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Not Fade Away
Classic American issue 173

 
In past articles we have discussed brake problems such as judder and warped discs. We also discussed how to correct such problems. However, we did not discuss what to do if you are simply not happy with the way your stock braking system is functioning. So what do you do if your American dream has all the get up and go of an F1 racing car, but the stopping power of a toasted cheese sandwich. Well to answer this we need to look at a problem we have not yet touched on when dealing with braking systems, brake fade. This is most commonly seen on earlier vehicles that are fitted with drum brakes all round. Most people get around drum brake fade by fitting a disc brake conversion to the front of the vehicle. However, with engines becoming more powerful the problem of brake fade in disc brake systems is on the increase.

Commonly there are three types of brake fade, pad fade, green fade and fluid fade.Pad fade can occur for several reasons; the most common is over heating. The material that brake pads are made from has what is known as a coefficient of friction curve over temperature. That is to say the amount of grip the pad has changes at different temperatures. All friction materials have a temperature where they are at their most grippy. This is known as the optimum friction coefficient. There are times that may find you using the brakes so hard that the temperature rises above this optimum working temperature, it is at this point that pad fade may begin to occur. To put it simply the pads become less grippy. The results of this over heating are of often quite varied. The most common is the glazed pad. This is where the pad reaches a high enough temperature for the bonding material of the pad to melt and smear across the surface of the friction material. When this bonding material cools it causes a glaze on the surface of the pad that covers the friction material, thus reducing the efficiency of the pads. If the temperature keeps rising even the friction material itself will begin to melt and possibly vaporize. When this occurs the pad rides above the disc on a layer of vaporized metal and friction material. This layer then acts just like a lubricant again reducing braking efficiency to almost nothing. Brake fade will usually build up over time and you are able to compensate for it. However, there are some friction materials in which the coefficient of friction drops off suddenly when a certain temperature is reached, in extreme situations this can result in dangerous and sudden brake fade.
Green fade is most commonly associated with new pads. The friction material of brake pads is usually held together with a phenolic resin binder. These binders are thermosetting plastic resins that have a high resistance to heat. When new brake pads are fitted and used for the first time, these resins will expel gas or cure if used hard during the initial bedding in period. Once again this can cause a cushion of gas between the new pad and the disc. As with pad fade this will obviously reduce braking efficiency. The real danger lies in the fact that many people will fit new pads and assume their braking system is back to normal. Correctly bedding the pads in is the easiest solution to this problem. Again the results of this type of fade is a glazed pad. The pads in fig.1 had only been fitted to the vehicle for two or three days, but in that time it has become glazed. As a comparison fig.2 shows a pad in new condition. Notice the literal shiny glazed appearance of the pad in fig.1, this is typical of a pad that has been exposed to temperatures higher than the optimum operating temperature. In this case the resulting glaze is probably due to green fade.

Fig 1. Pads have become glazed

Fig 2. New sinted apperence
As mention earlier, fade commonly occurs in drum brake systems. The glazed condition of the brake shoe shown in fig.3 is the result of excessive heat in a drum brake system; a new shoe is shown in fig.4 for comparison. The system this shoe was part of would almost certainly of suffered brake fade. Again these shoes had only been fitted to the vehicle for two to three days, so this condition is probably also due to green fade. On closer inspection of fig.3 you may notice some lighter coloured bands running across the shoe. This is damage caused to the surface of the shoe lining by a drum with imperfection in its braking surface. The chances are that the drum probably requires changing. Similar damage can occur to brake pads when a brake disc needs replacing.

Fig 3. Used and glazed brake shoes

Fig 4. New brake shoes

Moving on to fluid fade, this is caused by the brake fluid overheating and actually boiling in the caliper. This boiling produces gas bubbles in the brake lines. Because air is compressible the brake pedal will become spongy. In a worst case scenario the pedal will become so spongy that it will travel to the floor with very little effect on braking. You’ll be pleased to know that there are several simple ways to improve your braking system and cure any brake fade problems that you may have. The obvious thing we have to do is get heat away from the brakes quickly and efficiently. The first thing to do here will be to upgrade the vehicles brake pads. Pads can be divided, roughly, into four groups.
Firstly we have the organic pads, fig.5. These are what we would call an old school pad; the sort of thing cars and trucks from the 60’s and early 1970’s would have been fitted with out of the factory, or on the rear of some later vehicles. They are often made of a ground up cellulose material. This is held together with a heat resistant phenolic binder. There is usually another material added, known as the filler, to improve the pads high temperature properties. Up to the early 1970’s this material would have been asbestos, but since this has been found to be carcinogenic, materials such as kevlar and fiberglass are now used as so-called fillers. Organic pads have a really good friction coefficient for light pedal use and work very well at low temperatures, giving a very good brake pedal feel. These are not recommended for any performance applications, they wear quickly, oxidize, crumble and are almost guaranteed to cause brake fade at high temperatures. These pads are easily identified by their light brown or tan colour.
Next we have the group of pads known as semi-metallic, fig.6. These pads have powdered metal added, this helps to stabilise the friction coefficient at higher temperatures. The metal that is added is usually brass, iron or bronze, some manufacturers will also add chopped brass or bronze wire just to give the pad more strength. The stock pads fitted to most new cars are usually of a semi-metallic compound. These are a good all-round pad and can be anything from light tan with metal flecks in to dark gray in colour. The darker colour will normally indicate a high metal content in the pad. A higher metal content will mean that the pad has a better friction coefficient at higher temperatures, but will cause more wear to the brake discs than an equivalent organic pad.

 

Fig 5. Oganic brake pads

Fig 6. Semi-metallic brake pads

 
Thirdly there are the full metallic pads, fig.7. These are made of a sintered metal using very little binder. They may be made from brass, bronze or copper or a combination of these. At the extreme end of the full metallic market are the iron pads, these are for use in high temperature situations. These pads are aimed more at the race market and have very poor grip in the wet and the cold. They also wear rotors out quickly. These pads generate large quantities of dark brake dust, so you if you’re using these, be prepared to clean your wheels regularly. The normal colour for these is a dark gray to black, some even have a slight copper sheen to them. It should be noted that some racing full metallics have ceramic powder add to the mix, this again will increase the friction coefficient of the compound.Finally we have the carbon pads, fig.8. Full on race carbon pads are not the sort of thing that many people would even consider for street use. The carbon pads that are designed for performance street use are usually a semi-metallic pad that have carbon powder added to the mix, again this improve the high temperature qualities of the pad. Carbon pads tend to have a good wide effective heat range, working well from cold through to high temperatures almost equally well. The pads also create a lot of brake dust, but they are a little easier as far as wear on the brake discs is concerned. They are a flat dark gray in colour.

Fig 7. Full metallic pads

Fig 8. Carbon brake pads
Moving away from pads, we could also consider using performance brake discs to help remove heat from the system and reduce the risk of fade. There are a few different types of disc that can be used. When brake discs were first introduced, most were simply a solid disc. It was common to try and upgrade to a ventilated disc. As time has gone by ventilated discs are now fitted as a stock item to most vehicles. These discs have vents radiating out between the inner and outer braking surfaces. They could almost be thought of as air fans. Cool air is drawn from an area around the center of the disc. This air then flows up the vents, because of the large amount of surface area in the vents the air very effectively dissipates the heat that has built up in the disc. The vents can be clearly seen on the disc in fig.9.

Fig 9. Vented disc

Fig 10. Slotted directional disc
The next step up from these is the slotted disc. As well as being ventilated these also have grooves radiating from the center to the edge of the brake surface. The grooves help to disperse the gases that lead to brake fade, as well as help to evenly cool the braking surface. This even cooling helps to reduce the chance of hot spots on the brake disc, and thus reduces the chance of having the discs warp. The slotted disc shown in fig.10 is not only slotted but also cadmium plated. So this disc should not only brake well but should also stay looking good. Top of the line street performance discs as well as having slots will also be either cross-drilled or have blind drilled holes on the braking surface. Cross-drilled discs have holes going completely through their surface, where as blind drilled holes only go part way through the disc. These holes help to expel even more of the gases that cause brake fade and reduce the temperature of the disc even more than slots. Whilst the cross-drilled discs do look extremely good, under very high temperature conditions they should be avoided. The reason for this is that at very high temperatures it is possible for cracks to start radiating from these holes. Where as discs that have blind drilled holes disperse the gases but do not crack, because the holes do not go all the way through the disc. Many of these slotted and drilled discs are actually designed to be direction specific. In other words there are specific left and right hand discs for a vehicle application, even if the vehicles stock discs are not left or right handed. Installing these discs on the wrong side of the vehicle can cause a dramatic reduction in their cooling ability and can reduce pad and disc life by as much as 75% of what would normally be expected. The disc shown in fig.11 is being fitted to a C5 Corvette, this is a directional, slotted and drilled disc, and it doesn’t get much better than this.

Fig 11.----
If the discs mentioned above are not up to solving your braking problems, you may need to think about upgrading the size of your brake discs. This does, however, lead us to a whole new set of problems. Whilst larger discs will help keep the brakes cooler, you may have to remount the calipers or use non-stock calipers. Also if the discs are considerably larger than stock they may not fit inside your wheels and so these will have to be replaced as well. Keep in mind this will also mean new tyres.

Finally a word on fluid fade. If after all of this you find you are suffering from fluid fade, don’t despair, you have some options. Regularly changing your brake fluid every three to four years is a good idea. Newer fluid is less likely to boil than old. Of course your fluid may require changing more often if you have a particularly high performance vehicle. Alternatively you could switch to using a high grade racing type fluid; these have a higher boiling point than regular fluids. You could also try changing your pads before the friction material gets exceptionally thin. The idea here is that the extra thickness of material will act as insulation between the pad and the caliper. Finally there are some companies that supply a thermal barrier spray that can be applied to the back of the pad for extra insulation, although we wouldn’t like to say just how effective these actually are.

It should be remembered that overheating in a braking system does not always mean that the system is not up to scratch. There is always the chance that there are other contributing factors leading to the over heating problems. It is possible that there is a partial brake caliper failure and that caliper is binding on to the disc. Obviously a similar binding could also take place in the wheel cylinders of a drum brake system. This binding is particularly common in systems with a floating caliper. Another item to check is the brake hose. It has been mentioned in our other articles that these can collapse internally and cause both caliper and drum brake systems to bind.


-------------------Fig12.

So remember that whether you are fitting stock or performance pads or shoes it is always worth checking the actual braking surface of your discs or drums. You should look for any sign of grooves or what we would call ‘tram lines’, if present your new pads or shoes will wear out extremely fast. A worst case scenario will have the friction material cracking because of uneven pressure. Also check the braking surface for any signs of discolouring, what is known as bluing, or cracks. If either is present it is a sure sign that the disc or drum has been exposed to some very high temperatures. Cracking and discolouration can also be a sign of a very old disc or drum, either way you should seriously consider immediate replacement. The disc in fig.12 shows signs of both discolouration and tram lines.

You have no excuses now. If your brakes are not working the way you want them to, hopefully this will have pointed you in the right direction. Try a brake pad upgrade if that’s not enough you could try discs as well. If you are driving a vehicle that still has drums all round you might even consider a disc brake conversion. Front initially and possibly converting the rear as well at a later date. Finally, if after reading this you’re still not sure which way to go, ask. Ask for advice from your local club or parts supplier. The chances are they will be able to help, or at least know someone who can. So as far as stopping is concerned, now could be the time to get going with that brake upgrade. Just remember when you’re traveling at warp factor 8 Scotty is no longer going to be there to help slow you down.