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I need to replace my rear brake pads. A mechanic suggested I also replace my rotors because they are rusted around the edges. The rotors are in fact quite rusted, however, there is no rust on the actual surface that touches the pads. Running my fingers over the surface of the rotors, there does not appear to be any buildup of brake pad deposits and the surface is relatively smooth. There is however one small circular scratch around one of the rotors. While the pads are ready to be replaced, they are still within standard tolerances, so it's not like there is metal-on-metal going on. The mechanic warned that replacing the pads without replacing the rotors at the same time would be a waste because the rusted rotors will just eat up the new pads. I find that hard to believe given that the rust is just on the extreme edges of the rotors, which are parts that do not appear to touch the pads. The brakes do not appear to make any abnormal noises at all. Therefore, according to this post, it doesn't seem like the rotors need to be replaced.

Is rust on the parts of the rotors that do not touch the pads enough to require replacement? Is there a way I can tell on my own if they need replacement?

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4 Answers 4

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It's not required that you replace your brake rotors at the same time you replace your brake pads, but there are many reasons why it's highly recommended.

Primarily, it's not the rust you should worry about, there most likely always going to be some rust around the edges, that's not at all out of the ordinary. The main problem is that your rotors are most likely grooved, and putting fresh pads on grooved rotors is going to wear the pads faster. Using those grooved brake rotors could also cause your wheels to wobble once you add new brake pads.

This is one of those instances where it makes more sense to spend a little more money to eliminate the possibility of a huge headache down the road. It's up to you whether you want to gamble.

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Is there a way for me to inspect the rotors myself to determine if they are grooved to the extent that they require replacement? –  ESultanik May 29 '13 at 0:37
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To clarify, the grooves themselves won't lead to wobbling. However, old grooved rotors are more susceptible to warping due to heating, which leads to a uneven braking (sometimes called shuddering, sometimes called wobbling). –  Brian Knoblauch May 29 '13 at 12:14
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The minimum acceptable thickness will usually be stamped on the brake rotor near the center. You can use a micrometer to measure the thickness of your rotor. Be sure to measure in different spots ( the grooves for example). youtube.com/watch?v=NpkoffI1co4 –  hillsons May 29 '13 at 17:43

If your rotors are thick enough and there are no defects in pads area (big grooves, buldgy edges) I would not worry at all. There's no need to replace rotors each time.

If there are some defects that can be removed and the rotors are thick enough - consider resurfacing the rotors, that might be cheaper.

All-in-all, given that the only downside is faster pads wear I would not bother as pads are generally quite cheap.

I my case set of pads and labor to replace them is almost twice cheaper than replacing rotors. So I would went for another round replacing only pads.

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It has been my experience lately, most rotors are not worth turning or resurfacing. Most resurfaced rotors are so thin that they tend to warp quickly even if thicker than the minimum thickness spec. –  mikes May 29 '13 at 10:36
    
I just did the front brakes on my car this weekend, and I noticed that the allowable rotor wear was only 2mm. I think resurfacing them after a normal amount of wear would probably take them over the limit. I tend to just replace them every other pad change -- they're not that expensive, considering how important they are. –  TMN Jun 3 '13 at 15:32

Can you feel or see deep grooves? Are your old pads irregularly worn? Have you already resurfaced the rotors? Do you feel shudder under braking? Are they close or under minimum thickness? Are they irregularly worn edge to edge?

Some folks do replace rotors at each pad change, most shops will at least resurface at each change (replacing them every other time or so when they can't be resurfaced anymore), but nobody does it because of exterior rust - which is pretty much normal. I imagine if your car has been sitting idle outdoors for 6 months, you may have a more serious rust issue - but that certainly doesn't sound like the case.

As with most preventative advice, you have to weigh the costs and benefits. If they're $20 rotors, it may be reasonable to go ahead. If they're $80 rotors, I'd probably think twice myself.

Either way, the "advice" of replacing rotors because of exterior rust would prompt me to replace the mechanic (assuming there's no other reason to replace them).

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you don't really need to replace the rotors unless the thickness is below the specs and damage to them can be seen with the naked eye. Your mechanic just want to make a bit more money. Brake rotors can last up to 100,000 miles on average before they need to be changed. This is what I suggest. Google " what is the average lifetime of brake rotors " and you will get a lot of sites with very useful information that will help you make up your mind. In your case, you have not provided enough information to get a meaningful answer. It all depends on how long they have been in use, how you drive, how often you change the pads...

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