I Want You - to Understand Brake Performance
By Larry Carley, Andrew Markel
Adapted from Larry Carley's and Gene Markel's article in Brake & Front End
How Uncle Sam Sets Braking Standards.
Everybody wants safe brakes, right? You want the assurance that any brake linings you install on a customers vehicle will provide adequate braking and meet all applicable safety standards. But guess what? There are no federal safety standards for aftermarket brake linings. Federal Motor Vehicle Safety Standards (FMVSS) 105 and 135, which are issued by the National Highway Traffic & Safety Administration/Department of Transportation (NHTSA/DOT) apply to new vehicles only. They do not apply to aftermarket replacement brake linings. So technically, aftermarket brake linings are unregulated and do not have to meet the same FMVSS standards as OEM brake linings.
How will this affect you?
The FMVSS standards are designed to assure new vehicles are capable of stopping within a certain distance deemed necessary for safe driving. FMVSS 135 is the current standard and applies to 2000 and newer cars, and 2002 and newer light trucks. Compared to the earlier FMVSS 105 standard, FMVSS 135 requires roughly a 25% reduction in pedal effort for the same stopping distance.
FMVSS 135 says all vehicles under 10,000 lbs. gross vehicle weight (GVW), except motorcycles, must be capable of stopping within a distance of no more than 230 feet (70 meters) from 62 mph (100 km/h) with cold brakes (under 212 F or 100 C) and with no more pedal effort than 368 ft. lbs. (500 N).
In July 2005, these same requirements were extended to trucks and buses weighing more than 10,000 lbs. Formerly, only school buses had to meet the same stopping requirements as passenger cars and light trucks.
The FMVSS 135 standard also specifies a required stopping distance for vehicles should the power brakes fail (no power assist), or if one of the two hydraulic circuits fail. Under these conditions, the maximum stopping distance from 62 mph (100 km/h) is not to exceed 551 feet (168 meters) with a maximum pedal effort of no more than 368 ft. lbs. (500 N).
FMVSS 135 also has a stopping requirement in the event of an anti-lock brake (ABS) system failure. The rules require the stopping distance not to exceed 279 feet (85 meters) with a maximum pedal effort of no more than 368 ft. lbs. (500 N).
There is also a hot performance stopping requirement for fade resistance. With the brakes hot, the maximum stopping distance for the second of two back-to-back panic stops is not to exceed 292 ft. (89 meters) with the same pedal effort as before (368 ft. lbs. or 500 N).
The parking brakes are also covered by FMVSS 135. The rules specify conditions under which the parking brake must be able to hold the vehicle on both an uphill and downhill incline.
The Brake Police
With no federal regulations, aftermarket brake suppliers have to police themselves and each other to assure their products are safe. No brake supplier in their right mind would sell brake linings they know are not capable of providing adequate stopping power under normal driving conditions. Even so, whats adequate is subject to interpretation, and some suppliers take a more liberal view of the bottom line requirements than others.
If you want to maintain like new brake performance, you should be installing application engineered or premium brake linings that have been tested and certified to meet standards similar to FMVSS 105 and 135. The aftermarket currently uses two such test standards: BEEP and D3EA.
In the Shop
All of this means that as a future technician, you need to pay close attention to the brake work youre doing and the kind of replacement linings and other parts you are installing on your customers vehicles. The hydraulic brakes and friction linings on todays vehicles are closely integrated with the anti-lock brake, traction control and/or stability control system on the vehicle. Consequently, any change that significantly alters the hot and cold friction characteristics of the linings has the potential of upsetting not only braking performance, but also the operation of the ABS, traction control and stability control systems.
Given the fact that aftermarket brake linings are essentially unregulated, how can you be sure the linings you are installing on your customers vehicles meet these criteria? The best advice here is to buy name-brand products from supplies who stand behind their products.
Real World Results
For the most part, the quality and performance of most aftermarket brake linings is not in question. BEEP and D3EA testing are accomplishing what they are designed to do, and virtually any name-brand brake lining that meets these criteria will provide safe braking and deliver performance that is similar to or even better than the OEM brakes on your customers vehicles. But there are some concerns with no-name offshore suppliers who are selling products that are of questionable quality or contain asbestos.
Many third world countries are not overly concerned about brake safety, the quality of their products or what kind of ingredients are used in their brake linings. They just want to sell their products at the lowest possible price. Asbestos disappeared from U.S-made brake linings years ago out of concerns over potential health hazards and litigation. But asbestos is still used in many brake linings outside the U.S., and some of these asbestos-laden brake shoes and pads are still being imported into this country.
Some say the only way to guarantee aftermarket brake linings meet the same safety requirements as new vehicles is to have FMVSS 105 and 135 apply to the aftermarket too. The brake manufacturers do not want to see that happen because it would require a lot of expensive vehicle testing. Imagine the cost of having to test brake linings for every year, make and model of vehicle in an entire product line, and how that cost would have to be passed along the distribution chain to you and your customers. It could increase the cost of brake linings significantly. Thats why the brake manufacturers prefer to use voluntary test procedures, such as BEEP or D3EA, to evaluate their aftermarket products.
Fortunately, no federal regulations are in the works for aftermarket brake linings so the brake manufacturers will continue to regulate themselves and each other to make sure their products are safe and deliver like-new or better-than-new performance. The bottom line, after all, is to have a happy brake customer.
It is often said by technicians in the field that if an engineer was placed in the field for a day he would change the way he designs cars. But, if you reverse the statement and have the technician design and engineer a new car, it would change the way technicians fix cars.
Trying to see a brake system through the eyes of the OEM engineer can make you a better technician. Just like any technician, the OEM engineer has to work under certain constraints to achieve a goal that is laid out in quantifiable performance objectives, like meeting FMVSS and internal standards. Understanding how they go about this can help to make you a better technician.
Before you pitch the pads and rotors, it would pay to take a look at the evidence and history. Think of yourself as a CSI (Crime Scene Investigator) looking for clues. What was the surface condition of the friction material? Was there any type of shim or other material on the back of the pad? If the rotors were replaced, why? In todays market, it is worth it to take some digital pictures and collect some information from the customer. Like, how many times have the pads been replaced.
An engineers version of CSI is FMEA (Failure Modes and Effects Analysis). It is the if then analysis. This means that they walk through all possible scenarios. For example, if the engine stalls, you have the brakes and steering to bring the vehicle to a safe stop, even if it is a 110 lbs. weakling behind the wheel.
How OEMs Meet Government Standards
Brakes are the opposite of an engine. Brakes take the motion energy created by the fuel and turns it into friction heat energy to stop the vehicle. If a vehicle has 200 horse power at the wheels, then it should have brakes capable of the same capacity to stop the vehicle. The bigger the engine, the bigger the brakes if you are going to stop as fast as you can go. Just as engine size and performance enhancements, such as supercharging, determine the amount of power to the wheels, the drum and rotor diameter and swept area determine the stopping power.
The vehicles ability to stop and go is greatly influenced by the wheels, tires and suspension, but that is about tire construction and chassis design. But there is one CSI/FMEA to consider and that is wheel and tire diameter. If a replacement wheel and tire assembly is significantly larger than the original factory set, it can reduce the stopping power of the brake. The distance from the ground to the center of the wheel increases the leverage on the brake. This results in an increase in the stopping distances and possible thermal damage to the pads and rotors.
This is about where the friction material meets the rotor or drum. Friction material is like fuel. It creates the heat energy and is designed to use it up just like fuel. In relative terms, friction material is inexpensive in comparison to rotors and calipers. The base material for a pad or lining should have the following qualities: thermally stable, insulates, good wear characteristics, easy to process and has a reasonable cost.
There are many recipes for friction material, just as there are recipes for baked goods. There are hundreds of ingredients to make bread, and there are more than 2,000 different materials, from the common to the exotic, to make friction material. Limitless combinations can be used to produce a pad or lining to meet specifications for noise, coefficient of friction, fade, pad wear, rotor wear, compressibility and operating temperatures. There are no set standards for the quantity and types of material to produce a given specification.
Structural fibers maintain the strength of the pad or lining. These fibers include asbestos and ceramic fibers. Binders are what hold the pad or lining material together. The most common are usually synthetic resins derived from hydro-carbons known as a polymers. The phenolic resin is the most common resin used in pads and linings. Phenolic resins are derived from the hydro-carbon, benzene.
|Did You Know
Compounding pads is sometimes referred to as a black art. But, every pad is based on three or four basic ingredients that include: binding materials, abrasive, lubricants and structural fibers.
Some binders are as exotic as the resins from cashew nut shells. Each resin has a set of characteristics that can change the performance of a given type of braking system.
The concentration of this resin can change the performance of the pad or linings material strength and coefficient of friction. The amount of heat generated during braking has the greatest affect of the binding resin. Fade occurs when the heat generated causes the resin to melt and reduce the friction between the pad and rotor. Overheating of the friction material to have a glazed surface. Extreme overheating can cause the friction material to crack.
Abrasives are the materials that help to clean the rotor surface and reduce the glazing of the pads or lining. They also increase the friction when the brakes are first applied. The most common used today are aluminum oxide, iron oxides, quartz, silica and zirconium silicate.
Friction modifiers are what make the construction of pads and linings an art. These materials can raise the friction and react with the oxygen in the air to reduce glassing of the rotor and pad. These can be as simple as metal chips or complex as ceramic microspheres. The majority of the materials are metal sulfides and oxides. Fillers are as exotic as cashew nut shells and powdered iron or as simple as common as scrap rubber.
Mixing the right components in the right quantities can greatly influence if the vehicle will meet FMVSS 105 and 135. But, the engineer has other concerns besides FMVSS standards. He also has to strike a balance between material and warranty costs. This is why some of the best compounders are treated like gods by their employers. Why? Because they can save their employers millions of dollars by eliminating trial and error with their years of experience.
|The most important point to keep in mind with respect to the FMVSS rules is that the rules are a performance standard and are based on how well the vehicle can actually stop. The rules do not specify what kind of brake linings, rotors or calipers the vehicle must be equipped with, or any friction, fade, noise or wear characteristics for the brakes. The only requirement is that the vehicle is able to stop within the specified distance under the specified conditions. Period.|
In that respect, compliance is fairly easy to verify. Either the vehicle stops within the specified distance or it doesnt. But this requires field testing each and every make and model of vehicle to make sure they meet the standards. The vehicle manufacturers do their own testing and certification, and NHTSA reviews the results to make sure the vehicle manufacturers are complying with the rules.
In the case of the OE manufacturer, a lot of time and money are spent to produce a brake pad or lining to meet FMVSS, plus warranty and performance specifications, per a specific vehicle. If the bumper-to-bumper warranty is 36,000 miles, those pads and linings are designed to last through the warranty period. A quality brake job should meet or exceed those warranty standards. If the vehicle has specified stopping distances, check the owners manual, the vehicle should perform as stated in a court of law.