The recent rise is popularity of these discs has brought some confusion as to the performance of these discs, and which are suited for which purpose.
As we know normal brake discs have a smooth, flat surface. Holes and slots in the face of the disc have a number of purposes.
Firstly removing gases from the face of the disc, these gases greatly reduce the coefficient of friction. Disc pads, when hot, expel gases. These gases form a cushion between the face of the disc and the pad.
It takes a tenth of a second to squeeze these gases out on normal rotors. Now this does not sound like a long time, but consider this. When a vehicle is travelling at 100km/h, it is moving at a rate of 30 metres per second, therefore a tenth of a second is three metres. So in essence when the brakes are applied the vehicle travels for three metres squeezing out gases, and not creating friction to slow the vehicle.
Another problem that occurs when the build up of gases is not released is that the pad material becomes hardened and glazed, greatly reducing the amount of grip between the pad and disc. Cross-drilling and slotting allow these gases to be removed immediately, also helps to deglaze the pads, increasing the grip between the pad and disc, hence shortening the braking distance.
Cross-drilling and slotting makes the disc surface uneven so water and dust cannot develop into a thin layer that becomes a smooth, glass like surface and can greatly reduce the coefficient of friction.
Cross-drilling and slotting work effectively to reduce the main problems that occur in brake systems.
But there are some tradeoffs, such as a shorter pad life of approx. 10% so if your getting 40,000kms from a set of pads this can be reduced to 36,000 kms, a small price to pay for better braking performance. Also the cross-drilled rotors are more prone to cracking under extreme conditions, such as racing.
Both the cross-drilled and slotted and just slotted discs have the same performance qualities.
So the question is do I fit cross-drilled and slotted discs, or just slotted discs?
This question has to be asked of the driver, what is the main use of the vehicle?
Is the vehicle used for racing, or driven extremely hard?
Does the vehicle go off road?
If the answer is yes to either of these questions, slotted only discs should be recommended.
If the driver of the vehicle only drives on the street, but drives it hard occasionally, and has nice open wheels where the rotors would look good as well as perform, recommend the cross-drilled and slotted.