gt40mkii at gmail.com
Fri Feb 8 21:10:04 PST 2008
Sit back, grab a beer, and get comfortable. This is a long one --
brakes are important.
As some of you may have noticed, I race cars, and if there's one
thing racers know, it's their brakes. Short of the driver, it's the
most important system on a race car. As such, I've been studying
brakes for about 20 years now. Here are a few of my thoughts, both
founded and unfounded...
At 07:26 PM 2/8/2008, Derek Pulvino wrote:
>I don't see
>a reason to change them. I've now put several hundred miles on the
>pads and while I did experience my first need to "bed in" the pads,
>no other problems.
Bedding in the pads is no myth. It serves an important purpose.
In the Bad Old Days of organic pads, racers bedded in pads mainly to
burn off volatile organic compounds in the friction material. They'd
do this so that the compounds wouldn't burn off during a race (racers
call this "green fade"). With modern pad compounds, particularly
high performance street compounds and racing compounds, this is no
longer needed. A high performance pad of any description has
negligible green fade.
Note that when I say "high-performance pad" I do NOT men the
super-duper-whatever pad from your corner auto parts store -- I'm
talking about pads from reputable manufacturers such as Pagid,
Performance Friction, EBC, Hawk, Ferodo, Porterfield, etc... More
importantly I do NOT mean any pad from Albany, or Duralast or any
discount parts store's house brand. Those are merely brake-pad-like
objects that more or less slows down the car, but only just. I
recommend those pads to people I do not like.
What the old timers may not have realized is that this "bedding in"
procedure also transferred a small amount of friction material onto
the rotor's surface. The presence of this material greatly improves
the friction between the rotor and the pad's surface.
When you put on a fresh set of pads these days. they still need to be
bedded in, particularly if you're changing pad compounds or
manufacturers. (The same holds true for rotors -- though you should
avoid bedding new pads in on new rotors -- for reasons never
adequately described to me.) Initially, of you pay close enough
attention, you'll notice that your new brakes don't seem to work as
well as the old ones did (unless you completely devastated the old
pads.) Over the next day or two, if you're observant, you'll notice
that the brakes gradually get better. This is because the pads are
getting bedded in with normal use. Going through a specific bedding
procedure just accelerates this process.
StopTech has an excellent white paper on their web site on the
bedding process and why it's still needed, even with "fade-proof"
modern brake pads.
>Don't know if you've spent much time on the T44 side of this list,
>but if you have I'm sure you're familiar the Bernie Benz method. He
>basically runs with the idea that rotors are going to wear true,
I disagree. If a rotor is well and truly warped, it will re-warp,
even after being turned. Especially if it gets hot. Turning a
warped rotor squares up the outside face, but the internal structure
of the rotor's disc is still asymmetrical (this holds true more for
vented rotors than for solid rotors.) Because the rotor face is
thicker on one half of the rotor and thinner on the other half,
180-degrees away, the two halves will expand differently when hot and re-warp.
This does not happen often. Read on...
>turning or changing the rotor "because" is a racket.
I think its a racket, too, but for other reasons. Read on...
I think 90% of "warped" rotors aren't warped at all. I think they
have irregular deposits of the pad's friction material on them.
I have a pair of very expensive racing rotors (they run about $550 a
pair.) When I was racing last summer, they developed a horrendous
shudder that got worse the hotter the rotors got. It was so bad that
the steering wheel was almost being ripped from my hands.
We got out a dial caliper and checked the rotors, both the inside and
outside faces -- not warped. No run-out, either. Dimensionally and
geometrically, they were perfect, but they shuddered horribly. We
replaced these super-duper rotors with a pair of old factory rotors
and they worked flawlessly -- glass-smooth.
Then I read an article from Carrol Smith, a well-respected chief
mechanic highly respected in racing circles (again, from the StopTech
web site,) who presents a good argument that warped rotors are a myth
-- that the "warp: was merely uneven friction material deposits from
improper care of the brake rotor. Turning these "warped" rotors
worked because the pad deposits were removed in the turning process.
So, I put a wire wheel on my drill and proceeded to clean both
surfaces of my expensive rotors. Next I carefully bedded in my
racing brake pads. The next practice session, I took the car out and
beat on the brakes as hard as I could for a 20-minute session. The
brakes worked FLAWLESSLY!
Thinking back, I remember being careful to bed these rotors in, since
they were expensive. I was running Performance Friction Z-rated pads
at the time. This was at a race outside of Ft.Worth, Texas. The
next race, near Tulsa Oklahoma, I used up the Z-Rated pads in
qualifying, and had to replace them with Carbotech pads -- a hugely
different compound compared to the Z-Rated pads. With no opportunity
to bed in the new rotors, I went and raced on them, and I'm pretty
sure that's when the vibration started. I do know it was severe the
next time I took the car out after that race weekend.
Switching pad compounds in the middle of a race weekend -- not a good
idea unless you can take the time to bed in the new pads to the old
rotors (or vice-versa!)
> For my money, I
>subscribe to that methodology. I ran my last car up to about 240k, a
>200TQ with the infamous "UFO" rotors, and never changed those
>rotors. For all I know, they may have been the originals.
I just replaced the front and rear rotors in my '01 A4 -- at 215,000
miles. The brakes worked fine and felt perfect, but the lip at the
edge of the rotors (where the pads don't wear away the material,) had
gotten so large that I was having a hard time getting the old pads off!
(FWIW, I run EBC Greenstuff pads on the front and Hawk HPS pads on
the rear. The rotors are whatever I could get cheap from RockAuto --
Raybestos, I believe.)
-- Robert King
More information about the A4