14/09/2012
Understanding the Anatomy of Your Front Brake Pads
When it comes to your vehicle's braking system, the humble brake pad is a critical component. While often replaced as a single unit, a closer inspection, particularly of front brake pads, reveals a collection of smaller, yet significant, ancillary parts. These components play vital roles in ensuring smooth operation, preventing noise, and maximising the lifespan of your braking system. This article will delve into the specific parts that make up a typical front brake pad assembly, explaining their purpose and how they contribute to effective braking.
The Core Components: Pads and Backing Plates
At the heart of the system lies the brake pad itself. This friction material, designed to withstand extreme heat and pressure, is bonded to a sturdy metal backing plate. This backing plate provides structural integrity and a surface for the caliper to press against. However, modern front brake pad assemblies often include additional metal pieces that enhance their performance and longevity.
Ancillary Parts: The Unsung Heroes
Service manuals often detail several key ancillary parts that accompany the main brake pad. These are typically made of thin metal and are designed to work in conjunction with the pad and caliper. Let's break down the common ones:
- Inner Shim: This is a crucial piece, usually a thin metal plate that mirrors the shape of the brake pad. Its primary function is often related to noise suppression. The inner shim sits directly behind the brake pad material, against the caliper piston. You'll often find it features several rounded slots cut into its face. These slots, along with small tabs extending from the shim's edges, are designed to help it hug the pad and keep it securely in place. A critical step during installation is often the application of caliper grease between the inner shim and the back of the brake pad. This grease acts as a lubricant and further aids in dampening vibrations that can cause squealing.
- Outer Shim: This component is typically a completely flat, thin metal plate, lacking the slots and tabs found on the inner shim. It is usually the same thickness and material as the inner shim. The outer shim is positioned on the outside of the inner shim, meaning it directly interfaces with the caliper pistons. While it shares the length of the inner shim, it might be slightly narrower, often tapering diagonally towards the top. Its purpose, in conjunction with the inner shim, is to act as a precise spacer. More importantly, it provides a perfectly flat surface for the caliper pistons to press against, ensuring even pressure distribution across the brake pad.
- Spacer: This is often an L-shaped bracket or clip, distinct from any anti-squeal hardware that might be riveted to the pad. The spacer is usually small and corner-shaped, featuring various tabs. These tabs are designed to fit around the lower rear corner of the assembled brake pad, inner shim, and outer shim. It also typically has a hole that aligns with the lower caliper pin. As its name suggests, the spacer's primary role is to maintain a specific gap or 'space' between the brake pad assembly and the surfaces of the caliper body where it might otherwise rest due to gravity or assembly tolerances. This ensures the pad is not constantly in contact with the rotor when the brakes are not applied, preventing premature wear and noise.
The Role of Noise Suppression
One of the primary reasons for the inclusion of shims, particularly the inner shim, is to combat brake noise. Brake systems operate under significant mechanical stress and thermal expansion. These factors can lead to vibrations, which manifest as annoying squeals or rattles. Shims, especially those with specific designs and the application of specialised grease, help to absorb these vibrations. The inner shim, with its flexible tabs and slotted design, can conform to minor imperfections and dampen high-frequency vibrations. The outer shim provides a consistent, flat surface that ensures the force from the caliper piston is applied uniformly, reducing the likelihood of uneven wear and the associated noise.
Why These Parts Matter: A Comparative Look
It's understandable to question the necessity of these small parts, especially when faced with aftermarket pads that come with their own adhesive backing. However, these original ancillary parts are often engineered with specific materials and tolerances to work optimally with your vehicle's braking system. Let's consider a scenario where a modern aftermarket pad comes with a pre-applied adhesive backing. While this backing might offer some cushioning, it may not replicate the precise dampening characteristics of a dedicated metal inner shim. Similarly, the flat surface provided by an outer shim ensures a direct and even contact with the caliper piston, which might be compromised by a less rigid or differently shaped adhesive backing.
| Component | Primary Function(s) | Material | Key Features |
|---|---|---|---|
| Brake Pad | Friction, transfer kinetic energy to heat | Composite | Friction material bonded to a metal backing plate |
| Inner Shim | Noise suppression, pad retention | Thin Metal | Slots, tabs, interfaces with pad and piston |
| Outer Shim | Spacing, direct piston contact, noise suppression | Thin Metal | Flat, no slots/tabs, interfaces with piston |
| Spacer | Maintains clearance, prevents unwanted contact | Metal | L-shaped bracket, tabs, aligns with caliper pin |
Common Installation Dilemmas and Considerations
A frequent point of confusion for DIY mechanics is whether to reuse old ancillary parts or rely on those supplied with new brake pads, or if alternative solutions like adhesive backings are sufficient. As observed, some aftermarket pads might come with an adhesive backing designed to serve a similar purpose to the shims. However, the effectiveness of these can vary significantly. The original equipment manufacturer (OEM) shims are often specifically designed and tested for your vehicle model to provide optimal noise reduction and performance. If the new pads lack adequate shims or if their provided solutions seem less robust, it's often prudent to retain and reinstall the original shims if they are in good condition. The spacer, being a structural element that ensures correct clearances, is almost always essential. Its absence can lead to improper brake function and premature wear.
A Note on Squeal Bars
It's important to distinguish the 'spacer' from a 'squeal bar' or 'anti-squeal bracket'. Sometimes, these are integrated into the brake pad (permanently riveted), while other times they are separate components. Squeal bars are typically spring-steel clips that apply a slight outward pressure to the brake pad, helping to hold it firmly in place and prevent it from vibrating against the caliper hardware.
The Waiting Game: Observing Brake Behaviour
When installing new brake pads, especially if you've deviated from the original assembly by omitting shims or using alternative components, it's natural to monitor the brake system's behaviour closely. New brake pads, regardless of the ancillary parts used, often require a 'bedding-in' period. During this phase, the friction material gradually conforms to the rotor surface, and initial noises like creaking or light squealing under light braking are common and expected. This is a normal process as the surfaces mate. However, if persistent or loud noises develop after this initial period, it might indicate that the omitted shims were indeed playing a crucial role in noise suppression. In such cases, reintroducing the original inner and outer shims, along with the correct application of caliper grease, could resolve the issue. It's a trade-off between the immediate convenience of installation and the potential long-term benefits of adhering strictly to the original design.
Frequently Asked Questions
Q1: Do I always need to use the shims that come with new brake pads?A1: It's generally recommended to use the shims provided with your new pads if they appear to be direct replacements for your old ones. However, if you're unsure or if the new shims seem of lower quality, using the original shims (if in good condition) is often a safer bet for optimal performance and noise reduction. Q2: What happens if I don't install the inner and outer shims?A2: Omitting the shims can lead to increased brake noise (squealing, rattling), uneven pad wear, and potentially reduced braking efficiency as the force from the caliper piston might not be applied as evenly. Q3: Is caliper grease necessary, and where does it go?A3: Yes, caliper grease is essential. It should be applied sparingly to the back of the brake pad and the mating surface of the inner shim, and on the caliper slide pins. It lubricates and dampens vibrations, preventing noise and ensuring smooth caliper operation. Avoid getting grease on the friction material or the rotor surface. Q4: How do I know if my brake pads need replacing?A4: Signs include a high-pitched squealing or grinding noise when braking, a longer stopping distance, a spongy brake pedal, or visible wear indicators on the pads (a small metal tab designed to scrape the rotor when the pad material is low). Q5: What is the purpose of the 'spacer' clip?A5: The spacer clip ensures the correct clearance between the brake pad assembly and the caliper housing, preventing unwanted contact and potential noise when the brakes are not applied.
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