In contrast, the holes remain effective in "out-gassing" for the whole life of the disc. As with slots, the pattern is unique for each rotor type; typically we have around 36 holes per disc face, each with a diameter of 6.5mm. To counter the tendency for cracks to form between holes, we normally put just one hole per vane on ventilated discs. This ensures the vane rib will act as a barrier between the holes. Each hole is chamfered around its opening to reduce pad abrasion and provide a smooth transition between hole and friction area (the latter to help combat surface cracking).
The feedback from our customers suggested that in certain circumstances a clicking noise occurs. We'd never noticed it; further research showed it is the sound of the pads running across the holes.
Are drilled-and slotted- rotors hard on brake pads?
A more powerful engine uses more fuel, and it stands to reason that if brakes do more work, or generate more energy, they must suffer more wear. In the original planning stage we predicated a 10 per cent increase in pad wear. After all, the holes and slots would be abrasive on the pads and, with less metal-to-pad friction area, we expected more as well. Yet the on-road results have surprised us. In normal usage, pad wear is generally no greater, and rotor wear may be extended.
Testing on high-mileage taxis over the past 12 months has shown a totally unexpected benefit. One leading taxi company using DBA Longlife Gold rotors on its fleet has reported that they can last up to 150,000 kilometres with no machining.
This is three times the company norm, and has been achieved without any additional pad wear (the pads continued to be changed at the normal 25,000km intervals). The reason for the improvement relates to the shaving effect of the slots, which ensure improved contact between pad and rotors, plus the lower running temperatures.
Should high-mileage DBA Longlife Gold rotors require machining, this must be performed on a modern, high-speed, low-feed brake lathe. The correct procedure is to machine from the hub to the outer edge, taking 0.25MM (.001") in each pass. A word of caution: Machine in the one direction only; when feeding back, withdraw the tools and recommence from the hub to avoid tip damage. Repeat until required finish is achieved.
Cross-drilling and slotting removes not only weight, but also friction surface (or "swept" area). The figures are relatively small and the enhanced performance shows that the reduction in swept area is more than made up for by the improved ventilation that cross-drilling and slotting achieves.
For the record, the four disc rotors on a typical big sedan have a total swept area of 351.5cm2. With the cross-drilling we remove 22.9cm2, while the slots take away another 5.4cms. That's a total of 28.3cm3, or 8.1 per cent of the surface area.
|