Testing coil springs comes down to a combination of visual inspection, physical measurements, and on-vehicle checks that together reveal whether a spring has lost strength, developed damage, or is nearing failure. You don’t need expensive equipment for most of these tests. A tape measure, a caliper, and a careful eye will catch the majority of problems.
Start With a Visual Inspection
The simplest and most important test is looking at the spring itself. You’re checking for three things: cracks or breaks in the coil wire, rust or corrosion on the surface, and any visible distortion or bending in the coil shape. Corrosion is especially worth paying attention to because it weakens the steel over time, making the spring brittle. A spring with deep pitting or flaking rust has already lost structural integrity, even if it hasn’t cracked yet.
Cracks often start small and hide near the ends of the coil where stress concentrates. If you can safely access the springs (with the vehicle on jack stands and wheels removed), run your fingers along the coils while wearing gloves. You’ll sometimes feel a hairline fracture before you see one. Overloading, repeated pothole impacts, and road salt exposure are the most common causes of visible spring damage.
Measure Ride Height for Spring Sag
A spring that’s lost its strength will sag, and the most reliable way to detect this is by measuring your vehicle’s ride height and comparing it to the manufacturer’s specification. Here’s the procedure:
- Park on level ground with the vehicle fully unloaded (no passengers or cargo).
- Fill the tank halfway to three-quarters full to replicate average vehicle weight.
- Roll the vehicle forward and back a few feet to let the suspension settle into its natural resting position.
- Measure from the center of the wheel hub to the center of the fender opening on each corner of the vehicle.
This hub-to-fender method is preferred because it removes tire wear or different tire sizes from the equation. Write down all four measurements. If one corner sits noticeably lower than its opposite side, that spring has likely sagged. Even a difference of half an inch between left and right on the same axle points to a weakened spring. Compare your numbers to the OEM ride height spec in your vehicle’s service manual.
Measure Free Length and Wire Diameter
If the spring is already off the vehicle, you can test it more precisely with a caliper. The two key measurements are free length (the uncompressed height of the spring) and wire diameter (the thickness of the coil wire itself).
To measure free length, place the spring upright and use the outside jaws of your caliper to span from end to end. For wire diameter, measure the thickness of the wire at several points around the coil. Before taking any measurement, clean the caliper jaws and the spring surface to remove dirt and debris. Take readings at multiple locations and average them together for accuracy, and make sure the caliper jaws sit square against the surface you’re measuring.
Compare both numbers to the original spring specifications. A free length that’s shorter than spec confirms the spring has taken a permanent set, meaning it compressed under load and never fully returned. Even a quarter inch of lost free length can translate to noticeable ride height loss and handling changes once installed. Wire diameter that’s thinner than spec in spots indicates corrosion has eaten into the material.
How to Check Spring Rate
Spring rate is the amount of force needed to compress a spring by one inch. Professional shops test this with hydraulic load testers that compress the spring at a controlled speed while measuring force. The machine applies a small preload first, then begins measuring deflection from the spring’s true free height to build an accurate force-versus-deflection profile. A healthy spring produces a nearly constant force rate throughout its travel, meaning each inch of compression requires roughly the same additional force. That rate climbs sharply only when the coils approach full compression (called solid height).
Most people don’t have access to a load tester, but you can do a rough bench test. Place the spring on a flat surface, stack known weights on top of it (or use a hydraulic press with a bathroom scale), and measure how much it compresses per unit of weight added. If the manufacturer says the spring rate should be 200 pounds per inch but your spring compresses an inch under 160 pounds, it’s lost about 20% of its stiffness.
One detail that matters for installation: spring rate changes depending on the mounting angle. A spring rated at 200 lbs/inch when standing straight up needs to be rated at about 230 lbs/inch if it’s mounted at a 30-degree angle to deliver the same effective resistance. This correction factor increases as the angle gets steeper.
Rule Out Shock Absorber Problems First
Springs and shock absorbers fail in ways that can look similar from behind the wheel, so it helps to distinguish between them before pulling springs off the car. Springs support the vehicle’s weight and determine ride height. Shocks control how quickly the suspension moves, preventing the vehicle from bouncing. A worn shock will never cause a vehicle to sit lower on one side.
The classic shock test is the bounce test: push down hard on one corner of the vehicle, then release. The body should rise back up and settle within one or two oscillations. If it keeps bouncing, the shock absorber is failing, not the spring. If the vehicle sits low but doesn’t bounce excessively, the spring is the likely culprit. A vehicle can have both problems simultaneously, especially on higher-mileage cars where everything wears together.
Safety When Working With Coil Springs
Coil springs store enormous energy when compressed, and they’re one of the most dangerous components in a vehicle’s suspension to work with. If a spring slips off a compressor tool, it can cause serious injury. Roughly half of spring-related workshop injuries trace back to using the wrong tool or using the right tool incorrectly.
Wear safety glasses, heavy gloves, and steel-toed boots whenever you handle compressed springs. Before starting, check your spring compressor for worn threads, cracked hooks, or damaged clamps. Equipment breakdowns contribute to about 30% of workshop accidents, so inspect the tool every time you use it. Make sure your compressor is rated for the type of spring you’re working on. A coil spring compressor designed for standalone springs can be overwhelmed if you try to use it on a strut assembly, which combines a spring and shock into one unit with different force requirements.
Always secure the vehicle on a flat surface with wheel chocks and jack stands before removing any suspension components. Vehicles that shift during spring removal account for over half of reported workshop accidents. Compress the spring gradually and evenly, alternating sides if your compressor uses two hooks, to keep the spring centered and the load balanced.