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Brake Pad Wear

October 1, 2007

Brake pads are one of the few remaining accepted wear items in a modern vehicle. As a result, it represents a significant revenue opportunity for the service community.

Drivers would surely love to have brake pads last for life. Service shop owners may have a different opinion. I’m sure that shop owners must deal daily with the question from the customer regarding the life of the pads. How many miles should I get out of a set of pads? Why did they wear out so quickly? My buddy got at least 10,000 more miles on his brakes than I did?

What determines pad life and how can it be influenced? At its root, the amount of pad wear is established by the particular wear characteristics of the friction material (and rotor), the number of stops and how aggressively the vehicle is braked.

The pad wear rate is established by some standard tests. These tests are conducted on a brake dynamometer. The pad is first run through a burnish and then the brake is subjected to a series of stops at a range of temperatures. The test will run stops at 100ºC, 200ºC, 300ºC and 400ºC and then repeated at 100ºC. The wear rate at each temperature is established. Generally, the wear at the higher temperatures is significantly more than at lower temperatures.

Wear rate at 400ºC can easily be four or more times the lower temperatures. The chemistry of the pad determines the wear characteristic. Semi-metallic materials usually have lower wear rates and less wear sensitivity to temperature than NAO (Non Asbestos Organic) or low metallic classes of materials. On occasion, the brake wear rate can have a sweet spot in the temperature range and actually achieve the lowest wear at the mid-range temperatures.

To complicate matters, the other factor that determines the wear of a pad is the amount of energy it is exposed to. Pad chemistries are used on a broad range of vehicles. As a result, they will be exposed to vastly different energy levels even though the recipe is the same. As a result, trying to compare the wear from one vehicle to another is difficult at best.

At the engineering level, we attempt to standardize the analysis by imposing the pad shape, caliper, rotor and inertia of the vehicle. If all of these factors are consistent, then it is reasonable to compare wear rates between friction materials.

Once the material is selected for the vehicle, the operating temperature of the brake becomes the determinate of the ultimate wear. Operating temperature will be established by the overall thermal capacity of the brake system, the amount of cooling, the brake balance (how much work the fronts do over the rears) and the driving characteristic.

The wear performance of the vehicle is evaluated generally with a city traffic schedule. Popular schedules in the U.S. are Los Angeles and Phoenix. The tests are similar and have been a standard in the industry for a generation. A route through the metropolitan area is established and is generally about 200 miles/day. Normal drivers are used and no special technique is required. These cities are the popular choice due to their high traffic density (lots of stops) and their generally high ambient temperatures.

Typical tests have a duration of 5,000 miles (or 25 days) and generally run 1st shift during weekdays to encounter the typical driving conditions. Pads are measured at the beginning of test and end of test. The amount of actual wear is divided by the 5,000 miles and a wear rate is determined. For example, a pad that starts at 10 mm thickness and wears to 8 mm after 5,000 miles has a wear rate of 2,500 miles/mm. This 10mm pad would have a “Projected Life” of 25,000 miles.

Back to the question at the service counter: “How long should a pad last?” Like most things in life, the real answer is, “It depends.” A general rule of thumb is that a “normal driver” can expect an approximate pad life of about two times the LA or Phoenix number. Normal usually means somebody that doesn’t live in LA or Phoenix. Normal in these cities is about what the test predicts and brake life will generally be in the range of 12,000 to 20,000 miles.

Driving habits can vary widely from individual to individual. As a result, it is unlikely that two people driving the same route in the same car would get equivalent life out of the brake pads. To illustrate, I once had an experience where the pad life in one of these city traffic tests dropped by almost 50% about one year after the original test. Engineers generally try and apply the “5 WHYs” problem solving technique to determine the root cause of the change in behavior.

After evaluating close to 100 possible causes and after eliminating most, we found the average brake temperature during the second test was about 30ºC hotter than the first test. We know wear increases with temperature. (1st Why). Why did we run hotter? Which test was the normal and which was the abnormal? Ambient temperatures were consistent during the two tests (same month a year later).

We noticed the number of stops in Test 2 was about 40% higher. (2nd Why). Which is the typical answer and why such a difference? Looking at the daily test details we noticed on certain days there were about 500 stops/day and on others close to 1,000 stops/day. On Test 1, the variation day-to-day was between 400-600 stops. (3rd Why). Was traffic exceptionally bad on those days?

We decided to ask the drivers what they remembered. In doing so, we learned that the days with the large number of stops were all driven by the same person (4th Why). Last question, why does this person drive so differently than the other people.

Was this a response to traffic levels or was this the way the driver behaves naturally? When we asked the driver why they might have more stops per day then some of the counterparts, we learned the driver was new. The driver knew they were hired to “Test Brakes.” So this person wanted to make sure they were giving the brakes a “good workout.”

They were hitting the brakes where and whenever they could. (5th Why). To be double sure, we reviewed the driver that followed this person on those days. Sure enough, this driver also had a significant increase in stops on the particular day they followed this driver.

We used a 6th why to get to the real root cause. (Insufficient Driver Orientation and Monitoring). A confirmation test confirmed the original. Along the way, we had to convince ourselves we had data to rule out any other more commonly considered root causes. For example, the brake cooling had changed or the rear brakes had stopped working.

The traditional service provider will have a much more difficult time sorting out excessive wear problems. A few questions and some good vehicle analysis can often times identify a probable cause:

  1. What is the driver comparing his wear too? His buddy? The last set of pads he had? His last car?

  2. Have his driving habits change? Did he move? Get a new job on a different side of town? Has someone else been driving the car significantly?

  3. Has the vehicle usage changed? Any evidence of significant loading? What’s in the trunk or bed? Is their a trailer hitch? Is the ball clean or rusty?

  4. Confirm all brakes are functioning properly. Are the rear brakes working effectively? Any evidence of drag? Evidence of significant heat at the fronts or rears?

  5. Are other parts (tires, shocks, struts and steering) showing significant wear? How about the seat or the pad on the brake pedal?
I will share one more experience. I once received a complaint from a fleet owner that said he was getting much lower life now that he had changed vehicles used in his operation. I asked what his old life was and what he was seeing now. The answer was that pad life had dropped from about 1,000 miles to about 200 miles!!!!! Numbers like that get you a quick plane ticket. Turns out the vehicles are used in a strip mining operation. The vehicles accumulated about 250 miles a week. And once a week, it got a brake job. Astonished, I asked to watch the next job.

This facility was doing brake jobs like NASCAR pit stops. I noticed the mechanic using a large “persuader” to get the drum off. This suggested two things — significant drum wear and an improperly functioning adjustor. The mechanic confirmed the adjustor was frozen and would not back off.

Once the drum was beaten into submission and removed, we immediately noticed that the brake was full of highly abrasive grit from the mining operation. The combination of the frozen adjustor (in the over-adjust position) and the grit wore pads out at an excessive rate (five times what he considered reasonable for his environment). Some preventive maintenance with aggressive anti-seize coupled with a more aggressive adjuster design offered some relief.

Some carefully chosen discussions with the owner, combined with a detailed inspection of the vehicle to fill in the gaps and confirm the story can go a long way to finding a true root cause to the owner’s perception.

A little extra effort during write up and inspection can be crucial to insuring a long lasting and quality job. Think how convincing you can be if you even have answered only three Why’s for the owner.

 

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