Strut suspensions are named after Earl S. MacPherson who invented the first strut suspension back in the 1950s. The strut suspension is an independent front suspension with a lower control arm and a strut assembly in place of the upper control arm. It is an alternative to the SLA (Short Long Arm) suspension that was used up until the 1980s on most cars. Struts suspensions came into widespread use in the 1980s along with front-wheel drive, and is used on most passenger cars and minivans today. SLA suspensions are still used on most fullsize pickup trucks and SUVs which have heavier loads to carry.
The strut's role in a MacPherson strut suspension is to (1) support the weight of the vehicle via the spring around the strut, and (2) to serve as the upper steering pivot for the knuckle. This eliminates the need for an upper control arm and ball joint (except for "wishbone" strut suspensions which still use upper control arms). In a wishbone suspension, the strut is attached to the lower control arm instead of the steering knuckle, and an upper control arm serves as the upper steering pivot for the knuckle. This setup allows better control over camber changes as the suspension moves up and down.
Another variation is the "Modified" strut suspension where the spring is not around the strut but is mounted on the lower control arm. The strut still serves as the upper steering pivot, but the weight of the vehicle is supported by the coil spring and carried by the lower control arm and ball joint. In this type of suspension, the lower ball joints carry the load and typically wear faster than the lower joints on a MacPherson strut suspension.
The lower end of the strut is bolted to or part of the steering knuckle. On most cars, the lower end of the strut is attached to the knuckle with two large bolts, and one of the bolts has an eccentric cam on the end for making camber alignment adjustments.
The top of the strut attaches to the strut tower inside the fender, usually with two or three bolts.
The position of the strut relative to the wheels and chassis determines both Camber and Caster alignment of the front wheels. On rear strut suspensions, the position of the strut relative to the wheels and chassis determines rear wheel Camber.
For more information about basic wheel alignment, see Camber, Caster & Toe Alignment.
Alignment adjustments for camber are made by moving the top of the strut in or out, or by rotating an eccentric camber bolt at the bottom of the strut to move the bottom of the strut in or out. Some vehicles do not have factory camber adjustments and require modifying the upper or lower strut mount with an aftermarket alignment kit.
Alignment adjustments for caster are made by moving the top of the strut fore or aft. Some vehicles do not have factory caster adjustments and require modifying the upper strut mount with an aftermarket alignment kit.
The components in a strut assembly include the damper (shock absorber), coil spring and upper bearing plate. The unit may also have a rubber bumper at the top of the strut rod and a protective rubber dust boot to protect the strut rod.
The lower end of the strut is bolted to or part of the steering knuckle. The top of the strut bolts to the strut tower inside the fender. On some suspensions, the steering arms are on the knuckle while on others the arms are on the strut housing.
Like shock absorbers, struts gradually wear out over time. A leaky shaft seal can allow fluid from inside the strut to leak out. In gas pressurized struts, the gas charge may slowly dissipate over time, too, causing the strut to lose some of its shock dampening ability. A weak strut will produce the same symptoms as a weak shock absorber: a bouncy ride (especially after hitting bumps and potholes), excessive body sway and roll when cornering or driving in strong crosswinds, suspension bottoming, nose dive when braking hard, and possibly cupping wear on the tires.
A traditional bounce test is still a good way to diagnose a bad strut. Rock your vehicle up and down several times then let go. If the body continues to bounce, the struts are weak and should be replaced.
Worn upper bearing plates are another common strut problem. If the upper bearing plate that supports all the weight pressing down on the top of the strut is worn or badly corroded, it may bind the steering and make the steering feel stiff or slow to return after making a turn. Play or looseness in the bearing place can cause clunking or rattling noises when the wheels hit a bump. It way also change camber alignment and contribute to uneven tire wear.
Another reason for replacing a strut would be if it is bent due to collision damage (either from an accident or hitting a curb with sufficient force to actually bend the strut). A bent strut will change camber/caster alignment which may cause uneven tire wear and/or a steering pull. The recommended fix for this kind of problem is to replace the strut, although some repair shops may attempt to straighten a bent strut with special equipment if the strut is not bent too badly.
The struts on some cars (primarily older imports) are rebuildable and can often be disassembled without having to completely remove the strut from the vehicle. By unbolting the top of the strut and allowing the suspension to hang free, the strut can be pulled out from under the fender and disassembled using a spring compressor. The top of the strut housing can then be unscrewed (or cut open in some cases) so the internal components can be removed and replaced with a cartridge.
On most vehicles, the struts are NOT rebuildable and must be replaced if they are weak, damaged or leaking. The entire strut assembly must be removed from the vehicle and replaced with a new or rebuilt strut assembly. This can be accomplished by disassembling the old strut with a spring compressor once it has been removed from the vehicle, and installing the original spring and upper bearing on a new strut. If the original spring is sagging or badly corroded it should NOT be reused. Replace it with a new spring. If the original upper bearing plate is badly corroded or has play, it should also be discarded and replaced with a new upper bearing plate.
The easiest way to replace a worn or damaged strut is with a complete preassembled strut assembly (such as a Monroe "Quick-Strut" or Gabriel "ReadyMount" strut). These ready-to-install struts have all new parts including the spring and upper bearing plate. They do cost more than a bare strut, but do not require using a spring compressor or disassembing/reassembling the strut itself.
WARNING: DO NOT attempt to disassemble a MacPherson strut without a spring compressor! The spring under the upper bearing plate exerts considerable pressure against the plate. If the big rod nut in the center of the bearing plate is loosened without first compressing and holding the spring with the proper spring compressor, the plate and spring may fly off with considerable force causing injury or death!
The basic procedure for replacing a strut goes as follows:
(Note: Worn struts are usually replaced in pairs.)
NOTE: On some vehicles, a brake line may be routed through a clip or fitting on the strut. On many vehicles the line can be separated from the strut by removing or cutting the clip. But on some, the brake line may have to be disconnected. This will require bleeding the brakes after the strut has been replaced and the line has been reconnected to remove air bubbles from the brake line. If this is not done, the brake pedal may feel soft and spongy because of the air in the line.
NOTE: If the lower end of the strut is part of the knuckle, remove the strut by disconnecting the lower ball joint and tie rod end so the strut can be removed from the lower control arm.
DO NOT reuse a spring that is badly corroded, has a damaged plastic coating or is sagging.
DO NOT reuse a worn or badly corroded upper bearing plate.
Alignment Guide is a quick reference program that covers all the basics of wheel alignment and steering/suspension inspection.
It covers basic toe, camber and caster alignment, causes of various kinds of steering and tire wear problems, and tells how to correct these conditions.