By Ralph Kalal
With rare exceptions, suspensions on modern cars fall into one of three groups: “parallel arms,” “MacPherson struts,” or “solid axle.” Your car almost certainly employs one of these systems and may use two of them, as the suspension system for the steered front wheels is often different than that used for the rear wheels.
Parallel arm systems employ upper and lower suspension arms parallel to each other. One end of each arm is connected to the car and the other end is connected to the upright that bears the spindle on which the wheel is mounted. The suspension arms are often called “A-arms” or “wishbones” because they have that shape. In practice, the upper suspension arm is shorter than the lower arm to maintain the wheel’s angle to the road as the body rolls in a corner. A coil spring is mounted so that it bears against either the upper or lower arm. The shock absorber can be mounted separately from the spring, or centrally within the spring, in what is known as a coil-over-shock or “coil over” configuration.
The MacPherson strut system—so named because it was invented by Earl S. MacPherson—employs only a lower suspension arm, which is connected to the bottom of the upright holding the wheel, called the “steering knuckle.” The top of the steering knuckle, which will be not much higher than the level of the spindle, is connected to the lower end of a strut. The strut extends almost to the level of the hood, where it bolts to the car body at the side of the engine compartment. Essentially, a MacPherson strut is a tube sliding in a shaft and combines the functions of upper suspension arm and shock absorber into one unit. The top of the strut sits in a bearing, so that the whole assembly rotates as the wheels are steered. Typically, the strut is located inside a coil spring, which acts against collars located at the top and near the bottom of the strut. However, there is also a “modified MacPherson strut” design. In the modified design, the coil spring is separate from the strut assembly.
Both parallel arm and MacPherson strut systems are “independent suspensions”: the movement of one wheel has no effect on any other wheel. That is not true of the third system: the solid axle.
A solid axle is exactly what the name implies: a solid connection of one wheel to the wheel on the opposite side. The term usually applies to rear axles consisting of a housing that contains a center differential (the unit that splits the power from the engine and sends it to each driven wheel) connected to the wheels by driveshafts. That arrangement is also called a “live axle.” But a “beam axle,” which is simply a large steel beam connecting the opposite wheels but with no driveshafts, has the same attributes. Solid axle systems generally use a coil spring and a separately mounted shock absorber, though solid axle systems on older cars and many trucks use leaf springs.
Each system has its advantages. Parallel arm systems are preferred for performance cars because they maintain better wheel geometry during cornering. MacPherson struts leave more space in the engine compartment, which makes them ideal for front wheel drive cars, especially those with transverse engines. Solid axles are far less expensive to manufacture than independent systems.
Original equipment struts and shock absorbers should always be replaced in axle sets (i.e., both front struts) so that the car’s handling and braking characteristics are the same on both sides of the car.
However, struts and shock absorbers are two suspension components frequently upgraded with aftermarket parts to enhance handling. Replacing springs and struts with shorter units also will lower ride height. Whenever performance components are substituted for original equipment springs, struts, or shock absorbers, corresponding parts should be replaced at both ends of the car to maintain safe handling dynamics.
Parts for these jobs:
- Struts (as applicable)
Shock absorbers (as applicable)
Tools for these jobs:
- Floor jack (if applicable, to support suspension arm)
- Coil spring compressor (if applicable, to compress strut spring)
- 1/2-inch drive breaker bar, 1/2-inch drive socket ratchet, and 1/2-inch drive sockets of appropriate sizes
- Box end wrenches of appropriate sizes
- Hex or Allen socket (if applicable)
- Penetrating oil
Time for these jobs:
At least the first time, these are jobs that require an open-ended time commitment. Don’t even start unless you know you won’t need the car that day. In fact, this is a good Saturday job. That way, if you encounter problems, there’s still Sunday.
NOTE: If you replace struts, you must have those wheels realigned. Replacing struts inevitably alters the existing alignment. So, you should also plan to take the car to an alignment shop on Monday.
First, you need to know what type of suspension system you have. If you have coil-overs, i.e., the shock or strut is inside the coil spring, you’ll need a coil spring compressor. You will not need that tool to replace shock absorbers or struts in a non-coil-over system.
The most basic and common type of coil spring compressor consists of two long bolts, each of which has a hook at both ends, as shown in the picture at the beginning of this chapter. One unit is used on each side of the spring. Turning the bolt pulls the hooks toward the center. Putting one on each side of the spring and turning the bolts alternately compresses the spring. These retail for about $50, but can be rented for much less and are sometimes available for loan at parts stores without charge.
If any of the nuts that must be removed are rusted (and it’s likely some are), put the car on jack stands the night before and thoroughly soak the rusted parts with penetrating oil.
You will need the torque specifications for the various nuts and bolts that you will be removing during disassembly, so that they may be properly tightened after installing the new struts or shock absorbers.
Finally, check to see if you have all of the tools you will need. Suspension components use large nuts and bolts. To do the job, you may need a wrench or socket larger than any you have in your tool chest. Also, a special tool may be required to remove the “damper shaft nut” at the top of the strut. If so, the factory shop manual will state the tool required and probably provide an illustration of it.
NOTE: While most MacPherson struts are non-serviceable and must be replaced as a unit, on some older cars the shock absorber function is performed by a cartridge that slides into the upper part of the strut and can be replaced without replacing the entire strut. In some instances, this cartridge can be replaced without removing the strut from the car.
Compressed springs contain enough power and tension to support the car, so anything that suddenly releases one creates a large projectile moving at high speed that can cause serious injury. Be very, very careful while using a spring compressor and handling the compressed spring. Do not allow children to be present.
Getting under a car is dangerous. Always use secondary supports and ensure that the car is securely positioned on jack stands.
MacPherson Front Strut Replacement
MacPherson struts are removed as an assembly with the spring. The spring is then removed from the strut at the workbench.
As soon as you have removed the wheels, examine the various bolts holding the strut in place, or holding things to the strut, and apply (or reapply) penetrating oil to any that are rusty. On a front wheel drive car, there will be a rubber boot for the constant-velocity joints in the front axle located directly under the strut. Drape a shop rag over the assembly to catch any dripping penetrating oil, so that oil does not get on this rubber boot. Use a wire brush to clean rust off any exposed threats. Otherwise, the nut may stall as it is removed because it cannot cut through the accumulated rust.
A word about penetrating oil: patience. Penetrating oil may not work in thirty minutes or an hour, or two hours, or three. It took over 6-hours before penetrating oil freed the stabilizer link bolt on the car in the pictures. That’s one of the reasons this is a Saturday job and why it pays to apply penetrating oil the night before you’re going to start the job.
If it is necessary to replace a nut or bolt used in the suspension, be absolutely sure that the new part is of at least the same strength grade as the fastener being replaced. Both standard and metric bolts and nuts are labeled with strength grades. Standard fasteners use short radial lines on bolt heads and dots on the face of nuts to indicate grade; the more lines or dots, the higher its strength. Metric fasteners use tiny numerals; again, the higher the number, the stronger the fastener.
If a new fastener is needed, you may be able to purchase it as a replacement part from the dealership parts department or a parts store, which should provide assurance that the new fastener will meet the old fastener’s strength standard. You can also get nuts and bolts of the proper strength grade at Fastenal (online at fastenal.com), which has almost 2000 locations in the United States.
On the top of the strut tower there will be two or three small nuts and a removable plastic cover. The nuts hold the top of the MacPherson strut in place. The strut’s “damper shaft nut” is under the plastic cover.
The damper shaft cover can be easily pried off with a screwdriver.
Now, look at the top of the strut tower. You’ll see two or three small nuts. These hold the top of the strut in place in the strut tower. Gently pry the plastic cover from the top of the strut tower. Underneath, you can see the top of the strut and the large “damper shaft nut” that holds the “upper spring seat,” i.e., the collar that restrains the top of the spring. That nut will be removed only after the strut and spring assembly has been removed from the car.
Examine the damper shaft nut before proceeding further, to ascertain whether it will require a special tool to remove it. If a special tool is necessary, acquire before starting the job.
Take a close look at the damper shaft and its nut. Once the strut has been removed from the car and the spring compressed so that it is no longer applying pressure, the normal way of removing the damper shaft nut is to prevent the shaft from rotating. Do this by holding it with a wrench or inserting a socket ratchet with the appropriate hex or Torx socket bit into a receptacle in the end of the shaft, depending on design, and then loosen the bolt with a box end wrench. If the method for removing the nut is not obvious, consult the factory shop manual to determine if a special tool is required. Now to begin:
The strut extends from the top of the steering knuckle to the top of the strut tower. The spring is contained by a lower spring seat that is part of the strut and an upper spring seat that is attached to the top of the strut. There is a bearing assembly attached to the top of the strut above the spring seat and directly below the top of the strut tower. This allows the entire spring and strut assembly to rotate as the front wheels are steered.
The bottom of the strut attaches to the steering knuckle with large bolts. The bottom of the strut is close to the level of the wheel center to minimize changes in wheel geometry during cornering.
Remove any wiring or hoses that are held to the strut by brackets, such as this anti-lock brake wiring, which is routed through a bracket on the strut to the brake caliper.
Remove the bracket that holds the brake hose to the strut.
On most cars, the brake hose does not need to be disconnected from the caliper to remove the strut. However, be careful to prevent the strut from falling on the hose when the bolts holding the strut to the strut tower are removed.
On the strut itself, inside the wheel well, there will be brackets or clips holding the brake hose and anti-lock brake (ABS) wiring, if so equipped. These route the hoses and wiring past the strut to the brake caliper. There also may be an electrical connector to the strut itself and, in the case of rear struts, an air hose connection. All of these must be removed from the strut. Remove the brackets, unclip the hoses, and disconnect the electrical and air connectors to the strut. These may simply press on, or there may be a clip holding the connector in place.
If, for some reason, you plan to reinstall the same strut that you are removing, before doing anything else use a sharp tool to scribe a clearly visible line around the strut bracket, around the nuts holding the bracket to the steering knuckle, and along the edges of the strut at any other place the strut contacts the knuckle. That way, you can reinstall the strut exactly as it was previously installed and, hopefully, avoid the need for a wheel alignment.
Disconnect the link to the stabilizer bar. In on this car, there is no hexagonal bolt head. To remove the bolt, a Torx socket is inserted into the end of the bolt to prevent rotation as the nut is removed with a box-end wrench.
The nut holding the stabilizer bar link to the strut was badly rusted and would not turn. After repeated applications of penetrating oil over a period of 6 hours, it turned and was removed. As the saying goes, patience is a virtue.
Disconnect the link from the stabilizer bar (also called an anti-sway bar) to the strut, if the vehicle is equipped with one that is connected to the strut. The connecting bolt may be the typical hex bolt and nut combination. Or, it may be more like a carriage bolt: rounded at one end, but with a receptacle in the threaded end of the bolt that accepts a Torx or hex socket bit, so that you can prevent the bolt from rotating as you remove the nut.
It is now time to remove the bolts holding the bottom of the strut to the steering knuckle.
Remove the first of the two bolts holding the bottom of the strut to the steering knuckle, starting with the top bolt. Hold the bolt head with a socket wrench and breaker bar while turning the nut with a box-end wrench.
Remove the second bolt from the bottom of the strut. These are large bolts, so be sure you have wrenches that are big enough before starting this job.
When the second bolt is removed, the strut may shift slightly, putting enough weight on the bolt that it can’t be easily pulled out. If so, it can be unscrewed from the strut with a socket ratchet.
Using two wrenches, one on the bolt and the other on the nut, remove the nut from one of the bolts. Then remove the bolt. Remove the nut from the other bolt. This second bolt may not slide out, because the weight of the strut may have shifted and be applying pressure to the bolt. If so, after the nut has been removed use a wrench to unscrew the bolt from the steering knuckle and strut.
The bottom of the strut can now be pushed away from the steering knuckle by prying with a large screwdriver.
With the bolts removed, you should be able to pull the bottom of the strut away from the steering knuckle. You should also be able to tilt the disc brake assembly outward, from the top. This will provide clearance for removing the strut.
Remove the nuts that hold the top of the strut to the strut tower. Either have someone hold the strut while this is done, or, brace it with a block of wood so that it doesn’t drop out of these holes when the last nut is removed.
The last step is removing the nuts holding the top of the strut in the strut tower. If you can arrange to have a helper at this stage, do so. You can do this job alone, but it’s easier with help. Have the helper remove the nuts while you hold the strut in place. Then have the helper hold the disc brake assembly out of the way as you gracefully and smoothly guide the strut from the wheel well. If you don’t have a helper, wedge a block of wood under the strut so that it does not drop when the last nut is removed from the strut tower.
It is easier to remove a strut if you can swing the bottom out from the car while the bolts on the top are still sticking through the strut tower bolt holes. This gives you the maximum amount of clearance possible between the strut and the brake assembly components.
Removing the strut from the car is easiest if you start with the strut’s top bolts still in their holes in the strut tower. That maximizes clearance at the bottom of the strut to move around the brake caliper and rotor as you remove the strut.
Remove the strut. Direct the bottom end through the opening and then let the strut come out of the holes in the strut tower as you lower it out of the wheel well.
You’ve completed Phase One. Phase Two is removing the spring from the strut, so that you can install the new strut.
Before beginning to disassemble the strut, use a felt tip pen to mark (on the insulators and upper and lower spring seats) the exact position of the spring’s ends, both top and bottom. The spring must be positioned in the new strut exactly as it had been positioned in the old one.
Before you begin disassembly, use a felt tip pen to mark the location of the spring ends on the insulators (the pads between the spring and the spring seats), on the upper and lower spring seats, and on the bearing assembly at the top of the strut. That way, you can easily reassemble them in their original positions.
Attach the spring compressor to the spring. Be sure to use the safety pins or clamps supplied with the compressor. Tighten the bolt finger tight and then tighten it with a wrench just enough that it holds itself in place on the spring.
Attach the other spring compressor to the other side of the spring.
Then install the spring compressor, following the manufacturer’s directions. Spring compressors come with pins or clamps designed to prevent the compressor from slipping off of the coils. Be sure to use them. All springs have an anti-corrosion coating and you should try to minimize to damage it during compression.
Tighten each of the compressor bolts, alternating at reasonable intervals so the spring is compressed evenly.
Compressing the spring is real exercise. The spring is designed to support more than 1/4 of the car’s weight. Moreover, it’s probably a progressive rate spring, which means it gets stiffer as it is compressed. Brace the strut assembly with a block of wood so that it is stable or put the bottom end of the strut in a vise. Using a socket ratchet, turn the compressor’s bolts, alternating between the two bolts at reasonable intervals, to compress the spring. During this process, you may have to switch to using a breaker bar rather than the socket ratchet to get additional leverage.
Compress the spring until it pulls away from one of the spring seats.
The objective is to compress the spring sufficiently that it pulls away from the insulators and spring seats. Once you have created a gap between the spring and either of the spring seats, you can stop.
With the spring compressed, the damper shaft nut can be removed. This requires two wrenches, one to prevent the shaft from rotating and the other to loosen the nut. On most cars, the nut is removed with a box-end wrench, but this car required the dreaded special tool. The special tool was improvised by using a Vice-Grip to hold a socket of the proper size and routing a Torx bit thought the hole in the socket to the shaft.
Next, remove the damper shaft nut—the large nut at the end of the strut. It will be necessary to keep the shaft from turning as the nut is removed. Ordinarily, the shaft is designed with a head that can be held by wrench as another wrench turns the nut or with a receptacle for a hex or Torx socket bit. On some struts, the nut is recessed into the bearing assembly. If so, it may require a special tool – a “strut rod nut socket,” which is a socket with a hole in the back and an ear to the side that accepts a socket ratchet – to remove it.
Remove the bearing assembly, upper spring seat, insulators, dust cover, and spring from the old strut.
With the damper shaft nut removed, the spring and strut assembly can be completely taken apart. Remove the bearing assembly, the upper spring seat and insulator, the dust cover, the spring, and the lower insulator.
Before transferring components to the new strut, place a mark on the spring seat of the new strut that corresponds in location to the mark you placed on the old strut’s spring seat before disassembly. Inspect the insulators and replace them if they are not in good condition (or else you may have suspension noises) and inspect the bearing that holds the top of the strut. If it shows signs of wear, it should also be replaced.
To install the new strut, reverse the removal and disassembly process.
Start by installing the lower insulator onto the new strut, positioned exactly as it was on the old strut. Then slide the spring onto the strut. Install the dust cover, the upper insulator, the spring seat and bearing. Install the damper shaft nut and tighten it to the manufacturer’s torque specification. To accomplish this, install the torque wrench in place of the wrench that was preventing the damper shaft from turning, and then tighten the damper shaft nut against the torque wrench.
Carefully decompress the spring. As you do so, periodically check that the spring is seating in the proper position on the insulators and spring seats. Then remove the spring compressor.
It is now just a matter of reinstalling the strut and spring assembly into the car.
Have a helper hold the strut and spring assembly in place through the holes at the top of the strut tower and install the nuts that hold the upper end of the strut to the car, finger tight. By leaving these nuts finger tight, you will be able to rotate the bottom of the strut slightly, as necessary to seat the strut onto the suspension arm or steering knuckle.
Install the bottom of the strut and the nuts and bolts that attach it. Reconnect the stabilizer bar link. Tighten all of those nuts to the manufacturer’s torque specification. Reattach any electrical or air connection and reinsert any hoses or harness wires into any brackets on the strut. Now tighten the nuts holding the upper end of the strut assembly to the manufacturer’s specification and reinstall the plastic cover.
Installation of the new strut is now complete. Put the tire back on, lower the car, after removing your secondary supports. Tighten the lug nuts to the manufacturer’s torque specification, repeat the procedure on the other side, and then take the car to an alignment shop.
Rear Strut and Modified MacPherson Strut Replacement
Though they are conceptually similar to front MacPherson struts, the procedure for replacement of rear struts may be different than the front strut replacement procedure. Similarly, replacing modified MacPherson struts always requires a different procedure. This is because in many rear strut systems and all modified MacPherson strut systems the spring is not part of the strut assembly, but is mounted separately on the suspension arm. Consequently, the suspension arm must be supported with a floor jack or jack stands strong enough to support the vehicle’s weight before the strut is removed to counteract the pressure applied by the spring.
Shock Absorber Replacement
Shock absorber replacement is conceptually simple: remove the bolt or bolts holding the bottom of the shock to the suspension arm, remove the nut or nuts holding the top in place, then remove the shock absorber. It’s the execution that can be a problem. Shock absorber mounting bolts live in a hostile environment, exposed to dirt and water. So, they rust.
The shock absorber in this rear suspension is visible behind the brake rotor.
Shock absorbers, unlike struts, are separate from the spring. Shock absorbers do not support the car—springs do that. Rather, they dampen the spring’s oscillations by forcing fluid in one chamber of the shock absorber through a valve into another chamber.
Soak any rusted parts with penetrating oil well in advance of the time you intend to begin the job.
Remove any hoses or wiring connected to the shock absorber, such as this air hose. Removing the clip releases the hose.
With the clip removed, the hose pulls off. Air hose connections are found on cars with automatic leveling. A number of luxury cars have shock absorbers with electronically adjusted valves between the fluid chambers.
Begin by removing any air hoses and electrical wiring connectors from the old shock absorber that can be removed without having to manipulate the shock absorber body. In some instances, the body of the shock absorber may need to be moved to disengage connections from a retaining bracket on the shock absorber body.
Remove the lower shock absorber mounting bolt.
Because shock absorbers don’t support the weight of the car, there is no need to jack the suspension when removing them. But using a jack to slightly lift the suspension can make it easier to pull the mounting bolt out of the shock absorber and suspension arm.
You can also use socket extension as a drift to tap the bolt out with a hammer. But be careful not to damage the threads if the bolt is to be reinstalled.
Remove the bolt or bolts holding the lower shock absorber mount in place. If the bottom mount uses a single bolt sliding sideways through an eye at the end of the shock, you can use a floor jack to slightly compress the shock absorber should removing the bolt be difficult.
Now compress the shock absorber by pushing on it with your hands to make it shorter. This will make it easier to remove from the car once the top mount has been disconnected. Compressing the shock absorber is easier while the top is bolted in place.
The last step is disconnecting the upper shock absorber mounting. Shock absorbers mount at the top in several ways: the shaft of the shock absorber may be threaded and held in place by a rubber insulator and a nut threaded onto the shaft. The top of the shock absorber may be a bracket that is bolted to a mounting plate. There may be a large eye at the top of the shock absorber accepting a large laterally positioned bolt and nut.
If the shock absorber has a threaded shaft, attempting to loosen the nut will merely rotate the shaft unless the shaft is held in position. The end of the shaft is usually squared off, so it can be gripped with a wrench or a Vice-Grip. Another way to do it is with Lisle’s number 20400 “shock absorber tool.” It consists of a socket that fits over the shock absorber nut and can be turned with a standard open-end or box-end wrench, and a second socket, designed to fits a standard socket ratchet, which inserts through the first socket and fits over the square end of the shock absorber stem. This tool lists for under $15.
A nut splitter cuts through the side of a frozen nut. Expect, however, to replace the bolt also. Even if it isn’t rusted, the splitter will probably damage the bolt threads as it cuts through the nut.
If a shock absorber nut is so badly rusted that it cannot be removed with a wrench, even after liberally using penetrating oil, it can usually be removed with a nut splitter, a tool that costs less than $15.. A nut splitter is a bolt in a frame that, when the bolt is turned, pulls a hard steel wedge-shaped blade into the side of a frozen nut, breaking it.
Install a new shock absorber at the top mounting first and then extend it, by pulling down, to install the bottom mount. Except for shock absorbers that mount with a large bolt laterally through an eye at the end of the shock, new shock absorbers will include new mounting hardware. Mounting bolts should, of course, be tightened to the torque specification provided in the factory shop manual, unless the shock absorber manufacturer specifies a different value.
Suspension Upgrades and Modifications
Replacing original equipment shocks, struts, and springs is one of the most popular and effective ways to enhance the handling of a car. Through the possible resulting lowering of the car, it is also one of the most popular appearance enhancements.
Gas charged shock absorbers and struts are one of the most basic suspension upgrades, although they are original equipment on some vehicles. These use pressurized nitrogen gas instead of air inside the shock absorber. This reduces aeration and foaming of the shock absorber’s hydraulic fluid, which quickens the response to cornering and road conditions. Gas shocks and struts tend to be stiffer than original equipment shocks and decrease body lean during cornering.
Replacing springs, however, is the most fundamental suspension modification. By replacing springs, the car can be lowered, the ride and cornering characteristics changed, and the handling tailored to the driver’s specifications. But these modifications, if not properly done, can make a car virtually undriveable. If a car is lowered too much, for example, available suspension travel can be reduced to the point that handling is seriously degraded.
A number of aftermarket suppliers specialize in performance suspension components and offer complete packages, at various levels of performance, that are tailored to a specific make and model of car. These packages include all of the parts necessary to complete the modifications and have been engineered and tested by people who know what they’re doing.
A quick and easy way to locate reputable aftermarket performance suppliers is to check the website of the Specialty Equipment Market Association (SEMA), the automotive aftermarket’s trade association, (online at www.sema.org.)