|02-10-2005 06:52 PM|
|02-10-2005 06:03 PM|
Understanding brake pad compound to fit your application
Here is a good read copied directly from GRMotorsports
Increasing your braking performance is one of the easiest-and cheapest-ways to lower lap times. By shortening your braking distances, you can stay on the power longer and drive deeper into the corners. This may sound academic, but too many racers take brakes for granted and thus neglect one of the most important aspects of their car's performance. On the track, as well as on the street, good brakes can also be the difference between life and death.
One of the easiest ways to increase your braking potential is to select the right compounds for your brake pads and shoes. Spending lots of time and money retrofitting trick, aftermarket big brakes could be a waste if you haven't first maximized your current brake setup.
Proper pad and shoe selection can make the difference between being a back marker and being a contender. Carbotech Engineering's Larry Narcus-a chemist with 30 years experience in plastics who has since decided to devote his energies to manufacturing better brake pads and shoes-tells of a customer with an Improved Touring Corvair came to him looking for some shoes. Now, if you're familiar with Corvairs, then you know that they have big drum brakes at all four corners. Not exactly high-tech, but according to Larry, they can be made to stop well with the proper shoe selection. Next time out, with the proper shoes in place, the Corvair driver dropped six seconds off his lap times. Think about that: six seconds. How much would you pay to drop that much from your lap times?
Not All Compounds are Created Equal
What's the secret to choosing the right pad or shoe? Obtaining a basic understanding of your needs and then matching them to the proper compounds. It's also useful to listen to the experts (hopefully, the people selling the stuff).
Brake pads and shoes, like a lot of things in this world, are available in a large number of flavors. Some are good, and others are so-so. Understanding this will make life easier. Let's take a look at what kinds of compounds are available:
· Organic: These are the $6.99 pads that you can get at the local parts shop; essentially, you get what you pay for. Organic pads are made up of compressed wood and/or paper, and perhaps some low-grade metal has been thrown in. Their friction coefficient (how well they grip against the rotor or drum surface) is very low, and they can't handle the high temperatures associated with any sort of performance driving. Basically, these pads not appropriate for competition use.
· Semi-metallic: These are definitely a step in the right direction. Expect to find much more metal in the mix (probably iron, but maybe brass or bronze) along with a better binder (what holds the pad together). The friction coefficient is higher (meaning the pads will provide more bite against the rotor) and they will wear better. Semi-metallic pads and shoes are the basic $30/pair-type found down the street. Usually a better choice over organic compounds, but you could do better.
· Carbon-based metallic: This is the good stuff that we'll be talking about today. The price goes up a bit, but performance rises many times over. Carbon and various metals (iron, nickel, brass and bronze) are thrown into the mix, and a better high-temperature epoxy binder holds everything together. The friction coefficient is much higher than semi-metallic pads, and in general they are also less dusty. Carbon-based metallic pads are offered by several companies in many different compound mixes. Hawk, Performance Friction, Porterfield, Cool Carbon and Wilwood (Polymatrix) all offer quality carbon-based metallic pads. Besides selling brake pads, Carbotech and TS Imported can also reline the backing plates from drum brakes with these modern materials.
· Carbon-carbon: Odds are you don't have these brakes (carbon-fiber rotors with carbon-fiber pads), unless you run in FIA races in Europe.
Choosing a Compound
Even carbon-based metallic pads are available in different compounds; choosing the right one may require some research and soul-searching. Basically, you need to match the right pad to your application. If you're worried that no one has pads or shoes for your oddball application, rest assured that some suppliers can do custom installations or reline your existing backing plates.
But how do you know which pad is right for you? Well, since no pad can do it all, you need to look at several different areas of concern when shopping for brake pads and shoes. By discussing these concerns with the different suppliers, you should be able to match your needs to the right product.
· Your first concern should be stopping power. This is measured by a friction coefficient, and the higher the coefficient, the more stopping power available from that compound. However, compounds with very high friction coefficients tend to be hard on rotors and drums, which may not be desirable in some situations (like on the street or in an endurance race). Like most things in life (brake pads included), it's a trade-off where you must look at the whole picture and decide which concerns are most important for you and your application.
· Modulation, the lack of the tendency for the brakes to lock up, is also a high concern for most of us. Modulation helps control the car, as locking up the brakes can put you in a spin. Again, different racers need different degrees of modulation. An autocrosser, to whom every hundredth of a second is valuable, simply cannot afford any lockup. Therefore, lots of modulation is important to him. An endurance racer, on the other hand, can deal with less modulation if it means getting longer pad life.
· Fade resistance is another big factor. Every compound has a temperature range in which it likes to operate; matching the compound to your brake temperature range is crucial. Most racing pads have a 600 to 800 degree window for their operating temperature, while brake temperatures generated by street driving tend to be much lower. So there is no way a great road race pad designed to work under high temperatures is going to stop well on the street. Nevertheless, many people seem to labor under a major misconception as they buy "racing" brake pads for their street cars and, when these pads fail to meet their expectations, they feel cheated. Be honest with yourself and buy wisely.
Likewise, a heavier car is going to generate more heat than a lighter car. By working with your vendor, you should be able to match your driving conditions to the proper compound. Tell them what conditions you race under, what tracks you visit and how hard you are on the brakes.
· Rotor and drum friendliness can also be a factor to some drivers, especially those with older cars. Some compounds, while they'll stop you on a dime, can be very hard on rotor and drum surfaces. If you race a car for which these replacement parts are hard to find, like a vintage Bugatti for example, then having friendly pads and shoes may be a prime concern-perhaps even more important than absolute stopping power. Likewise, a set of pads that will chew up a set of rotors every three hours will be of little use to a racer about to run a four-hour enduro.
· For a lot of us, economics also need to be addressed. However, don't simply look at the final price when shopping for new pads and shoes. If a pair of brake pads costs twice as much as the competition but lasts three times as long, which is the better deal?
· While not a huge concern for everybody, brake dust and squeal can be a factor for some people. If neglected, dust can attack the finish on a set of wheels, quickly turning a prized set of rims into an eyesore. At the extreme end, hot flakes from the brake pads can land on the fenders of a car, singing the paint. Now, a road racer may be willing to pay that price, whereas a street driver may not.
For a lot of people who only drive on the street, squealing brakes can be a major inconvenience. If you don't like being tortured by a set of wailing brake pads, move this consideration to near the top of your list.
· The rate of wear is also something to think about, although it is related to many of the factors we already discussed. But to bring back our endurance driver, it may be the most important factor when selecting new shoes and pads. Again, be sure to discuss this factor with the guys selling the goods.
Speaking of wear, some companies offer slightly thicker pads designed for endurance racing. As a side benefit, these thicker pads seem to handle heat better and provide better braking characteristics. (The more pad material, the bigger the heat sink). While extra-thick pads may seem like the answer to everyone's problems, there is a limit-make the pads too thick, and you'll need thinner rotors, which is not a good move.
Matching the Pad to Your Application
The biggest part of buying brake pads is matching the compound to your application. So, let's take a look at some common applications and what kind of braking compound characteristics they require.
· Though they may seem different at first, autocrossers can require the same compounds as high-performance street drivers. Both need effective cold stopping power and lots of modulation.
In autocross, there is no time to bring the brakes up to the temperatures required by many "racing" brake pads. You need full stopping power right off the line, and you need it right away. Likewise, when out on the street, you also have no time to wait for brakes to heat up, and here the stakes are a lot higher. On the street, your brakes have a lot of time to cool off between stops, so it is important to pick a pad that will be happy to work while cool. For both applications, look for compounds designed to work in the 100- to 800-degree range.
Modulation is also important, both when autocrossing and on the street. It's the kiss of death for an autocrosser to lock up a brake. A skid means lost time, which means the run is junk. With only three or four runs at an event, that can be the end of any chances of a trophy. On a road course, a racer can usually overcome or bounce back from a skid on the same lap; autocrossers, by the nature of their sport, don't have that luxury.
Cost and a lack of audible squeal can also be factors, depending upon the individual. Some of us are also willing to live with more dust than others.
· Track events and drivers schools are gaining popularity; these drivers also need to select the right compounds for their needs. Generally, they will be looking for good modulation and lots of stopping power. The main difference between them and the autocrosser is the temperature range of their brakes. Generally, and depending upon the car, driver and track, they'll need to select a compound that works in the 300- to 1100-degree range.
These pads and shoes require some heat to work properly, so you should strongly consider swapping to these "track pads" before heading out on course-just as you mount sticky tires before an event.
· Road racers, whether they be sedan or formula car drivers, also have to look at more at fade resistance. Stopping power and modulation is still very important, but fade resistance throws a new factor into the equation. This will require a pad that can work in the 400- to 1400-degree range, depending on track, car type, length of event, level of brake cooling and driving style.
As road racing conditions can change from event to event, you may want to consider using different pads for varying conditions. Remember that a track like Sebring is a lot harder on brakes than Roebling Road.
Weather conditions can also cause a change in braking compound requirements. In the rain, you may need more modulation than all-out stopping power. Racers are eager to change tires when faced with rain, but why not brake pads? In some situations, like at last year's rainy ARRC, rain and its effect on stopping power is a deciding factor for many drivers.
Economics and rotor/drum friendliness can also be factors for road racers, especially those running endurance events or driving older cars for which replacement brake parts are getting harder to find. As we discussed before, rotor and drum friendliness may be top priority for a vintage car racer.
I hope this will help some people to understand and help resolve all of the brake pad questions.