How often should I be replacing my brake rotors?
October 1, 2018 11:59 AM Subscribe
We have two cars: a 2012 Honda CR-V and a 2013 Toyota Prius. The brake pads on both of the cars wear out with time (as brake pads do) and have needed to be replaced. Our mechanic, who comes highly recommended, has recommended that we replace the brake rotors every time we've replaced the pads. Is this reasonable or am I a sucker?
I've questioned him about it and he has said something about the new brake pads not seating well onto the old rotors. This sounds strange to me based on what I know about brakes but, on the other hand, if I knew enough to be an expert on this, I'd probably just replace the pads myself. I've asked a mechanically-inclined friend and he also thought this sounded wrong. On the other hand, the cars he works on are older than mine.
So, I'm left with three options: 1) I'm a sucker and this mechanic is fleecing me. 2) This is completely normal and neither me nor my friend know what we're talking about. 3) This is normal on my cars, but not my friend's cars.
Mechanics of Metafilter: what say you?
I've questioned him about it and he has said something about the new brake pads not seating well onto the old rotors. This sounds strange to me based on what I know about brakes but, on the other hand, if I knew enough to be an expert on this, I'd probably just replace the pads myself. I've asked a mechanically-inclined friend and he also thought this sounded wrong. On the other hand, the cars he works on are older than mine.
So, I'm left with three options: 1) I'm a sucker and this mechanic is fleecing me. 2) This is completely normal and neither me nor my friend know what we're talking about. 3) This is normal on my cars, but not my friend's cars.
Mechanics of Metafilter: what say you?
Best answer: When I worked as a mechanic a few years ago, the theory was "if in doubt, replace the rotors too". If a customer balked, we didn't force the issue. Modern rotors seem to be a lot more prone to warping and are just, generally, not nearly so robust as older rotors.
The big problem was--even if the pads would bed into the old rotors pretty quick-- until that time, there would be brake noise. And for something as low-margin as a basic brake job, having to throw the car back on the rack for brake noise (even once) was not something you wanted to do.
posted by notsnot at 12:29 PM on October 1, 2018 [1 favorite]
The big problem was--even if the pads would bed into the old rotors pretty quick-- until that time, there would be brake noise. And for something as low-margin as a basic brake job, having to throw the car back on the rack for brake noise (even once) was not something you wanted to do.
posted by notsnot at 12:29 PM on October 1, 2018 [1 favorite]
Best answer: Not a mechanic, but a former owner of a Honda with disc brakes. Here's how it was explained to me: braking generates heat and excessive heat causes brake rotors to warp. The whole brake assembly (rotors, pads, and everything else) is supposed to be able to cool itself enough to prevent damaging heat buildup. Apparently (according to both an independent mechanic and a mechanic at a Honda dealer) when the pads are new they are able to dissipate heat enough to prevent warping. As pads get thin (but not quite worn out) the brake assembly generates heat more quickly than it can dissipate, increasing the chances that the heat will cause the rotors to warp. And if the pads completely wear out that can cause mechanical damage to the rotors. Rotors can be resurfaced to some extent, but if they are warped they might not be saved. Even if they're not warped, resurfacing removes material, so it's also possible that the mechanic might have concluded there wouldn't be enough thickness left after resurfacing.
There were a couple times I went in for a brake service only to get a phone call that my rotors also needed to be replaced. If I went in for brake service and the mechanic told me the rotors couldn't be resurfaced because they were warped, I'd buy that (and the new rotors). If the mechanic said resurfacing would cost more than all new rotors, or that I could resurface the rotors but after that I'd probably still need to replace them before the next pad replacement, then replacing the rotors without resurfacing them first saves the cost of resurfacing and it's a better deal. If I went in for brake service and the mechanic said I needed to replace the rotors simply because of the new pads, I'd want more of an explanation, or a new mechanic.
posted by fedward at 12:32 PM on October 1, 2018 [1 favorite]
There were a couple times I went in for a brake service only to get a phone call that my rotors also needed to be replaced. If I went in for brake service and the mechanic told me the rotors couldn't be resurfaced because they were warped, I'd buy that (and the new rotors). If the mechanic said resurfacing would cost more than all new rotors, or that I could resurface the rotors but after that I'd probably still need to replace them before the next pad replacement, then replacing the rotors without resurfacing them first saves the cost of resurfacing and it's a better deal. If I went in for brake service and the mechanic said I needed to replace the rotors simply because of the new pads, I'd want more of an explanation, or a new mechanic.
posted by fedward at 12:32 PM on October 1, 2018 [1 favorite]
(and yeah, Brockles, i know rotors don't actually warp, just get localized differences in heat dissipation.)
posted by notsnot at 12:34 PM on October 1, 2018 [2 favorites]
posted by notsnot at 12:34 PM on October 1, 2018 [2 favorites]
/stops typing comment about how brake rotors don't actually warp
posted by Brockles at 12:58 PM on October 1, 2018 [5 favorites]
posted by Brockles at 12:58 PM on October 1, 2018 [5 favorites]
rotors DO warp if the caliper seize up & things get hot.
posted by patnok at 1:13 PM on October 1, 2018
posted by patnok at 1:13 PM on October 1, 2018
No, they do not. They exhibit a behaviour that people interpret as a warp or deformation of the disc but it is not in any way warping. From a previous AskMe about brakes (my reply):
Just for reference - brake discs do not actually warp, that's incorrect terminology/erroneous. They do not physically buckle or warp out of true. What does happen, however, is that as the brake disc/pads get thinner, they are less able to manage localised heat dissipation from the pad contact area and in extreme cases will allow 'smearing' of pad material onto a small part of the disc face. So, for instance, at the very end of a long sustained stop with the brakes at high temp, you may get pad transfer during the last second or so of the stop. This is often known as 'glazing the discs', and if it happens all around the surface you just lose braking efficiency. If it happens for only a section of the disc then you get the vibration effect.
The 'juddering' you feel in the pedal that people assume is a disc that has warped is actually the vibration caused by the difference in friction between the change between brake disc surface then (brake disc+pad material smear section) then brake disc again. The resurfacing of the disc is intended to remove all traces of the old pad material and machine down to a clean and new surface. However, as this reduces disc thickness it makes the machined discs proportionally more likely to allow pad material transfer to happen again.
posted by Brockles at 1:27 PM on October 1, 2018 [3 favorites]
Just for reference - brake discs do not actually warp, that's incorrect terminology/erroneous. They do not physically buckle or warp out of true. What does happen, however, is that as the brake disc/pads get thinner, they are less able to manage localised heat dissipation from the pad contact area and in extreme cases will allow 'smearing' of pad material onto a small part of the disc face. So, for instance, at the very end of a long sustained stop with the brakes at high temp, you may get pad transfer during the last second or so of the stop. This is often known as 'glazing the discs', and if it happens all around the surface you just lose braking efficiency. If it happens for only a section of the disc then you get the vibration effect.
The 'juddering' you feel in the pedal that people assume is a disc that has warped is actually the vibration caused by the difference in friction between the change between brake disc surface then (brake disc+pad material smear section) then brake disc again. The resurfacing of the disc is intended to remove all traces of the old pad material and machine down to a clean and new surface. However, as this reduces disc thickness it makes the machined discs proportionally more likely to allow pad material transfer to happen again.
posted by Brockles at 1:27 PM on October 1, 2018 [3 favorites]
If you are wearing out brakes at all quickly on a Prius, you are habitually braking too late and too hard. That's a safety problem and a waste of fuel. The regenerative braking system handles light braking and should be doing almost all of the stopping.
posted by w0mbat at 1:35 PM on October 1, 2018 [3 favorites]
posted by w0mbat at 1:35 PM on October 1, 2018 [3 favorites]
Best answer: ...recommended that we replace the brake rotors every time we've replaced the pads.
Oh, hells NO. Dude must have big boat payments to make. While rotors certainly wear over time, they do not wear anywhere near as quickly as the pads. As long as there’s no visible damage or warping, you should be able to go several pad changes before needing rotor work.
Now, in the past, mechanics would re-face or “turn” a rotor to make it true again, but manufacturers have gone to using some fairly thin rotors these days, so you usually just replace them when they’ve reached their end of life.
But, replace them with every pad change? Nope, nope, nope.
posted by Thorzdad at 1:39 PM on October 1, 2018 [4 favorites]
Oh, hells NO. Dude must have big boat payments to make. While rotors certainly wear over time, they do not wear anywhere near as quickly as the pads. As long as there’s no visible damage or warping, you should be able to go several pad changes before needing rotor work.
Now, in the past, mechanics would re-face or “turn” a rotor to make it true again, but manufacturers have gone to using some fairly thin rotors these days, so you usually just replace them when they’ve reached their end of life.
But, replace them with every pad change? Nope, nope, nope.
posted by Thorzdad at 1:39 PM on October 1, 2018 [4 favorites]
Best answer: I'm 100% team replace with every pad change, unless the car is an absolute beater. They're cheap. Your brakes will perform as new. The new discs have a specially machined cross hatch surface which helps the new pads bed in. Sure, you can scuff up the existing discs to break the glaze, but it's not a factory-grade finish. Also, if you live in the rust belt, discs/rotors will often end up with weird rust-ridges near the inside and outside edges of the disc and effectively reduce the working surface area of the brakes.
posted by Larry David Syndrome at 4:01 PM on October 1, 2018
posted by Larry David Syndrome at 4:01 PM on October 1, 2018
Best answer: If rotors don't warp, what is the cause of a pulsing brake pedal?
The last truck I had made it 150K miles on the original rotors. If there are irregularities in the surface of the rotor, the pads wear to them quickly. Any hatching or surface roughness on a new rotor (what machinists would call the "as-turned" surface) is gone very quickly.
Seconding that the mechanic has a boat.
posted by rudd135 at 4:11 AM on October 2, 2018
The last truck I had made it 150K miles on the original rotors. If there are irregularities in the surface of the rotor, the pads wear to them quickly. Any hatching or surface roughness on a new rotor (what machinists would call the "as-turned" surface) is gone very quickly.
Seconding that the mechanic has a boat.
posted by rudd135 at 4:11 AM on October 2, 2018
Response by poster: Glad to know that there are at least some of you who think I'm not a total rube. That said, I may try another mechanic and see what they say. I feel like when I was going to the dealership (when they were paying for the oil changes), the rotors weren't replaced nearly as much as they are with this independent mechanic. I'll give another independent shop a try. I'm certainly firmly in favor spending money on things that need to be addressed, but I'm annoyed by paying for things that I don't need.
posted by Betelgeuse at 10:56 AM on October 3, 2018
posted by Betelgeuse at 10:56 AM on October 3, 2018
This thread is closed to new comments.
Pads will wear into and profile to the rotors pretty easily in a very short time. It's a perfectly normal process. I don't think he is necessarily fleecing you as much as he isjust wrong. If the rotors are badly worn? Sure, but I don't think it is ever a blanket issue like he is saying.
posted by Brockles at 12:03 PM on October 1, 2018