Brake Judder

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24 Aug 2020

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Brake judder can be felt as a vibration through the brake pedal, the steering wheel and potentially through the seat. If the judder is particularly severe, the car may shake.

Drivers and mechanics often assume that the brake discs have warped. To diagnose the causes, it is worth understanding whether the brake judder is immediate or has developed over time.

Brake Judder Symptoms and Causes

If a new set of brake discs are fitted and brake judder is felt immediately, this may be a result of a mechanical issue that hasn’t been diagnosed. For example:

• Loose wheel bolts
• A severely worn or damaged hub, hub flange and/or wheel bearing
• Severely worn or loose steering and suspension components
• Incorrectly seated wheel/discs
• Buckled rims
• Damaged tyres
• ABS fault
• Loose brake components (e.g. carrier)

This list is not exhaustive and if these underlying issues are not resolved, the issue will remain and potentially lead to a mechanical failure. Most frequently, brake judder develops over time and it is assumed that it is a poorly manufactured brake disc that has warped over time. However, due to modern materials and manufacturing processes, it is extremely rare for any brake disc to warp. For warping to occur, a combination of extreme conditions will need to be present. For example, hot discs when driving through standing water, or excessive heating and insufficient cooling. The most common cause of brake judder is disc thickness variation.

What is Disc Thickness Variation?

Disc thickness variation (DTV) occurs when the disc has worn unevenly, causing the friction surfaces of the disc and pad to no longer be parallel with one another (see fig.1). This is commonly mistaken for disc warping – which is defined as distortion with uniform thickness (see fig.2).

If DTV is misdiagnosed as disc warping, replacing the discs will only solve the problem temporarily (approximately 2,000 – 5,000 miles). DVT occurs over time, but what causes it? To understand this, we must first understand the bedding-in process.

Definition

The “bedding-in” for brakes is essential for performance optimisation. Friction is key for effective braking and is a force resistant to movement between contacting areas. In braking, there are two types of friction processes that occur, which have been outlined below which must be understood to comprehend the bedding in and material transfer processes.

 

 

 

 

 

 

 

 

 

ABRASIVE FRICTION PROCESS

When braking, the contact between the brake pads and brake discs will generate heat and cause the particles on the material’s surface to break down. If looked at under a microscope, tiny particles would be seen breaking away from the surfaces as they made contact. During this process, the heat generated physically and chemically changes the exposed friction material, leading to mechanical wear of the discs and pads. Some particles become part of the friction surface of the disc, whereas others form brake dust that stick to wheels. This dust does wash off and has the potential of polluting the environment.

ADHERENT FRICTION (BEDDING-IN) PROCESS

The particles that become part of the friction surface leads to the friction or bedding in process. This process transfers a layer of brake pad material onto the disc and increases the coefficient of friction (i.e. makes the brakes stickier). As there are now two forces at work (adherent friction and pressure), stopping distances can be improved by up to 20%.

The transferred material acts like an adhesive and sticks to the brake pad as it’s compressed against the disc, creating a barrier between them. This prevents the brake pads and discs to grind each other down, meaning your brakes can last significantly longer.

Adherent friction isn’t as destructive as abrasive friction due to that protective later. However, the material transfer must be evenly distributed on the disc surface to ensure a continuous smooth operation of the brakes. This is an evolving process as heavy braking can remove this protective layer which will need to be replaced under normal braking conditions.

BEDDING-IN PROCEDURE

Bedding-in new pads and discs must be done with care to ensure even transfer of materials and doing this correctly ensured they will work flawlessly together. Vehicles should be driven at a moderate speed (30-35mph) during the test drive and brakes should be applied gradually, without coming to a complete stop, to start the transfer process. This should be repeated 8-10 times and should be done less than a minute between each cycle to maintain the temperatures required. It is essential to advise customers to brake gradually and to avoid feathering their brakes or braking heavily for about 200 miles to continue the bedding in process.

POTENTIAL PROBLEMS

Not enough heat during the bedding-in process keeps the pad material from transferring to the disc face. This is why it is important to avoid feathering the brakes (or over-heating the system with heavy braking). Both can lead to uneven pad deposits causing a stick/slip situation. Heavy braking will then lead to uneven heat build-up, with high spots heating disproportionally to the rest of the disc. If the temperatures exceed 650°C, the cast iron within the product changes structurally and transforms into a harder substance: cementite.

The disc is now composed of varying materials of different hardness and these materials wear at different rates. This will not immediately induce brake judder but it will cause vibrations that will lead to irreparable damage to the brake disc, although this may not be apparent for over 2,000 – 5,000 miles. This may explain why customers experience brake judder months after the discs were installed.

To check DTV, measure (at least) eight equal points around the brake disc with a micrometer. A DTV of >30μm (more than 0.03mm) causes the brake pads to pulsate under the uneven surface, resulting in the symptom of brake judder. The pistons then kick back over every high spot, causing the pedal to pulsate and replicate the brake judder symptoms.

Although skimming the discs will temporarily remove the high spots and judder, the hard spots of cementite will remain under the surface and the symptoms will return after several thousand miles.

NAPA QUALITY ASSURANCE

All NAPA brake pads benefit from a high pressure treatment scorching process (to enable easier bedding-in and provide superior initial performance) and are manufactured from the latest generation friction materials. NAPA brake discs are tested before they leave the factory to ensure they have a thickness variation of <13μm (less than 0.013mm).

NAPA partially coated discs are made to match OE standards and have coated hubs and edges. However, there is no coating on the friction surface because this would cause contamination, disrupting the material transfer process, reducing efficiency. After the initial bedding-in, material transfer is a continual process and anything affecting the even transfer of these materials will cause issues. The biggest cause of uneven material transfer is excessive lateral runout, which is explained below. Other causes may simply be poor technique repeated emergency-style braking without adequate cooling time, using the brakes to slow the vehicle on serious inclines instead of engine braking, or even a mechanical issue such as a slider, caliper or a sticking pad that can affect the transfer process.

What is Lateral Runout?

Lateral runout is when the disc deviates from its true axis due to not being parallel to the hub surface (see fig.3).

What causes Lateral Runout?*

• Not following best practice during fitment
• Insufficient hub surface cleaning (excessive lateral runout can be caused by miniscule particles of debris and dust caught between the disc face and hub)
• Low quality refurbished alloy wheels (uneven paint thickness)
• Worn steering and suspension components
• Work or damaged hub/hub flange/wheel bearings
• Corrosion build-up after disc fitment
• Not replacing a missing disc location screw
• Using copper grease on the hub surface
• Over-torqueing of the disc location screw and/or wheel bolt

 

What else can cause Judder?

Static Spots

If a vehicle has been parked for an extended period, especially in areas like the coast where there is high humidity and salt content in the air, corrosion can build up between the disc and pad and they can stick together, causing severe judder when braking. They can be freed by softly twitching the vehicle (by quickly releasing the clutch). If the corrosion is particularly severe, it will cause damage to the brake pads. The corrosion may cause a temporary judder when braking but if this continues for an extended period, the brake disc should be replaced.

Brake Disc Thickness is Low

Brake discs are exposed to compressive and centrifugal forces, which creates a traction force. The compressive strength of the disc is measured by its capacity to withstand clamp load from the brake pads. This strength decreases and the brake disc becomes worn and if it exceeds the material resilient limit, cracks and arping can appear when the disc cools. Therefore, it is crucial to replace the brake discs when the surface thickness is less thanthe recommended minimum thickness.

*This is not an exhaustive list.

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