To avoid a hijack in this thread:
It seems to me that if the rotor is warped, the crystalline structure of the metal is also warped. If you machine that down, you’re getting a flat surface, but the underlying structure is still warped. I know this was a very common practice but was it a good practice? Is there any downside of grinding down a warped rotor? Would the subsequent wear be even, or tend to reconform to the warpage contour? Would this practice encourage or discourage further warping?
The rotors are designed to retain their shape up to some temperature and exceeding that causes them to warp. If they’re ground down then less material is available and they should warp more easily if overheated again.
At the point a rotor is warped, it has gone through many hot/cool cycles. I suspect it has now assumed a very stable point. Turning it to a flat surface now, may lead to a very long period of it keeping true during more cycles. This may not be true if it is very warped and a lot of material has to be removed. Too much difference between thick and thin areas now. Of course there is a maximum of allowed removal of material.
I wonder if the cast in vented discs are more prone to warping due to imprecise casting?
If your rotor was warped you would experience a lower pedal than normal even if you did not feel the pulsations on the pedal when stopping.
Ordinary heating and cooling cycles are built in to the design. Turning a rotor after it’s broken in should result in a very clean flat shape with minimal removal of material. But it isn’t warped from overheating as described in the OP.
Steel doesn’t suffer from this sort of “memory effect”. It will not spring back to a warped shape
Warped rotors is a misnomer. Rotors do not “warp” from excessive heat – every weekend, thousands of amateur race car drivers put bog-standard parts-store rotors through dozens of heat cycles with no discernable effect on how true the rotors are.
A pulsating pedal, the cause of which is colloquially referred to as warped rotors, is actually caused by uneven wear. This can be caused by a the brake pad leaving friction material stuck to the rotor, and yes, confusingly, this can be caused by a hot pad being held against the rotor surface (say, at a stop light). Subsequent braking events will cause the caliper to pulse as the pad slides over the high spot of pad residue and is then pressed back into the rotor. If you catch a pulsating pad early, you can actually see the stuck pad material.
The effect is not dissimilar to how a feedback loop of car suspensions causes washboarding in dirt or gravel roads.
So rest assured, turning the rotors will remove all of the high spots and return them to serviceable condition, as the internal “shape” of the rotor never changes.
I suspect that in most Western countries, the cost of machining used rotors would exceed the cost of a new replacement.
A long time ago I would take my rotors to a shop to be machined. I don’t do that anymore. For the cars I drive, they have gotten so cheap that I don’t think it’s worth it anymore.
But machining it does not flatten out the warping, it just grinds the surface down flat. The structure is still warped. However:
So this is the missing fact. If they are not actually warped then that renders the premise of my question moot.
It’s the uneven deposition of pad material that causes the pulsating.
Here and many other sites:
Usually when the pads are shot, the rotors are near the minimum thickness and get replaced too. Rotors are heavy and are unsprung weight on the suspension. This weight negatively affects handling so rotors have gotten lighter in later production years. Honda was an early example. They stressed lightness in their cars for performance and economy. The rotors were “always” down to the minimum.
If you go too long on worn pads, screeching from the wear indicators (How could you not notice that???) is the indication; the rivets holding the pad and backing together start to carve grooves in the rotors - not good.
As said, rotors are relatively cheap, get a matched set with the pads, and if you do the work yourself, use plenty of brake cleaner on the rotors before assembly and test drive/break-in period. The rotors have a non-corrosive coating you need to get off or else it will melt and foul the pads (and the rotors again). And don’t get the brake cleaner on any paint surfaces.
A brake lathe is a pretty simple mechanical machine that runs by itself, no machinist required. If as stated above, newer rotors are too thin to turn then perhaps it’s just not done now for that reason. There are more causes to flatten the rotor surface, mainly due to scoring from material caught between the pad and the rotor, and wearing pads out an scoring the rotor. The pads all have a metal strip that starts squealing in contact with the rotor when the pad is worn down too far but if that’s ignored the rivets in the pad could cut all through a rotor causing the pad to extend too far and all the fluid can leak out of the caliper. It is far beyond the point of repair when that happens, even with thick rotors. I did manage to get a set a rusty set of rotors from a junkyard smoothed out with a hand sander long ago in order to pass inspection.
No, you are missing my point. When steel has been heated to the point that it changes shape, its new shape is its shape. It doesn’t have a “natural” shape from which it has warped. It is the new shape. It does not keep a memory of how it was before its shape was changed by heat.
Maybe I am misunderstanding, or not making myself clear. Here is what I had imagined is going on. The dotted blue lines are the internal structure of the metal. My question was whether the third configuration here has some inherent weakness, like a tendency to wear unevenly or continue to warp further in the same way.
LIke @smithsb pointed out, they don’t actually warp. It’s kind of absurd when you think about it- even annealing cast iron is done pretty hot - 700F or more.
Brakes don’t get that hot typically. Even track brakes only get into the 1000F range, which isn’t enough to actually affect the strength of the material (that takes place around 1500F). Maybe if you got your rotors that hot, they might warp.
What does happen though, is that you get uneven spots of friction material that adheres to the rotors, which cause a pulsating feeling as if the brake rotor itself is uneven.
Turning your rotors removes the friction material and evens out the surface for the best braking friction. If the rotors were truly warped, they’d be more like a warped record- no amount of lathe time is going to even that out.
Yeah, I saw that and concluded that my question was moot, but in the interests of discussion with @Princhester I was following through on the original idea to clarify what was in my head.