Rotors


#1

Can rotors warp by being hit directly with cold water. If you have a car that has been sitting in the sun for a while and then was the inside of the wheels can that warp your rotors?


#2

Washing rotors just in the sun? I wouldn’t think so. Spray cold water right after heavy use, maybe. Many rotors are warped by over/incorrect torquing of the lug nuts.


#3

They may also warp if you had a recent brake job and they turned the rotors. Many rotors and so thin to start out with now, that you have to replace them instead of turn them.


#4

It was always (at least for me) hard to determine what the rotor thickness would end up being after turning, you are faced with the problem of telling the customer that the rotors will be in spec. after turning, then when you get done they are right at min. or even slightly under, I suspect many undersized rotors are in service.

I would bite the bullet and tell the Advisor if the rotors ended up undersize, no one was happy, but I had to sleep at night.

There is a difference between undersize for service and undersize for a cutting attempt.


#5

What you’re really alluding to is stress relieving of the metal. The temperatures required to perform this function are much higher than you’ll ever reach by letting the car sit in the sun. My ASME membership (and my memory) have been lapsed for too many years now, but perhaps one of our more up to date regulars can look up the required temperatures.


#6

The sun’s rays have a really hard time hitting the rotors, since the wheels are in the way.

Your car could be sitting outside in the blazing sun in Phoenix, and you could wash the car with cold water without worrying about damage to the brake rotors.

Whoever came up with this theory is full of something, but it’s not common sense.


#7

I agree with the consensus here. Spraying cold water on a sun-heated rotor might distort it a thousandth or so for a few seconds, but it will not warp the rotor. The sun cannot warm rotors enough to have any metallurgical effect on them.

Warped rotors come from non-homogeneous metal going through heating and cooling cycles. Uneven tightening of lug nuts can theoretically cause problems on some cars, depending on the design of the hubs, but I doubt that is as common as people assume. If it does happen, it is probably more from uneven heat transfer due to uneven tightening than from actually distorting the hub and rotor.


#8

The eutectoid temperature for iron/carbon is 700C, and that’s probably hotter than the rotors will get, even under heavy use.


#9

Right or wrong, this is the method I used to use; if necessary, snug up rotor using lug nuts, snap on magnet base dial indicator, spin wheel and measure runout. Multiply runout * 2 and subtract that from the rotor thickness. If close to min, no sense doing it.

Since then, rotors are too thin to begin with and replacements cheap enough that I simply replace them if they have any runout. YMMV.


#10

Thanks. I make that to be over 1200 degrees F.


#11

Xackly! (1292F, at least)


#12

Yup. Hotter than frying pan!