03 Jan 2013
January 3, 2013

What is brake fade?

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Brake fade is a term used to describe the temporary reduction or complete loss of braking power of a vehicle’s braking system. Brake fade occurs when the brake pad and the brake rotor no longer generate sufficient mutual friction to stop the vehicle at its preferred rate of deceleration. The end result being inconsistent or unexpected braking system behaviour, often resulting in increased stopping distances.

 

Brake fade is caused by overheating of the brake pad, therefore any vehicle which uses the action of a brake pad rubbing on a brake rotor to convert the vehicle’s kinetic energy into heat has the potential to develop brake fade, including motorcycles, cars and trucks. Because brake fade occurs when the brake pads are overheated, the phenomenon is only temporary and braking performance usually returns once the brakes have cooled down.

 

(Note: it is worth knowing that brake fade is not the cause for a long or spongey brake pedal/lever. If you experience a spongey pedal/lever you’ve probably got air in your brake hydraulics or have boiled your brake fluid and should read our article on fluid vapour lock to resolve your issue. Or another article on bleeding brakes.)

 

The brake pad in any brake system is designed to work within certain operating temperatures and if used outside this operating window the braking system will underperform. The friction compound of modern organic brake pads is a precise mix of many different materials and these individual materials perform differently under temperature. How these constituent materials respond at elevated temperatures defines the performance characteristics of that brake pad, including the temperature at which brake fade occurs (we’ll go into more detail on the mechanisms of brake fade later in this article). Different formulations of brake pad can therefore perform very differently from each other and it is important to use the right brake pad for the right application, and not to use a brake pad of general low quality.  Buying and fitting new brake pads for your car/

 

Firstly, it is important to recognise there two very different kinds of brake fade:

1. Green fade or early life brake fade

This is quite common and almost normal when new brake pads are fitted. It is merely a settling down of the components after install and can be gone in a few brake applications. To minimise or avoid this effect it is wise to drive cautiously when new brakes are fitted and give yourself a little extra braking distance for the bedding in period. How to bed in your new brakes The bedding in period for new pads can be anywhere up to 500 miles urban use.

 

You can shorten the green fade period by using the brakes positively on a quiet and safe stretch of road, getting the pads up to a moderate temperature and thus burning off some of the volatiles that cause early life brake fade. But before doing this, ensure the brake pads have seated properly and established full contact across the rotor face (this is easy to see with EBC rotors as it is the point when all the black coating has been scrubbed off the pad track). Take care not to be over aggressive on your newly installed pads during this process, otherwise you can do more harm than good, potentially resulting in glazed rotors article on this? or the development of ‘hot spots’ on the rotor face. A few moderate stops from 50mph to 10mph should be adequate to push through the green fade period.

 

Its worth noting that some brake pads are now “surface scorched”, a process which burns off surface organics and volatiles in the pad and almost completely eliminates green fade. For any serious fast road driver or race driver, having a pad that they can drop into a caliper and perform straight away brings significant advantages. For this reason EBC Brakes surface scorch all our track day and race grade brake pads, YellowStuff, BlueStuff and OrangeStuff.

 

It is also worth considering that brake pads are to a degree porous, hence they will absorb a small percentage of water vapour from the surrounding air between the date of manufacture and the date they are first fitted to a vehicle. Water of course boils and turns to gas at much lower temperatures than the other constituents of the brake pad, hence early instances of green fade which are observed at very low temperatures are a result of the emission of water content in the pad. This water vapour will quickly burn off as soon as you get some temperature into the pads.

 

2. Dynamic fade or in stop brake fade

This is more serious and means that you have either chosen or been sold the wrong grade of pad for your vehicle or driving style, or that the brake pad is of a general low quality. Brake fade could also be caused by caliper drag if the caliper (particularly sliding pins) has not been properly maintained, which does not allow the caliper to fully release after brake application. Dynamic brake fade, which is best described as loss of brake during the stop outside the bed in period, is dangerous and needs to be remedied. Dynamic brake fade is particularly undesirable during fast road or track driving, since once the driver has committed to stopping their vehicle within a certain distance, there is very little they can do mid braking zone to make the correction.

 

In the early days of drum braked vehicles, brake fade was more prevalent. Cooling of drum brakes was minimal and meant that heavily loaded or towing vehicles braking down a long descent could superheat the brake shoes, causing the surface of the friction material to vaporize inside the brake drum and leading to an almost complete loss of braking power. Traffic authorities even built vehicle run off sand traps as an emergency route for vehicles descending out of control.

 

Modern disc brake systems (particularly those with vented discs) exhibit much better cooling and use modern advances in metallurgy and brake materials which all contribute to vastly improved braking performance. But there are of course many different brake pad qualities in today’s world markets and if not careful with their brake pad selection, drivers can still be subjected to in stop brake fade, even when driving at reasonable speeds on the public highway.

 

For either heavily loaded vehicles, fast road drivers or race drivers who frequently reach high speeds, having a brake pad which can handle the extra braking demand is absolutely critical in order to achieve acceptable levels of performance. Remember, when you are decelerating your vehicle’s brakes are converting the kinetic energy into friction and then heat. Since kinetic energy rises with the square of velocity, a stop from high speed puts an exponentially increased demand on the brake system and this generates a great deal more heat. For example, doubling vehicle speed would not double the demand on the braking system, it actually increases demand by a factor of 22 = 4! Similarly, kinetic energy also rises in proportion to vehicle mass, so a heavily laden vehicle that is loaded to double the weight actually doubles the amount of heat generated during each braking event. If not dissipated effectively or if the vehicle is fitted with brake pads that cannot cope with the extra heat, the heat build up could cause undesirable in stop brake fade, resulting in a decrease in brake effectiveness. Drivers who regularly make several high energy stops in quick succession (such as fast road drivers or race drivers) should absolutely consider using performance pads and grooved rotors, or even consider completely upgrading over the stock braking system by installing an oversize brake kit. (Introducing EBC Brakes brand new range of ultimate performance, precision UK made oversize brake kits for automotive applications). Oversize MX disc rotors

 

3. Mechanisms of Brake Fade

So now you know the 2 types of fade, and the scenarios under which brake fade can manifest itself, lets go into more detail on the underlying mechanisms and principles that actually cause brake fade…

 

There are principally 2 common types of formulation for a brake pad friction material, organics and sintered metallics (there are also brake pads known as ‘semi-metallics’ but these are a ‘hybrid blend’ of the two aforementioned friction types and thus have properties that typically lie somewhere in the middle). For more information on the different brake pad constructions read our article ‘How to make brakes’.
Organic brake pads inherit their name from the organic phenolic resins used to bind together the different compounds used in the pads construction. There are countless different types of thermoset phenolic resin, but they can all be generally considered to have a maximum temperature up to which they are thermally stable. Above its intended maximum operating temperature, just like any organic matter, the phenolic resin used as the binding agent becomes altered by the heat and effectively ‘boils’, expelling an appreciable volume of gas as it degrades. (The actual technical term for this process is sublimation, since once the phenolic material reaches the critical temperature it jumps from its original solid state and changes instantaneously to a gas, with no detectable liquid phase).

 

The dominant mechanism causing brake fade is this thermal degradation of the phenolic resins and other materials in the friction lining, which create a film of gas at the pad-rotor interface and effectively causes the brake pad to skid off the disc. As these gasses build up at the pad-rotor interface, they produce an appreciable backpressure which creates an opposing force to the brake caliper that is trying to hold the pads against the rotor. If there is no way for the gasses to escape, the opposing force as a result of the outgassing can become large enough to prize the pads away from the rotor, reducing the area of pad in contact with the rotor and thus reducing braking power (i.e. brake fade).

 

Providing a means for these gasses to vent is an effective method to reduce the severity of brake fade. For this reason, many brake manufacturers offer slotted and dimpled brake rotors that help sweep away the gas build up every time a slot or dimple passes over the pad surface. Why choose EBC grooved & dimpled rotors. Interestingly, experimental testing on dynos shows that drilling several small holes through the brake pad can drastically reduce brake fade, by providing a venting network to effectively allow the build up of gas at the pad-rotor interface to escape. However, these holes only relieve pressure from the immediate surrounding area and hence the number of tiny holes required is so vast that it makes this solution completely impractical for mass production. Cost, coupled to the fact that the vent holes then rapidly become clogged up with brake dust, makes this concept unusable for braking applications. Consequently, the best and most effective way to prevent the gaseous build up that causes brake fade is to choose the correct pad that is designed to work at the operating temperatures it will regularly be subjected to, using vented/dimpled discs if additional improvements in performance and aesthetics are desired. Better ducting of air to the braking system can also be an effective way to improve cooling.

 

Additionally, if you are experiencing brake fade it does not necessarily mean you are using a brake pad of general poor quality. What it does mean is that you are operating the brake pad at conditions it was not designed to cope with and you consider upgrading to a higher performance pad (also check the brake caliper isn’t dragging). One of the key differences between low and high performance organic brake pads is the maximum temperature they can withstand before thermal deterioration of the friction lining becomes significant. I.e. the maximum temperature the brake pad can cope with before it begins to rapidly outgas, causing brake fade. For guidance on what type of brake pad you should select for your individual application you should find the following article of interest – Advantages and disadvantages of different pad types, Part 2.

 

4. Avoiding Brake Fade on Track

Trackday driving is a common example of vehicles overloading the braking system. A weekend warrior takes their street based car to a race track and drives at speeds not seen on the highway and brakes till his eyes pop out of his head at a corner instead of smoothly decelerating the vehicle into the corner as a professional race driver would do. This immediately superheats the pad and kills the friction compound, causing it to operate at conditions it absolutely was not designed for. Brake temperatures shoot past 1000 degrees F and few standard brake pads will tolerate that.

 

There is also good scientific evidence to support why smoothly decelerating the vehicle into a corner yields the best braking performance, rather than a driver playing last of the late brakers. Harsh and abrupt stops require a greater pedal force which in turn applies a greater clamping force on the pads. If too great, this excessive clamping force over compresses the brake pad and prevents the build up of gasses from escaping, and we know that these gaseous build-ups are the dominant contributor to brake fade in organic pads. On the other hand, a longer and more controlled stop reduces the clamping pressure on the pads, allowing more of the gasses to escape. This helps to control and minimise the severity of brake fade, allowing the natural adhesion of pad to rotor to do the work.

 

Brake fade is also frequently problematic on tuned vehicles, since many of us will admit that when considering tuning our vehicles the default thought process is to scream “power, power power” and then installing upgrades to suit. When that driver takes their beast of a ride to a track, approaches a braking zone travelling at 120+ mph and slams on the brakes they will have a real surprise when they quickly discover their rate of deceleration can not match their rate of acceleration. Our advice, upgrades should always be carried out in proportion to one another. Your first stop should be a quality set of tyres, closely followed by appropriate upgrades to the braking system. Leave the power upgrades till last. That way you’ll keep your pride and joy out of the barriers and wont be riding a cab home. EBC has a broad range of high performance braking products for the track day enthusiast and racer, click here for advice on what products we recommend ‘race products’.

 

if you experience brake fade on the road or on the track, consider installing an EBC oversize brake kit with larger diameter rotors and curved internal vanes to help with heat dissipation and eliminate unwanted brake fade.