How to adjust adjustable shocks and cure your car's ...



How To Adjust Adjustable Shocks And Cure Your Car's Wayward Suspension

BY WAYNE SCRABA

If you have a skinny-tired doorslammer (a legal Stocker or ET bracket car), you probably don't have much suspension adjustment. That's a bad thing because these are the kinds of cars that need it the most. Perplexing? It doesn't have to be. In simple terms, a car without a big tire "crutch" responds favorably to adjustable shock absorbers.

Before we begin, you should know that there are two schools of thought regarding shocks and front-end setup. Most seasoned "skinny tire" Stock Eliminator racers (who run on 9-inch wide rear slicks) agree that every piece on the car's front end should be new-from ball joints to control arm bushings. Here's where the controversy begins.

Some racers prefer to use a non-serrated front bushing on each side of the control arm so the front end does not bind as it travels up and down. In other words, the front end is "loose" and all motion is controlled by the spring and shock absorber. Other racers have a slightly different point of view: They use a regular bushing in the front control arm. With a non-serrated bushing, they feel the nose becomes too loose and tends to bounce when it comes down after launch. Instead, they use stock or jobber replacement control arm bushings. According to AVO, high horsepower race cars (like a big-block Camaro) are also better off with conventional serrated bushings. In either case, remember that a spring or torsion bar with a lower "spring rate" will make weight transfer easier than a heavier spring. The rate of the spring and the use of serrated or non-serrated bushings will affect the shock absorber valuing. In a car that needs more weight transfer (such as one with small slicks), use a lower rate spring.

How do you figure out which shock valving is right for your car? Simple. Use an adjustable shock. In the following photos we'll show you how to adjust two different styles of shocks and how the adjustments compensate for varying conditions. What we won't show you is how to install the shocks that's pretty basic and we're certain anyone with a 9/16" wrench can handle the job. So follow along as we show you how shock adjustments can lower your elapsed time and make your car more competitive.

12 SHOCK TIPS

#1 Lengthy Matters-Any time you add a traction device to a narrow-tired car, the idea is to gain body separation, which in turn plants the slick. In simple terms, the body and wheels are moving further apart as the car launches. Generally, an OEM shock absorber is too short for this action. As the car launches, a short shock will reach full extension before the body and rear axle reach maximum separation. The shock extends rapidly but comes to an abrupt (and premature) stop. The axle housing has no choice but to go back up which creates a condition

known as "wheel hop." The solution? Lengthen the shock. If you don't, the shock internals will eventually be torn up by the over-extension.

Longer than stock shocks usually aren't available (including race shocks like our examples). Instead, add a set of Mr. Gasket shock extensions to your existing shock absorbers. How much "extension" will you need? With externally adjustable shocks like AVOs, remember the adjustment hardware (knob) consumes approximately 1" of stroke. Thus you'll need an inch of shock extension.

Nonetheless, here's an easy length test for leaf spring cars: Support the rear frame rails of the vehicle with axle stands. Then allow the axle housing/wheel-tire combination to completely drop down. At full extension, the shock absorber should be capable of meeting this length or separation criteria. If not, a longer extension is required (several shock extension lengths are available from Mr. Gasket for various leaf spring applications). The extensions are simply a bolt-on proposition. Leaf spring jobs typically install at the top of the shock. Cars with coil springs can use this type or bottom-of-the-shock extensions. When playing with extensions, begin with the shortest. If the car still wheel hops, go to longer extensions. Simple as that.

#2 Loosie Goosie-Cars with non-serrated front control arm bushings usually work best with shock valving stiffer than the typical 90/10, something around 60/40 is a good starting point. According to racers, a 90/10 shock comes up too fast and with too much force. This tends to unload the rear and promote tire spin. The idea, move the car smoothly forward and up.

#3 Good Tracks V. Bad-If you race on slick tracks, soften the back shocks to a lighter setting. This allows the chassis to separate quickly and (hopefully) plant the tires with sufficient force to hook the car up. On an AVO shock, that means setting the adjuster on the rear shock counterclockwise, while CE shocks should use the 50/50 rear valving. If the track has teeth, go the other way with rear shock settings.

#4 Total Package-The shock can't do it's job alone, and neither can the springs (or torsion bars). The idea is to have a total integrated spring and shock absorber package in which the shock controls the rate at which the springs move (torsion bars too). This has a direct relationship on how fast weight transfers from front to back in the car. It just makes sense to have some form of adjustment with the spring-shock absorber package. And since changing springs on a Detroit-built door car isn't all that practical in testing, you can see why shocks play such an important role in chassis tuning.

#5 Cheap Thrills-If you're a bucks down racer who can't afford adjustable front shocks, try a set of very conventional 50/50 hydraulics. In other words, no high tech shock absorbers, just simple, Monro-Matic bargain bin specials-the cheaper the better.. These work to some degree, but don't expect the level of adjustability found with drag race hardware.

#6 Gas Attack-Bucks down racers can also get away with dirt-cheap shocks on the back. Some cars work relatively well with a conventional 50/50 gas filled street shock. Which gas shock is right for you? Experience has shown that an inexpensive shock such as the KYB models work well in this application. What about air shocks or load levelers? Air shocks shouldn't be considered for any drag car; they're designed for load compensation and have little or no value in a race car. Generally, the same applies to the many "helper" spring equipped shock absorbers.

#7 Softly Sprung-How do you know if your rear shocks are too soft, too firm or too short? Simply put a nylon tie-wrap tightly around the center of the shock shaft and make a pass. If the tie-wrap is shoved to the top of the shock, it's probably set too soft. If the tie-wrap hasn't moved, chances are the shock is too stiff. What you want is a tie-wrap that's about 2/3 of the way up the shaft. This tip can be used to check rear shock travel as well. If the tie-wrap is moved all the way to the end of the shock, the car definitely needs a shock extension.

#8 Extension Test-Just how much should your shocks extend (using the "tie-wrap" test)? According to AVO, the front shocks should show 70% piston rod extension while the rears 60%. Some cars with wayward suspensions (i.e. a set of long rear spring shackles used for tire clearance) go beyond this point. If that's the case with your car, bring it down to earth and make it work without the extra hardware.

#9 Locked In Place-How do you hold the top of the shock when tightening the nut on stud-mounted units? Once you start cranking on the nut, the piston rod turns. Vice-Grips do nasty things to piston rods, but there is an answer: a special sockets from Snap-On and other tool suppliers. As you can see, the 3/8" drive socket is shaped like the top of the stud, which allows you to hold the stud solid while turning down the nut with a wrench.

#10 Carry A Big Stick-When setting up drag shocks, there is a difference between stick and automatic cars. A stick or trans-brake car will generally hit the tires on launch harder than a foot-brake automatic, so it needs more damping. A straight foot-brake automatic needs softer settings. As for the nose, both cars should begin their testing on the softest setting.

#11 Snub This-Some Stock racers set their cars up initially without the factory front suspension limiter or "stop". The stop rather than the shock are then used to control wheelstands. If you're at a track with excellent bite and the car wheelstands, install the front snubber, which bolts to the upper control arm. Having several sets of different limiters (each with a different height) makes it easy to "tune" the wheelstand.

#12 Slot Cars-This is an AVO shock designed for an early Camaro. Due to the design of the front suspension, the shock mounts inside the front coil spring. You might find that turning the knob is difficult in certain applications (especially if the spring coils are tight). To solve the problem, simply slot the knob with a hacksaw and use a screwdriver for easy adjustment. We should point out that most late AVO shocks have pre-cut slots.

BUMP AND REBOUND: What's it all mean?

We've all heard the terms "bump, rebound, compression and extension". They sound a bit confusing but they're not. We should point out the expression "shock absorber" probably isn't right either. The Brits call them "dampers", which is a more appropriate name. Shocks simply reduce the suspension spring oscillation to zero.

To picture this, think of the goofs who drive cars down a road with completely worn shocks or no shocks at all. Once the car hits a bump, it never stops bouncing. You see, a good spring will oscillate for a long period of time if left unchecked. If some sort of friction (preferably controlled) is added to the spring, the oscillation time (up and down bouncing motion) is kept to a minimum. In a nutshell, a shock dampens the spring oscillation in a controlled fashion.

A shock absorber travels in two directions: It gets shorter (compresses) and it gets longer (extends). Most shock manufacturers call this "bump" and "rebound". To grasp this, pretend you drive your car over a 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. Racers usually call this movement compression and extension.

When looking at shocks you'll run across a ratio like 90/10. This is called the "bump/rebound ratio". It's used to compare the peak forces in either direction over a series of cycles until the suspension stops oscillating. The force required to extend or compress a shock absorber is proportional to the shock speed. The higher the speed, the higher the force. Believe it or not, most stock passenger cars do not incorporate shocks that have a 50/50 ratio. Instead, these shocks usually have a 30/70 ratio-30% of the suspension travel is resisted in bump (compression) while 70% of suspension travel is resisted in rebound (extension). It's a conservative setting that basically allows for a smooth ride.

One more pertinent race shock term is "jerk," which is the instantaneous change in shock acceleration. Jerk is something that relates more to passenger car comfort.

What does this have to do with drag race shocks? When you look at a drag car front end, the rebound or extension force of the shock controls how easily the nose rises. If the extension is made softer, the nose rises quicker, transferring more weight to the rear. Conversely, the bump or compression force of the front shock controls how easily the nose settles down. If the car nose drops too rapidly or bounces during a gear change, the bump should be increased (made stiffer).

At the back end of a drag car, the rebound or extension force of the shock determines how much force initially hits the tire. If the tire initially spins too much, soften rear shock rebound. The compression force or bump of the rear shock also controls how the back of the car settles as weight is transferred. Typically, a car that squats on the rear during the launch will need a higher bump or compression force.-WS

RISING RESISTANCE CURVES: Dyno testing shock absorber valving

Everyone who's anyone in drag racing uses a dynamometer to test and refine engine combinations long before the car hits the strip. What was once the domain of Pro Stock has now trickled down to us. Not so common is testing shock absorbers on a shock dyno, a practice that's been popular for years in Europe and most recently a service provided in this country by Koni and AVO.

In AVO's case, "revalvable" shocks are those, which can be dyno tested and custom tailored to a customer's application. For example, you buy a set of shocks like AVO's Stocker Stars and put them on your car. If after the dozen shock adjustments you still can't get the car to work, send the shocks back to AVO. They'll install them on their in-house shock dyno and check valving. Based on your in-car testing, valving is then modified and the shocks dyno tested again.

In each case a printout or "dyno chart" is generated which shows the before and after results of the revalving. It's like having a baseline engine combination. You test the engine on the dyno. If it looks good, you test the engine in the car at the track. You think it might work better with a different cam, but instead of testing it blindly in the race car, you yank the engine out and dyno the combo first.

The dyno charts for shock absorbers are far different than those spit out by a SuperFlow 901-engine dyno. In this case, the horizontal lines on the chart are used to measure the load of the shock for each adjustment. The lines on the chart represent 350 (+) pounds total in rebound and 250 (-) pounds total of compression. Each of these lines equates to 50 pounds of force. Also note a series of numbered hash marks on the centerline (marked "0"). These represent piston shaft velocity or shock speed, measured in inch/second, which in turn correspond to the chart accompanying the dyno graph.

In essence, what the shock dyno chart shows is how the various external valving adjustments (clicks on the knob) correspond to both the bump and rebound settings versus the load (in pounds) placed upon the shock. Typically, a drag shock builds up force loads quickly represented by the sharp rise as it approaches maximum rebound (compression). The quick buildup of force loads is accomplished by the shock absorber's low speed circuits, sometimes called a "nose angle" on the dyno's "rising resistance curve". The overlaid rising resistance curves show how the shock responds to different valve settings (including rebound and bump). In other words, it shows how each "click" on the adjuster knob affects shock performance.

With this information in hand, you can now adjust your adjustable suspension for the quickest elapsed time possible. Remember the rebound or extension forces control body-suspension separation. The bump or compression forces control the frontal "crash landings," and have a lot to to with planting the rear slicks into the asphalt.-WS

SOURCES

AVO USA Perf. Shocks

Box 1129, Dept. SSDI

Palm City, FL 34990

407/221-0164

Competition Engineering

80 Carter Dr., Dept. SSDI

Guilford, CT 06437

203/453-5200

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