My car has cross drilled rotors, and I notice that the AMG Mercedes have this factory, as do some other cars.

Is there any real gain to this? Or it is just a item to help sell a vehicle? I figured I should ask here because there are a lot of race orientated people here.

Besides eating pads twice as fast and the extra BLING BLING points...

Slotting *should* give you more gripping surface area on the rotor, cross-drilling *should* give it better cooling.

More often on solid rotors the slotting just eats your pads and the cross-drilling leads to bad stress fractures.

factory cars usually have cast holes.....then drilled

cross drilling *apparently* weakens the rotor... watch for cracks

newer cars have the holes cast in -- this wasn't done in the '70's. the drilling isn't done for cooling -- the air ducted to the eye of the vented rotor handles that.

the holes are to prevent fade during hard braking when the brake pad material vaporises, and to prevent "green fade" as solvents and goop bake out of new pads. nowadays, slots do that as well, or better, especially the log-spiral grooves. in the early days, before CNC machines, grooves like that were a costly hand operation by a skilled machinist. now they can be cranked out on huge quantity.

but MOSTLY (IMO) holes are there to improve braking in the wet, because water will be driven into the holes and out through the vents instantly.

i like them. been a while since i've had them on a track, but in the events i did run, i had no cracking issues. i do expect cracks to form eventually. i'll probably replace them drilled disks again, but there are some interesting disks that have dimples (not through-holes) that should at least displace the water and slots to wipe the fire-band.

it's NOT something you can do for yourself. there is a science to the hole placement, they need to be carefully countersunk, and ideally, heat-treated and Blanchard ground after.

if you do the research, you will find cases where the Porsche factory race teams took OFF drilled rotors at dry tracks that were hard on brakes -- to increase the amount of heat-sink mass. this is a team that can be depended to come to the track with bedded pads :-)

According to Wilwood Engineering (the brake caliper and rotor company)

QUOTE

Caution on drilled rotors: There is a common mis-perception that rotors are drilled to improve cooling. The reduced mass of a drilled rotor will dissipate its retained heat quicker, but it also builds up heat at a much faster rate. The decision to use drilled rotors should be solely based on the merits of the lower rotating and unsprung weight, and not for improved cooling. It is not wise to use drilled rotors in sustained high heat on hard braking tracks unless the team budget affords a high frequency of rotor and brake pad replacement.

QUOTE (ArtechnikA @ May 13 2005, 04:50 AM)

the holes are to prevent fade during hard braking when the brake pad material vaporises

If you drive a lot and are in a wet climate (like me) then the cross-drilling and/or slotting helps with initial brake bite. Cracking isn't a real issue unless the car is track driven i.e. driven HARD and thus braked HARD. I've had cross-drilled rotors on my solid rotors and cross-drilled on my vented rotors and neither have cracking issues. However, for my vented rotors I am going to go to slotted rotors. After talking to the Saturn race team brake specialist the slotted will still give great wet bite yet not be so aggressive on pad wear compared to cross-drilled.