Re: cross drilled rotors

[Toby Erkson <Toby_Erkson@ccm2.hf.intel.com>]

Read "European Car" or other hi-performance car magazines as this is 
extremely common.  As a rule a VW won't need this treatment since most are 
driven modestly and they don't put out enough horsepower.  But when you 
upgrade your engine to produce greater power you need to have improved 
braking as well.

Cross-drilling helps in cooling the disc and, like the channels in the 
tread of a tire, helps to displace water more quickly so the brake pads can 
make contact to the disc.  Drive thru a deep puddle and once out 
immediately try to use your brakes.  You'll notice a delay before the car 
actually begins to stop.  Scary -- I've had it happen.  This is because the 
pads are hydroplaning on the disc surface, much like a tire hydroplaning on 
the road surface.  The holes in a cross-drilled disc provide space for the 
water to escape to thus giving a quicker braking response.

You may notice that some brake pads have a channel in the middle, so it 
looks like two braking surfaces on one pad.  This little channel is to help 
remove water as well as allow one to see pad wear.

If you live in a particularly wet climate you may want to have channels (I 
forgot the proper term so we'll just go with 'channel').  Channels basically run
tangently from the disc hub to the rim of the disc.  This allows improved wet 
weather braking because the water is pushed into the channel and is slung out 
due to centripetal force.  It's becoming more common amongst autocross racers 
and performance enthusiasts to have their rotors cross-drilled and channeled 
(many modern cars now have vented discs as this helps keep the disc cool).  

For those who don't quite understand channeling here's a *rough* picture:
                 <----- direction of rotation
                  _ ______ 
                /  |       \
              /    |         \
             /     | __======== <--- channel
             |      o  o      |
             |     |hub | disc|
             |      o__o      |
             ========   |    /
              \         |   /
                \_______|_/
                        ^---channel
                 -----> direction of rotation

             -- Facing front left rotor --

Don't bother trying to have local shops try this as many won't do it because of 
insurance reasons and they are just not set up to perform the procedure.  Just 
contact the company I mentioned (or any other that does this) and deal with them
-- it'll save you from a whole lotta negative, ignorant comments.

     Toby Erkson
     air_cooled_nut@pobox.com
     '72 VW Squareback 1.6L bored and stroked to 2.0L
     '75 Porsche 914 stock 1.8L for sale

______________________________ Reply Separator _________________________________
Subject: cross drilled rotors
Author:  type-3-errors@umich.edu at SMTPGATE
Date:    4/8/97 9:12 AM

>From: WALTEE <WALTEE@prodigy.net>

>Hey Toby, more on the crossed drilled rotor saga.  I had been checking
>around town here for a machine shop to cross drill my front rotors like
>we discussed, and I have gotten nothing but flack.  Everything from "why
>would you want to do that for?" to "you don't want that, you'll never be
>able to ever turn them after they're drilled" to " they'll break" to
>"sure, but since they are a special hardened steel, it'll cost you $10
>per hole to have it done, and we won't guarentee the work".  I'll keep
>looking, or maybe explore the possibility of using your shop like you
>suggested...

While I really don't have any facts on this, I was under the impression that
cross drilled rotors were a fad a decade ago that had now been shown to have
no real benefits.

On the other hand, there shouldn't be any problem drilling them.  After all
they can be turned without difficulty and if you take a file to them you
will find that they are dead soft.
...