[A4] Brakes

[Derek Pulvino]

Since I'm not sure on the exact best way to go point by point, I'll  
just add input as I have input, and were I don't I'll "expunge it  
from the google search record."

On Feb 8, 2008, at 9:10 PM, Robert wrote:

> 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.
>

What I noticed in this run was the pads did get better over those  
miles, and this is actually the first time I'd experienced this.    
Everything was pretty much back to normal brake feel after about two  
tanks of gas (about 600-miles), or right about the time I was well on  
my way down the back side of an Oregon mountain pass.  First stop  
after changing the pads was rather alarming, but luckily that was in  
the city going slow so figured out what was going on and drove even  
more conservatively wrt to following distance.  At that first stop, I  
thought maybe the system needed to be bled, but that made no sense as  
there was not a way to have introduced air into the system (brake  
fluid was only higher, not lower in changing).  That mountain pass  
was many hundreds of miles away from the first city stop.

My theory is the worn-in path on the rotors didn't exactly match with  
the pad dimensions.  This was more or less beared out by seeing a bit  
of gap between the center of the pad and the rotor, and then after a  
couple hundred miles still seeing a bit of corrosion on the center of  
the rotor (aka no rotor/pad contact).  So "bedding in" may not be the  
most correct term, but it seemed to fit.  So in essence, my decreased  
braking was directly related to the contact surface area.

> 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...

I think there may be a bit of a disconnect here.  The idea is at root  
based on the idea of the rotor/pad contact in essence being a very  
slow acting machining tool that runs at a constant and precisely set  
up alignment.  Rotors out of the box are (or should be) true.   
Calipers are mounted very precisely relative to the rotor...if  
calipers aren't binding, the wear of the system will continue to be  
true.  As he always sums it up, "if it aint' broke..."  AKA, brakes  
aren't pulsing, leave the rotors alone.  If they are pulsing, it's a  
different situation.

>
>
>>  and
>> 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.
>
> WTF?!?
>
> 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!
>

Interesting, first I'd heard that.  Makes wonder what would lead to  
those irregular deposits.

>
>>   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 

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