Replacing the brake pads on your car is likely going to be one of the more expensive maintenance items you’ll experience in owning your car, with the average brake pad and rotor replacement costing between $300-$700 or even more on some luxury cars.
Because of this, many workshops will also give you the option of either machining or replacing your brake rotors, depending on your budget. But what is better for your car? And what is brake rotor machining anyway?
Brakes on a car operate in the same way as a skater slows down on a skateboard. By dragging their foot along the ground and creating friction. In this scenario, the ground is like the brake rotor (the moving element) and the brake pads are like the skater’s foot, creating friction to slow the car. (We explain this better in this blog post)
This friction causes 2 big factors that lead to the machining or replacement of the brake rotors, heat and wear.
Though the pads material and the rotor are (usually) both made out of metal-based compounds, the friction of the pad against the rotor actually causes the rotor to wear and become thinner, the same way that sandpaper removes wood for a woodworker. Because of this, most brake rotors are designed with a minimum thickness which the rotors must be replaced at.
The other reason that many rotors need to be replaced or machined is heat. As the metal gets heated and cooled over time, the metal rotors tend to warp and become not-round. This “runout” (technical term) causes the brakes to pulse and vibrate while slowing down and can be bought on faster by rapidly cooling the rotors. For example, if you drove through a puddle.
This only part answers the question of should you replace or machine your brake rotors though and, doesn’t tell us what machining even is.
Machining rotors is the process of removing metal to make the rotor surface flat again. The rotor is mounted in a lathe, which turns the rotor past a cutting tooth that removes runout and high spots on the rotor.
However, if a rotor is too thin, machining is not meant to be performed and the rotor should immediately be replaced. Additionally, If a rotor is close to minimum thickness before machining, you must allow for the amount you need to cut off to make the rotor flat again and the amount a new set of brake pads will wear the rotors. If you expect that the rotor will be under minimum thickness before the next set of pads, you should always replace them.
The final nails in the coffin of machining are the cost and inaccuracies involved.
Most workshops lathes are really quite old and are often not serviced regularly. Because of this, it’s not uncommon for a rotor to be machined to “flat” according to the lathe and to still have runout on the car. This can mean an extra visit to the mechanics to rectify this, which is just lost time.
Additionally, if we take a fairly normal car (a 2007 Mazda 3) and compare machining to replacing. The price to replace the pads and machine the rotors is likely going to be $300 (approx.), whereas the price to replace the brake pads and rotors on this car is likely around $360.
Overall, it often works out better as a vehicle owner to just replace rotors. Machining rotors may be the cheaper option, but in saving $50-$100, you’re often increasing the likelihood that a problem occurs further along. If you want some more information or a different opinion, always feel free to ask our Kashy mechanics.