Brake fade: What it is, how it happens and how to avoid it

Comline advises technicians on brake fade in its latest technical bulletin

Brake fade: What it is, how it happens and how to avoid it
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In its latest technical bulletin, Comline has defined the causes of brake fade and provides practical advice for how best to avoid it.

The term itself is largely self-explanatory and applies when the braking power of a vehicle reduces or ‘fades’.

Regardless of the component failure which is causing brake fade, the underlying cause is the same – excessively high temperature resulting from heavy braking and/or a heavy load.

Pads and discs

One of the main reasons for brake fade that results from excessively high temperatures is the reduction in braking coefficient between pads and discs.

Brake pads and discs are designed, manufactured and tested to perform across a temperature range which will prove more than adequate for typical, on-road driving conditions.

For example, Comline brake pads are tested up to 700°C as part of our ‘R90-plus’ test protocol.

This is a temperature far beyond the realms of everyday driving in the majority of passenger cars.

However, add a heavy, over-loaded trailer to your car, or take it onto a racetrack, and you could generate the sort of temperatures required to experience brake fade.

This is precisely why high-performance vehicles feature highly specialised and costly braking components – the McLaren P1 for example features ceramic discs coated in Silicon Carbide which are designed to operate at temperatures far beyond a typical cast-iron disc.

Pushing a cast-iron disc beyond its limits can see brake fade for two main reasons.

Comline braking parts.

Material transfer

Firstly, friction material of the brake pad can begin to break down at very high temperatures and adhere to the braking surface of the disc.

This means that the pad is no longer contacting with a flat surface but instead something that is much more irregular.

In addition to causing premature wear of both the pad and disc, material transfer can result in brake judder and impact the friction coefficient to cause brake fade.

Gas slide

Secondly, ‘gas slide’ is a term we have coined to explain what happens when the resins within a brake pad turn to gas due to excessively high temperatures.

In this unavoidable chemical reaction triggered by heat, a layer of gas forms between the pad and disc which quite literally causes the pad to slide off the disc and the brakes to fade.

It is comparable to worn tyres “aquaplaning” on a road surface with a lot of water on it.

As temperature returns to a more normal level this gas will dissipate and reliable braking will return but constant exposure to such temperatures would irreparably damage the brake pad as its essential resin content would all ‘burn off’ into gas.

It should be noted that ‘gas slide’ can occur with newly fitted brake pads at more everyday temperatures.

This early life fade occurs for the same reasons as highlighted above because all new brake pads will release a small amount of resin gas the first few times they are brought up to temperature.

‘Bedding-in’

To avoid this issue, technicians fitting new brake pads to a vehicle should always conduct a controlled ‘bedding-in’ process.

It is also worth noting that brake discs should also be bed-in for the first 300 miles after fitment to avoid ‘warping’.

It should also be noted that a modern high-quality brake pad is unlikely to result in any noticeable brake fade if bedded correctly and used within normal driving conditions.

Vapour Lock

Another reason for experiencing brake fade is so-called ‘vapour lock’, a problem caused by the water content of a vehicle’s brake fluid being turned to steam inside the braking system at high temperatures.

All brake fluid is ‘hygroscopic’ and naturally absorbs water from the atmosphere over time.

The more water content, the lower the boiling point and the greater the risk of steam accumulating inside the braking system causing vapour lock.

Gases are compressible and, unlike incompressible brake fluids, are unsuitable for delivering consistent, reliable braking.

If a vehicle has steam inside the brake system the driver will notice increased pedal travel is required and that braking isn’t nearly as consistent, powerful or immediate.

To avoid vapour lock, it is important to regularly replace a vehicle’s brake fluid in-line with recommended service intervals.

Preventing brake fade

To prevent brake fade, Comline recommends following the proper vehicle maintenance schedule.

Drivers should also be advised that the harder they work the brakes, the higher the temperatures and the greater the likelihood of experiencing brake fade.

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