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one caveat (archive)

[ Follow Ups ] [ 7-series (E32) Message Board ] [ Msg. Board FAQ ]

Posted by txhedg on August 06, 2001 at 17:58:56:

In Reply to: Anyone have pics of posted by Ben 88 750il on August 06, 2001 at 17:15:43:

I don't have pics, but thought I would offer my $0.02 with regard to cross drilled and slotted rotors..........

From my experience with track cars, in many cases cross drilled rotors are NOT worth the extra expense/hassle/risks associated with the cross-drilling process. I guess they can look "cool", but IMHO that is not worth sacrificing performance/reliability/durability.

I would agree with most of the "conventional wisdom" of the benefits of cross drilling (IMHO a cross drilled rotor will perform better i.e. less fade, than a similar non-drilled rotor), but I wanted to add one big issue that is not often mentioned.

Basically, there are two ways to create the "holes" in a cross drilled rotor. One way (and the way most aftermarket crossdrilled rotors are made)is after the rotor is cast, holes are drilled through the rotor. These holes allow better cooling of the rotors and venting of the "off-gas" creating in the friction process. However, this drilling process creates weak spots in the rotor, and can lead to stress cracks forming near at the edges of the holes in the rotor under hard use conditions (especially track time, but also other non-track hard braking conditions in a heavy car e.g E32s).

The way to avoid this cracking problem is to CAST the holes into the rotors at the time the rotors are originally made. This process has been shown in track cars to eliminate the tendancy for rotors to develop cracks at the edges of the holes. Creating the holes thru the casting process is much more difficult and costly, so few aftermarket rotors are made this way (Porchase OE is one of the few sources of cast crossdrilled rotors I know of.....I'm sure there are others, but in my experience most you find in the market are NOT cast.)

Again, from track experience, we have found a good alternative to cross drilled rotors are slotted rotors. The slots are only "superficial" on the face of the rotor, do not seem to weaken the rotor or promote cracking, and the slots provide sufficient escape routes for the built-up gasses. They don't look as cool (the slots are barely noticeable), but are less $$ and better performing than non-cast crossdrilled.

So, JMHO, but if you do elect to go with the non-cast crossdrilled, makes sure you pay carefull attention to the condition of the rotors.

1991 750iL 98k......non-slotted, non-crossdrilled rotors, still stops great.

their E32 with cross drilled or slotted rotors? Trying to decide what I'm going to upgrade to. Thanks.


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