance than any other single component in your race car.
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John Coffey
johnc@betamotorsports.com
