Non-directional finish is a process to help mechanics ensure a quality rotor job is done. When a non-direction finish is not applied, it is not uncommon for a customer to experience poor stopping power or brake noise after leaving the shop. In fact, many times the rotors friction surface isn’t correct. As a result, poor contact is made with the brake pads. Likewise, the customer is likely to experience less stopping power and more brake noise.
Poor surface contact reduces the surface area available for creating friction. As a result, the brake pedal has to be pushed harder in order to stop the vehicle. Additionally, the poor surface contact can also cause the pads to vibrate while stopping, which causes the brake noise.
Non-Directional Finish: An Insurance Policy
Before beginning, it is important to note that performing a rotor service consists of more than a non-directional finish. In effect, non-directional finishes should be looked upon as an insurance policy. For example, it should enhance the rotor’s finish, NOT try and fix it. The following steps will go over the process of completing the rotors finish:
1. Resurface the Rotor
If you are resurfacing the rotor(s), use the proper machining techniques and make sure that the lathe is in good working condition. On hub-less rotors, make sure to clean the mating surfaces with the appropriate tool. Next, scratch cut the rotor(s) to ensure an accurate setup. Always use sharp bits and a vibration damper when machining.
2. Non-Directional Finish
After machining, apply a non-directional finish by using 120 grit drywall sandpaper on a rubber sanding block for 60 seconds per side. Apply the finish by applying moderate pressure. At the same time, use a slight rocking motion. (See Figure 31.1)
3. Clean the Rotor
Finally, clean the rotor before installing it onto the vehicle. This is done in order to prevent machining dust from contaminating the brake pads. To begin, using a mild soap and water solution, wash both friction surfaces and wipe everything dry with a clean, lint free rag. Likewise, if using brake cleaner, us more than usual and wipe the surface down with a clean lint free rag or paper towels while still wet. In contrast, never use petroleum based cleaners as they will leave a residue.
There are many theories and opinions about non-directional finish. Typically, they center around the reasons and methods of completing the process. In this case, if we start by defining what is meant, we can clarify the two areas. For example, most lathes produce a directional finish. In this case, directional means that the finish has a pattern to it. Usually, the pattern produced is a spiral similar to an old vinyl record. If too pronounced this spiral could cause brake noise or even a loss of stopping power.
On the other hand, a finish that is non-directional will not have a pattern or “direction” to it. One common myth is that the finish must be a swirl finish. This myth is born from some new rotors coming out of the box with a pronounced swirl finish. In reality, you don’t have to see the finish for it to be effective.
Non-Directional Finish Improves Results
With this understanding we can now discuss why and how to perform a non-directional finish. First, it is important to know that by correctly applying a finish with no direction the surface finish can be improved by up to 20%! As a result, the improved surface will provide a better mating surface for the friction material resulting in less chances for brake noise and reduced stopping power. And the best part? It only takes 2 minutes! That being said, it should be mentioned that most quality brake lathes produce a surface finish that falls well within acceptable limits. So, why bother applying a non-directional one? As stated above, performing this process may not always be necessary, but it is an insurance policy that only takes a few minutes to perform.
To Help, Not To Fix
At this point, it is important to emphasize that this process will not correct a poor surface finish. For example, if the base finish is substandard, you will gain little by applying a bit of sandpaper. In fact, proper surface finish is accomplished by a combination of a well maintained lathe, sharp cutting tips, and having the correct setup. Only after a quality base finish is obtained can the benefits of a non-directional finish be realized.
Don't Use an Angle Grinder
Contrary to popular belief, the angle grinder is not the best tool for the job. In fact, it is not even an acceptable method! Not one friction or rotor manufacturer, or any of the OEM’s endorse the use of an angle grinder as a method of applying non-directional finishes. This is because there are too many variables involved with angle grinders to perform the process correctly. Because the speed, type of disc, disc condition, time spent, and angle of the tool can be changed, results will always be inconsistent when using an angle grinder.
In figure 31.2 a rotor is shown that had an angle grinder used on it. The surface finish was measured before and after the application of the finish. Believe it or not, the surface finish was actually rougher after the finish was applied! For example, the smoothness of a rotor is measured in what is referred to as its RA factor. The lower the number, the better. The typical specification range is 15 to 80. On the rotor depicted above the RA started at 42, and ended at 45!
The image below shows a rotor having a finished applied via a sanding block with 120 grit. Similar to the above rotor, this one was also measured. In fact, before the finish was applied it had an RA factor of 42. After sanding the rotor, the measurement dropped down to 33. As a result, the RA changed by 9, or roughly 21%. The non-directional finish is applied by using a full contact rocking motion while applying moderate pressure to the sanding block.
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