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      04-14-2014, 02:53 PM   #1
MightyMouseTech
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For Those Who Don't Believe in Warped Rotors

For some reason, there are a lot of people who don't believe there is such a thing as excessive runout, commonly called warped rotors.

Have a car in today, all four rotors warped due to over tightened wheel nuts. Can't believe there are still shops that just pound on the nuts with an impact without a torque stick.

Anyways, huge vibration while braking, measured 0.5 mm of run out.

Pic after the first cut to show the on car lathe taking off the high points, inside of the rotor is similar, but high point on outside is low point on the inside. Means entire rotor face is twisted, or warped.

And a pic of the Pro Cut on car lathe.
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Last edited by MightyMouseTech; 04-14-2014 at 02:59 PM.
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      04-14-2014, 03:07 PM   #2
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Thanks for posting.

I have cracked a few rotors from overheating, and had pad deposits, but haven't ever warped any. Those do look clearly warped
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      04-14-2014, 03:10 PM   #3
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Ya, they were pretty bad, that is why I took the pics.

Nice thing about the on car lathe is that it will compensate for the runout in the hubs as well as the rotor. Machining off the car can not get anywhere near as straight. After initial setup, the lathe was showing final runout as 0.0002" (that is the 0.2 you can see in the pic of the lathe) Not going to get much straighter than that. Most new rotors after install are not that good.
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      04-14-2014, 04:53 PM   #4
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What was the cost of resurfacing all 4?
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      04-14-2014, 05:09 PM   #5
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We charge 1.9 hrs per axle. So, 3.8 hours for all four. We are at $102/hr, so $387.

Gotta pay for the $15k lathe.
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      04-14-2014, 05:18 PM   #6
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I don't know very many people who say it isn't possible to "warp" a rotor (defined: bend or twist it out of shape). But such a feat would require immense heat and/or forces. The issue with the term is that 99% of the people that claim they have "warped rotors" actually just have uneven pad deposits or other rotor surface imperfections.

I might even still argue that if you can fix it with a lathe, is it really a "warped" rotor? Using a lathe indicates you can restore it to the right shape by simply shaving off the high points. A truly warped rotor would have a misshaped structure altogether (it would be twisted or bent), not just on the rotor surface.
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      04-14-2014, 05:23 PM   #7
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Cool pics @MightyMouseTech I always enjoy reading your posts. I use to work at a VW Subbie store... its weird how the Japanese always want to "turn" the rotors while the German's just replace them. I think it also has to do with the type of steel(used) in the rotors. I think the German's use a higher quality of steel. Maybe thats why our rotors get surface rust so fast.

Btw... another reason why brake rotors warp or the brake pedal pulsates is due to the rotor not sitting flush on the hub. Most guys don't take the time to use a 3M "cookie" to clean off old rust on the hubs before mounting new rotors. Not doing so can make your pricey new rotors feel like worn out ones in no time.
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      04-14-2014, 05:24 PM   #8
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Yes, the internal structure is still "warped", just the outer surfaces are straight. Makes it easier to warp them in the future. Also due to the reduced mass of the rotor. Less mass means rotor gets hotter with same amount of heat.

Although, it is not heat that will do it. I have had rotors glowing red, and as long as the pads are cooled down before stopping, they are good.

It is quite easy to do. Overtighten/unevenly tighten the wheel nuts and it is pretty much instant. Also, going through a carwash with really hot rotors will do it too due to uneven cooling.

If the car is driven hard, pad deposits are much more likely.
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      04-14-2014, 05:40 PM   #9
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Ah, so the lathe is making is usable again, but isn't really correcting the full extent of the warping. Makes sense. Sounds like new rotors might have been a cheaper and/or better fix (depending on model of the car)?
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      04-14-2014, 05:48 PM   #10
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Yes, new rotors would be better, and is what I would do to my own car. But that would add about $6-800 to the $387.
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      04-14-2014, 06:05 PM   #11
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0.5mm run-out is huge. Performance rotors should never be cut on a lathe. Replace only.
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      04-15-2014, 07:28 AM   #12
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Quote:
Originally Posted by Dackelone View Post
Cool pics @MightyMouseTech I always enjoy reading your posts. I use to work at a VW Subbie store... its weird how the Japanese always want to "turn" the rotors while the German's just replace them. I think it also has to do with the type of steel(used) in the rotors. I think the German's use a higher quality of steel. Maybe thats why our rotors get surface rust so fast.

Btw... another reason why brake rotors warp or the brake pedal pulsates is due to the rotor not sitting flush on the hub. Most guys don't take the time to use a 3M "cookie" to clean off old rust on the hubs before mounting new rotors. Not doing so can make your pricey new rotors feel like worn out ones in no time.
Most OE rotors are made from Cast Iron, not steel. The entire rotor does warp due to phase changes in the microstructure of the cast iron rotor.

When rotors are initially cast, upon cooling they will have "retained austenite". Cryo cooled rotors will transform this structure, so the rotor is more uniform and this phase transformation does not take place. As you apply heat and stress to a rotor that has excessive retained austenite, it transforms to martensite, causing a change in volume, which results in a rotor warping to compensate.

Some rotor manufacturers may cryo cool or control cool their rotors after casting to minimize retained austenite, but it is an added expense, so I imagine few if any OE manufacturers have their rotors treated as such.

All cast iron rotors will rust and actually the quality of the cast iron has nothing to do with it. The only way to stop the rusting is to coat the rotor or use stainless steel or ceramics.
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      04-16-2014, 09:00 AM   #13
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Quote:
Originally Posted by MightyMouseTech View Post
Yes, new rotors would be better, and is what I would do to my own car. But that would add about $6-800 to the $387.
? What the hell kind of rotors are you buying?

You should be able to buy more than decent rotors for less than the $387 one would pay to have them resurfaced.

Jesus Christ, who would pay $400 to resurface 4 cast iron 1-piece rotors?
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      04-16-2014, 09:43 AM   #14
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Quote:
Originally Posted by PrematureApex View Post
? What the hell kind of rotors are you buying?

You should be able to buy more than decent rotors for less than the $387 one would pay to have them resurfaced.

Jesus Christ, who would pay $400 to resurface 4 cast iron 1-piece rotors?
apparently Canadians
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      04-17-2014, 11:14 PM   #15
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Cryogenic,s 550.00 for 4 new rotors. Still should be torqued though. Been a believer in cryogenics for almost 30 years for a lot of stuff. including firearm parts.
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      04-21-2014, 09:21 PM   #16
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i cant believe shops still turn rotors. I havent done that shit in years
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