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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
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Some suspension experts contend that shocks are the single most important set of components in the entire suspension. In light of all the fancy hardware on the market today—such as four-links, splined sway bars, aluminum spindles and control arms, and aftermarket subframe assembles that boast fully reworked geometry—this seems like a rather bold proclamation. Once analyzing basic suspension dynamics in closer detail, however, it all makes sense. While the springs support the weight of a chassis and absorb cornering loads and bumps, it's up to the shocks to control the motion of the springs. That means the shocks determine how quickly weight is transferred from front to back and side to side. Since getting the suspension to play nice with the tires—whether it's in a straight line or a cornering application—is all about optimizing weight transfer, optimizing the shock package is critical. Furthermore, since it's tough to swap out sway bars or springs at the track, adjusting shock valving is one of the most effective means of chassis tuning on race day.

Considering the substantial role shock absorbers play in the overall acceleration, braking, and handling equation, it's not the least bit surprising that manufacturers and race teams invest staggering sums of hours and research dollars into shock development. AFCO is one of the biggest players in the industry, manufacturing shocks for everything from 6-second door-slammers, to circle track cars, to street/strip warriors, to road race machines. To get the lowdown on the fundamentals of how shocks work and the cutting-edge technology that goes into their development, we sat down with AFCO's Eric Saffell for a chat. As an added bonus, since AFCO falls under the same umbrella of companies like Dynatech, we threw in some exhaust system tech as well. Here's the scoop straight from Saffell.

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Shock Anatomy

The fundamentals of how a shock absorber works are very easy to understand. Most hot rodders are already familiar with the body and shaft of a shock, which is what you can see from the outside. On the inside of the shock, there is a piston attached to the end of the shaft. Hydraulic fluid on both sides of the piston provides resistance to compression and rebound loads, and rings seal the piston to the inside of the shock body. While a tight seal ensures long shock life, the rings will grab onto the shaft if the seal is too tight, thereby increasing stiffness. Likewise, shims and valves on each side of piston control how much fluid passes through the piston, which determines the stiffness or softness of the shock. Shock damping can be manipulated depending how much oil passes through the deflective discs, valves, and shims. When you turn the knob clockwise on an AFCO adjustable shock, it puts more preload on the valves. This restricts the flow of fluid, which makes the shock stiffer. When you turn the knob counterclockwise, it removes preload from the valves. This allows more fluid to pass, which makes the shock softer.

Mono-Tube vs. Twin-Tube

Shocks utilize either a mono-tube or twin-tube construction, and each have their benefits and drawbacks. Twin-tube shocks have been around for a very long time, and have an inner body that slides inside an outer body. It's the workhorse of the industry, and the type of shock most people start out with in their racing careers. They can be used in entry-level circle track or drag cars, and double-adjustable twin-tubes can be used in high-end racing. While less expensive than mono-tubes, twin-tube shocks have a smaller-diameter piston, which limits damping precision. On the other hand, mono-tube shocks use newer technology and allow for more precise damping. Since mono-tubes don't have a small inner body, the increase in space inside the shock body allows for a larger-diameter piston. This extra surface area provides better response time. Overall, mono-tube shocks dampen more crisply and at a higher frequency. Furthermore, since they have a column of pressurized gas on top of the hydraulic fluid, they perform better in higher heat applications, like circle track cars that have their grille openings taped off. The downside of mono-tubes is that they are more expensive, and since they don't have an inner body, they are less forgiving of dents.

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Data Acquisition

The information gathered from data acquisition systems on race cars plays a very important role in helping AFCO design state-of-the art shocks. By monitoring shock loads and shaft speed, we are able to capture the dynamic weight transfer on circle track, road race, and drag cars. Once we map what's happening on all four corners of a car during a race, we can then plug that data into the shock dyno, and put a set of shocks through the exact same cycling that they experience on the track. This allows us to chart the force values, shaft speed, response time, and gas pressure inside the shock. Once this information has been gathered, we can then dissect each aspect shock of performance, and make adjustments to improve performance. In essence, data acquisition allows taking track data, bringing it into the lab, and optimizing the shock valving for a specific track and track conditions. The goal is to sculpt the damping curve to generate the performance we're looking for on the track, which might involve developing new pistons that have differently sized ports and orifices. The beauty of data acquisition is that you can gather lots of information from a very brief duration of time. In a drag car, for instance, you can break down a pass by how the shocks react during various stages of a launch, then string all that data together. This allows us to truly develop a shock package for a specific type of car on a specific type of track.

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.

Single-Adjustable vs. Double-Adjustable

One of the most popular questions people ask all the time is whether they need a double-adjustable shock, or if a single-adjustable shock is sufficient for their needs. This is an issue we take very seriously, and it all comes down to the level of performance you expect from your car. AFCO single-adjustable shocks adjust the rebound only, or in other words, how quickly the shock pulls apart to control body roll. For street cars that will see occasional use at the dragstrip or at Pro Touring events, single-adjustable shocks are a great value. They're easy to use, provide great ride quality, and allow fine-tuning the valving to stiffen the suspension up at the track. Double-adjustable shocks control both compression and rebound, and are intended for max effort cars used in very competitive environments where you expect a higher level of expectation from your race car. The more power you have, the greater the need for double-adjustable shocks.

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Remote Reservoirs

Many race cars and an increasing number of high-end street cars utilize shocks with remote-mounted shock reservoirs. With this type of setup, a large quantity of fluid is held in a remote canister, and fed to the shock body with a high-pressure hose. The purpose of this arrangement is to isolate the effects of the shock's internal gas pressure from the shock valving. Mono-tube shocks are pressurized at 50 to 200 psi with nitrogen gas to prevent cavitation, but this pressure can have the adverse effect of increasing spring rate. Pro Touring cars are constantly transferring weight from front to back and side to side, so an increase in spring rate is undesirable. By holding the fluid in a remote canister, we can decrease rod pressure, which is the difference in force it takes to compress a shock with and without gas pressure.

Better Headers

Dynatech offers a diverse range of products for muscle cars, late-models, and race cars. For muscle cars, we have great-fitting line of stepped headers for first- and second-gen Camaros, and '68-and-later Novas, Chevelles, and El Caminos. They feature thick 3/16-inch flanges for good sealing, and a stepped tube design. Starting with a smaller-diameter tube near the header flange keeps exhaust velocity up, and from there the header primary increases in diameter for improved scavenging. Additionally, Dynatech headers are coated inside and out for looks and longevity, and also utilize a four-bolt ball-and-socket connector at the collector for leak-free operation. Traditional three-bolt flanges work themselves loose over time, and are much more prone to leaks. By using a sintered-metal gasket inside a ball-and-socket connection, which is identical to what the OEs use in late-models, we can ensure leak and maintenance-free performance. Furthermore, our MuscleMaxx headers for muscle cars include header bolts, gaskets, and a color-coded instructions. Our SuperMaxx headers for late-models feature stainless steel construction, triple-layer factory style gaskets, and oxygen sensor extension cables.

Stepped Advantage

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Stepped header primaries are often used in drag cars, and offer several advantages over non-stepped designs in street cars as well. If you think about an engine as an air pump, you can't put a fresh air/fuel charge into the cylinders until you have evacuated the spent gases out of them. That said, having properly sized primary tube gives you the best opportunity to remove these gases. If the header primaries are too large, the exhaust can become very lazy. Conversely, if you keep the exhaust compressed, it speeds up velocity and helps pull the exhaust out. At Dynatech, we have experimented with different lengths of smaller tubing in order to maximize scavenging. Despite their benefits, they're not ideal for every application. For instance, if you use stepped headers on a motor with lots of cam overlap or efficient exhaust ports, you can evacuate the air/fuel charge out too quickly before it has a chance to ignite. That's why it's important to have dialogue between the engine builder and header builder when dealing with a pro-built engine with great heads and a big cam.


To complement our line of headers, Dynatech offers both auger and split-flow–style mufflers. The baffling inside our auger mufflers looks like a coarse thread screw. They were developed to meet some specific rules requirements in Sprint Cars parts where race officials have implemented very strict maximum decibel levels. The tradeoff with mufflers is the more you dampen the decibel level, the more backpressure you create. Since the auger mufflers create minimal backpressure, they would probably be too loud for most street cars. For street applications, Dynatach's split-flow mufflers are a better alternative. They're essentially a straight-through–style muffler with a perforated core that's surround by stainless steel packing for absorption-style damping. The end result is the best of both words: high flow and noise reduction.

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Collector Tricks

In the past, collectors were simply seen as a tube that the header primaries dumped into, but collector design plays an important role in overall exhaust scavenging. Dynatech race headers are offered with merge collectors that start out at the same diameter as a standard collector, but then neck down in diameter before it hourglasses back to its original diameter. This creates a venturi or vortex effect to draw exhaust out of the engine more effectively. By speeding up the exhaust flow with merge collectors, they can make an engine more efficient. Similarly, if you want to run an open header exhaust but are concerned about the noise, Dynatech's Vortex insert cones are a great solution. These cone-shaped inserts go into the end of a collector, and utilize a perforated core to reduce sound by three decibels with little to no loss in horsepower. The core has a bunch of small holes in it, and noise is reduced as the exhaust hits the edges of the holes. Since the surface area of the holes is greater than surface area of collector outlet, there is little to no increase in backpressure.



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