Brake rotor warping

Ryan from Edmonton Mechanic Posted   Latest  
Tech Tip
Braking
Myth

I have been at this career for almost twenty years now. Since the beginning I was told by the journeymen that a pulsating brake pedal meant that the brake rotors were warped. I was even taught this in trade school. Only when I stumbled across an article on the subject a couple of years ago did I learn of this widespread mistake. Brake rotors don't warp.

Rather than try and explain it, I'll just post a link to the article.

stoptech​.​com/technical-supp…

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Thomas from Manassas Park

 

Technician
 

Now here is my question because at my current place of work my coworkers and I debate on this, you find a vehicle that was excessive run out, do you replace both the brake pads and rotors or do you turn the rotors and if there is plenty of friction material keep the same brake pads on after you have cut the rotors? Now from what I have been taught from my last job when i started in this field was you replace both rotor and brake pads, but this was a European shop so the rotors on Mercedes and BMW where softer then the domestics I'm currently working on. I ve told at my place of work now to turn the rotors and keep the same pads on just to later on see that in couple hundred miles they are warped again.

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Ryan from Edmonton

 

Mechanic
 

A hard pad will give you thickness variation on the rotor. This is because it wears the rotor down. A soft pad will leave deposits on the rotor. As long as there was adequate brake material left I would leave the pads as they aren't the issue.

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Michael from Clinton

 

Mobile Technician
 

Hi Ryan,

That article is very interesting. It explains how a brake job can go bad right out of the gate. Proper break in for the customer is something most techs do not do. Taking a test drive eats into the flat rate. So far we don't add time for test drives. Maybe it is time we do.

Over the last few months I have been training a multi store chain. We are doing ASE prep. The common attitude of the techs was that turning rotors took off too much material and allowed the rotors to warp. They all want to slap new rotors on with every job. Their statement was that they did not want to do the job twice.

Rotors work by changing the energy of motion to heat. The mass of the rotor is important. Most of the time we are only taking off five to ten thousandths of a inch of material off. If you weigh the rotor before and after the cut, how much mass is really lost?

It is important to perform proper preparation of the rotor and hub surface. Mistakes many make are to ignore the rust on the inside and outside of the rotor hat, the rust on the hub surface. Ignoring this may create wobble. Improper wheel torque? They may improperly mount the rotor on to the brake lathe. The brake lathe shaft could be bent. How many use Brake Clean to take the shavings off the rotor surface? This leaves plenty of embedded metal to cause issue later. Soap and water is the best practice for cleaning the rotor. Personally I train the techs to put a non directional surface on the rotor. This helps with noise.

The cost of rotors has come down. Many are abandoning turning rotors to save time. This also increases the ticket total. Does it really help margin? A local shop offered an Accuturn lathe for sale with all the attachments. The selling price was $500. I would have bought it if I had a place to put it. It cost around $6k when new. What a fantastic deal.

+3

Ryan from Edmonton

 

Mechanic
 

Exactly. A few more minutes at the job will prevent future comebacks. I have always been a fan of the on car brake lathes. Your run out is bang on every time.

+1

John from Beaver

 

Instructor
 

We had to learn the hard way that cooling fin corrosion inside the rotors creates varying heat ranges in the rotor under heavy use which is the source of a lot of brake pulsation conditions. If not that the other cause is delamination. Those two conditions combined in the rust belt make machining rotors a poor choice and just asking for a vehicle to have to come back under a warranty repair. So today we replace instead of machine. Now maybe in other parts of the country machining rotors still works out well for everyone involved but not in the rust belt.

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John from Beaver

 

Instructor
 

It's a good article from a racing perspective but has questionable application from normal vehicle service and repair. For one, it does not take into account corrosion of the rotor surface, and especially the rotor's cooling fins. He is correct about pad material transferring unequally around the surface of the rotor as a cause for brake pulsation, but he never mentions a brake pulsation caused by uneven cooling of the assembly resulting in temporary change in runout or thickness variation. We would recognize that as a pulsation that occurs when the owner is slowing down coming off the interstate highway, but doesn't seem to occur any other time.

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Ryan from Edmonton

 

Mechanic
 

When I was working in the rust belt, I found that rotors would need to be replaced from surface pitting well before vent rusting was ever an issue.

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John from Beaver

 

Instructor
 

Totally agree, where techs ran into trouble is when someone else didn't understand the complete scope of the corrosion issue and were told to cut the rotors. Now the cooling fin issue becomes dominant and the tech would get accused of machining the rotor incorrectly when the car returned with another pulsation complaint.

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