30/08/2008
A vehicle's shock absorbers, also known as dampers, play a vital role in maintaining control and stability on the road. They work in conjunction with the suspension system to absorb the impact from bumps and uneven surfaces, preventing excessive bouncing and ensuring a comfortable ride. During an MOT test, the condition and effectiveness of your shock absorbers are thoroughly checked to ensure they meet the required safety standards. Failure to have adequately functioning shock absorbers can have serious implications for your vehicle's handling, braking, and overall safety.
What is an MOT Test?
The Ministry of Transport (MOT) test is an annual inspection in the UK that ensures vehicles are roadworthy and meet environmental and safety standards. It's a mandatory requirement for most vehicles over three years old. The test covers various components of a vehicle, including brakes, lights, tyres, steering, and, importantly, the suspension system.
Shock Absorbers: The Basics
Shock absorbers are hydraulic devices designed to control the movement of your car's springs. When you hit a bump, the spring compresses and then tries to expand. The shock absorber resists this expansion, dissipating the energy and preventing the car from continuing to bounce. Without effective shock absorbers, your car would continue to oscillate after encountering any irregularity on the road, leading to a loss of tyre contact with the surface and significantly reduced control.
How Shock Absorbers are Tested During an MOT
The MOT test for shock absorbers focuses on two main aspects: condition and effectiveness.
1. Visual Inspection (Condition)
During the visual inspection, the MOT tester will carefully examine the shock absorbers for any signs of damage or wear. This includes looking for: * Leaks: Any oil or fluid leaking from the shock absorber body is a clear indication of a seal failure and will result in a fail. * Corrosion: Significant corrosion on the shock absorber body or mounting points can weaken the component and affect its performance. While minor surface rust might be acceptable, deep pitting or structural compromise will lead to a failure. * Damage: Cracks, dents, or bent shafts on the shock absorber can impair its function and are considered dangerous. * Mountings: The brackets and mountings that secure the shock absorbers to the vehicle's chassis are also inspected. Loose or damaged mountings will cause the vehicle to fail the test.
2. Functional Test (Effectiveness)
This is where the 'bounce test' comes in, although it's a bit more sophisticated than just pushing down on the car. The tester will use a special piece of equipment, often a 'shaker plate' or 'suspension tester', which is designed to oscillate the wheel and suspension. The machine measures the rate at which the oscillations are dampened. * The Bounce Test (Simplified): In a less technologically advanced but still valid method, the tester might manually push down on each corner of the vehicle and observe how many times it bounces back. A properly functioning shock absorber should return the car to its normal position with only one or two gentle oscillations. If the car bounces excessively (more than two bounces), it suggests the shock absorbers are worn and not effectively dampening the movement. * Electronic Suspension Testers: Modern MOT centres use sophisticated electronic testers. These machines lift each wheel slightly off the ground and then vibrate it at a specific frequency. Sensors measure the amount of vibration that is transmitted through the suspension. If the vibration is not adequately dampened by the shock absorbers, the tester will register a failure. This test is more precise and can detect even subtle wear that might not be apparent with a manual bounce test. Key Indicators of Worn Shock Absorbers during the MOT:* Excessive Bouncing: As mentioned, more than two bounces after a manual push. * Uneven Ride Height: One corner of the car sitting lower than the others can indicate a failed shock absorber. * Poor Handling: The vehicle may feel "floaty" or unstable, especially at higher speeds or when cornering. * Increased Braking Distances: Worn shocks can reduce tyre grip, leading to longer stopping distances. * Visible Leaks: Any sign of fluid escaping from the shock absorber body.
Why are Effective Shock Absorbers Crucial?
Beyond passing your MOT, properly functioning shock absorbers are essential for several reasons: * Safety: They are critical for maintaining tyre contact with the road surface, which directly impacts braking and steering control. Worn shocks significantly increase stopping distances and can make it harder to manoeuvre the vehicle safely, especially in emergency situations. * Handling and Stability: Shock absorbers help to keep the car stable when cornering, braking, or accelerating. They prevent excessive body roll and keep the vehicle planted. * Comfort: While not a primary safety concern for the MOT, effective shocks contribute to a smoother and more comfortable ride by absorbing road imperfections. * Tyre Wear: Severely worn shock absorbers can cause tyres to bounce, leading to uneven and premature tyre wear, such as "cupping" or "scalloping."
Common Causes of Shock Absorber Failure
Several factors can contribute to shock absorber wear and tear: * Age and Mileage: Like all components, shock absorbers degrade over time and with use. * Driving Conditions: Driving on rough roads, potholes, and frequently carrying heavy loads will accelerate wear. * Corrosion: Exposure to road salt and moisture can lead to corrosion, particularly in the UK's climate. * Impacts: Hitting potholes or kerbs forcefully can damage shock absorbers.
What Happens if My Shock Absorbers Fail the MOT?
If your shock absorbers are found to be faulty or ineffective during the MOT test, your vehicle will fail. The specific reasons for failure will be listed on the VT30 (Refusal of MOT Test Certificate). You will then need to have the faulty shock absorbers repaired or replaced by a qualified mechanic. Once the necessary work has been completed, the vehicle will need to be re-tested to confirm it now meets the required standards.
Can I Test My Shock Absorbers Myself?
While a professional MOT test is the definitive way to know if your shocks are up to scratch, you can perform some basic checks at home: * The Bounce Test: As described earlier, push down firmly on each corner of your car. If it bounces more than once or twice, the shocks may be worn. * Visual Inspection: Look for any obvious leaks or damage to the shock absorber bodies and their mountings. * Listen for Noises: Knocking or clunking sounds from the suspension when driving over bumps can indicate worn shocks or other suspension issues. * Check for Uneven Tyre Wear: As mentioned, this can be a symptom of faulty shocks. However, these home checks are not a substitute for the thorough testing conducted during an MOT. The MOT test uses calibrated equipment to provide a precise assessment of the shock absorbers' damping capability.
Replacing Shock Absorbers
If your shock absorbers need replacing, it's generally recommended to replace them in pairs on the same axle (both front or both rear). This ensures consistent damping and handling characteristics across the axle. Using a reputable mechanic is crucial to ensure the correct parts are fitted and the job is done to a high standard.
Frequently Asked Questions (FAQs)
Q1: How often should shock absorbers be replaced?A1: There's no fixed schedule, but many manufacturers recommend checking them every 20,000 miles and replacing them around 50,000 to 70,000 miles, depending on driving conditions and usage. However, the MOT test is your primary indicator of their condition. Q2: Can I drive with worn shock absorbers?A2: While your car might still be drivable, it's highly unsafe. Worn shock absorbers compromise your vehicle's ability to steer, brake, and maintain stability, increasing the risk of accidents. Q3: Do all cars have shock absorbers?A3: Most modern cars have shock absorbers as part of their suspension system. Some specialised vehicles might use different damping mechanisms, but for the vast majority of passenger cars, shock absorbers are a standard and essential component. Q4: What's the difference between shock absorbers and struts?A4: While often used interchangeably, a strut is a type of shock absorber that is also a structural part of the suspension system, often incorporating the spring seat and steering pivot. A standard shock absorber is typically a separate unit from the spring. Q5: Is a minor oil leak on a shock absorber a fail?A5: Yes, any visible leak of fluid (oil or hydraulic fluid) from a shock absorber is generally considered a reason for failure during an MOT test, as it indicates a seal has failed. In conclusion, the MOT test rigorously examines your vehicle's shock absorbers to ensure they are in good condition and functioning effectively. Regular checks and prompt replacement of worn shock absorbers are paramount for maintaining your vehicle's safety, handling, and roadworthiness. Don't underestimate the importance of these often-overlooked components – they are key to a safe and controlled driving experience.
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