By
Counterman
It’s a question – and answer of many factors. One to keep in mind is why this is a common question. It’s because decades ago, we always resurfaced rotors and only replaced them when they had been resurfaced too many times. Why did this concept change? Let’s start by looking at rotor resurfacing, a process typically referred to as “turning” the rotor. Turning is the general name of the machining process where a workpiece is rotated against a fixed cutting tool. In the case of a rotor, any surface irregularities, including any grooves formed from normal service and also any rust or pitting, can be removed by this machining process.
In addition to surface condition, rotors often suffer from different forms of distortion. Lateral runout is the side-to-side movement of a rotor, measured with a dial indicator while rotating it by hand. Parallelism is the thickness of a rotor measured at multiple spots around the circumference for comparison. When describing this to a customer, we generally use the basic term of “warped” rotor. These conditions will cause a vibration during braking, and, in some cases, just driving at higher speeds.
Either one can be caused by normal wear or by incorrect mounting or installation of the rotor and wheels. Customers know what it means to have a warped rotor because of the symptoms, but few of them care about the technical terminology or reasons. They just want it fixed. Turning a rotor will correct these problems as long as an underlying cause, such as incorrect rotor installation has been addressed.
Turning a rotor involves several steps, the first of which is measuring it to determine if it will still be above the minimum thickness afterward. In most cases, the minimum thickness is cast or stamped into the rotor, but often it’s rusty and difficult to find, so we generally must look up the specification in service information.
Typically, when you turn a rotor, you’re going to remove a total of about .015 in. to .020 in. (15 to 20 thousandths of an inch) of material. It may be less on a clean rotor, or more on a rusty, pitted or warped one. After measuring the thickness of the rotor and assessing the condition, you’ll know whether you’ve got plenty of material left to turn it, or whether it’ll be too thin when you’re done.
If you determine the rotor can be turned, the next step is to remove it from the car and mount it on the brake lathe. This is where the type of rotor, hubbed or hat, starts to become part of the equation. Hat rotors require a thorough cleaning and rust removal from the mounting surface to ensure they seat properly when mounting on the brake lathe spindle. The mounting surfaces for a hubbed rotor are the wheel bearing races, from which you can just wipe away the excess grease for quick and easy mounting.
When the turning is complete and you’ve taken a final measurement to ensure the rotor is still at or above minimum thickness, the next step is to put a non-directional finish on the brake rotor, which aids in proper break-in of the brake pads. The most popular method is to use an angle-grinder with a cleaning disc, and it literally only takes a few seconds per side.
The final step includes washing the rotor in a mild soap and water solution. Though not visible, small metal particles remain on the rotor after turning, and these particles will embed themselves in the pad and prevent an effective “break-in.” Washing the rotor removes these particles. Hubbed rotors will require removing all the old grease, since a wheel bearing clean and repack is a normal part of this service.
If this sounds like a lot of work, for a technician it quickly becomes routine and many of us enjoy the process, but it does take time, which plays another part in answering the question.
There was a time when the hum of a brake lathe was almost as constant as the ticking of the clock on the shop wall, but this was also primarily in the days of the hubbed rotor. Hubbed rotors, so-called because they were cast as a large one-piece unit consisting of the outer ring and an integrated center hub to house the wheel bearings, were big, heavy and expensive. But they lasted a long time, because they could be turned and reused multiple times before they were too thin to put back in service. The expectation of customers during this era was that their rotors would be “turned” during brake service. Even with the additional cost of labor, it was still far more expensive to replace them.
Hat rotors earned their name due to their similar look to a formal top hat. They have no integrated hub to locate wheel bearings. As the hat rotor slowly became the predominant rotor in use, many other changes were taking place in the automotive industry. New auto parts stores were opening up to meet the demands of the increasing number of cars on the road, and parts were being manufactured overseas. Price competition was high, and the more parts that were produced (hat rotors included,) the less expensive they became.
At the same time, technician salaries were increasing, and suddenly, the labor cost to turn rotors was increasing. Then, there was the process of turning the rotors. It can be done wrong as easily as it can be done right. Traditional hubbed rotors were very heavy, and as a result easier to turn because the weight inherently reduced vibration and mounting them on the lathe was easy and straightforward.
Two things kill a rotor when turning it. One, vibration, and two, incorrect mounting. Guess what? You probably figured this, hat rotors are lightweight, so it’s much more difficult to prevent vibration and they are commonly mounted incorrectly on the lathe. Most of this happens because of incorrect training, or simply a shop not having the proper lathe adapters, or both, but that subject can be reserved for a whole different article.
The trouble involved with turning hat rotors was sort of a nail in the coffin for the whole process. In today’s shops, you rarely hear the sound of a brake lathe. A good majority of the rotors that are scrapped could, in reality be turned and returned to service. But a new set of rotors is less expensive than the labor to resurface an old set (hubbed rotors being the exception). Factor in the reality that they quite possibly could be machined incorrectly causing a comeback, and it doesn’t make as much sense to turn them.
Replacing them is quicker, a shop makes money on the parts, technicians make more money on labor, and they can get onto the next job quicker. Techs and shops like these factors.
The bottom line is hubbed rotors are often the only ones we can justify resurfacing when you compare cost versus time. But your customer may not care about all these technicalities. They likely just want a quick answer about replacement. Here’s an easy approach: Due to the critical importance of breaking in new pads, which relies on the surface of the rotor, any time you are replacing pads, the rotors should be replaced as well, unless it makes economic sense to turn them. And, that’s the key. Economics. With any rotor problems, unless it makes economic sense to resurface, replacing them is the answer that most will choose.
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