Designing Building and Tuning Shocks - How It Works

AFCO Explains the Basics of Designing, Building, and Tuning Shocks for the Street and the Track.

Stephen Kim Aug 6, 2013 0 Comment(s)
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How Shock Dynos Work

Many people have seen shock dynos in action, but aren't familiar with how they work or how they're used during the R&D process. To put in simply, the dyno captures the resistance in a shock absorber at any given shaft speed by applying load to the shock with a large ram assembly. The dyno is programmed to input force that mimics the shaft speeds we know the shocks see on the racetrack. The dyno moves the shock shaft up and down, the load cell measures force and resistance, and that data is used to create a graph of the shaft speed and damping rate. That enables measuring how stiff a shock really is at a given shaft speed. You can make a great shock that doesn't perform well because you're asking it to perform in a range it wasn't designed. Consequently, the goal is to optimize shock performance for a very specific operating range. Race teams that are on the ball know exactly how much damping they want at a given shaft speed, and having this dialogue allows us to tailor a shock to for their specific needs. In essence, the dyno gives you the ability to tune a shock to the specific range the suspension needs it to operate in.

Quality Control

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The dyno not only enables designing custom-tailored shock packages, but it serves as a vital tool in quality control as well. AFCO dyno tests all of its shocks to make sure they fall within certain performance parameters. Unless they perform in a certain window, we throw them out. Many mass-produced shocks aren't dyno tested, which can lead to inconsistent valving and compromised performance. Identical shocks should have identical valving, but without tight quality control, you might get one that's stiffer or softer than it should be. If a drag car is pulling to the left or right and nothing is wrong with the corner weights and chassis tuning, the shocks might be the culprit. In this scenario, if you swap the left and right side shocks, and the problem reverses, then the shocks are to blame. In fact, race teams ask us to dyno their shocks all the time as a means of troubleshooting their chassis setup.

Cavitation and Hydraulic Lock

In a drag application with big power adders, a shock goes from static to traveling at over 25 inches per second at launch instantaneously. In extreme situations like this, you need shocks that can handle these brutal forces quickly without cavitating or going into hydraulic lock. If a shock is asked to move at a rate that's too fast, it won't be able to pass fluid through its piston quickly enough. At this point, the shock will hydro-lock, become solid, and lag behind the speed requirement of the suspension. This is why getting the shock valving right is critical in high-end drag racing applications. In contrast, cavitation occurs when a shock moves at such a high rate that the fluid shears and leaves an air gap. The shock then passes through air pockets in the fluid and is unable to provide any damping. To combat this, mono-tube shocks pressurize the shock internals with gas, which forces the air molecules to stay separated and prevents them from aerating the fluid. If the air molecules can't bunch up, then they can't create a void.

Straight-Line Performance

Some people think shock performance isn't that important in a drag car since it only has to go straight, but this logic is inherently flawed. The sport has evolved dramatically in the past few decades. Looking back to 30 years ago, there was limited horsepower, limited traction, and limited track prep. Now there is a ton of horsepower, clutch and torque converter packages are much more aggressive, the tires have never been better, and track prep has improved as well. There's more opportunity for lower e.t.'s and higher miles per hour than ever before, and all these improvements add up to put more force than ever on the shock absorbers. With all this added power and traction comes faster suspension movement and more force than the tires can handle. This is compounded by racing classes that limit tire size. Now you can "drive through" a shock very easily, whereas it just wasn't likely in the past. Granted most of these issues apply to high-level competition, but the shock technology used to make these cars faster trickles down to bracket race level as well. Competition has never been higher than the level it's at today, and even local weekend track events are very competitive. That's why optimizing shock performance is so important.

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Tuning Valving

Managing the flow of fluid inside a shock determines its stiffness. The shock piston features ports that allow fluid to pass through them. The number of ports, placement of the ports relative to the piston face, and the thickness and makeup of the valving determines how quickly fluid passes through the ports, thereby affecting a shock's damping characteristics. AFCO shocks incorporate Velocity Sensitive Valving. In street applications, these shocks will increase damping rate relative to the speed they're moving at. If you hit a bump, as you go over the bump and compress the shock at a high rate, the shock reacts by increasing the valving, which keeps suspension movement under control. This not only improves ride quality, but it also translates to better track performance as well.