Cross-Drilled and Slotted Disc Rotors

The recent rise in popularity of these discs has brought some confusion as to the right products for particular applications.

As we know, most brake discs have a smooth, flat surfaces. Holes and slots in the face of the disc have a number of purposes.

The first is to remove gases from the face of the disc. Disc pads, when hot, expel gases. These gases form a cushion between the face of the disc and the pad, and can greatly reduce the coefficient of friction.

It takes a tenth of a second to squeeze these gases out on normal rotors. This does not sound like a long time, but consider this. When a vehicle is traveling at 100 km/h, it is moving at a rate of 30 metres per second, therefore a tenth of a second is three metres. So 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, and 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 works effectively to reduce the main problems that occur in brake systems. But there are some tradeoffs, such as reducing pad life by approximately 10 per cent (if your getting 40,000 km from a set of pads this can be reduced to 36,000 km, though most would agree this is a 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.

In normal motoring the slotted-only and cross-drilled and slotted rotors have similar performances qualities. So the question is: do I fit cross-drilled and slotted discs, or just slotted discs?

This question has to be asked: what is the main purpose 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 are the best choice.

If a vehicle is only used on the street, but is occasionally driven hard, and has nice open wheels where the rotors need to look good as well as perform, cross-drilled and slotted rotors are the way to go.