Non-Directional Rotor Finishes

Oct 14, 2024

Back in the day, a non-directional rotor finish was the method used to solve a common problem that occurred on bench brake lathes. If the crossfeed speed was too fast, the rotor became like a vinyl record, and the pads became the needle that followed the grooves in the record. This would cause a clicking noise as the pads moved in the caliper as it followed the concentric grooves.

The solution was to apply a non-directional finish. A non-directional finish breaks up the grooves cut by the lathe. These are typically cut with a rotating abrasive disc that moves across the face of the rotor. The finish looks like cross-hatch marks on a honed cylinder. In the 1970s and 1980s, the pages of Brake & Front End had ads for lathes accessories to apply a non-directional finish.

Why was it such a big deal? The reality was that the crossfeed on some lathes was set to what was used for drum brakes. The faster setting made more pronounced concentric grooves on brake rotors. Typically, the solution was to reduce the crossfeed speed to reduce the grooves. Also, many floating caliper designs were not great at holding the brake pads steady in the bracket.

Today, non-directional finishes on new brake rotors still serve the same purpose, but they also help in the bedding of some friction formulations.

The surface finish of a new or resurfaced rotor should meet OEM specifications for good braking performance, pedal feel and quiet operation. Brand-new OEM rotors and aftermarket rotors from a quality supplier typically have a surface finish that can vary from 15 to 80 microinches. Most brake experts say the best finish is 50 microinches or less, though a finish in the 60- to 80-microinch range is acceptable.

When a rotor is turned on a brake lathe with sharp bits (we emphasize the word “sharp” because it is absolutely essential for a quality rotor finish) and a feed rate that is not too fast, the rotors will have a finish that meets these recommendations. Dull bits and fast feed rates tear chunks of metal from the rotor, instead of properly cutting it as they should.

If you turn your rotors with sharp bits and the proper feed rate and depth of cut, using a hone to apply a non-directional finish can help to reduce noise and shorten burnishing times.

As a final step, any rotor should be cleaned so metal debris, oil and anti-corrosion chemicals are removed from the braking surface. Not washing the rotors after they have been turned can leave a lot of junk on the surface that can embed in the pads and possibly cause braking issues, as well as noise when the rotors are installed.

Non-directional rotor finishes can be applied in a number of ways. One way is by using an abrasive disc in a drill or a special rotor refinishing brush. As with the sanding block, you want to give each side about one minute of sanding while the rotor is rotating on the lathe. Also, follow the manufacturer’s recommendation for rotational speeds. Another method is to hold a pair of sanding blocks wrapped with 120-grit sandpaper firmly against both sides of the rotor for about one minute while it turns on the lathe.

Sanding knocks off the sharp peaks on the surface of the rotor and generally improves the surface finish by 15 to 20.

A non-directional finish can reduce initial break-in noise and help suppress noise for a while; however, brake noise can still occur if there are vibrations between the pads and rotors.

Brake squeal is caused by undampened high-frequency vibrations. When the brakes are applied, and the pads contact the rotors, tiny surface irregularities in the rotors act like speed bumps, causing the pads to jump and skip as they rub against the rotors. If the pads are not dampened by shims (external or internal) or are loose in the caliper mounts, they shake and vibrate and may produce an annoying high-pitched squeal.

The vibration of the pads against the rotors can also create harmonic vibrations in the rotors that cause them to ring like cymbals. Depending on the metallurgy of the rotors and the design of the cooling fins, some rotors may ring louder than others, regardless of the type of surface finish.

So, even if you do everything right, you can still end up with a noise problem if the pads or rotors themselves are inherently noisy. Switching to a different brand of brake pads or substituting a different type of friction material may be necessary to get rid of the noise.

A tip for reducing noise-producing vibrations is to apply a high-temperature brake lubricant to the backs of the pads and the points where the pads contact the caliper. Lubricating the caliper mounts, shims and bushings is also recommended to dampen vibrations here, as the lubricant acts as a cushion. It also helps the parts slide smoothly so the pads wear evenly (uneven pad wear is a classic symptom of a floating caliper that is sticking and not centering itself over the rotor).

The type of rotors used on the vehicle can also affect noise. Some grades of cast iron are quieter than others. That’s one of the reasons why composite rotors have been used on various vehicles over the years. Besides being lighter, composite rotors can also be quieter when the right grade of cast iron is used for the rotor disc. Replacing a composite rotor with a solid cast-iron rotor changes the harmonics and frequency of the brake system and may increase the risk of brake noise on some vehicles. Also, some low-price rotors may use a lower grade of cast iron that is noisier than the OEM rotors they replace.