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Old 12-09-2006, 12:20 PM   #4
Butt Dyno
Street's closed, pizza boy
Member#: 17301
Join Date: Apr 2002
Chapter/Region: MAIC
Location: Why do they always say the Evo
is the "dark side"
06 Evo #7 STU, 05 Saabaru


How do I swap from drums to discs?

Upgrading Rear Brankes '03 TS - Drum to Disc

I have a late 2002/early 2003 WRX. What pads do I need?

StopTech has a handy guide here:

Drilled rotors *must* be good for track use! Porsche/BMW/Lamborghini use them on their cars!

Here's some things I've found from the major brake manufacturers:

Drilled or cross-drilled rotors : Discs that have been drilled through with a non-intersecting pattern of radial holes. The objects are to provide a number of paths to get rid of the boundary layer of out gassed volatiles and incandescent particles of friction material and to increase "bite" through the provision of many leading edges. The advent of carbon metallic friction materials with their increased temperatures and thermal shock characteristics ended the day of the drilled disc in professional racing. They are still seen (mainly as cosmetic items) on motorbikes and some road going sports cars. Typically in original equipment road car applications these holes are cast then finished machined to provide the best possible conditions by which to resist cracking in use. But they will crack eventually under the circumstances described in another section (see Cracking). Properly designed, drilled discs tend to operate cooler than non-drilled ventilated discs of the same design due the higher flow rates through the vents from the supplemental inlets and increased surface area in the hole. That's right, inlets. The flow is into the hole and out through the vent to the OD of the disc. If discs are to be drilled, the external edges of the holes must be chamfered (or, better yet, radiused) and should also be peened.
We have countless street cars and full race cars running our products on the track. StopTech never recommends drilled rotors to customers who plan to track their car. All of our testing and data collection has shown that drilled rotors do not perform as well as slotted or plain rotors at the track. The drilled rotors crack more easily, they develop terrible concentric grooves, and just don't last as long.

We've gone through TUV approval on a number of our applications in Germany. This testing involves putting some severe strain and endurance testing on rotors. The results are always the same...the drilled rotors don't take the same level of punishment. We just don't view the benefit of a bit more pad bite and water evacuation as worth the tradeoffs for most of our customers.
In years past, cross-drilling and/or slotting the rotor for racing purposes was beneficial by providing a way to expel the gasses created when the bonding agents employed to manufacture the pads began to break down at extreme temperatures. This condition is often referred to as "green pad fade" or "outgassing". When it does occur, the driver still has a good firm brake pedal, but simply little or no friction. Since this normally happens only at temperatures witnessed in racing, this can be very exciting! However, with today´s race pad technology, “outgassing” is no longer much of a concern. When shopping for races pads, or even ultra high performance road pads, look for the phrases, "dynamic surface treatment", "race ready", and/or, "pre-burnished". When these or similar statements are made by the pad manufacturer, the pad in question will likely have little or no problem with “outgassing”. Ironically more pedestrian pads used on most streetcars will still exhibit “outgassing”, but only when used at temperatures normally only encountered on the racetrack. Although cross-drilling and/or slotting will provide a welcome path to expend any gasses when and if they develop, it is primarily a visual enhancement behind today’s often wide-open wheel designs. Cross-drilling offers the greatest gas relief pathway, but creates potential "stress risers" from which cracks can occur. Baer´s rotors are cast with cross-drilling in mind, from the material specified, to curved vanes, behind which the holes are placed to minimize potential crack migration. Slotted surfaces are what Baer recommends for track only use. Slotted only rotors are offered as an option for any of Baer’s offerings.
Q: Why are some rotors drilled or slotted?
A: Rotors are drilled to reduce rotating weight, an issue near and dear to racers searching for ways to minimize unsprung weight. Drilling diminishes a rotor's durability and cooling capacity.

Slots or grooves in rotor faces are partly a carryover from the days of asbestos pads. Asbestos and other organic pads were prone to "glazing" and the slots tended to help "scrape or de-glaze" them. Drilling and slotting rotors has become popular in street applications for their pure aesthetic value. Wilwood has a large selection of drilled and slotted rotors for a wide range of applications.
Caution on drilled rotors: There is a common mis-perception that rotors are drilled to improve cooling. The reduced mass of a drilled rotor will dissipate its retained heat quicker, but it also builds up heat at a much faster rate. The decision to use drilled rotors should be solely based on the merits of the lower rotating and unsprung weight, and not for improved cooling. It is not wise to use drilled rotors in sustained high heat on hard braking tracks unless the team budget affords a high frequency of rotor and brake pad replacement.
AP Racing:
Drilled discs.

1. Lighter weight than a grooved equivalent

2. Good brake bite due to:

A forcing pad material into the holes

B increased braking surface (many leading edges in theory)

3. allowing gases formed during the braking cycle to escape from the disc braking face.


Discs can be prone to cracking unless care taken when bedding in

The use of hard race pads(high friction level) can cause cracking giving the disc a limited life.

You will find today that most forms of car motor sport will use grooved discs.
Anyhoo - I'd happily admit that I'm not technical enough to speak to all the intricacies of heat transfer as long as everyone else admits that drilled rotors are not a good idea on a track car

But if you cast the holes in place, it should be better than drilling them, right?

No From Brembo:
Are discs with cast-in-place holes better than cross-drilled discs?

Brembo has extensively studied and tested cross-drilling versus casting the holes in place and found no significant effect on performance or durability.
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Last edited by Butt Dyno; 01-22-2009 at 06:04 PM.
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