Originally Posted by Kestas
The rotor shown looks to have a lot of meat left, a smooth shiny braking surface, and a good surface for wheel mounting. If it passes thickness and has no vibration during breaking, it should be good for continued service once the scale is chipped out from between the fins.
Yes the braking surfaces look fine, the mounting surface looks good also, all true but the mounting surface is covered by the wheel and the surfaces are constantly being cleaned off due to braking action, the thickness may also be okay but that's not the problem.
These are cooled rotors meaning the two braking surfaces are separated and held together from the hub section out only by the cooling fins. as I said a small to moderate amount of rust is usually not an issue but severe rust in this internal area weakens iron in the rotor. during normal driving where line psi is around 500 psi you probably wouldn't see a failure but..
Consider during a panic stop there can be 1500-2000 psi line pressure driving the pistons and pads against the rotor, this translates to lots of energy at the rotor.
Granted stopping from 100mph in NA is unlikely but 70-80 is certainly possible on the highway.
Quote
The amount of heat produced in context with a brake system needs to be considered with reference to time meaning rate of work done or power. Looking at only one side of a front brake assembly, the rate of work done by stopping a 3500-pound car traveling at 100 Mph in eight seconds is 30,600 calories/sec or 437,100 BTU/hr or is equivalent to 128 kW or 172 Hp.
Think about the amount of pressure/force required to generate that much energy power for each front brake. If the cooling fins fail the rotor will initially crack and the brake pedal will go to the floor because all the fluid in that stroke has been used, release the pedal and push it again and it will push the pads once again against the rotor, with luck it will stop and tear the pads to pieces if unlucky the two rotor surfaces can collapse even more, not the scenario you really want to be in in an emergency situation.
Considering new rotors for most cars are very inexpensive IMO any rotor that is suspect should be replaced, you cant go wrong erring on the side of safety.
The miserable old geezer that taught me everything after 4 years of trade school said many things that have stayed with me for 40 years, one of them is..
"A rotten (not surface or light rust) stressed part is a dangerous part". He was right.