31/01/2021
Brakes are arguably the most critical safety system in any vehicle, and your Mazda CX-5 is no exception. Ensuring they are in optimal working condition isn't just about passing your MOT; it's about your safety, the safety of your passengers, and everyone else on the road. While the question of 'when to replace brake pads' might seem straightforward, it involves understanding various factors, from physical wear to subtle performance indicators that demand your attention. Let's delve into the specifics for your CX-5, ensuring you're well-informed to make timely maintenance decisions.
The Crucial Question: When to Replace Brake Pads?
The most direct answer to when you should replace your brake pads comes down to their physical thickness. For your Mazda CX-5, specifically the rear brake pads, the minimum thickness is a critical measurement. According to service guidelines, if the remaining thickness of your rear brake pads is at or less than 2.0 mm (approximately 0.079 inches), they must be replaced. This isn't just a suggestion; it's a safety imperative. Worn pads drastically reduce braking efficiency, increase stopping distances, and can lead to damage to other, more expensive brake components like your brake discs.
It's important to remember that brake pads should always be replaced as a set across an axle – meaning both the right and left wheels should have their pads replaced simultaneously, even if only one side appears to be below the minimum threshold. This ensures even braking performance and prevents uneven wear or pulling to one side when you apply the brakes.
Beyond Thickness: Recognising Brake Judder
While pad thickness is a primary indicator, your car often communicates issues through other sensations. One significant problem is 'brake judder,' a disconcerting feeling that indicates a problem with your brake discs or their interaction with the pads. Brake judder manifests in three distinct ways:
- Steering Wheel Vibration: You might notice the steering wheel vibrating in a rotational direction. This is often most pronounced when braking at higher speeds, typically between 100–140 km/h (62.2–86.9 mph). It's a clear sign that something is amiss in the front brake assembly.
- Floor Vibration: When you apply the brakes, the entire vehicle body can shake back and forth. Unlike steering wheel vibration, the severity of this shaking isn't necessarily influenced by vehicle speed, indicating a broader issue affecting the vehicle's stability during braking.
- Brake Pedal Vibration: A pulsating force pushing back against your foot on the brake pedal is a classic symptom of brake judder. This pulsation is directly transmitted from the brake disc as it rotates unevenly against the pad.
These sensations are not merely uncomfortable; they signal underlying mechanical issues that compromise your braking system's integrity and safety. Ignoring brake judder can lead to further damage and an increased risk of accident.
Understanding the Causes of Brake Judder
Brake judder doesn't appear out of nowhere; it's a symptom with specific causes, usually related to the brake discs themselves:
- Excessive Disc Plate Runout: This refers to a side-to-side wobble of the brake disc as it rotates. If the runout exceeds 0.05 mm (0.002 inches) at 10 mm from the disc plate edge, the brake pad will contact the disc unevenly, leading to uneven wear and thickness variation over time. This uneven contact is a primary cause of pulsation and vibration.
- Disc Plate Deformation by Heat: Repeated or severe braking, such as during panic stops or spirited driving, can cause localised temperature spikes on the disc plate, sometimes reaching as high as 1,000 °C (1,832 °F). Such extreme heat can permanently deform the disc plate, leading to uneven surfaces that cause judder.
- Corrosion Affecting Thickness and Friction: If your CX-5 is parked for extended periods in damp or humid conditions, corrosion can develop on the friction surface of the brake discs. This corrosion can be uneven, sometimes appearing in a wave pattern, which alters the disc's thickness and friction coefficient, leading to an inconsistent braking force and reaction.
Understanding Your Brake Discs (Rotors)
While brake pads are the consumable friction material, the brake discs (often called rotors) are equally vital. Their condition directly impacts braking performance and the longevity of your pads. Regular inspection of the discs is therefore paramount.
Lateral Runout Inspection
Lateral runout, as mentioned, is the side-to-side wobble of the disc. To accurately measure this, a dial gauge is used. For the rear disc plate on your Mazda CX-5, the maximum allowable lateral runout limit is 0.1 mm (0.004 inches). If the runout exceeds this, it's a strong indicator that the disc needs attention, potentially machining or replacement.
Thickness Variation Inspection
Even if the disc doesn't wobble excessively, its thickness might vary across its surface due to uneven wear or heat. This is measured with a caliper or micrometer at several points around the disc. The thickness variation limit for your CX-5's rear discs is a mere 0.015 mm (0.00059 inches). If the variation exceeds this, the disc should be machined on a brake lathe or replaced to restore a smooth, consistent braking surface.
Disc Plate Thickness Inspection
Finally, the overall thickness of the brake disc is crucial. Discs wear down over time, just like pads. For the rear disc plate, the minimum rear disc plate thickness is 8.0 mm (0.31 inches). If the disc has been machined to correct runout or thickness variation, there's a separate minimum thickness after machining: 8.8 mm (0.35 inches). Exceeding these minimums can lead to the disc overheating, warping, or even cracking, severely compromising braking safety. If your discs are below these minimums, they must be replaced.
The Inspection Process: What Your Mechanic Does
During a routine service or brake inspection, a qualified mechanic will perform several checks:
- Wheel Removal: The vehicle is safely jacked up and supported, and the wheels are removed to gain access to the brake components.
- Visual Pad Inspection: The remaining thickness of the brake pads is visually verified. If it looks low, precise measurements will be taken.
- Disc Inspection: The brake discs are inspected for visible signs of wear, scoring, cracks, or discoloration (indicating overheating).
- Measurement: Using specialised tools like a dial gauge and micrometer, the mechanic will measure the disc's lateral runout, thickness variation, and overall thickness against the manufacturer's specifications.
- Fluid Check: Although not directly related to pad wear, brake fluid level and condition are also typically checked as part of a comprehensive brake service. Low or contaminated fluid can impact braking performance.
- Recommendation: Based on these inspections and measurements, the mechanic will advise on whether pads, discs, or both need replacing, always recommending replacement in sets for optimal performance and safety.
Remember, while it might be tempting to save money by only replacing the most worn component, compromising on brake maintenance is never advisable. Quality replacement parts and professional installation are key to ensuring your CX-5's braking system performs as intended.
Mazda CX-5 Rear Brake Specifications at a Glance
To help you keep track of the critical limits for your Mazda CX-5's rear brakes, here's a handy reference:
| Component | Specification/Limit | Notes |
|---|---|---|
| Minimum Rear Disc Pad Thickness | 2.0 mm (0.079 in) | Replace pads as a set if at or below this limit. |
| Rear Disc Plate Runout Limit | 0.1 mm (0.004 in) | Measured 10 mm from disc edge. |
| Rear Disc Plate Thickness Variation Limit | 0.015 mm (0.00059 in) | If exceeded, machine or replace disc. |
| Minimum Rear Disc Plate Thickness | 8.0 mm (0.31 in) | Replace disc if at or below this limit. |
| Minimum Rear Disc Plate Thickness (after machining) | 8.8 mm (0.35 in) | Do not machine below this value. |
Frequently Asked Questions About Brake Maintenance
How often should I check my brake pads?
It's generally recommended to have your brake pads and discs inspected at least once a year or every 10,000 to 12,000 miles, whichever comes first. However, if you notice any changes in braking performance or hear unusual noises, have them checked immediately.
Can I replace just one brake pad or disc?
No, brake pads and discs should always be replaced in pairs across an axle (both front or both rear). This ensures balanced braking force and prevents uneven wear, which can lead to handling issues and premature failure of other components.
What happens if I don't replace my brake pads when they're worn?
Ignoring worn brake pads can lead to metal-on-metal contact, causing significant damage to your brake discs, requiring more expensive repairs. It also severely compromises your stopping ability, increasing stopping distances and the risk of an accident.
What's the difference between brake pads and discs?
Brake pads are the friction material that presses against the brake disc to slow your vehicle. The brake disc (or rotor) is the metal plate that the pads squeeze. Pads wear out faster than discs and are designed to be replaced more frequently.
Why is brake fluid important, and how does it relate to pads?
Brake fluid transmits the force from your brake pedal to the brake calipers, which then push the pads against the discs. While not directly part of the pad/disc wear, old or low brake fluid can reduce braking efficiency and feel, complementing the need for well-maintained pads and discs.
Conclusion
Understanding when to replace your Mazda CX-5's brake pads and discs is fundamental to maintaining your vehicle's safety and performance. Pay close attention to the minimum thickness limits for both pads and discs, and never ignore the warning signs of brake judder. Regular inspections by a qualified mechanic are your best defence against unexpected brake failures and costly repairs. Prioritising your braking system means prioritising your safety on every journey.
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