But if you read the posts I linked to above, modern brake pads no longer produce the "gasses" that they used to many, many years ago, so drilled/slotted rotors don't even perform that function anymore.
From the link above:
Then from Grassroots Motorsports:
QUOTE
"Crossdrilling your rotors might look neat, but what is it really doing for you? Well, unless your car is using brake pads from the '40s and 50s, not a whole lot. Rotors were first drilled because early brake pad materials gave off gasses when heated to racing temperatures, a process known as "gassing out." ...It was an effective solution, but today's friction materials do not exhibit the some gassing out phenomenon as the early pads. Contrary to popular belief, they don't lower temperatures. (In fact, by removing weight from the rotor, they can actually cause temperatures to increase a little.) These holes create stress risers that allow the rotor to crack sooner, and make a mess of brake pads--sort of like a cheese grater rubbing against them at every stop. Want more evidence? Look at NASCAR or F1. You would think that if drilling holes in the rotor was the hot ticket, these teams would be doing it...Slotting rotors, on the other hand, might be a consideration if your sanctioning body allows for it. Cutting thin slots across the face of the rotor can actually help to clean the face of the brake pads over time, helping to reduce the glazing often found during high-speed use which can lower the coefficient of friction. While there may still be a small concern over creating stress risers in the face of the rotor, if the slots are shallow and cut properly, the trade-off appears to be worth the risk. (Have you looked at a NASCAR rotor lately?)
I also agree about "warped" rotors not actually being warped. What people call "warped" rotors are just rotors with deposits "baked into" them from the brake pad material... From the second link in my post above:
So what really happens when the rotor is “warped”?
When you’re on the brakes and there’s a pulsing sensation or vibration, we typical call this situation warped rotors. Surprisingly,*I personally have never seen a rotor that actually has physical distortion. I have even placed a dial indicator on so-called warped rotors to find that they have no run-out what-so-ever.
What does happen is that the layer of pad material on the rotor surface builds up unevenly, and the metallurgy*of the rotor can change states. The layer of pad material on the rotors surface, if it is unevenly distributed, will create hot spots. If these spots get hot enough, it can form cementite in the rotors metal – a rough iron carbide formation that creates a lot of friction, but is terrible at dissipating*heat. The cementite formation can get so bad and cause so much friction that even when you are off the brake pedal completely, because your pads are always in contact with the rotor ever so slightly, it can create a vibration when driving normally. I have even mistaken this vibration as my tires being out of balance.
Uneven transfer layer buildup and cementite is what produces “warped” rotors, not run-out or distortion. Cementite is a problem with iron rotors, rotors made of other materials like carbon do not suffer from this problem.
Vibrating can also be caused by a crack in the rotors surface. If you have a vibration that only appears during hard or extended braking, it may be a crack. The reason for this is that the cracks opens up when the rotor is hot and closes when it’s cold. You may never even know that there is a crack if you never build heat in the system…
Let me digress a little bit – There is surely some uneven*dimensional change (warping) to the rotor if you have a localized hot spot. But this seems to be only temporary and when the rotor cools, it returns to its normal flat state. I suppose you could drive through a puddle with very hot brakes and cause a*permanent measurable change, but it must be rare.
If I have my rotors resurfaced, will that fix the problem?
In my experiences, no. When I have had my rotors resurfaced, it only cured the vibrations temporarily. Within a few months, they return even if I haven’t been doing any hard driving. So what gives? Most likely, parts of my rotors had turned to*cementite and it was thick enough where resurfacing did not remove it all. Even if there was a small area left after resurfacing, that one spot will create a hot spot which will grow in fairly short order.
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