I found a vendor on eBay that offers slotted and drilled rotors for just $25
or so over the price of standard rotors, this is for the complete set of
rotors and semi-metallic pads for both the front and rear axles. The cost-up
also includes nickle plating that I hope will keep the rust at bay. The
rotors LOOK really good, how they perform is yet to be seen. I'm not really
a speed demon, but I like stuff that looks good -- assuming it works good
too, which is really the more important quality of stuff.
I decided that if slotted and drilled rotors are good enough for an M3, they
should be good enough for my '94 convertible. My Bentley manual states that
the the slotted rotors are directional, but does not describe which rotor
goes on which side.
My instinct is that the slots and holes should slant toward the rear as I am
looking at the top half of the rotor. Does anybody dispute that, and why?
Interesting question. My first thought would be to start looking at
motorcycles but I did find this in an ad for Brembo Rotors on the internet.
Which direction should the discs rotate?
It is a popular misconception that the slots or drillings in a disc
determine the direction of rotation. In truth, for an internally vented
disc, the geometry of the vanes dictates the direction of rotation. There
are three vane types in use:
b.. Pillar vane (comprised of many small posts)
c.. Curved vane
The first two vane types are non-directional, and can be used on either side
of the vehicle. The curved vane disc, however, is directional. A curved vane
disc must be installed with the vanes running back from the inside to
outside diameters in the direction of rotation. Please see figure. Orienting
the disc in the manner creates a centrifugal pump. The rotation of the disc
causes air to be pumped from the center of the disc, through the vanes, and
out through the outside diameter of the disc. This greatly enhances the
disc's ability to dissipate heat.
Additionally, all of Brembo's slotted discs are directional as well,
regardless of the vane geometry. The discs should be installed such that the
end of the slot nearest the outer edge of the disc contacts the pad first.
Please see figure.."
Interesting. This says that I should install the rotors opposite of what my
intuition told me.
POINT OF ORDER
BMW uses vented front rotors that have straight vanes.
I wish the figures referred to in the link actually existed ...
So your intuition doesn't know physics well. At any given point in the
travel of your wheel, the top is always passing through the air faster
than the bottom (which is, technically, momentarily stopped). If you
want to 'scoop' cool air, the vanes at the top should facilitate that.
This makes more sense than trying to expel hot air from the center of
the disk since the brake backing plate will tend to block the intake of
Then it doesn't really matter ...
I'm hoping your considerations don't include noiselessness in your
cabrio because drilled or slotted rotors tend to make some noise
compared to solid face types.
(Been there; done that)
If you were a physicist, you probably would not have bought slotted rotors
(disks)UNLESS your BMW is a motorbike. On a bike the disks get wet in the
rain and the slots allow water to be dispersed more quickly (less than a
single rotation of the wheel). On a car they simply reduce the contact area
and probably muck up the wear profile of the disk and the pad.
That may be true, but lots of high performance cars get slotted and drilled
rotors as factory fitment, so I suspect it is not true. At least it is not
true to be significantly adverse to the quality of the braking experience.
There may be no particular benefit to me since I am not a Ricky Racer, but I
have 'em now and I want to put them on in the correct orientation.
The M3 gets directional discs on the front and rear as factory fitment. I am
not sure if the directional nature is due to slots and drill holes, or if it
is due to the cooling vanes between the disc surfaces. Perhaps somebody with
an M3 will chime in.
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