All About Shocks and the Seven Deadly Signs It's Time to Replace Yours

Awhile back, we published a story on diagnosing front suspension issues on your daily driver or work truck. We addressed common problems and the parts required to fix them, but one item we didn’t mention was shocks. We’re going to remedy that shortfall right now with this article on shocks and struts. We’ll also explain how to know when yours need to be replaced. Grab a beverage and read on.

What Do Shocks Do, Anyhow?
We went to the experts at Monroe Shocks to find out what shock absorbers actually do. The primary purpose of the shock—or in more modern parlance, a damper—is to control spring and suspension movement and keep the tires in constant contact with the road surface. It does this by turning the kinetic energy of spring movement into thermal, or heat energy, and dissipating it through the shock’s hydraulic fluid. A piston valve and rod assembly is located in the fluid chamber. As the suspension travels up and down, a small amount of hydraulic fluid is forced through tiny holes inside the piston. This controls piston and rod movement, which in turn controls how fast the springs move up and down.

The amount of resistance, or damping, a shock absorber develops depends on how fast the suspension moves and the number and size of the orifices in the piston valve. All modern shocks are velocity-sensitive—the faster the suspension moves, the more resistance the shock provides. This allows the shock to control suspension bounce, body roll, brake dive, and squat under acceleration.

Struts function much the same as shocks, but they are also a structural component of the suspension. Struts can take the place of two or three conventional suspension components, and are often used as a pivot point for steering and to adjust the position of the wheels for alignment purposes.

What shocks can’t do is compensate for sagging coil or leaf springs. If your vehicle’s tail is dragging, you will need to install new springs along with the shocks to restore proper ride height.

Twin Tube and Monotube Shocks
There are two basic types of shocks:

Twin tube, or telescoping shocks, have two cylinders. The inner, or working cylinder, is where the piston valve and rod move up and down. The outer cylinder serves as a reservoir for the hydraulic fluid. A stationary base valve controls fluid flow between both cylinders. The piston and base valves control the shock’s compression and rebound by controlling how fast the fluid flows back and forth between the cylinders.

Some twin-tube shocks have low pressure nitrogen gas instead of air in the outer cylinder. The gas compresses slightly as the shock moves to reduce fluid aeration and absorb vibration for a softer ride. On the downside, twin-tube shocks don’t dissipate heat quickly and have a smaller piston valve and less fluid than a monotube shock, so they have less damping force and slower responsiveness.

Monotube shocks have a single cylinder that houses a hydraulic fluid chamber, a pressurized gas chamber, and a piston valve/rod assembly. A free-floating piston and seal separates the oil from the gas. The piston assembly resides in the fluid chamber, which has no air or gas. That allows the valve to adjust quickly without aerating (creating air in) the fluid. That helps prevent the shock from ‘fade,’ or reduced performance.

Monotube shocks have a larger piston valve and hold more fluid than twin-tubes, so they create more damping force and respond faster to suspension movement. The downsides to monotubes are price, a stiffer ride thanks to the high-pressure gas, and increased susceptibility to damage from road debris. If you want to improve handling, Summit Racing has monotube shock options for many vehicles that came from the factory with twin-tube shocks.

Then there are air shocks. Instead of hydraulic fluid, the shock’s chambers are filled with air. By adjusting the air pressure, air shocks can help tailor a vehicle’s ride height to compensate for heavy loads or towing a trailer. Enterprising hot rodders in the 1970s bolted air shocks to their muscle cars to jack up the rear ends and make room for big tires. Aired up to maximum pressure, the shocks provided little to no damping and made the car ride like a dump truck. But it looked cool.

On later model vehicles with factory-installed air suspension, the shock or strut is really more of an air spring. They have a pressurized, flexible rubber bellows and are hooked to an electronically controlled onboard compressor. The system monitors and continuously adjusts air pressure in the bellows based on road conditions, keeping vehicle ride height at factory specifications and providing a smoother ride than a suspension with conventional springs and shocks.

If you have to replace a set of struts, consider getting loaded struts. These have the strut, spring, mounts, and hardware factory-assembled as a unit—just slip the old strut assembly out and bolt the new one in. It’s easier and also safer as you don’t have to deal with removing the old coil spring.

The Seven Deadly Signs
It can be hard to know when shocks and struts need to be replaced. They tend to wear out gradually, and you get used to the way the vehicle drives over time. Look for these Seven Deadly Signs of shocks and struts going bad:

1. Excessive vibration in the steering wheel and constant steering corrections due to instability at highway speeds
2. The vehicle leans or “tips” to the outside of a sharp turn
3. Excessive front end diving or swerving during hard or panic braking
4. Front end rise and/or rear-end squat under hard acceleration
5. Excessive shaking or suspension rattles after hitting small bumps or potholes
6. Unusual or irregular tire wear because the tires are not in constant contact with the road
7. Fluid leaks that indicate the seals have broken and hydraulic fluid is escaping

Who Makes What
Summit Racing carries stock replacement shocks and struts from these manufacturers, many of which are also OEM suppliers.

• Gabriel: Shocks and struts, air shocks, loaded strut assemblies
• Monroe: Shocks and struts, air shocks, loaded strut assemblies
• KYB: Shocks and struts, electronically controlled shocks, loaded strut assemblies
• Arnott: Air shocks and struts, air to spring-based shock conversion kits
• ACDelco: Shocks and struts, specialty shocks and struts for GM vehicles
• Motorcraft: Loaded strut assemblies
• Mopar Replacement: Shocks and struts for Chrysler, Dodge, RAM, and Jeep vehicles
• Summit Racing: Loaded strut assemblies
• Dorman: Air shocks and struts, air to spring-based shock conversion kits