Re: Slotted / Grooved rotors

[Ed Lutz <edlutz@domain.elided>]

There seems to be some confusion on all this slotting, grooving,
drilling, and venting of rotors so what the hell, here's my
contribution...
First I have to say that I have never seen rotors grooved as described
by Brian and Michael as used by Saab. My brake engineering has all
been in the field of motorcycle racing and its not a method I have
seen used there..yet, I hear as of yesterday that MV Agusta brakes
have "circumferential grooving" but as of yet I don't have an
explanation for what that really is referring too (quote is from a
friend who writes for one of the big US bike mags, lucky bastard has a
MV test bike in his mitts...).
Rotors are drilled to increase surface area but not brake frictional
area. The need is to increase cooling effect and by drilling holes you
gain the surface area of the inside of the hole. The trick, which many
manufacturers and machinists seem to not understand is that the hole
size can quickly get to be too big. If you remove more surface with
the hole than you gain with the hole walls, then you decrease cooling
effect and end up with what may look pretty, but in reality is a junk
rotor.
Slots cut through the disk, radiating from the center either in
a curving pattern or straight out are meant mostly to control warping
by allowing the disk to expand without forcing bends in the material.
Think of them as expansion joints. Added cooling area may also exist
depending on the width of the slot. They don't need to be very
wide and again many manufactures go more for style than function.
Milling, which is a groove that runs like the slots described above
but without cutting through the disk, just a shallow groove, is
something I've never had much faith in. There is probably some added
cooling surface area but not enough to be worthwhile. Some claim it helps
water dispersion but in my experience I've seen no difference. Others
claim it will help keep the pads from glazing but again I've never
seen it and prefer to take care of my brakes as opposed to purposely
grinding them down. They can look nice though so what the hell.
Venting is great for cooling and has I think been discussed already.
As vented rotors are not used on bikes I've not worked with any
outside of a few street cars. They work great though just don't fit a
bike application very well.
As for water dispersion, if you want to have a good exciting time with
no brakes then hop on an older (or newer for that matter) bike in a
heavy rain storm. It can get you real close to whatever god or maker
you may believe in really quickly. The problem is that unlike cars the
bikes rotors are right out there in the breeze, right in the water
with no nice comfy wheel wrapped around it to help keep it somewhat
dry. That coupled with the junk stainless steel rotors that the
Japanese used to use; rain meant the brakes would just go by by, until
you held them long enough to wipe them clean when they would grab
RIGHT NOW. Newer rotors work much better and slotting especially seems
to help but the trick, which may work on cars too (no reason it
shouldn't, I've just never tried) is to cut the pads. Using a narrow
triangular file, cut grooves in the pad following the direction of the
rotors rotation. Just a few, it shouldn't take much. Three or four
groves spaced evenly across the pad. This gives the water someplace to
go when you press the brakes. Its not a miracle cure but does help on
the bikes I've done it to and used to be standard procedure on our
endurance racers if we thought rain was a possibility.

Ed

Monday, April 08, 2002, 11:10:42 AM, you wrote:

SI> I was confused about the surface area too, untill
SI> someone said "concentric rings". I've only seen
SI> the grooves that spiral out from the middle;
SI> looking somewhat like fan blades.

SI> Concentric grooves are totally new to me. My discs
SI> appear to have them though, but they were not
SI> created during the manufacturing process. It took
SI> years to create. I better not hone them flat!!

SI> Stefano
SI> Oakland, CA

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