02-22-2011, 09:21 AM
Seek to understand,^Value
Location: San Jose, California
Join Date: Mar 2008
Mein Auto: 02 BMW 525i M54 auto 130K
What people "think" is rotor "warp" is actually something else altogether (whether it be runout or, more often, brake pad deposition building up from the driver's braking habits).
Originally Posted by fadetogrey
maybe I've been braking super hard
To the OP, it's not braking hard that leaves pad deposition, but braking hard and leaving your foot on the pedal where and when you stop. And all you have to do is do this just once!
From reading the references (and I've read them many times ... and they're not wholly consistent ... nor is my memory) ... but from reading the references, once you have a pad imprint, the deposition grows upon that single pad imprint (for chemical reasons).
- Avoiding Brake Judder at the Track, by Dave Zeckhausen
- Brake Rotor Shake Demystified, by Dali Racing
- The "Warped" Brake Disc and Other Myths of the Braking System, by Carroll Smith
- 8 Myths ... Holding You Back from Performing the Best Brake Job, by Andrew Markel
- Brake Roughness and Vibration: Causes & Solutions, by Dave Mann
- There's no such thing as 'warped' brake rotors, by Michael Block
- Raybestos Brake Tech School, Part One: Rotors Don't Warp, by Police Fleet Manager Magazine
So, if you stopped once, the theory goes, under just the wrong conditions, and left your foot on the pedal, unless you "scrape" that pad imprint off the rotor, it "could" grow to a ten thousandths of an inch mound (or whatever).
That teeny tiny mound of growing pad deposition shows up as highway speed vibration when you brake. In fact, it happened to me, when I did my first brake job in my life (and my first post ever on Bimmerfest) simply because I was ignorant of these facts at that time:
- A user's very first complete 4-wheel brake DIY experience on the BMW E39 (1)
In that job, I bought, begged, and borrowed the tools necessary to check rotor & pad thickness and runout specifications (see photo below). There was no appreciable runout (note, runout is not warp!), and the rotor thickness was well within specifications. Doing my research, I easily solved the problem of severe brake-related vibration at highway speeds.
What eliminated the very severe brake-related vibration was merely a proper rebedding (not easy to do in crowded San Jose California!):
- How to Bed-in Your Brakes, by Dave Zeckhausen
- Removing the Mystery from Brake Pad Bed-In, by Matt Weiss
And, what keeps deposition-related vibration from coming back is daily good braking hygiene!
Think about that. If my rotors had actually 'warped', no amount of rebedding would have fixed them so thoroughly. However, to be fair, as Jared said, it can't be all that simple because you wear down metal when you brake. So where does THAT factor in?
Also, maybe it takes a LOT of hard stops where you hold down your foot on the pedal? Maybe you have to land in the same spot more than once? I do not know (and the articles don't really say).
But, here's my heartfelt advice for good braking hygiene ...
As soon as you get the HINT of brake-related highway-speed vibration ...
- If you have to brake hard
- Lift your lead foot off the brake pedal as soon as you can!
- When you finally stop ... let the wheels roll a bit (if you safely can)
As for why it doesn't always happen ... I do not know (reference Jared's comments). My procedure above might be akin to the "miracle in a can". I don't know.
- Get rid of that pad deposition as soon as you can!
- Do a quickie rebedding procedure (eight serial ~60-to-10 mph)
- And then go back to good braking hygiene
All I know is that it makes sense (somewhat); and that it doesn't cost anything; and that it seems to work; and that I'm not the only one proposing it (it's in all the rebedding procedures).
Up to you what to believe ... but "warp" didn't happen (to me anyway).
And, for those who say they've seen brakes "plenty of rotors warped, right out of the box" ... I say if you put me on a lathe ... with all those lovely cast-iron curlicues of hot metal writhing off the carbide bit ... at the end of the day ... you'd have a lot of lousy rotors (I'd compete with China!).
But badly machined and horribly out of spec rotors with poor quality control is not what we're talking about when someone says their rotors 'warped' in use.
In fact ...
I'd LOVE for one person on the entire Internet to post a picture and/or specs of a truly warped rotor from street use. I'm sure it happens (especially in racing). But not very often. Certainly it's not the first, second, or even fifth thing I'd look at if I had severe brake-related vibration!
Let it be known I have searched the net as mush as anyone and I can't find any reliable measurements of "true non-racing brake related warp" that are believable. To be believable they need to come from a source who knows what they're talking about and that source must discuss both sides of the story (i.e., most warp-related threads clearly show the writer is ignorant of the alternate view).
If you can find believable threads showing rotors often warp in street use, I'd love to see it posted right here!
Last edited by bluebee; 02-23-2011 at 09:42 AM.