This is the final segment in our look at drag race brake systems (much of the information presented in the series can be applied to other forms of racing too). In past portions of the series, we looked at rotors, calipers, hats, mounts and hubs. This time around, we’ll wrap up the brake dance investigation with a focus upon brake pads and brake fluid. You might be surprised at what you discover. Check it out. This stuff is important.
You could write a book about brake pad or lining material (and few if any folks would actually read it – L-O-L). Friction material is normally mounted on a brake pad backing plate (most often constructed from steel). The friction material is physically bonded to the pad that rests upon the caliper pistons. In some OEM brakes, an anti-squeak shim is positioned between the pad and the piston. In some race pads, an insulator is placed between the pad(s) and the piston(s) in an effort to reduce brake fluid and piston seal temperature.
So which pad compound is best? Different companies have different theories, but remember, something like a drag race pad starts cold and ends up hot. This places special demands upon the pads, since there is no “warm up lap” to bring the brakes to operating temperature. Carbon/Carbon brakes in drag racing need to have some heat to increase the “on the line” holding power. Most competitors have learned they have to apply a light pressure on the brake during the burn out to heat the Carbon pads and discs. What’s needed is a pad with a very high coefficient of friction. The pad also has to work with the rotor material you’ve selected for your car (iron, steel or Carbon). In certain cases (a good example is a Funny Car or Top Fuel Dragster) with considerable top speed capability, there is a need for pads that can live under severe braking forces. The pads can reach extreme temperatures when slowing down from a pass. Obviously, the pad material must be able to withstand these temperatures.
Some brake pads need to be “bedded” before use (not all pads require this process). According to some manufacturers, this “conditions” the pads and rotor surfaces. This bedding in procedure will result in greater performance, longer wear and less likelihood of rotor cracking due to thermal shock. Just how are pads “bedded in”? Several manufacturers offer this advice (keep in mind, it’s geared for all sorts of motorsports): “to bed in brake pads, start by pumping your brakes at a very low speed to assure proper brake system operation. On the racetrack, make a series of hard stops from moderate speeds until some brake fade is experienced. Allow brakes to cool while driving at moderate speeds, avoiding use of the brakes. Your brake pads should now be properly bedded in.”
In the end, the choice of pad compounds depends upon the car, the brake system and of course, the type of racing you’re actually doing. This is one of those cases where you should consult with your brake component manufacturer to determine which compound is best suited for your application.
Brake Fluid…Want to open a can of worms with a group of racers? Start talking brake fluid. What? Brake fluid is brake fluid. Right? Not exactly. Over the past several years, there have been many advances made in brake fluid — not the least of which are the silicone-based fluids. Unfortunately the fluid situation isn’t as simple as swapping one type of fluid for another.
For some time, the Department of Transportation (DOT) has issued a set of stringent specifications for brake fluid types. The more common glycol-based or “conventional” fluids fall under DOT 3 and DOT 4 specifications. DOT 3 has a minimum dry boiling point of 401 degrees F while DOT 4 has a minimum dry boiling point of 446 degrees F. On the other hand, silicone fluids fall under DOT 5 that has a dry boiling point of 500 degrees F minimum. Something to watch for is this (and don’t get this confused with DOT 5): Another DOT fluid is 5.1. This is a Glycol based fluid that has a dry boiling point of 518 degrees F. In a typical passenger car, the conventional fluid boiling point diminishes with time. How long does it take for this to happen? In most passenger cars that are operate under damp conditions there can be a rapid deterioration of the brake fluid boiling point in as little as six months. Obviously, something like a show car isn’t used in the rain, but it does go to show that brake fluid definitely is affected by time and weather. That’s why some aftermarket brake manufacturers recommend you flush the system with fresh fluid on a regular basis (as an example, some road racers change fluid after each event!).
So what’s the big deal in regard to brake fluid boiling points? In reality, one of the most critical factors in regard to a hydraulic brake system is the dry and wet boiling point. If the fluid boils, small gas bubbles occur. These bubbles don’t disappear. Instead, they’re trapped in the system. Since gas bubbles are compressible, then the brake pedal becomes spongy.
Normally, the brake system will function with a percentage of moisture in the system. Over time, this moisture content increases and as a result, brake performance decreases. That’s why “wet boiling point” specifications are also provided by the Department of Transportation. These “wet boiling points” are outlined as follows:
DOT 3: 284 degrees F
DOT 4: 311 degrees F
DOT 5: 356 degrees F
DOT 5.1: 375 degrees F
As you can easily see, the wet boiling point of silicone brake fluid is significantly higher than it’s glycol-based counterparts. That’s because silicone fluid does not absorb moisture while glycol-based brake fluid does. When you take a look at a can of conventional glycol-based brake fluid, you’ll almost always find a warning with regard to using the fluid immediately. The reason, of course, is moisture absorption. Once the seal is broken, then moisture will attack the fluid in the can.
Further to the decrease in brake efficiency when the fluid is intermixed with moisture, the water found in the system can attack a number of internal brake components. Things like disc brake pistons can corrode beyond repair, which in turn can create both a dangerous situation and an expensive repair bill. On a similar note, it’s always a good idea to use the same brand of brake fluid in the system (or the brand you’re planning to use) as a lubricant when assembling brake calipers and assorted brake components.
So why don’t the OEM manufacturers make the big switch to silicone-based brake fluids? Silicone brake fluids have some admirable qualities: They don’t absorb moisture and they don’t harm paint finishes if spilled. Unfortunately they aren’t perfect. One problem is slight compressibility under high temperatures. If the brake system is exposed to very high temperatures, the silicone fluid can compress slightly with the end result being a spongy pedal. In addition, silicone fluid is affected by atmospheric pressure. When a silicone brake fluid-equipped vehicle is driven in high altitude conditions, the fluid can expand significantly — again contributing to a spongy pedal. Further to this, many of the rubber components used in brake systems are manufactured from “ethylene propylene rubber”. Some silicone fluids are not compatible with the “EPR” (it causes the rubber components to expand).
So what’s the answer? Always use a brake fluid that is recommended by the brake manufacturer. Never mix brake fluid brands or types and make every effort possible to keep contaminants out of the brake fluid (as well as the brake system). Once the protective seal is broken on a can of brake fluid, use it immediately. Never re-use brake fluid that has been bled from a system. It’s a disposable item. Silicone brake fluid can be used in cars that are driven very little. Finally, to maintain the health of your system, try flushing out the old glycol fluid in your race car braking system on a regular basis — replacing it with fresh DOT 3 or DOT 4 fluid. Your brake pedal will be much happier.
Source: Brake Dance – 4
Mark Williams Enterprises
765 South Pierce Avenue
Louisville, Colorado 80027
About the author: Wayne Scraba is a racecar junky. His background includes operation of his own speed shop, building racecars (some you might be familiar with, but we won’t get into it here), building hot rod street machines; he’s built custom motorcycles, restored muscle cars and played with off road bikes. Scraba has a sundry background in writing too: He’s toiled as a magazine Editor, a technical Editor, a free-lance magazine contributor and authored five automotive books. His byline (well over 4,500 articles worth) has appeared in well over sixty different high performance automotive, motorcycle and aviation magazines worldwide. Racecar junky? Most likely!