Cupcar#12

also we need to think about the difference in true from the Hat as it relates to the disc - on stock rotors these are one peice and can "Warp" as well - leading to an out of true situation with the entire unit. this is caused by differing expansion due to diffeing amounts of heat being transfered (hotter to cooler).

I never turned my rotors - if they warped (and i was always very carful to cool them propperly in the pits - i would replace them. they usually needed replacement yearly along with the other wear items in the brake system (pads and fluid) - just part of the maintiance routie when i was tracking 20+ times a year

My 2 cents (as always i reserve the right to get smarter )

Arjan B.

Thanks Rob for your input.

Bill Verburg

The following is a direct quote from Brembo
"If the vibrations appear after or during exposure to high temperature then the phenomenon is known as hot judder. In an attempt to cause such vibration a series of average level deceleration braking actions are performed while speed is increased up to 90% of top speed. For instance this type of test resembles leaving a motorway: speed is high while braking action is average but prolonged since it continues until the vehicle comes to a halt. As a consequence the temperature increases considerably. If friction is uniform over the entire disc braking surface, then energy is evenly distributed and nothing critical happens. If friction is more accentuated in one or more points then the energy exchanged at those points will be greatet at the outset and there will be a rapid, localised temperature increase. Vibrations appear Simultaneously at the hot points, which are normally distributed in a uniform manner over the disc, and these, on cooling, create more or less visible dark patches, or blue points. This transformation mechanism, the cause of which can be attributed to pad material (rapid friction coefficient variation, encrustation, etc.) is extremely detrimental and the disc is damaged beyond repair.
In fact the blue points are a localised conversion of cast iron into cementite an extremely hard substance. This transformation takes place at very high temperature and is non-reversible: as the blue points will be lesss subject to wear than the rest of the surface, the phenamenon will spread with each braking action of the type described. Transformation of the cast iron affects it to such a depth that a reworking of the surface would not resolve the problem. During development of pads for a new vehicle the possibility that a certain composition may cause hot judder is sufficient reason to discard it. As a result, pads homologated by the original equipment manufacturer do not have this defect. It should be noted however that use of discs worn down below their minimum thickness can be the cause of this type of deterioration since the temperatures reached tend to be much higher. The "snatching vibration" phenomenon is the onset of noises that are sometimes accompanied by vibrations. This is due to a variation in the braking torque, the cause of which can be attributed to instability of the pad material's friction coefficient. Incorrect running-in may at the same time be both the cause and the consequence of this. This problem appears in cases of heavy, though not sharp, braking -once again, when the temperature is very high."

Here's the reason all rotors will warp and eventually crack if abused. Each app will have a different level at which this occurs.

When two bodies slide against each other, frictional heat is generated and the resulting thermoelastic deformation alters the contact pressure distribution, the data shown here is on a properly functioning system in ~4mins of normal street use

For further reading check out some of the work done by J. R. barber