Brake warping revisited

[C M Smith <cmsmith@domain.elided>]

Being unafraid to go where even angels fear to tread, I suggest brake rotor 
warping is caused by uneven cooling of the rotor from very hot temperatures 
combined with inadequate thickness to resist the internal forces causing 
the rotor to bend. I very much doubt that pad pressure could cause even red 
hot cast iron to flow in the manner suggested by our esteemed Fred.

I have only experienced rotor warping once. A short while after my mechanic 
resurfaced (against my better judgment but he always tries to save me 
money) the solid rear rotors on my 86 SAAB 9000 Turbo I was driving at high 
speed on the Coquihalla highway on my way to Vancouver, in the rain. There 
is a steep downgrade well known to aficionados of this high speed highway, 
just West of the Summit. I might have been travelling in excess of 180 
km/hr in the rain down this steep grade ( officer) when some dim bulb in 
his Ford Aerostar van decided he really wanted the fast lane of three for 
no apparent reason, just as I arrived. I hit the brakes very hard as I 
understeered nicely into the middle lane (inside bend for the fast lane) 
and the rear rotor on the outside warped immediately, due to the cold rain 
hitting the hot discs. I got back on the loud pedal right away and as I 
regained speed I was amused to see the driver of the Aerostar attempt to 
increase his already dangerous speed presumably on the theory that if I 
could, he could. Bad mistake. He dropped back sharply at the next bend 
after a little bit of what must have been real white knuckle understeer on 
what was then an outside bend for him and an inside bend for me (the third 
slow lane disappears at this point as do the shoulders, replaced by nice 
sturdy New Jersey Barrier, adding to the excitement) BTW, the Pirelli P7000 
Performance tire is truly wonderful in the rain, very predictable with 
excellent steering and braking feel and impressive grip.

Having said all of that, I remain convinced that failing to release the 
brake pedal as one comes to a complete stop so as to allow the pad heat to 
be distributed around the rotor more evenly before clamping the brakes on 
to remain stopped may be a significant contributing factor to premature 
rotor warping. However, I doubt this has anything to do with solid metal 
flow but everything to do with differential cooling. The hot pads insulate 
the rotor at one spot when stopped allowing the rest of the rotor to cool 
faster. If the pads and rotors are very hot this differential cooling rate 
will be significant. As the front brakes get much hotter than the rears, 
applying the emergency or handbrake just as you come to a complete stop 
would be a good practice if the brakes had been heavily used as you came to 
a stop. this at least allows the hotter front brakes to cool without the 
pads actually clamped to the rotor faces.

Bottom line; buy only good quality rotors, do not bother to resurface them, 
just replace them when worn. Also,  vented rotors are pretty warp resistant 
as compared to solid discs. It's the cooling that warps the rotors, not the 
heating. That's why the best rotors are heated and then chilled before 
final surfacing.

Cheers


Michael Smith
White 1991 164L
Original owner 
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