Brake Fade
Brake fade can be the number one braking problem that is encountered on road or track. It is rare on road as the limits of the braking components are not exceeded but the following information may be of interest to road racers, track day enthusiasts and racers.
Brake fade occurs after braking hard several times in a row, for example during road racing or on track, and you lose braking effectiveness. This usually occurs gradually so you can compensate in your brake point by braking sooner, but sometimes happens so suddenly you can end up going on a wild off-road excursion with sometimes painful results.
There are three kinds of fade commonly encountered in fast riding; pad fade, green fade and fluid fade.
Pad Fade
Pad fade occurs for several reasons. All friction material (the materials brake pads are made of) has a coefficient of friction curve over temperature. Friction materials have an optimal working temperature where the coefficient of friction is the highest. Sometimes you can use the brakes so hard that you get the temperature over the point of maximum friction to where the coefficient of friction curve starts to decline.
The mechanics of this decline in the coefficient of friction are varied. At a certain temperature, certain elements of the pad can melt or smear causing a lubrication effect, this is the classic glazed pad. Usually the organic binder resin starts to go first, then even the metallic elements of the friction material can start to melt. At really high temperatures the friction material starts to vaporize and the pad can slide on a layer of vaporized metal and friction material which acts like a lubricant. Pad fade is felt on a bike that still has a firm, 'non spongy' feeling brake lever that won’t stop even if you are squeezing as hard as you can. Usually it builds somewhat slowly giving you time to compensate for it but some friction materials have a sudden drop off of friction when the heat is put on them resulting in sudden dangerous fade.
Green Fade
This is perhaps the most dangerous type of fade. Green fade is a type of fade that manifests itself on brand new brake pads. Brake pads are usually made of different types of heat resistant materials bound together with a phenolic resin binder. These are thermosetting plastic resins with a high heat resistance. On a new brake pad, these resins will cure when used hard on their first few heat cycles. The new pad can hydroplane on this layer of excreted gas. Green fade is dangerous because many people assume that new brakes are perfect and can be used hard right off the bat. Green fade typically will occur much earlier than normal fade so it can catch a rider that is used to a certain bikes characteristics unaware. Typically the onset of green fade is rather sudden, further increasing the danger factor. Some teams have a new pads warning sticker that they place on the top yoke to inform the rider to be careful on the first few laps.
Green fade can be prevented by bedding in the pads correctly. This is a simple procedure to boil off the resins and break in the pads under controlled conditions which has been explained below - Reducing Green Fade
Fluid Fade
Fluid fade is caused by heat induced boiling of the brake fluid in the calipers. This produces bubbles in the brake system. Since bubbles are compressible, this makes for a soft spongy lever. In worse cases, the lever can come all the way back to the grip without slowing the bike.
The major cause of fluid fade is absorbed atmospheric water in the brake fluid. DOT4 Brake fluid has a tremendous affinity for absorbing water from the atmosphere, especially at high temperatures and under humid conditions. Brake fluid can absorb atmospheric water just by sitting in the brake system of your bike. A container of brake fluid sitting open can literally go bad from water absorption in an hour on a hot humid day. It is important to keep your fluid in a tightly sealed container, keeping the cap on at all times except when pouring fluid out!
Fluid fade can be avoided by running a higher grade racing type brake fluid and most importantly frequent changes of brake fluid. Regularly changing the fluid is the most important thing you can do to avoid fluid fade - change your brake fluid every 3 months and fit braided brake hoses and you will have superb brakes.
Fortunately fluid fade usually has a gradual onset, usually enabling you to either slow down or pit before a total loss of brakes occurs.
The reason we are explaining these forms of fade to you is so that you can identify what kind of fade that you are suffering from and do the proper thing to fix the type of fade that you have with the correct countermeasure. If you are experiencing pad fade, switching brands of brake fluid won’t help. If you are getting fluid fade, the trickiest carbon pads won’t stop you a bit sooner. If you have the finest brake parts available, you could still fall prey to green fade.
Reducing Pad Fade
Pad fade is easily reduced by getting pads with a higher coefficient of friction at higher temperatures. On most bikes the stock pads, genuine Nissin, are remarkably good but in our experience Ferodo Brake Pads ( as used by most Isle Of Man TT competitors since 1924 and also by Duke, Agostini, Surtees, Sheene, Lawson, Dunlop, Fogarty..) have a great range of pads available for most motorcycles. They offer Platinum Organic, SinterGrip and Racing CP911 (Strictly for Track Use Only)
Brake pads can be roughly broken down into about 4 types:
Organic - (Ferodo : Platinum Pads)
Made of stuff like cellulose, which is like ground up cardboard! The cellulose is held together with a phenolic resin binder which is a heat resistant thermosetting resin. Organic pads used to have asbestos to give better high temperature properties but since asbestos is now a carcinogenic, kevlar, fiberglass and mineral fillers are now also used. Organic pads have a good coefficient of friction for a light lever effort, work well at low temperatures and are very quiet. They are not as good for high performance use as they quickly wear, fade, oxidize and crumble. Organic pads are kind of old school and are common on cheap aftermarket replacement pads for older and sometimes new bikes. These pads do not wear the discs very much. Organic pads are usually a light brown or tan in color.
Semi-Metallic
These have some powdered metal added to the mix to help stabilize the coefficient of friction at higher temperatures. Typically powdered Brass, iron or Bronze is added. Chopped brass or bronze wire is sometimes added to help give the pad more mechanical strength. Usually these pads are excellent for all-around use. The more metal added usually means better high temperature properties, more noise, more rotor wear and less effective cold braking. Semi-Metallic can be light tan with metal flecks in them to a dark gray in color. The darker pads usually indicate a higher metal content. A higher metallic content, semi metallic is usually a good all around high performance street pad.
Full Metallic - (Ferodo : SinterGrip Pads)
These pads are made of sintered metal with very little binder. Sintered metal is powdered metal that is pressed into a mold at high temperatures until it becomes a more or less homogeneous piece. Pads of this type are pretty aggressive with ones made of brass, bronze or copper or a mix of metals being more for street use and ones using iron being more high temperature oriented. For very high temperature use, ceramic powder is added to the pad material. Full metal pads usually require more lever effort to stop the bike. These pads produce corrosive black brake dust so clean your rims frequently. These pads are usually a dark gray to black and sometimes even copper-looking with a lustrous sheen.
Carbon
Carbon pads available to us mere mortals are not the amorphous carbon-carbon exotica that F-1 cars, the space shuttle and high performance jets use. They are not "carbon-fiber" either. Carbon pads that are available over the counter are semi-metallic pads that have powdered carbon added to them to improve the high and low temperature properties. Mostly they have the cold friction of a good mild semi-metallic with the high temp properties of medium aggressive full metallic. Even the full race, high metal/carbon pads seem to have a fairly wide effective heat range. They are fairly good on the discs too. Since they work so well over a broad range, carbon pads seem to have taken over the high-performance street pad market. The only drawback that these pads have is cost. They are pretty pricey. They also leave lots of black, corrosive, sticky, brake dust so clean your rims regularly. The full race carbon pads seem to eat discs pretty well too. Carbon pads are a flat dark gray to black with a flinty look.
Reducing Green Fade
The way to eliminate green fade is to properly bed or break in your pads before you have to use them hard. The key is to get rid of the volatile elements of the binder resin without overheating or glazing the pad. Ever seen your brakes smoke ? - That smelly stuff is the volatile resins being cooked out of your pads. Bedded pads will not smoke very easily.
It is better to bed new pads in on older discs. Older discs are seasoned and more dimensionally stable making them less likely to warp or crack while bedding. Older discs for some reason are less likely to glaze new pads. You should always run a new rotor in with bedded pads also for the same reasons.
When replacing your pads, you could lightly sand your discs with an electric drill and a 220 grit sanding disc, putting a light cross hatch pattern on them. This helps break the glaze on the disc and aides in bedding the new pads quickly. Install your new pads and go for your bedding run. Before making the first stop after changing pads pump the brake lever carefully before you really need to stop. The pistons are fully retracted into the caliper when you change the pads and the lever will feel long at the first brake application.
When bedding in the pads, be very careful as the brakes will not work at their optimum until fully bedded in…
Harder, high temperature pads usually have an overall lower coefficient of friction even when they are in their ideal operating temperature. Because of this you can expect having to pull on the brake lever much harder with them installed unless you go to a bigger brake system with more pad area. Softer, lower temperature pads generally have more initial bite and require less lever effort but they will fade much more quickly.
Through proper selection of brake pad material and careful bedding in you should be able to reduce pad fade to a manageable level except in the most extreme racing conditions.
Reducing Fluid Fade
Fluid fade is caused by the boiling of the hydraulic fluid in the brake system usually in the calipers and even sometimes the lines under hard use. This localized boiling allows bubbles to form in the brake’s hydraulic system. Since air bubbles are compressible, the end result is a long and mushy brake lever / pedal. In extreme conditions the lever will go all the way to the grip without much deceleration of the bike…
Brake fluid is hydroscopic which means it has an affinity to water and absorbs water from the air. When brake fluid absorbs water it’s boiling point drops rapidly. That why it is important to use only very fresh brake fluid, preferably from a recently opened bottle where the factory seal has just been broken. When bleeding brakes, keep the bottle capped except when you are pouring the fluid out. It is also a good practice to keep the cap of the master cylinder reservoir on, but only loosely screwed about ½ turn while you are bleeding, as the brake fluid pulls in the humidity from the air thus you want to minimize its exposure to the air.
You should also bleed your system and change your fluid at least once a year to get the moisture laden old fluid out. Your brake system will last much longer this way as the moisture in old fluid causes corrosion of the brake systems internal parts. If you are racing the fluid changes should be much more frequent than that.
Fluid fade can be avoided nowadays to a large degree with modern high-performance brake fluid, upgarding to DOT 5.1 from the standard DOT 4 but mostly by frequently changing the brake fluid.
