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Drilled rotors: several interesting issues
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Posted by Dean [Email] (more from Dean) on Fri, 29 Apr 2005 08:44:03 Share Post by Email
In Reply to: Certianly not true, ckpaden, Thu, 28 Apr 2005 10:41:07
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Putting on my heat transfer hat...

For any given braking event, the kinetic energy of the vehicle is turned into heat which is generated at the pad-rotor interface. Many braking events are brief and the heat generation is thus also of brief duration. The temperature rise will be the that energy divided by the thermal mass of the rotor. A drilled rotor has less thermal mass and thus it will rise to a higher temperature than an equivalant plain rotor. Likewise, the pad surface will then also run hotter in a cross drilled application. The surface temperatures will be higher than the average calculation as the surface will be hotter as a result of the time delay for the heat to conduct into the bulk of the iron.

Compared to the cast iron rotor, the pad is a thermal insulator, and the bulk of the heat will be delivered into the iron, not the pad.

After a braking event, the rotor then cools. The heat loss is to to the air. Vehicle speed aids in this. A hard stop to a stop light really reduces the abiltiy of the heat to escape. The holes do increase the heat transfer surface available, but how significant this is, is not known. For serious track use one would want to duct some air flow to the brakes to keep the temperatures under control.

The pads can generate gases as the organics in the pads are roasted. This can cause pads to float on a gas layer (on a plain rotor). Slots, drilled holes or dimples releave the gases from the pad/rotor interface. So gas-fade is prevented. So drilled rotors will have higher peak temperatures and increase fas generation when very hot, but the gas-fade is eliminated. And wet or damp pads can gas-fade on a layer of steam under conditions where the brakes really are not very hot at all.

A performance pad can have a higher fricton factor and also be designed to be less prone to out gassing. Pads for track use are designed for high temperature operation, but often have poor friction factors when not heated up, thus these can be a very poor choise for use on the street. There are some pads which are good for street and lighter track use.

So drilled/dimpled/slotted rotors will increase rotor and pad peak temperatures, and pad outgassing will increase, but the gas-fade or float effects of the outgassing are avoided.

The major complaint about drilled rotors is cracking initiated by the weaknesses from the drilling. Remember the heat transients? The surface temps are very high and the inner temps of the rotor are lower. This creates tensile loads at the roots of the holes and compression of the swept surface. Dimpled rotors are effective at reducing gas-fade effects, but avoid much of the cracking tendancy. They maintain most of the visual appearance that many seek. There are some slotted and drilled rotors available too.

For repeated hard stops, temperatures keep climbing. A drilled rotor might run cooler as a result of its increased surface area. But that would depend on other variables as well as speed and duration of runs between braking events. But I think that one should discount this increased heat transfer consideration as it is not a known quantity. The main benefit is avoidance of gas fade, even though peak (transient) temperatures may be higher than with a plain rotor.

Drilled/dimpled/slotted rotors do increase pad wear compared to a plain rotor. This may have off setting advantages from a performance point of view by exposing newer pad material. But this certainly does not decrease the cost of ownership.

My preference would be for dimpled vs drilled from a durability point of view. And for single braking events on the street (dry pads), the plain rotors are probably every bit as good as the alteratives. Only when the pads are hot enough (repeated agressive braking) to be generating gasses do the other rotors show any significant advantage. In the wet, a good driver is aware of the implications of wet brakes and will dry the brakes or brake well ahead and early.
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