Brakes Part II

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Ok I asked the expert to give me a speil about "warpage" after explaining how I was being garotted for using big words and confronting the myth. I'm sure it won't make any difference to the closed minds, but it's worth reading for party conversation.

It's true that car makers have a spec for the runout of rotors on new cars. They would also have a published spec for the maximum allowable runout for workshops doing brake jobs. In crude terms, I guess you could refer to runout as "warpage" - it's a measure of how far the rotor is distorted from perfectly flat.

The reason for this is that runout is a key contributor to DTV generation. And as we have discussed before, DTV (disk thickness variation) is what causes brake shudder. Runout ("warpage") doesn't cause shudder - the caliper is mounted on slide pins, which allow it to move so as to follow the surface of the rotor. You'd need a rotor "warped" to the shape of a Pringle before it started causing shudder.

Different types of friction material have different DTV characteristics. The Australian and Asian markets are obsessed with "organic". These are becoming more common in the USA, although their marketroids are calling them "ceramic" formulations.

With all materials, but particularly organics, DTV is generated while driving off-brake. The caliper is released, but the pads only retract as far as they are knocked off by the rotor. Because of this, the rotor grazes past the pad every revolution - that is, the pad makes light contact with the extremity of the rotor's runout.

Over thousands and millions of revolutions, this grazing pad contact causes rotor wear. Tiny amounts of wear - tens of microns at worst. But because this wear is only on one side, and only on one part of the rotor, it causes a measurable thin section of rotor, where the contact surfaces are slightly closer together than on the rest of the rotor.

When the brakes are applied with ~constant pressure, you get ~constant friction at the pad-rotor interface - except for this thin section of rotor, where there's less contact pressure so less friction. Say hello to brake torque variation and shudder.

In order to not get DTV, you need one of three things: no pad contact off-brake, "corrective" rotor wear on-brak, or low (no?) rotor wear at very low contact pressure.

To get rid of off-brake pad grazing, you can try for zero rotor runout (how flat can you get? Typical OE specs are about 50 microns), or use positive pad retraction (springs and clips) combined with lots of piston retraction (which leads to terrible pedal feel).

Europeans take the approach of correcting DTV, by using abrasive friction materials. Driving off-brake down the Autobahn, you generate a tiny amount of DTV. Then you use the brakes around town, and wear a uniform quarter micron of rotor away, so you end up with flat rotors again.

You also end up with short rotor life (European cars typically need new rotors every second pad change), and dusty wheels (most wheel dust is actually rotor steel, worn off the rotor by the brake pads). Ever noticed how European cars always have dirty black wheels?

Reduction of low-pressure wear is the holy grail of high-friction organic friction materials. Everybody is trying, nobody is there yet.

Older style American materials (semi-mets) also have a different form of DTV, caused by corrosion and material transfer. Particularly in salty winter conditions, the metal in the pads reacts with the metal in the rotor when the car is parked.

Depending on the chemistry, the rotor can be pitted (leading to thin parts of the rotor = DTV), the rotor can get an oxide layer (leading to thick parts of the rotor = DTV), or the pad can actually stick to the rotor and leave some friction material behind when the bond is broken (thick rotor = DTV, sticky friction material on rotor + DTV = shudder like a bastard).

In any case, I've ranted enough. The fact of the matter is - shudder is not caused by "warped rotors". Anyone who claims otherwise is uninformed.


posted by  Wally

Ok, but it would be fair to say that if you had warpage, you would have a greater chance of DTV? Perhaps even a more severe case of DTV. And with that greater chance and severity of DTV, you would have a greater chance for brake shudder.

I am sure there is brake shudder in all vehicles. Most of the time it is too small to actually feel. It isn't until you start to have a problem with your brake rotors, like warpage, that the shudder starts to become noticable and annoying.

So while the warpage may not be the direct cause of the shudder, it is a large factor in setting up the conditions for brake shudder. If rotors wouldn't warp, chances are we would rarely ever feel brake shudder.

In general, the public doesn't need an explanation like the one you provided. They only need to know that their rotors are warped and should be replaced.

posted by  theman352001

THANK YOU SOOOOO MUCH "theman" ARE the man!!!! :thumbs: This has been my POINT all along. NO ONE comes in here to hear someone's (Wally's) expert engineering explaination of "warped" rotors. As an engineer I TOTALLY agree with everything TIM stated. I never said that DTV is BS, I said that Wally's blanket statement starting off in this forum that "warpage does not exist" is Bullshit. And it is!!! Now he tries to cover his ass with this scientific explaination that know one cares about. I tell you what wally, I will take a car into the shop here soon and tell them I have brake shudder and I suspect it is DTV and see how stupid of a look they get on their face. That will be the all-telling sign of how lame this whole thing has turned out to be. I don't think anyone in here wants to read scientific findings,,,,most of them that come in for help want answers fast and don't otherwise give a damn.

Disc Thickness Variation is exactly that, and if you look at runout, it directly equates to the basic explaination that the disc(rotor) has WARPED. So Wally, you are not impressing a sole on this site trying to be the rocket scientist that you are clearly not. No one in here would make a stupid statement to a forum such as "there is no such thing as warpage" but you mate. I think that is all I need to say. Consider you are a fairly new member of this forum and how it looks when you come in and say things such as that, and then go on to try and prove yourself higher than anyone else with scientific bullshit. NO ONE CARES! We all know that brake pedal pulsations are caused by rotor warpage due to excessive heating and or accompanied with rapid cooling due to parts quality, laws of physics and the like. END OF STORY!

posted by  cmeseadoin

I fail to see how brake dust is actually the fillings of metal from the discs and not bits of brake pad...Also, that doesn't say anything about 'warpage' being a myth...unless I missed that bit lol

posted by  Cliffy

Stange fellow and you are definitely not an engineer as you claim, or there is a very poor standard of education in your state, because I know I wouldn't have passed using incorrect answers. You are like a dung beetle that spends it's time carefully crafting little balls of dung only to have someone bigger come along and ruin all that useless work.

If you can't read English and have a poor grasp of the language, take it out on your parents, not me. If you prefer to use slang proffered by grease monkeys over engineering do so, but don't slate me for being educated.

If you think your attempts at flaming are working, let me tell you that it will take a lot more than you can dish out. In the meantme you are denying others the opportunity to learn things the right bullies are apt to do.

Now get a thesaurus and read what Tim has written. Let me reiterate that he, like myself and many other experts do not subscribe to warped rotors. It's relatively simple to test with a set of verniers and a bit arithmetic.Capice?

posted by  Wally

I'm not usually one to condone flaming, and believe me...I dont mean any offence to you, Wally, but the quoted statement as absollute bollox mate, for starters, you can tell that cmeseaetcetc is educated from the language he uses, and secondly, I was a 'grease monkey' for a few years and I learned from some of the best people I could have hoped to learn from. Just because you have a better education than some people means absollutelly nothing.....For example, at school I was in the bottom class for everything, does it show? *Awaits the sarcastic remarks lol*

posted by  Cliffy

Cliffy your post jumped in front of mine. I was just giving cmeseadoin a bit of his own back. It was not directed at you and I couldn't get the link to work until now.

It wasn't me that raised the qualification bit, but two other members in a bid to make me unqualified to post. I couldn't care less if it was a bush native telling me a fact, a fact is a fact.

There must be a lot of cast iron electric hotplates out there that need replacing due to warping don't you think?

posted by  Wally

Did it ever occure to you the calipers are not easily moved on the pins when the brakes are applied and thats where some of the shudder comes in?


posted by  DodgeRida67

I think by now we have established a few things summing this flaming contest up in a nutshell

1. Warped rotors is a generic term for any rotor which is not completely parallell for whatever reason be it thermal, mechanical, or brake pad debris bonded to the rotor. I read a couple of sources Wally referenced. Some of it is good, some of it pertains to specialty applications and some of it boils down to what has been said. If I were go to a shop and start describing the metalurgical things which happen to the rotor when it overheats or the things Wally is describing, the technician would refer me to the nearest mental health facility for a head examination

2. We have also established at least three or four causes of DTV or warped rotors. As a professional mechanical; engineer, I can assure you there is no single cause of DTV or warped rotors. There are several interrelated causes which we have debated in this and in another post which I closed becaise it was becoming a flame-a-thon.

3. The an important outcome of this debate is certain actions contribute to the problem. These include choice of replacement friction material, brake wear in procedures, driving and braking techniques, running a hot rotor though puddles, etc.

4. The most important outcome is if the rotor has excessive DTV or warpage, the thing needs to be replaced, preferably as an axle set.

posted by  tbaxleyjr

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