Signs Your Shocks or Struts Are Worn: Bouncing, Nose Dive and Cupped Tires

Shocks vs struts: what each one does

Both are dampers. Their job is to control spring and suspension movement. Without that control, the car would keep bouncing on its springs after every bump.

The difference is structural:

• A conventional shock absorber only damps. It doesn't carry the vehicle's weight.
• A strut damps too, but it's also a structural part of the suspension. It supports the coil spring, holds the wheel in its aligned position, and takes the place of the upper control arm and upper ball joint used in other designs.

Every wheel has one or the other, and a vehicle can mix them, for example struts at the front and shocks at the back. Two practical things follow. Because a strut helps locate the wheel, replacing one can change the alignment. And because the coil spring on a strut is held under load, taking a strut assembly apart is work for a shop with the right equipment, not a driveway job.

The signs, and what each one means

Monroe's list of warning signs has eight:

These changes often come on gradually, which makes them easy to get used to. Ask yourself whether the car still rides and stops the way it did a few years ago.

Bouncing after bumps

A suspension in good shape lets the car settle quickly after a bump. With worn dampers, the body keeps bobbing. ZF, which makes Sachs dampers, says a car that keeps bouncing after a bump or during wind gusts is a clear sign the shock absorbers may be degraded. Monroe adds that the bouncing reduces the tires' road-holding force.

Nose dive, rear squat and body roll

When you brake, the car's weight shifts forward. When you accelerate, it shifts back, and in a corner it shifts to the outside. Dampers slow those shifts down. Worn units can lose their ability to control weight transfer, which is what makes a car feel as if it isn't fully under your control, including in a crosswind. Take nose dive seriously, because it can lengthen stops.

Steering-wheel vibration

Vibration is on the list, but it's also a classic sign of a wheel out of balance. Transport Canada's tire guide, Riding on Air, says an out-of-balance wheel can cause a pounding or shaking through the steering wheel or seat, and that the pounding can shorten the life of suspension parts and wear bald spots in the tires. A shake at highway speed needs both possibilities checked, and balancing and alignment fix different problems.

Clunks and knocks

Clunking or knocking is also on the list. A worn shock or strut, or its mounts, can cause it, but worn steering and suspension joints can knock too. If a clunk arrives with steering that suddenly feels loose, wanders or pulls hard, especially after a pothole or curb strike, pull over where it's safe and stop driving. Have the car towed or inspected before it goes back on the road.

How worn shocks and struts affect stopping

Tires brake and steer through their grip on the road. When a damper lets the wheel and body bounce, that grip comes and goes, and the car needs more room to stop.

Monroe's European site cites TÜV traffic-safety tests:

• A vehicle with 50%-worn shock absorbers, braking on an uneven surface from 80 km/h, needed an extra 2.6 m to come to a full stop compared with the same vehicle on new shocks.
• A front-wheel-drive car with 50%-worn shocks and new tires, driven at a steady speed over a 6 mm film of water, started aquaplaning at a 10% lower speed than the same car with new shocks.

Monroe also warns that longer stops "can be critical in an emergency braking situation," and that the risk of skidding on wet roads goes up. And worn units can make related steering, braking and suspension parts wear out faster.

If the symptoms started after a pothole or curb strike, mention it when you book: a hard pothole hit can damage more than the shocks and struts.

Leaking shock or strut: a film or a real leak?

Not every oily-looking shock has failed. Monroe's service bulletin on weepage vs leakage separates the two:

• Weepage is a light film of fluid. It comes from fluid clinging to the piston rod being pulled past the oil seal as the shock extends. The bulletin calls small amounts "highly acceptable" and says weepage doesn't require replacement.
• Leakage is when most of the shock or strut looks wet and oily, or fluid is dripping off it. When leakage is diagnosed, the bulletin says the unit needs replacing.

A light, dusty film is easy to mistake for a leak, so let a technician make the call up close, alongside the car's other symptoms.

Cupped tires: what causes cupping

Cupping, also called scalloping, is a pattern of dips worn at intervals around the tread instead of a smooth, even surface. Monroe says the bouncing from worn shocks "can also cause accelerated tire wear including cupping or scalloping of the tires," and that cupped or fast-wearing tires may mean the shocks or struts aren't holding the tires to the road as well as they should.

Worn dampers aren't the only cause of uneven wear, though. Transport Canada notes that out-of-balance wheels can wear bald spots, and that uneven tread wear can come from improper wheel alignment or a tire imbalance. A wear pattern is a clue about where to look, not a diagnosis on its own.

Whether a cupped tire can stay in service depends on how much tread it has left and what else a tire check turns up. Either way, have the cause found first, because a new tire on the same worn dampers can cup the same way.

About the bounce test

A common tip is to push down hard on one corner of the car and count the bounces. Treat it as a rough screen at most. The list of signs above doesn't include a bounce test: every sign on it is something you notice while driving or can see on the car, and Monroe advises consulting a certified technician. A push by hand in the driveway is nothing like what the dampers handle at road speed, and no bounce count proves a shock or strut is good or bad. If the car felt wrong on the road, a corner that settles nicely when parked doesn't cancel that out.

Getting the suspension checked

Monroe's Safety Triangle inspection advice is to have shocks and struts checked whenever the car is in for tire, brake or alignment service, because steering, stopping and stability depend on all of those systems working together. That makes a seasonal tire changeover a natural time to ask.

Worn dampers, mounts and joints can produce similar symptoms, which is why a suspension and steering inspection looks at the whole system rather than one part. To book one at West Yale, call the automotive line at 604-793-9310.

If shocks or struts need replacing

Because a strut holds its coil spring under load, replacing one is shop work, and the job often doesn't end with the new parts.

Monroe recommends an alignment whenever struts or other steering and suspension components are replaced. New struts can change camber, which can lead to uneven tire wear if it isn't set back to the manufacturer's specifications. A toe setting that's well off can wear tires quickly, and the wrong caster can make the car hard to keep on a straight course, one of the causes covered in why a car pulls to one side. There are exceptions, such as some shock-absorber and double-wishbone designs, where the new part doesn't change the angles or the manufacturer provides no adjustment. When it applies, a wheel alignment after the repair sets the angles back to spec.

Common questions

Is it safe to drive with worn shocks or struts?

There's no way to know without an inspection. Worn units can lengthen stops and raise the risk of skidding on wet roads, and you can't feel from behind the wheel how far that has gone. Treat nose dive, instability or a dripping unit as reasons to book an inspection soon. If the car ever feels hard to control, pull over where it's safe and stop driving it; have it towed or inspected before it goes back on the road.

How long do shocks and struts last?

There's no distance that tells you the condition of a particular car's dampers. Wear depends on the vehicle, how it's loaded and the roads it's driven on. The signs above and an inspection are a better guide than the odometer.