Rick's Free Auto Repair Advice

How to test struts and shocks

There’s only one way to test struts and shocks and it’s not what you think

Every time I see someone ask how to test struts and shocks I see the same reply; bounce the fender and see how many times it rebounds. That methods works for shocks, but it’s not the way to test a strut. It never was, never will be. In fact, if you use the bounce test for struts, you’ll ALWAYS get a false positive result.

So let’s tackle this common strut testing myth

You CANNOT test a strut by bouncing on the fender. The bounce and jounce test is NOT a legitimate test. Period. Anybody who tells you to test your struts by bouncing on the fender and counting the number of rebounds is someone who doesn’t understand the difference between a shock and a strut.

Unless the strut is completely worn out and has lost all it’s fluid, it will always pass the bounce and jounce test. So you’ll get a false positive result every time.

Why the bounce test for struts produces false results

A car strut is a structural component of your vehicle. It does far more than just dampen spring oscillations and keep your tires on the road. A car strut is mounted diagonally between the steering knuckle and the strut tower, eliminating the need for an upper control arm and ball joint. A complete car strut assembly includes the strut, jounce bumper, bellows, coil spring and strut plate (also called a strut mount) containing a bearing that allows the strut to turn as you steer.

The bounce test for shock absorbers is designed to test the dampening capabilities of the shock. If the shock is worn, it continues to rebound several times after you’ve stopped your movement, proving that it has lost its ability to dampen spring oscillations. But if you use that same bounce test for struts all you’re doing is confirming that coil spring and lower ball joint operation. Here’s why: When you apply downward motion on the fender that motion transfers to the strut tower, strut plate and top of the coil spring (see illustration below). Since the strut plate is attached to the strut piston rode it will push into the strut body, beginning the dampening cycle. However, the bottom of the coil is restrained by the spring retainer mounted on the bottom portion of the strut, so placing downward pressure on the fender also transmits to the strut body AND the steering knuckle.

Since the strut is mounted diagonally, pressure applied to the strut puts a certain amount of sideways thrust against the steering knuckle, forcing it outwards a bit. The steering knuckle pivots on the lower ball joint and exerts inward force on the lower control arm. That, in turn, distributes the force to the control arm bushings. So, in reality, you’re converting a portion of your downward motion into deflection of the rubber portion of the strut mount, outward movement of the steering knuckle and inward motion on the control arm bushings. So you never really get enough compression to actually test the strut’s dampening abilities. Perhaps you compress the strut ¼ of its normal travel.

That small amount of travel might be enough to test one of four valves inside the strut body—the valve that dampens minor road bumps. But you’ll never really apply enough downward motion to activate valves, 2, 3 and 4. Valves 2 and 3 handle the really heavy hits and dampen the most severe oscillations. Bottom line, Most struts will easily pass the bounce test even if valves 2, 3 and 4 are worn out.

struts, when to replace struts, shock absorber

Compare struts and shocks

test car struts

strut plate

Strut mount

The only valid car strut tests

Struts show wear at about 50,000 miles. At 80,000 miles that wear can affect drivability. So forget about the bounce test for struts and take it for a short road test. You don’t have to take it off road to check your car struts. In fact, a simply drive around the block should do it. Here’s how:

Performing a road test is the only way to test shocks and struts

1) Start by checking tire pressure and tire condition. Improper tire pressure will affect your test drive results. Run your hand around the tread to check for cupping. That’s a sure sign of shock and strut failure.

2) Check the condition of steering components. Check for looseness in tie rod ends, ball joints and idler arms.

3) Check the condition of the suspension components line control arm bushings, sway bar bushings and sway bar end links.

4) Locate a mile long stretch of quiet road where you can safely accelerate, stop and swerve without traffic interference. The road should have some bumps.

5) You’ll be looking for nose dive, rear squat and body roll. You’ll also be listening for clunking noise and rebound after shock, so turn off the music and close your windows to block out ambient noise.

6) Accelerate hard and watch the rear deck in your rear view mirror to check for squat during acceleration (especially noticeable on RWD vehicles).

7) Stop hard an check for nose dive by watching the leading edge of your hood.

8) Swerve hard and check for body roll by watching the left and right front pillars and comparing to the landscape.

9) Listen closely after hitting road bumps. You should only hear one bump. If you hear more than one bump per hit, your shocks and struts aren’t doing their job.

10) Pay attention during maneuvers to see if the vehicle closely follows your steering. If the vehicle tends to wander out of your lane on harsh turns with bumpy roads, it’s because your tires have lost traction and centrifugal force is allowing movement out of your lane.

11) Check braking effectiveness during a stop. Worn shocks and struts cause nose dive. Nose dive transfers more weight to the front wheels which decreases braking effectiveness.

Another symptom of worn car struts is premature front brake wear due to excessive nose dive and excess weight transfer to the front brakes.

Are the car struts leaking?

Most car struts seep a small amount of fluid where the piston leaking strutenters the strut body. Acceptable seepage appears as a light but dry oil stain. That, by itself, isn’t a reason to replace struts. However, if the seepage is wet and shiny, you have a worn piston seal and you’re losing more than an acceptable amount of fluid. The struts must be replaced.

Are the control arm bushings cracked or missing?

Rubber cushions affect the strut’s ability to properly dampen spring oscillations. Since you most likely have to disconnect control-arm-bushingthe strut to replace worn bushing, it’s a good idea to do a full suspension evaluation and replace all worn parts at the same time.

 

 

 

Difference between a strut and shock

A shock absorber is much different. When you bounce on a car with a conventional short arm/long arm suspension (SLA), both control arms move up and down. Due to the design of an SLA system, there’s always some shock absorber compression. When you stop bouncing an SLA system, a worn shock will allow the spring to continue oscillating. So bouncing an SLA system is a valid test of the shock absorber.

Strut without spring

Strut without spring

 

shock absorber

Shock absorber

©, 2013 Rick Muscoplat

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Posted on by Rick Muscoplat



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