Ever wonder why car brake rotors warp? The ones I’ve seen are heavy castings, and maybe a half inch thick with cooling passages cast into them. They look hard to bend, and their use seems to challenge them evenly all the way around. So why would it take on a nonplanar shape?
My pet theory is that it’s caused by making a long stop from high speed on downhills, like we’ve been doing to wait at the bridge repair project not far from here, which has reduced traffic to a single lane. Going down a really long and fairly steep hill, we have to slow down and stop at the bottom and wait for oncoming traffic to clear. I picture the brake pads getting fantastically overheated, and when we finally stop we have to keep the car still with pressure on the brake pedal, so we are clamping those pads to just one spot on the rotor. Could this be it?
On the other hand, if the active part of the rotor gets heated uniformly while the hub doesn’t, the rotor would like to adopt a wavy shape to accommodate the thermal expansion. So maybe that’s it?
What you describe seems to be the known reason, not a pet theory. Specifically after heating, which is fairly even, the brake pads bake in the resting spot, depositing a amount of brake material on the rotor, which has a self reinforcing tendency that eventually caused the metal to heat unevenly then it warps for real…
For this reason I usually after such a stop release the brake a bit and let her roll a bit, sometimes several times, perhaps even place in neutral with the brake off.
One solution that sometimes can work is long semi-hard braking but don’t come to a stop several times in a row from highway speed. The pet theory here is it redistributes any brake material on the rotor. As I said it sometimes works given personal experience.
While downshifting has it’s place on a automatic, or even a automatic CVT, and can help avoid a accident, there is the saying that brakes are cheaper then transmissions, I would add that brakes and rotors are cheaper then transmissions and loss of a tranny could spell the end of many cars. If it’s a manual, brakes are cheaper then clutches however it’s not as serious.
I would agree. My metallurgy training for my apprenticeship touched on cast iron, which is what rotors are made from. If you chill a specific region of a cast iron casting, you will change its hardness in that area. So the spot cooling could be just the start of a greater wear/ lesser wear discrepancy for the rotor, which would show up as a pulsing brake pedal, a condition seen as a warped rotor.
I have yet to come across a convincing argument how or why downshifting to control speed would wear out an automatic transmission any sooner than otherwise.
I would say that the increased shear on the fluid would raise the temperature and if the tranny cooler was not large enough to dissipate the heat, then the higher temperature the transmission was exposed to would cook the gaskets and seals.
I’ve yet to see any convincing evidence that rotors “warp.”
As someone who regularly beats the snot out of conventional, OEM cast rotors on racetracks, I’ve never had rotors warp on a race car. I HAVE had brake pulsation on street cars. I don’t think excessive heat has anything to do with it.
My theory is similar to MrMrmrmr’s, except it has to do with a small amount of brake pad material gluing itself to the rotor. As the rotor spins around and comes into contact with the pad again, that small amount of pad material knocks the pad back a little bit, and then the pad bounces back into the rotor. Again and again and again, eventually building up more pad material ahead of the scallop and a deeper area behind it. Hence, a pulsing brake pedal without any actual warping.
This assumes the torque converter is in slip mode instead of locked.
This is a known issue that is solved in high-performance vehicles by using “floating” rotors: the disc is a separate piece that’s attached to the hub in a way that allows it to expand/contract radially as its temperature changes, preventing the application of forces (from the hub) that would cause lateral warp when the braking surface gets very hot.
I already do this. My car’s a manual, and the instruction manual says to downshift to assist in braking, so I do it. I also give the accelerator a tap while downshifting at speed to match the clutch plate speed, so I don’t feel much when I release the clutch. On the long hill I was thinking of, I sometimes do this all the way down through first gear on a six gear transmission (which, luxuriously, is synchromesh).
This doesn’t necessarily wear the clutch much. It isn’t making up much of a speed difference at all, if I tap the accelerator right. The engine is pumping and compressing air, which dissipates energy, but the clutch shouldn’t be dissipating much energy unless I’m clumsy about it.
Brakes, on the other hand, do nothing but wear out.
There’s a question of scale here. Descending a mountain range with a roughly similar downhill slope for several miles only requires one clutch actuation, whereas every second of that descent would be wearing away on brakes. Stop and go driving in a traffic jam may favor the brake method, on the other hand.
On trucks, using the engine for braking on long downhill runs is a must; without it, the brakes overheat/fade in a hurry, resulting in dangerous loss of control.