24/03/2007
Understanding Brake Judder: A Comprehensive Guide
Brake judder is a common and often unsettling phenomenon experienced by drivers. It manifests as a vibration that can be felt through the steering wheel, the brake pedal, and sometimes even the entire car. This disconcerting sensation can lead drivers to reduce braking pressure, inadvertently increasing stopping distances. While many immediately suspect 'warped' brake discs, the reality is often more complex. This guide will delve into the various causes of brake judder, differentiating between immediate symptoms and those that develop over time, and provide insights into effective diagnosis and prevention.
Immediate Brake Judder: Beyond Disc Warping
If you experience brake judder immediately after fitting new brake discs, the initial assumption might be a manufacturing defect. However, reputable manufacturers employ rigorous testing to ensure their discs meet quality standards. Immediate judder is typically a symptom of an underlying mechanical issue that was present before or overlooked during the disc replacement. These can include:
- Severely worn or damaged hub, hub flange, and/or wheel bearing: These components are crucial for the proper alignment and stability of the braking system.
- Severely worn or loose steering and suspension components: Play in these areas can amplify even minor imperfections in the braking system.
- Loose wheel bolts: Inadequate tightening can lead to uneven pressure and vibration.
- Incorrectly seated wheel/discs: Improper alignment of the disc to the hub can cause immediate issues.
- Damaged tyres: Out-of-balance or damaged tyres can contribute to vibrations felt during braking.
- Buckled rims: Similar to tyre damage, a deformed wheel rim will cause imbalance.
- ABS fault: While less common, an issue with the Anti-lock Braking System can sometimes manifest as judder.
- Loose brake components (i.e., carrier): Any looseness in the brake caliper or its carrier can lead to instability.
It's crucial to address these fundamental mechanical issues. Failing to do so not only perpetuates the judder but can also lead to more serious, potentially catastrophic, mechanical failures.
Brake Judder Developing Over Time: The Truth About 'Warped' Discs
The most frequent scenario is brake judder that develops gradually. The common belief is that brake discs warp due to heat. However, modern manufacturing processes and materials make true disc warping extremely rare. For a disc to warp, it typically requires a combination of extreme conditions, such as overheating to the point of glowing followed by driving through deep water, or prolonged overheating with insufficient cooling. The more prevalent cause of judder developing over time is Disc Thickness Variation (DTV).
Disc Thickness Variation (DTV): The Real Culprit
DTV occurs when the brake disc wears unevenly, causing its two friction surfaces to no longer be perfectly parallel. This is often misdiagnosed as warping, which is a distortion of the disc's shape. DTV is a gradual process, and the common practice of simply replacing discs when judder appears often provides only a temporary fix, with the judder returning within a few thousand miles.
What Causes DTV?
The primary driver of DTV is uneven material transfer. During braking, a microscopic layer of brake pad friction material should transfer evenly onto the surface of the brake disc. This process, known as the adherent friction process, is essential for optimal braking performance. Anything that disrupts this even transfer can initiate DTV.
The Process of Uneven Material Transfer
When friction material transfers unevenly, 'high spots' are created on the disc surface. These high spots experience excessive heat build-up. If the temperature at these spots exceeds approximately 650°C, the cast iron undergoes a structural change, forming a harder substance called cementite. As the disc is now composed of materials with different hardnesses, they wear at different rates. While this doesn't immediately cause judder, it lays the groundwork for vibrations that can eventually lead to irreparable damage to the brake disc over time. This explains why customers often return with judder issues months after a disc replacement.
Measuring DTV
A disc with DTV will not appear visually warped. However, measuring its thickness at multiple points around its circumference will reveal variations. A DTV exceeding the manufacturer's specified tolerance will cause the brake pads to pulsate as they encounter these uneven surfaces. This pulsation can force the brake pistons back, leading to a pulsating brake pedal that mimics the sensation of judder.
The acceptable threshold for DTV varies between vehicle models, depending on the design of the braking, suspension, and steering systems. Reputable manufacturers like Apec ensure their new discs have a DTV of less than 13µm (<0.013mm) before leaving the factory.
Skimming brake discs can temporarily alleviate judder by removing the high spots, but the underlying hardened cementite remains, and the judder is likely to return.
Lateral Run-out: A Major Contributor to DTV
Lateral run-out is a significant cause of uneven material transfer. It occurs when the brake disc is not perfectly parallel to the hub surface, causing it to deviate from its true rotational axis.
Causes of Lateral Run-out:
- Not following best practice during installation: This is a broad category encompassing several specific errors.
- Insufficient cleaning of the hub surface: Even tiny particles of rust or debris trapped between the hub and the disc can cause run-out.
- Corrosion build-up after fitting: Failure to use copper grease on the hub surface or not replacing a missing disc location screw can lead to corrosion.
- Worn or damaged hub, hub flange, and/or wheel bearing: These issues compromise the hub's integrity.
- Worn steering and suspension components: Similar to immediate judder causes, these can exacerbate run-out.
- Over-torquing of the disc location screw or wheel bolts: Excessive tightening can distort the disc.
- Low-quality refurbished alloy wheels: Uneven paint thickness on refurbished wheels can affect seating.
Other Causes of Uneven Material Transfer and Judder:
- Sticking brake pads, slider bolts, or calipers: This can cause the pads to rub on the disc when not braking or prevent even contact during braking.
- Failure to adhere to the correct bedding-in process: Proper bedding-in ensures even material transfer from the start.
- Stationary periods with the brake applied: Prolonged braking while stationary can lead to 'pad etching', an uneven deposit of friction material.
- Severe overheating of the discs: Extreme temperatures can contribute to uneven wear patterns.
Less Common Causes of Brake Judder
Static Spots
If a vehicle is left parked for extended periods, especially in humid or salty environments, corrosion can form between the brake disc and pads. This adhesion, or 'static spot', can cause significant judder when braking is first applied. Often, gently moving the vehicle can free the brakes. If the corrosion is severe, it can also damage the brake pads. Persistent judder due to static spots may necessitate brake disc replacement.
Brake Disc Below Minimum Thickness
Brake discs are subjected to considerable forces. As a disc wears down, its structural integrity decreases. When a disc becomes too thin, it can deform under the clamping load of the brake pads and centrifugal forces. This deformation can lead to cracks or even warping, especially upon cooling. It is crucial to replace brake discs when their thickness reaches the minimum limit specified by the manufacturer, which is usually engraved on the outer diameter of the disc.
Preventative Measures and Best Practices
To avoid brake judder, adhering to best practices during brake maintenance is paramount:
- Thorough hub cleaning: Ensure the hub surface is meticulously clean before fitting new discs.
- Correct torque settings: Always use a torque wrench to tighten wheel bolts and disc location screws to the manufacturer's specifications.
- Use of appropriate lubricants: Apply copper grease sparingly to the hub surface, avoiding contact with the friction material of the disc.
- Proper bedding-in procedure: Follow the manufacturer's recommended bedding-in process for new brake pads and discs.
- Regular inspection: Periodically check steering and suspension components for wear.
- Replace discs when worn: Do not exceed the minimum thickness or ignore signs of excessive wear.
Frequently Asked Questions
Q1: What does brake judder feel like?
Brake judder feels like a vibration or pulsing that can be felt through the steering wheel, brake pedal, or the entire car during braking. It can range from a subtle shimmy to a violent shaking.
Q2: Are warped brake discs the only cause of judder?
No, while often blamed, true disc warping is rare. Disc Thickness Variation (DTV), caused by uneven material transfer, lateral run-out, and other factors, is a more common cause of judder that develops over time.
Q3: How can I prevent brake judder?
Proper installation, including thorough hub cleaning, correct torqueing, following bedding-in procedures, and replacing discs when they reach minimum thickness, are key preventative measures.
Q4: Can skimming brake discs fix judder permanently?
Skimming can offer a temporary solution by removing surface imperfections, but if DTV is present due to underlying issues or hardened material, the judder will likely return.
Q5: What should I do if I experience brake judder?
If you experience brake judder, it's advisable to have your braking system inspected by a qualified mechanic to diagnose the root cause and perform the necessary repairs.
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