Alfa Romeo Forum - View Single Post - Front disc replacement...drilled, slots or std
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Join Date: Mar 2016
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Posts: 340
My understanding:

I can't see how it could be correct that grooves or holes will affect pedal feel in any significant way.

Grooves and holes don't significantly reduce heat. To a very minor degree thay can help the disc get rid of heat because both grooves and holes can increase the surface area of the disc. But, with holes this can go the other way, i.e. if the holes are not large enough relative to the depth of the metal through which the hole is drilled, then the surface area of the disc can actually be slightly reduced. This would have the opposite effect, i.e. cause the disc to retain more heat (though either way the effect will be very slight).

The primary purpose of holes and grooves is to fascillitate the evacuation of gasses which are a product of the pads overheating (or at least getting very hot). When the pads get hot enough they 'outgas', i.e. produce gasses which are vented from the pad faces against the disc faces. When the gas quantity is low the gasses can escape quickly enough that it has little affect on braking performance, but when a lot of gas is being produced it cannot escape quickly enough (from between the working surfaces of the pads and disc). When this occurs, the pads are lifted off the disc faces on a microscopically thin film of highly pressurised gas. Since gas has an extremely low coefficient of friction, the result is a dramatic loss of braking effect, often very sudden. This effect is known as 'pad fade'. It's often visible as 'smoking' brake pads, but you may have problems before smoke is actually seen.

If the disc has holes drilled from the working faces through to the internal vane voids, this provides an escape route for the gasses. If the disc has grooves machined into the face surfaces, then gasses can escape along the grooves to where the pads are not covering the grooves, and vent more easily to atmosphere. Either way the gasses will vent more readily, and so not as easily prevent the pads from properly contacting the disc faces (for friction to occur, the pads must be contacting the metal of the disc faces, not riding on a film of pressurised gas). This is why even stock brake pads very often (usually) have a groove machined into the pad material, i.e. to help vent gasses from between the pad and disc faces.

Disc grooves can help to remove glazed pad material from the pads, but in doing so can contribute to accelerated pad wear. For this to happen, the edges of the grooves must be fairly 'sharp', so that the groove edge can very lightly 'machine' the pads every time the brake is used. If the edges of the grooves are 'rounded', then the machining affect won't occur (or at least much less so).

Disc holes / grooves may well be useful if the brakes are used hard enough that the pads 'outgas' significantly and cause pad fade. If not then they probably only increase pad wear. If the car never experiences sudden brake loss (due to pad fade) when using stock ungrooved / undrilled discs, then it doesn't need grooved / drilled discs. If it does experience pad fade, then it may benefit from drilled / grooved discs, but equally, 'harder' pads may work just as well. Using grooved / holed discs, it may be possible to use softer pads.

Grooves / holes will reduce unsprung and rotational mass, but only to an insignificant degree.

Grooves can be stress risers, and so can initiate cracks. This is why the grooves should never extend right to the edge of the disc, i.e. this makes cracks more likely to form. It is also why some disc grooves are kind of 'J' shaped, i.e. the recurved end of the groove lessens the stress raising characteristic of the groove.

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