When coming to a hard stop, the brake disc heats up significantly. By maintaining pedal pressure after stop, the disc area directly under the pads experiences heat soak by not being exposed to the air and afforded an opportunity to cool at the same rate as the uncovered areas. My theory is that this contributes to accelerated disc warpage which can be avoided by allowing the vehicle to creep ahead after stopping to avoid localized heat soaking and thus allowing a more even cool-down. I?ve been practicing this for quite a number of years and have never had to replace discs due to being too warped to resurface.
Any thoughts on the validity of this or am I just being obsessive/compulsive?
I would say that is a good theory. The only question is how much.
Considering that the pads are usually very close to the rotors normally, I don’t believe the difference is significant. However that is not based on imperical data imperical data, so my guess is no better than anyone else’s.
Some real-life observations from one of those infrared thermometers would be handy here.
There is some validity to it, but It takes some pretty hard driving to heat the rotors to a point where this is a significant factor. On the street, good defensive driving, such as looking 10-15 seconds ahead (Smith system “Aim high in steering”), will largely eliminate the need for such hard stopping.
I think that the quality of the rotors, as well as the lug nuts being torqued properly, is a bigger factor in preventing warpage. I had a lot of trouble with rotor warpage in my Saturn SL 2 with the OE parts. When I finally started doing my own brakes I got the highest grade NAPA rotors, which were half the price of the rotors that were sold at the dealer. The NAPA rotors never warped.
Rotors from the dealership are guaranteed to be priced higher than anywhere else because there is no such thing as a discount there. They sell them to you at list price, unless you own a shop, then you might get them for five to ten dollars under list. NAPA’s list price would probably be comparable to the dealership price.
Interesting theory. Let me preface my reply by saying that metallurgy is not my area of expertise.
My understanding of the cause of warpage is that the heat level in the metal reaches a point where the residual internal stresses in the grain structure of the metal are relieved, leaving variation in the resultant profile of the metal. Residual stresses are introduced primarily by machine operations.
The factors involved are (1) the amount of residual stress in the metal, (2) the amount of heat generated, (3) the part’s ability to dissipate the heat successfully and keep the temperatures balow the level of stress relief, and (4) the amount of impurity in the metal (impurity exascerbates the problem).
It is my belief that a disc properly designed and manufactured with good quality castings (with few inclusions and exclusions) that is heat treated after cutting to relieve machining stresses and checked for flatness and parallellism, and perhaps even “final machined” if necessary, are the good discs. The cheap Chinese discs that tend to warp are probably not made of quality casting and probably not heat treated.
“Quality casting” is a whole 'nother. clarification. Casting processes vary from gravity sand casting to reverse-gravity lost-wax casting, the latter being by far the best quality.
In short, I don’t think a localized heated area cooling at a lesser rate than the rest of the disc causes warping.
I’d be interested in hearing from a metallurgist out there. Is there a metallurgist in the crowd?
Thanks for your thoughts, guys. You are correct about the castings, Mountainbike. Based on my experience with castings, brake rotors are made as sand castings. Cast iron is typically made from re-melted steel and iron and who knows what. The resulting molten mass is not always homogenous when poured in the molds, so the resulting casting may have ?hard? and ?soft? spots which typically occur in rotors made in China or wherever cheap parts are made. No amount of heat treating will fix that.
I believe the consensus here is that there does not appear to be a real contributing factor associated with heat soak. However that is based on anecdotal observations, including mine. Out of curiosity, I would really like to see a lab study performed on this.
I would too. Mine is only theory, no more valid than anyone elses.
Supertramp weighed in on this subject with The Logical Song. “Some they do and some they don’t, and some you just can’t tell. Some they will and some they won’t, and some it’s just as well.”
So did Bud Dry. They just asked “Why ask why?”
Meat Loaf just left the arena entirely. “Who am I, why am I here? Forget the questions, someone get me another beer.”
Disc brake rotors are quite effective heat sinks. It is difficult to get a single hot spot because the heat dissipates over a region of the rotor. To check this out, try to heat up a small section of a rotor without getting everything around it hot also. So, the worst case scenario would be an uneven temperature profile over the rotor area, but not a clearly defined hot spot under the brake pads. I think you will find that the area under the brake pad is not much hotter than the rotor 1" away from the brake pad.
"or am I just being obsessive/compulsive? "…Yes, I think so…Silvergo posted the correct answer…Cast iron conducts heat quickly, and can withstand tremendous heat without damage…