Shades of Grey

Shades of Grey

By Thomas Hall, Technical Contributor

As we experience more and more life situations, we generally evolve from very black and white thinkers to accepting that most situations have two sides or competing priorities.

One of the most common complaints about a vehicle’s brake system is the propensity to develop really ugly brake dust. Walk through any shopping mall parking lot and you can see many examples of nice wheels that are just black with dust. Sometimes it will be all four wheels and sometimes it might only be the front or rear wheels. How can it be so different from vehicle to vehicle and even on the same vehicle?

The significance of brake dust on customer perceptions has grown over the years. I would suspect part of this is the result of the prominence the styling of wheels has achieved. Drop a couple thousand dollars on a nice set of “double deuces” (22-inch rims) and have to spend every weekend with the most wicked cleaner you can find to keep them shining and us brake guys will quickly be in the cross hairs.

One measure of a vehicle’s dust propensity is now available in some of the New Car Quality Surveys. A few months after you purchase your new vehicle, you will likely get a survey asking about your impressions. Among the many questions is a specific question asking about brake dust.

The public has the chance to rate us. This data is collected and then published. Recent data on complaints for brake dust on light-duty pickups shows a range of 97 complaint/ 1,000vehicles to as low a 1/1,000 vehicles. Clearly some vehicles have this issue and some do not. So what is at the root of the problem? It would be simple to say that brake dust is just the dust generated by the brake pad as it wears. The more the wear, the more the dust. Unfortunately, like most things it’s not that simple. A chemical analysis of the dust itself can provide some clues. Using our best CSI techniques, we can determine the chemical constituents of the dust and then try to determine what component is the source of those constituents.

A couple of techniques are commonly used for this type of process. The simplest is a magnet. Drag a magnet through the sample and see what sticks. This will establish if there is Ferric content (iron/ steel). Generally, there is not a strong iron/steel content. However, in many cases this does not account for all the content nor does it determine the composition of the iron.

For greater understanding, a more sophisticated technique is used. A combination of techniques, such as x-ray diffraction and energy dispersive analysis can determine the elemental and primary compound construction of a sample. In very simple terms, these techniques bombard the sample with x-rays and electron beams and measure how the ray’s scatter when they hit the various materials in the sample. Due to the unique molecular structure of each compound, they will have a unique spray pattern. By comparing the results of the unknown samples to a library catalog of known samples, the composition can be determined. Two samples of brake dust are compared in the table below. While the elemental content is similar, the amount of each material is significantly different. In sample 1, the content is predominantly iron (Fe).

In sample 2, while iron is present it is in much smaller quantities and the primary content is salt based (sodium, calcium, chloride etc). Therefore it can be determined that the source of the dust is different. Neither pad material contains significant amounts of the salts or iron. Therefore to say that brake dust is just worn pads can not be confirmed.

The primary source of iron is the rotor. The iron is worn off the rotor during braking and remains as dust. The strong black color of the rotor can be explained by further analysis that would establish that the iron is in the form of Fe3O4. This is an Oxide of Iron and is more commonly known as “black rust.” Sample 2 shows a strong salt content and would suggest a significant presence of road debris, and more specifically road salt.

What I haven’t told you is that Sample 2 was generated during a vehicle exposure period of Dec – March in Michigan. Knowing this, the presence of a high salt content is understandable.

While all this is interesting (humor me), at the retail level dust is dust and the customer is disappointed and wants it fixed. Is there anything we can do about this at either the design stage or the service repair stage? In general the black dust is the result of rotor wear.

While the hardness of the rotor could be considered a factor, virtually all rotors come from very similar grades of iron and therefore have hardness that is quite similar. The abrasiveness of pads however can vary widely and it can be shown that certain pad materials will generate more dust than others.

Generally, high performance pads will have a stronger tendency to generate dust. European classes of friction materials tend to fall into this categories.

Brake dust on a BMW or Mercedes would be considered relatively common and in one case a “badge of honor” to a driver that participated in a customer clinic. This is an event where you ask a bunch of car owners to come in and answer some questions about their perceptions, preferences, likes and dislikes.

The Q/A is conducted by professional marketers that know how to ask questions in a way that doesn’t bias the answer. Our focus group was buyers that would generally consider American luxury cars. One of the questions posed asked about the reaction people had to brake dust.

Most people complained vigorously about dust and the effort required to clean it. Some thought the dust was an indication that something was wrong with the brakes, or an indication that the brakes were wearing out very quickly.

I recall one older lady that lit up when posed with this questioning. She had previously stated that she was not satisfied with the brakes on her American luxury car. She had not been asked to explain.

She stated in a very strong German accent that she had owned German motor cars for a great many years and she knew that her brakes were working hard because she could see the dust on her wheels right from the start. To this lady, brake dust was a sign of German engineering and was actually a positive indicator of the ruggedness of her brakes. While her opinions may not be widely held, it clearly shows that people can interpret the same thing in very different ways.

In the design domain, dust is one of the parameters of friction material selection that must be balanced in consideration with other factors, such as lining output and fade performance. The general rule is that higher output and improved fade sensitivity will increase the propensity for dust.

A vehicle manufacturer must make a judgment on the relative balance of these factors for their market segment. In the service environment, the shop owner tasked with solving the brake dust issue is kind of stuck in the middle. If you just put a set of factory replacement pads on the vehicle, the dust will surely comeback.

For vehicles with a strong history and reputation for dust, it is important to be up front with the owner at the time of writeup. Are they there because of the dust condition? Do they expect the condition to go away with the repair? Would they accept some small changes in brake performance in exchange for a reduction in the dust performance?

Helping them understand some of the fundamentals of dust may help alleviate some of their concerns. This isn’t to suggest that dust should be ignored. Excessive dust can be a strong indication that the linings and rotors are significantly worn.

The shop owner’s most effective solution for dust will be to offer an alternative friction material. Generally, if a material has performed well for dust on one vehicle, that performance will transfer to another vehicle. That’s not to suggest though that all braking performance attributes will transfer. It is still critical to use a material that is recommended for that vehicle or vehicle class.

By explaining the origins of the dust and that dust really has minimal bearing on the performance or safety of the brake system, you will hopefully reduce their concern from a safety concern to a nuisance concern.

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