Quick Tech With Mar­lan Davis

Hot Rod - - Contents -

Last time we dis­cussed pos­si­ble ben­e­fits of cross­drilled disc brake ro­tors, but there’s a down­side, par­tic­u­larly on street-driven cars sub­ject to repet­i­tive heat­ing and cool­ing cy­cles: it cre­ates stress-ris­ers, lead­ing to crack prop­a­ga­tion from the drilled holes. Gen­er­ally, a race car has less of a prob­lem be­cause it has less ex­treme heat-cy­cling (the brakes get hot and stay hot). High-end race ro­tors are made from bet­ter ma­te­ri­als, and rac­ers con­sider ro­tors a dis­pos­able item and pe­ri­od­i­cally re­place them just like an oil fil­ter.

To re­duce crack po­ten­tial, cross-holes should not be drilled with a uni­form “in­line” cir­cum­fer­en­tial pat­tern on equal radii. That’s more likely to ad­versely af­fect the struc­tural in­tegrity and lead to cracks. In­stead, drill holes in an over­lap­ping, stag­gered pat­tern.

As to hole lo­ca­tion, Porsche used to drill through the ven­ti­lated disc webs. AP Rac­ing drilled into the hol­low area be­tween the webs. The late race en­gi­neer Car­roll Smith rec­om­mended the Porsche pat­tern for curved-vane discs and the AP pat­tern for straight-vane discs. It is also nec­es­sary to care­fully cham­fer and ra­dius the hole edges to re­duce the chances of stress crack­ing.

There’s a way to ob­tain most of cross-drilling’s ben­e­fits with­out the dan­ger of cracks: Slot the ro­tor face in­stead. We’ll get into that next time.

Baer Brakes

[ Baer EradiSpeed ro­tors are avail­able both cross-drilled and slot­ted. Baer says it casts its ro­tors from crack-re­sis­tant ma­te­ri­als, and the holes are placed be­hind the curved vanes to min­i­mize crack mi­gra­tion. Note how the holes aren’t con­cen­tri­cally in line with ad­ja­cent holes, as well as how the slots point in op­po­site di­rec­tions on th­ese two di­rec­tional ro­tors.

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