New brake pads on old rotors — Can you reuse old rotors?
Whether you can slap new pads on old rotors depends on the condition of the old rotors. Many times old rotors have grooves and those grooves can quickly wear out the new pads. In essence the grooves have high and low spots. So the new pads with hit the high spots first. In other words, all your braking will be done with just a small portion of the pad which will cause overheating and pad shredding in those areas.
How to check your old brake rotors
• Are there any grooves on the old rotor deeper than 1/16″? If so, you can’t reuse that rotor.
• Does the rotor exceed the “Discard thickness” stamped into the rotor? If yes, it’s possible that it can be reused if it meets the other criteria.
• Is there flaking rust accumulation in the cooling vanes? If you can remove the majority of the rust, you can possibly re-use it if it meets the other criteria.
• Is you getting any brake pedal pulsation? If yes, you can’t reuse the old rotors.
• If the rotor face is shiny, you’ll have a much harder time bedding in the new brake pads and the new pads will most likely cause noise issues even after they’re “bedded.”
Rotor surface condition is important
There are two types of brake pad friction materials; abrasive and adherent. Semi-metallic pads are abrasive, while organic and ceramic are classified as adherent. Adherent friction material works by applying a transfer layer to the rotor, so braking is accomplished by rubbing friction material against friction material.
Abrasive braking is more like sandpaper against wood. In this case, the brake pad is the sandpaper and the wood is the rotor.
In both cases, the rotor face must have a smoothness.
How smooth is smooth when it comes to brake rotor surface conditions
Brake rotor surface “smoothness” is rated by its Roughness Average (RA). The proper RA is critical to achieving the proper transfer layer for adherent style brake organic and ceramic pads as well as abrasive semi-metallic brake pads.
If the surface is too smooth, it’s impossible to form a transfer layer onto the rotor face. If it’s too rough, the brakes will make noise, wear faster and even lose stopping power.
What happens when you install new pads on old rotors?
If the old rotors are shiny, you’re going to have a hard time establishing a new transfer layer. So using the proper bedding technique for the new pads is critical. You will most likely encounter noise and reduced braking until the rotors can re-establish the transfer layer.
In addition if the old rotors have grooves deeper than 1/16″, they can’t be reused. If the grooves are less than 1/16″, the new pads won’t provide full stopping power until the pads wear into the shape of the rotor. Since only a portion of the brake pad is touching the rotor, it will create noise.
Why don’t shops replace pads without replacing rotors?
Because they don’t want the noise comebacks. Putting new pads on used rotors can cause noise complaints. The reasoning can be complicated but it boils down to this; even though the rotor looks perfectly flat, it isn’t. So the new pads don’t sit flat on the rotor until they wear into the unevenness. Until that happens, it’s like a stylus playing the grooves on an old LP record. The brake pad becomes the stylus and the micro-grooves on the rotor contain the “music”.
What can you do to reduce noise when replacing brake pads without replacing rotors?
First, try to match the new brake pads to the old brake pads. Don’t switch from organic to ceramic or semi-metallic to ceramic. Next, perform a brake pad “bedding” procedure. This involves applying the brakes to come to a complete stop multiple times to “embed” a layer of brake pad material onto the face of the rotor and accelerate the wear of the pad to the shape of the grooves in the rotor.
If you still have noise, you may have to resurface the rotor or replace it.
©, 2021 Rick Muscoplat
Posted on by Rick Muscoplat