January 21, 2019
Chances are good that you've heard shocks described as struts, or struts described as shocks. Both are found behind the wheels of cars, and a car can have both, but not in the same place. Struts are used in front while shocks are used in the rear. Vehicles are designed to use either a strut or a shock as part of the suspension system, and it's not possible to use one in place of the other. A strut has a similar function as a shock absorber, but a shock absorber can't do the job of a strut. Here's a look at the differences between shocks and struts and why they matter.
The Basics of the Strut
A strut is an integral part of the car's suspension. It supports the vehicle, absorbs shock like a shock absorber, and functions as part of the steering system. The type of strut used on most modern cars is known as a MacPherson strut, which is a two part assembly. It consists of a spring with a shock absorber running through the center of the spring and has a flat top known as strut interface. The strut mounts into the steering knuckle behind the wheel hub and connects to the tie rod, control arm or wishbone link, and radius rod. All of these parts come together to aid with steering and absorb impact from the road surfaces.
The strut performs multiple jobs at once. It absorbs shock to the vehicle, helps the car stay upright over bumps, and supports the weight of the vehicle. When a vehicle goes over a bump, the struts move independently of the body and minimize how much shock is transferred to the vehicle. Struts keep a vehicle stable in just about every driving condition while also absorbing impact.
The MacPherson strut is named after its designer, Earl S. MacPherson. It was created shortly after the end of WWII, and its design has remained largely unchanged since then. Advances in technology and materials have improved upon the original design, but it still follows the basic shock-in-spring, or coil-over, configuration. The following will focus on the function of the MacPherson strut as it is the most commonly used strut on cars and trucks.
A standard MacPherson strut consists of:
- Upper mount and bearing assembly
- Spring seat
- Jounce bumper
- Dust cap
- Strut assembly
The strut assembly consists of:
- Shock absorber
- Gland nut
- Spring perch or lower spring seat
- Strut tube
Multiple variations on the MacPherson strut exist, but all will contain these parts in their basic construction.
The top of the strut mounts to the body of the car and the bottom is bolted into a carrier that is mounted to the wheel assembly. These connection points dictate the angle or orientation of the wheel assembly. A correctly aligned strut and wheel assembly means the tire will make correct contact with the underlying surface. In sum, the MacPherson strut design does an excellent job of keeping a car parallel to the road and creates a smoother ride than other types of suspension.
The genius of the MacPherson strut is that it replaced the bulky double wishbone suspension that was in common use up until WWII. The double wishbone configuration added width and weight to a car and was costly to manufacture. MacPherson's design allowed cars to become narrower while retaining all the qualities of the double wishbone suspension. The MacPherson strut has an additional feature in that it connects to the steering knuckle and has a ball joint that allows the entire strut to move when the steering wheel is turned.
The MacPherson strut is usually used in the front with shock absorbers on the rear wheels. Some vehicles use the MacPherson strut in front and back, but this use has become less common over time.
The Basics of the Shock Absorber
The use of the words shock absorber for this vehicle part is a bit of a misnomer in that it doesn't actually absorb shock; it's really a damper that counters the motion of the wheel spring. A spring is a heavy-duty coil that connects to the rear of the wheel, and the shock absorber runs through the center of the spring, which is known as a coil-over configuration. When you go over a bump or pothole, the spring absorbs the energy generated by the bump. In turn, the shock literally absorbs the energy from the spring and turns it into heat.
The most common shock absorber design consists of a twin tube configuration. The upper tube, or cylinder, fits over the smaller lower cylinder like a sleeve. The upper cylinder contains the piston rod, and the lower cylinder holds the oil or gas and other mechanisms that help the absorber do its job. A shock absorber consists of:
- Upper mount
- Piston rod
- Piston rod guide
- Pressure Tube
- Absorber oil
- Piston valve
- Bottom valve
Again, there are variations on how a piston is constructed as manufacturers put their own improvements and modifications on the basic design. However, modifications don't change the basic performance of the shock absorber and its conversion of excess energy into heat.
The twin-tube shock absorber connects to the wheel via a mount on the wheel hub assembly at its base and to the body mount of the car at the top. The shock absorber has no effect on the steering of the car if it's been placed on the front wheels.
A twin-tube shock absorber goes into action the moment a car goes over an obstacle of any size. The spring absorbs the motion of the shock and the shock absorber works to dampen that motion. The shock causes the shock absorbers to compress and push down the internal piston. Valves between the tubes allow the oil to flow back and forth at a steady rate of speed which in turn assists in control of the wheel.
The shock from the impact is transferred from the spring to the shock absorber, which then converts the energy into heat. The shock heats up and eventually cools down as the heat dissipates. This can become a disadvantage when driving over rough road surfaces because the shock absorber may not be up to the stress of constant impact. The oil inside the piston can overheat, which in turn reduces steering control.
Can a Strut be Replaced With a Shock?
A strut cannot be replaced with a shock, and a shock cannot be replaced with a strut. Both have a coil-over design, but that's where their similarities end. The strut plays a crucial role in the steering of the vehicle and keeps the car upright during turns and maneuvers. A shock is much simpler in design and has the sole function of dissipating or damping energy that was generated by a bump or pothole.
The shock absorber is a single piece that only has two points of connection. The strut is a multi-piece unit that has multiple connections to the body of the vehicle and the steering. Replacing one with the other is simply not possible without major modification to the suspension of the vehicle. Ultimately, it's not a good idea to replace the MacPherson strut with a double wishbone and coil-over shock absorption suspension unless you plan to go off-roading or engage in rally sports.
The strut provides safety through its various functions and capabilities and helps the driver maintain control of the vehicle at all times. A double wishbone suspension with coil-over shocks does have qualities that are superior to the MacPherson strut, but they don't override the safety features and qualities of the MacPherson.
The Difference Between Replacing Struts and Shock Absorbers
There comes a point when a strut or shock needs to be replaced. You might see fluid coming from the strut or shock, or you might notice that your car rocks a lot more during a sudden stop. The most common sign of strut or shock failure is a bouncy vehicle. If you're experiencing any of these conditions, chances are good that it's time to replace one or more of these parts. The following is a general overview of what's involved in replacing struts and shocks.
Of the two, replacing struts is more expensive and complicated. The entire strut assembly has to be removed from the vehicle and requires multiple hours of labor from start to finish. Some experts recommend struts be replaced every 100,000 miles while others suggest holding off until the struts are showing obvious wear and tear.
Check for fluid leaking from the top of the strut by feeling around for oil on the surface. If you can't detect oil, push down strongly on the hood of your vehicle. You want to push the car down far enough that it visibly bounces back up. Look at how many times the car bounces up. If it comes up once and stops, the struts are good. Multiple bounces can be a sign of strut failure. Tire wear causes low and high spots on the tires, and causes them to make a noise akin to a basketball being dribbled.
The average cost of replacing struts runs about $750 for a pair, but prices vary from location to location. A mechanic has to remove the tire, remove the brake assembly, drop brackets, and sway bar, and then take the bolts out of the steering knuckle. This is the simplified version as there may be structures unique to the car that have be disconnected from the strut. Once the strut has been removed from the vehicle, the mechanic can install the replacement.
Shocks are generally less expensive to replace than struts as they are typically found on the rear wheels of a vehicle and have fewer points of connection. It's also a good time to decide if you want to replace the shocks with a pair that offer a specific type of shock dampening properties such as better handling or a smoother ride. The type of shock you pick as a replacement adds to the final cost, but you can expect to pay anywhere between $300 and $500 depending on what part of the country you reside in and the amount of labor needed.
Replacing the rear shocks involves removing the tire from the vehicle and unbolting the shocks from their mounts on the vehicle. Some vehicles may require the additional removal of the rear brake assembly if it's in front of the shock and prevents easy access to the bolt heads. The anti-roll bar may also need to be removed from the area, as well. Once the shock is free from the bolts, it can be removed from the vehicle. After the old shock has been taken out, the new shock can be installed, and any other assemblies or structures are reconnected once the new shock is secured into place.
Shocks and struts are integral parts of every vehicle. They help support the weight of the vehicle, keep it upright, and make the ride that much more comfortable. Struts and shocks are also known for their longevity, which means that they're replaced infrequently. If you intend to own a vehicle for as long as possible, you might find that you have to replace them over time. But unlike other parts on a vehicle, you may only replace them once or twice if you own your car long enough.
When it comes time to replace your struts and shock absorbers, come check out our vast inventory of OEM parts. Search by part number, make and model, or even VIN to find the exact part that's made specifically for your car and restore it to its original performance standards. Not sure what part you need? Our team of part specialists can help you find the right part. Give us a call or email today.