myth of warped disks

[Danny D.]

Scott Dorsey wrote, on Wed, 05 Nov 2014 14:44:48 -0500:

You need a straight edge.

[clare]

Your police officer friend has a better understanding of the problem than you do. He defines warpage as thermally induced run-out (no variation in disk thickness) However, he only addresses ONE cause of brake vibration - faulty installation procedures.

The writer is an "ase certified mechanic" and a "former service writer" and he makes good points -and defines BTV and DTV quite adequately. He also covers the difference in pad and rotor quality - and the problem with machining rotors. He however does not define "warpage" - he just attempts to explain it away. He does note you cannot do proper brake repairs/diagnosis without the proper measurement tools - but does not tell you exactly how. He is also writing for a parts manufacturer and is deflecting blame for brake problems to the installer.

The first paragraph proves this guy's premice is not well though out

- as he allows the variation in thickness of the rotor to define "warpage"

However, even he shows a less serious case of disc "fracture" than the total brake loss I witnessed on the rallye car. He also show a very real case of "warpage" - which he calls "coning". It is, in reality, a case of symetric warpage - as it is thermally induced distortion which has no wear component or difference in thickness. This can happen from the outer circumfernce cooling faster than the inner part of the rotor, causing the outer circumfernce to shrink faster than the center. If the center cools faster and shrinks, causing the outer circumference to be larger, the rotor warps.

Figure three, if there is no corosion involved, is also, technically, a warpage problem because the metal is dformed under thermal stress. It is more generally referred to as "collapse" of a vented rotor.

Picture #4 is clasically known as "hot spotting" and it can be caused by DTV, or it can ve caused by bad metalurgy. It is (or at least WAS) very common on some cheap rotors in years past, where under the influence of heat and temperature change, the metal actually hardened

- to the point that a carbide cutter would "skate" accross it rather than cut it on a brake lathe. Grinding could remove the hard spot. I'm not saying that DTV could not contribute to this condition - but this happened way back in the days of asbestos pads too - and experimenting with heating the rotor red hot and slowly cooling it (annealing) removed the hardness from the spots. We did it as an experiment many long years ago. The fact that heat cracks often accompanied the hard spots prove it was a thermal condition.. The early use of recycled steel by japanese and chinese parts manufacturers in particular, caused this to be a problem most often eliminated by replacing with higher quality parts.

In this case, again, variations in thickness are being confused with warpage. Still does not prove warpage does not exist. He does give a good explanation of bedding in the pads

This is an advertisement for a piece of equipment I used extensively in my career as a mechanic.

The writer claims the only reason a rotor can be out of true is improper installation. He is writing for a brake component manufacturer and is deflecting the blame from possible poor materials to the installed. There IS a third reason for a rotor to be "out of true". We used to recieve rotors that were "out of true" right fresh out of the box.. No amount of cleaning, remounting, or retorquing would remove the measurable run-out of the rotor - replacing with another rotor invariable solved the problem - and this was BEFORE the vehicle was even test driven. In some cases we used the handy-dandy on-car lathe to true them up (particularly on first generation tercels

- where replacing a rotor was a major production. - but almost without fail they came back within weeks or months with the rotors running out again. No thickness variation - only run-out. The ONLY explanation was warpage due to poor metalurgy and/or quality control.. Going to a different manufacturer solved the problem - COMPLETELY.

A very basic explanation of "bedding in" issues.

Nowhere did I say what you claimed didn't happen -I FULLY understand your aguement. So no, it is not in "every" one of those cases. What YOU are not giving is a "balanced" discussion of the arguement. You are claiming, based ENTIRELY on what has been written by guys who in many cases have an agenda,, tha warpage NEVER happens.

I am claiming, from my experience, that actual warpage DOES occur, while agreeing that MOST cases of "warped rotors" in reality are NOT warpage.

What you need to learn is NEVER say never, and ALWAYS avoid always when you make an arguement. For every rule there is an exception. Just because someone says they have never seen something does not mean it cannot and does not happen.

I have personally experienced way too many cases of warpage of rotors

- with warpage beinf defined as runout without thickness variation (measured with accurate machinists tools including dial indicators reading to .0005" and or 0.005mm along with micrometers accurate to the same .) and also documented those (and other) rotors going from true to out of true without thickness change over a period of time - and documented solving the problem by switching to a different brand/quality of rotor.

I spent over 25 years actively involved in the automotive repair trade, including 10 years as service manager of a dealership and 3 years of teaching - 2 of those at the trade level (teaching the equivalent of ASE technicians) over a working life of over 40 years, and involved in the automotive "hobby" all of my life to the present. (including 3 years of regional navigational rallying, placing 4th,

3rd, and 2nd in the Ontario region 3 years running, campaining a 1972 Renault R12), scrutineering with the CHRA, and pit crewing for the Toyata factory rallye team (Taisto Heinonen and Tom Burgess) as well as building show quality street vehicles and working on high quality restorations as a consultant / trouble-shooter. I am currently building an airplane as well.

And you refuse to take anyone at their word unless they are "published" on the internet in a so-called "trade" paper.

You sound like a British or Indian engineer. Progress is what happens while the rest of us do what you attempt to prove cannot be done

[clare]

You still have not said how you measure warpage of a rotor. Measuring warpage of an engine block is totally different - and I know how to do that too. To measure warpage of a block you need to measure not only un-evenness of the block deck, but also twist and alighnment both vertically and horizontally of the bearing bores and trueness, roundness, and alignment of the cyl bores.

All of this is done without rotating the block.

Not how you check a rotor for warpage - not even close.

The rotor is a lot simpler.

You still have not explained how you measure warpage on a rotor - possibly because you don't believe it is possible to measure it because it does not exist..

How then do you measure to prove a rotor is NOT warped????

[clare]

The chances that you have actually experienced a warped rotor ARE pretty slim.

[clare]

So now you are admitting that actual warpage "can" exist???