A few months ago my brakes developed a wobble, and the mechanic explained that there is a high spot in the rotor so when I'm braking the brake pads grab that spot more than the rest and it makes the brakes wobble or throb. The fix he suggested is to take the rotor off, use a lathe to make it flat again, and reinstall it.

So my question is, why don't the brake pads effectively do the same thing? I would think that through the use of the brakes, the pads would naturally wear off the higher spots more than the lower, and level the surface of the rotor again automatically. What am I missing here?

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  • This site claims that material eroded from the pads is deposited onto the face of the rotors and, that the process happens faster at the "high spots" where the pads grab harder. brakeandfrontend.com/warped-rotors-myth – Solomon Slow Dec 5 at 15:20
  • The fix he suggested is to take the rotor off, use a lathe to make it flat again, and reinstall it Get a new one. Brakes are absolutely critical to you and your passengers' safety - don't depend on a hack like this. You could easily create a more severe danger if you try the lathe. – StephenG Dec 5 at 15:30
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    Brake disks are often machined by mechanics when changing the pads, it was normal to do this in the past , the mechanics have guidelines on rotor thickness they have to follow by law. But in general disks are cheaper now so many people opt to replace one for 75 -100 bucks instead of the labour ~ 50 bucks. The vibration can damage other parts so it does not make sense to try and let the pads smooth it out, it will take too long. – PhysicsDave Dec 5 at 15:44
  • @PhysicsDave the mechanics have guidelines on rotor thickness they have to follow by law I've known good and bad mechanics and the I'd be loath to rely on all mechanics following the law. It's also worth remembering that just because they try and follow the law does not mean they won't make mistakes. – StephenG Dec 5 at 16:50
  • Take your rotors to auto zone and they'll resurface your rotors for a small price – user38183 Dec 6 at 7:57

when brake pucks grab a little more at one spot on the disc than at another, the frictional heat generated is unevenly distributed about the disc: the grabby spot heats up more. Heat causes the disc to expand, and uneven heating causes the disc to expand unevenly, and it warps. The high spots get more wear, but they also get more heat, and so once the warpage sets in the disc will generally not self-level.

If car engineers made the pads as abrasive as your question implies, I would guess that your brake discs/rotors wouldn't last 6 months.

But the pads do have enough abrasive action (or more correctly, enough lateral force when applied), to cause the warp in the first place.

This occurs if the rotor is heated sufficiently and the alloy it is composed becomes soft enough, which it often does after heavy braking.

So it's a trade-off, in that you want the brake pads and their actuators to be capable of stopping a car quickly, but not to need replacement or repair within too short a time period.

If you have a high spot on a brake disc, repeatedly braking will not improve the situation. If anything, it will make it worse because the highspot will become a hotspot in the disc. Brake pad friction material is not suitable to use as a cutting blade to re-face the disc.

If you have a particularly rare or exotic car with hard to or expensive brake discs, find somewhere with a brake disc lathe. Some systems allow rotors to be turned on the car, no need to remove them from the vehicle. However, for most vehicles, price up the cost of replacing the discs as you may find that this works out cheaper than turning them.

There is no such thing as a 'high spot' on brake rotors. They can however deform when significantly worn down or when overheated and develop the "wobble" you describe. That wobble is not caused by uneven brake wear, but by a rotor that is warped.

It would be a good thing to check if your brake rotors are still above the minimum thickness as specified by the manufacturer, this also is a check for MOT. But in order to get rid of the wobble the brake rotors need to be replaced anyway.

You also mentioned using a lathe. Brake rotors sometimes are ground down on a lathe, but that is to remove a "lip" on the outer edge of a rotor that appears when a brake pad didn't properly contact all of the rotor surface. If the rotor still meets the minimum thickness requirements, the lip can be ground down when new brake pads are to be installed. This is to ensure that the new brake pads contact the rotor surface properly.

  • I'm not quite sure I agree with your assessment. If the only reason to machine the surface of a rotor was to take off the lip, then why is the entire surface machined (and not just the lip)? The reason it is machined is to ensure the entire surface is flat. Usually, if there's a lip, there's other surface variations as well, which would require surfacing. Also, if there's no "high spot", why would there be a "wobble" in the first place? There has to be something there which is grabbing. – Pᴀᴜʟsᴛᴇʀ2 Dec 6 at 12:20
  • @Pᴀᴜʟsᴛᴇʀ2 In his question Dylan is suggesting that rotors wear unevenly, which is not true if you look at the rotatiing surface. The wobble is there because the entire rotor is warped (think potato chips) and not because one part is thicker than the other because of uneven wear. – MadMarky Dec 6 at 13:19

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