Re: Cross-drilled rotors? (Long, sorry) (archive)

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There was an interesting article in a recent edition of the Audi Quattro Quarterly that talked about the various kinds of brake discs and what the advantages and disadvantages were.

Cross-drilling was originally conceived as a heat dissipation measure on race cars. The Porsche 917 was probably the first car I recall having them, and, as someone observed, they are MASSIVE (not so much in diameter but in their thickness.) The wheels on these cars even had large fans mounted on their faces, the idea being to draw air out through the wheels and aid brake cooling.

But the Quattro article also mentioned something else about the way the cross-drilling worked. The article said that as the puck comes in contact with the disc during the application of the brakes, there's a thin layer of vaporized gas between the two. This layer of gas essentially reduces the effectiveness of the braking by reducing friction in the system. Drilling holes in the disc provides a means of flushing this vaporized brake material from the point of contact on the disc, thus theoretically improving the effectiveness of the brakes. Theoretically. Now, there was someone who'd commented on how the braking of his vehicle worsened with the fitting of drilled discs, but I'll go with Porsche on this one: drilled rotors on racing cars are not fashion statements - - - they work and that's it, so long as they're properly engineered. (Sorry, drilling holes in street-car brakes just sounds dangerous.)

But there IS a down-side to the drilling. Knocking holes in something that heat-cycles as often and as dramatically as a brake disc is gonna cause problems. The holes are stress-risers that eventually lead to cracks. You can heat-treat the disc, champher the holes, space 'em, splay 'em, whatever - - - eventually they're gonna crack. Race cars can change discs every race; a street car has to go a little longer.

Cross-drilled rotors work but you probably don't drive hard enough on the street to justify the crack-checks and more-frequent replacements they'd require - - - and if you do drive that hard, brakes are probably the least of your worries.

A better alternative advocated by the article was what are known as 'gas-slotted' rotors; these are the ones that have radial grooves sliced into the face of the rotors and they have several advantages. First, they're cheaper; 5 to 10 passes with a mill instead of 30 or 40 or 50 some-odd individual drillings. Second, they don't crack. The grooves are scooped out and shallow, without the holes and sharp edges that creat stress-risers - - - and cracks. Third, the slots allow for the dissapation of hot gas from the face of the disc just like the holes in the cross-drilled rotors. And last, as the disc wears, the slots disappear; basically, you've got a built in wear-indicator. No, they're not as sexy-looking as a Godzilla-sized set of Alcons or Brembos . . . but they'll only cost you a hundred-or-so a corner, less if you do the work yourself. If you have a few more bucks to spend, get the cryogenically heat-treated discs; they'll probably out-last your enthusiasm for the car.

The short version is - - - drilling holes in street car brakes might look trick, but probably compromises the system as it was engineered to work. As for full-on racing-duty systems, go find a place that deals in Alcon and look at the prices; believe me, without a sponser's name on your car, you'll never do it. For a car you like to use and live with every day, get a good OEM rotor first, a slotted disc next, and a cryo rotor if you can find it. They may not exactly be eye candy - - - but they work.

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