Originally Posted by Voyager
After 28 years of engineering, I am no longer sure about anything and that is why you won't see me making the sort of absolute statements that you are so fond of. Everyone thought Newton had it nailed until Einstein came along. How many times have scientists claimed to have found the smallest particle of matter/energy, only to have someone else find evidence of something even smaller.
Sorry, you won't see me entering into absolutes as the world is just too complex for that. There is always one more factor that we don't know of or failed to consider. The only absolute I will state is that anyone who states absolutes is absolutely wrong most of the time. :-)
Well, wait just a cotton pickin' minute - you're not gonna' bail on me THAT easy - especially if you're an ENGINEER
- right? I want my $100.
I mean, after all, I'm just a lowly chemist.
You know - better living through chemistry? It's not really ALL that complex. I'll even attempt to distill it down to layman's terms.
How about some show and tell? Pictures always
seem to work.
Attached are a couple of thermal images of disc brake rotors and the associated hot spotting caused by thickness variation from cementite crystals on the surface. Other various forms of ceramic crystals can also form depending on temperature and alloy - like martensite which can be responsible for the "chipping" you mentioned. It is VERY brittle.
The first picture shows what a nice smooth disc brake feels like when the pads clamp down on the rotor. Notice the continual white area. Nice even pressure, temperature and no hot spots. A nice smooth stop.
The second picture shows what "brake judder" (what is commonly and mistakenly referred to as the warped disc brake myth) feels like when the pads clamp down on the rotor. You can actually SEE the high spots as much brighter white areas. Those bright white spots are the "Cementite hills on the plane" I was alluding to earlier. You're gonna have a whole lot o' shakin' going on here - and I'm not talkin' about listening to Jerry Lee on the box.
BUT EVEN MORE TELLING
is the difference in the hot spotting between the inner and outer surfaces, left versus right. They are out of phase
, meaning that the pads are riding up and down and contacting the disc surface at different times - and actually skipping over the tops
- of the "hills" caused by the depositions. That feeling, translated to the brake pedal is a distinct pulsing. The more out of phase the deeper the pulsing sensation. It is a vicious downward spiral - it just gets worse and worse - unless you remove the hills with a very hard abrasive that will not leave any residue behind - aka garnet paper. You can also blanchard grind, but that's only for advanced cases where the hills have literally turned into mountains. If you catch it early enough it can be easily removed - precisely when you start
to feel the judder. Unfortunately grinding is not an option on our stainless motorcycle discs. It might be available somewhere but it is certainly not common nor inexpensive compared to just buying a new rotor and slapping it on.
Interestingly enough, the thickness of the pads and the composition of the pads play a role in this as well - separate from the depositions. A thick pad has more ability to absorb the hit when slamming into the peak of the hill, a thin pad transfers more energy since it has less material to dissipate the energy. The composition is important too - racing pads (hard) will bounce off the hills, while street pads (soft) more or less skip off the top.
Now can I have my money?
Not yet - because - wait, there's more!
...we still have to discuss the possibility that some sort of magical thermal stress has happened to the rotor to cause it to distort.
The third image shows the thickness of a rotor prior to contact with the pads, during contact, and again after contact. Not surprisingly it DOES change thickness during contact - but literally snaps back to the previous thickness immediately. This happens probably a hundred thousand times during the normal life of rotor and pads. The interesting thing is that the difference in thickness here is measured in angstroms on an atomic level - which is pretty much a defunct method of measuring things these days. The variation is so small it's barely measurable, certainly not enough to cause any feeling at the brake pedal, and besides, this is only a temporary
condition at the temperatures involved. There just isn't enough energy here to cause a permanent change in the thickness. No distortion, no "warping".
I'm done... just how much does a box of good ceegars cost these days? Why, about $100!. As Jesse James was so fond of saying: "Pay Up Sucker."
...but I ain't holding my breath.