Understanding Shock Types and What Is Best for Your Project
The suspension under your muscle car, classic truck, or hot rod is made up of multiple components, but the two most critical pieces are your springs and shocks. Your springs support the vehicle’s weight. This weight support is in the form of a compressed spring, with that spring storing energy. As the vehicle moves down the road, encountering changes in surface grades, potholes, and the like, the springs compress or expand while supporting the vehicle’s weight. The shock absorber works with the spring to dampen this movement and keep the spring’s energy release in a more controlled environment. Without shock absorbers the vehicle would bounce needlessly over bumps and potholes, seriously affecting ride handling.
Car shocks have come a long way since the early 20th century when most vehicles didn’t even have shock absorbers! Initially, shocks were friction shocks, where they used friction discs in a “knee action” design to dampen the spring’s movement, as seen here. Hydraulic versions of these shocks would make their way onto vehicles in the late 1920s with the popular Ford Model A being the first American car to come with hydraulic shocks standard. The telescopic tube shock that is commonly used still to this day was invented by Monroe and was first used in 1951.
Upgrading your shocks should be considered whenever performance lowering springs and new suspension bits are installed. Not only are the stock shocks most likely not up to the task due to years of wear, leakage, and more that prevent them from controlling the spring’s energy, but a true performance aftermarket shock has better control over the vehicle’s handling and ride quality due to the higher quality shock components, special valving, adjustability, and other great features you will not find in auto parts store stock replacement car shock absorbers that are designed to be a cost effective replacement for your daily commuter car. Furthermore, many performance shocks are rebuildable, or can be sent in for valving changes to adjust the damping rate as well throughout their lifespan.
The easiest wat to determine the shock end type is to examine the existing shock absorber assembly on the vehicle. Often, both front and rear shocks will take different mounting ends, both for the top and bottom mounting. For custom applications you can often weld or bolt on shock mounting brackets and the bracket instructions will state what type of end mount is required. That said, we wanted to provide an overview of each mounting end style here.
- Round loop at the end of the shock with a rubber or polyurethane bushing
- Often uses a steel sleeve inside of the rubber bushing
- Bolt hole sizes from 7/16” to 5/8” ID are common
- A wide variety of bushing eyelet widths are available so be sure to match the mount width if using custom fit shocks
- A shock mounting stud can be used to install a bushed eyelet shock
Bearing Eyelet (Spherical Rod End)
- Like a bushed eyelet but uses a spherical bearing
- Allows for more misalignment while remaining bind free
- These are commonly used on racing and custom mounted shock applications
- Bolt hole sizes of ½-inch and 5/8-inch ID are common
- Bearing widths are available from 3/8-inch to 1-1/16-inch wide
Stem or Through Stud
- Threaded stud on the end of the shock that points straight up or straight down
- Uses two rubber or poly bushings and a nut with washer to clamp to the chassis or axle
T-Bar or Tie Bar
- Bushed eyelet at the end of the shock with a rubber or poly bushing that has a bar pressed through it
- The T-bar has two holes or slots in it to bolt to the chassis or suspension
When lowering a vehicle shock length becomes more critical than in a stock application. Often a suspension system will come with the proper length shocks to work with the springs, control arms, and other bits in the kit. However, when piecing together your suspension, or you are building a custom suspension system it is imperative that you get the shock length right.
Let’s start with how to measure shock absorbers. On a standard tube shock with eyelet ends you would measure from the center line of each eyelet. Where things get a little tricky is shocks with T-bar or stud mounting style ends. Most will measure from the stud where the threads end and the washer and bushing would seat. Same goes for the T-bar. However, some brands of car shocks with studs and/or T-bars are measured differently, so always confirm with the manufacturer how they measure/spec their shock lengths. Finally, there are three important measurements you will need to know, usually taken from your old shocks, to determine a suitable performance replacement.
Extended Length This is measured with the shock is fully extended to its maximum height off the vehicle
- Some car shocks will need to be pulled apart by hand before taking this measurement
Compressed Length This is measured with the shock fully collapsed or “bottomed out”
- When installed on the vehicle a shock should never reach its fully compressed length throughout the entire suspension travel
- If a shock is too long and fully compresses it can easily be damaged
Mounted length This is the ideal length that the shock should be installed at on the car when the car is sitting on a flat level surface at ride height
- Typically, a shock mounted length spec will allow 60 percent of the stroke for compression and 40 percent of the stroke for rebound travel
- When fabricating mounts to install a custom shock the mounted length is very helpful in determining the proper length shock (always move the suspension through its entire travel and verify the compressed and extended lengths will work properly before finish welding your shock mounts)
Speaking in broad terms there are three main shock absorber designs in use today. The telescopic tube type shock absorber, the strut (usually a MacPherson-style unit), and coilover shocks. Most will be familiar with the telescopic tube shock with the coilover shock being a close second as more and more coilover conversion kits hit the market. MacPherson struts are generally not found until you get into vehicles from the late 1970s and newer, but 3rd Gen Camaro and Fox Mustang enthusiasts are already well versed in front strut suspensions, so if you’re looking at one of those for your next project you might want to pay attention to our MacPherson strut section.
Telescopic Tube Shocks There are two main styles of telescopic tube shocks, the twin tube and mono tube designs. We’ll outline each one below for a better understanding of them. Both styles of car shocks are designed to work with your vehicle’s suspension springs to dampen their stored energy and do not affect ride height.
Twin Tube Shocks
- The twin tube design has two metal tubes of different sizes with one inside the other
- The smaller inner tube is filled with oil and houses the piston which moves up and down inside the inner tube when the shock is compressed and extended
- The piston has carefully designed passages and valves which allow the flow of oil through them as the shock is compressed and extended, creating the damping force
- As the oil flow is restricted more there will be more damping force and thus create a stiffer shock
- The inner and outer tubes have another valve positioned between them, which is called a base valve, which allows oil to be forced out of the inner tube and into the space between the inner and outer tube
- The base valve contributes to the compression damping force by metering or restricting the flow of oil between the two locations
- The outer tube is the exterior portion of the shock and is partially filled with oil, while the remaining amount of space between the inner and outer tubes is filled with air, pressurized nitrogen, or a gas bag (which is similar to a heavy duty bubble wrap)
- The pressurized nitrogen filled shocks will have pressure forcing the shaft of the shock out to full extension
- When atmospheric pressure air or a gas bag is used the shock will not extend the shaft on its own, requiring you to pull the shock apart to reach full extension
- One advantage of the gas bag design is that it can be mounted in any position such as horizontal or upside down
Advantages Of the Twin Tube Design
- Shorter compressed length can be achieved for a given extended length (as compared to a mono tube)
- Robust design with a large shaft size as well as minor dents to the outer tube will not affect the performance of the shock
- Typically, are easy to manufacture and can offer a lower price point
- No gas pressure forcing the shock apart (with gas bag design) which is advantageous for ride quality in some light weight vehicles
Mono Tube Shocks
- Has a single oil filled tube which houses the piston and valves, which allow the flow of oil through them as the shock is compressed and extended, creating the damping force - A second piston is used to divide the oil and pressurized nitrogen chamber and is called a divider piston or floating piston
- The floating piston will move slightly as the shock is compressed due to the shock shaft displacing volume in the oil portion of the shock
- The nitrogen chamber is pressurized as necessary to keep the oil from cavitating (the stiffer the compression damping a shock has the more nitrogen pressure that is typically required to prevent cavitation)
- Due to the nitrogen pressure a mono tube shock will always push the shaft out to be fully extended
- Many performance mono tube shocks will have a Schrader valve which can be used to maintain the pressure to specs
Advantages Of the Mono Tube Design
- Large piston size relative to the outside diameter of the shock body provides maximum control of damping forces
- Typically runs cooler and has less chance of cavitation due to heat in extreme conditions such as rough terrain
- Nitrogen pressure can be added without disassembling the shock when equipped with a Schrader valve
The coilover shock combines the dampening motion of a telescopic tube shock with a coil spring assembly around the shock body. This helps immensely with suspension packaging, but often standard shock mounts must be reinforced since now the shock assembly is not only doing the dampening, but it is supporting the vehicle weight as well now. While we do offer many direct fit applications, there are some build configurations, such as pre-war hotrod builds, where you will need to determine proper spring rate. Our coilover spring rate guide will help in those instances.
- Coilover shocks have a spring seat on the body of the shock which is able to be adjusted up and down to dial in the ride height of your vehicle
- When used on all four corners you will have the ability to get your car perfectly balanced using a four corner vehicle scale system
- Most coilover springs are 2-1/2-inch ID and approximately 3-1/2-inch OD, which makes the spring much smaller and lighter than typical OEM coil springs
- Springs are available in various lengths and spring rates which will allow you to choose the proper one for your application
- If the spring rate you choose is too stiff or too soft it can easily be changed for a different spring rate
- When installing coilovers the shock mounts will typically need to be upgraded or replaced
- There are many applications which have bolt-on adjustable coilover shocks available in a kit which include the proper mounts, such as our new 65-66 Mustang front adjustable coilover shocks
The MacPherson strut design is named after automotive engineer Earle S. MacPherson, who came up with the design which dates back to the 1940’s. It became a popular OEM suspension (especially front wheel drive applications) in the 1970’s and 1980’s. A large majority of today’s cars and SUV’s use the McPherson strut design or a similar iteration of it, often for the front, while a standard tube shock is used in the rear. With this suspension design you will find shocks and struts on the same vehicle.
- Struts are like a shock as far as damping is concerned, but also are a structural component of the suspension and upper steering pivot point
- A strut will eliminate the need for an upper control arm by bolting directly to the knuckle (spindle) at the lower mount and fastened to the chassis at the upper mount
- Struts typically have a coil spring mount built into it, but some applications use a “modified MacPherson strut” setup with a coil spring mounted between the lower control arm and the frame
- Since a strut eliminates the need for a shock, upper control arm, and spring seat they create a lighter and simpler suspension design
- Struts have heavier duty internal parts such as bushings, bearings, shafts, etc. than a standard shock does since the strut is required to handle the load that an upper control arm would have in a double wishbone suspension
- Some performance struts are available in adjustable valving and coil over designs as well
- Coilover struts are not vastly different than standard struts but typically use a smaller diameter spring and have an adjustable spring seat to set the ride height
You will find performance adjustable shock absorber offerings in both single and double adjustable valving. This allows you to adjust the amount of damping (or stiffness) of the adjustable shock absorber by either the turn of an adjustment knob or by rotating the shock body itself in some models. If the shock body requires rotation for adjustment at least one end of the shock absorber must be removed from its mount, while an adjustable shock absorber with an adjustment knob can be adjusted directly on the car. You’ll find many adjustable coilover shock kits as well.
Many adjustable shocks are tailored to a specific application such as the Afco Big Gun and Reactor series shocks, as well as the QA1 Drag R Series shocks, which are all for drag racing and street/strip applications. The great thing about adjustable shocks and struts for a car that is driven on the street to the track is that you can change your shock settings to aid ride quality on the street while still having the ability to dial in a perfect launch at the track. Then readjust the shocks for the drive home. Same goes for long distance driving. Many people enjoy a stiffer shock setting for fun around town on short drives, but you can easily soften the ride for that day trip to a show or car event out of town and not beat yourself up on the highway.
Single Adjustable Shocks and Struts
- One adjustment knob on the shock to adjust damping
- Can be rebound adjustable only, compression adjustable only, or simultaneous compression and rebound adjustments, such as this QA1 Proma Star coilover for example.
Double Adjustable Shocks and Struts
- Has two adjustment knobs on the shock, one for compression valving and one for rebound
- Allows maximum adjustability without the need to disassemble the shock or remove it from the vehicle
Having the proper shock setup for your vehicle not only enhances ride quality and handling, but ensures your suspension’s springs, control arms, sway bars, and more are all doing their job in concert to provide the best control whether you are simply tooling down the road or throwing your ride into a corner at the local autocross. While we’ve focused on street performance here, we do have additional racing specific info in our racing shock guide. Our huge selection of performance shocks, coilovers, and struts will surely allow you to dial in the exact suspension feel you are looking for.
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