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Brake Rotor Pulsation explained and repaired!

Posted by Doug99RS, Oct 22, 2004

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  1. Doug99RS

    Doug99RS DSM N/T Wiseman

    1,660
    15
    Joined Nov 10, 2002
    Raleigh, North Carolina
    No matter what car you drive or how hard you apply the brakes you will, at some point in your life, experience a shimmy and pulsation feeling. Alot of people think it's suspension related or maybe just tires but the fact of the matter is that if it happens when you press the brake pedal then its either drums or rotors that are warped.

    Most people can say they know what warped rotors are but not many people understand the term nor know what steps can be taken to fix the problem without having to replace any parts. Hopefully, I will be able to explain that all in this long-a$$ article about what I do at work for customers with pulsating brakes. The test subject for this article though is not an unsuspecting victim. It's my own car that has warped drums and rotors and so I took a few extra steps here to gather up enough pictures for your enjoyment.

    I'll start off with the fronts since everyone has at least one set of rotors on their car. Mine happen to be GSX front rotors (I upgraded the RS) made by Powerslot. They've got slots cut in them that help cool the rotor and vent gas build-up between the pad and rotor surface. First you see the rotor on the car in picture 1. I've marked the rotor with an "R" so that I know which side it goes back on. The rotors have to go back on a certain way in order for them to cool and vent gases. If the slots are going the wrong way then the gases and heat are directed towards the center of the rotor. When properly installed all of that is directed out towards the wheel where there is more air and the gases can escape. I've also marked the rotor to the hub so that it goes back on in the same spot it came off at. There's two reasons I do this only one of which is REALLY necessary. The first reason is that I'm just that anal and want to make sure that IF moving it clockwise or counterclockwise by one or two lug holes made a difference then I wouldn't have to worry about it. I don't think it makes a difference in this case but I try to put everything back the way I found it.

    The second reason (important) is because there are two types of brake lathes out there. One cuts a rotor while it's removed from the car. The other type of brake lathe cuts the rotor while it's still attached to the hub. What this does is matches up the rotor and the hub as a perfect fit. It's kinda like balancing a tire on the car. It's just more precise to do things on the car in their own environment. The important part here is that once a rotor is cut on the car it CANNOT be removed and put back on where ever you want. All the lug nut holes must line up with the lug nut studs they were cut next to. If you remove them and put them back on in a different way then clicking, vibrations, knocking or other sorts of noises will occur when braking. In some cases the variances are so great that the noise will show up even without hitting the brakes.
     

    Attached Files:

    841  0

    1999 Mitsubishi Eclipse N/T
    automatic · 2G DSM
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  2. Doug99RS

    Doug99RS DSM N/T Wiseman

    1,660
    15
    Joined Nov 10, 2002
    Raleigh, North Carolina
    The pictures 2 and 3 show the rotor on a brake lathe. This is a component designed to remove metal evenly from both sides of the rotor. The rotor is chucked up on to an arbor which spins around the same as it would on a car. The next picture shows the bits that cut the metal off of the rotor as it's spinning. The idea is that you remove as little as possible from each side but still have it completely flat/true/straight. The bits have knobs at the opposite end (closest to me taking the picture) that have fractions of an inch measurements on them to help determine how much you're cutting off. As the rotor spins, with the cutting bits touching the rotor, the bits are on a gear drive that pulls the bits towards the outer portion of the rotor. As such the metal is shaved off as the rotor spins and the bits more outward.

    Alot of brake noises are actually caused by vibrations. Metal in the brake pads themselves is touching the rotors causing a squeel. That squeel is a vibration. The brake lathe is also subject to vibrations as the rotor is being cut. To prevent this we wrap basically a rubber band around the rotor to absorb vibrations. Vibrations while cutting turn in to grooves in the rotor. Sometimes once grooves get cut in to a rotor, they cannot be removed.
     

    Attached Files:

    841  0

    1999 Mitsubishi Eclipse N/T
    automatic · 2G DSM
    Loading...
  3. Doug99RS

    Doug99RS DSM N/T Wiseman

    1,660
    15
    Joined Nov 10, 2002
    Raleigh, North Carolina
    Picture 4 shows my first cut on the rotor. As you can see there's alot of metal shavings all over the machine and in the catch pan. BUT what is more important for you to see is the shiny spot compared to the dull/grayish area. The shiny section is uncut rotor. As you can see the section of the rotor that has been cut is not even. There are high spots and low spots in the surface of the metal. These differences in thickness and shape is was causes the pulsation we feel in the pedal as brakes are applied. Pictures 5 and 6 are prime examples as to the differences in thickness that cause the problems.

    If you look at picture 5 you will see that I have marked the rotor with a 1, 2, 3, and 4. I marked the rotor the same on the opposite side as well. I'll refer to each number as a "position" for ease of interpretting. Notice how in position 2 in picture 5 shows an area that has been cut. But in picture 6 it is uncutt. This is an example of how warped these rotors are. Positions 1, 2, and 3 are cut on one side and completely uncut on the other.
     

    Attached Files:

    841  0

    1999 Mitsubishi Eclipse N/T
    automatic · 2G DSM
    Loading...
  4. Doug99RS

    Doug99RS DSM N/T Wiseman

    1,660
    15
    Joined Nov 10, 2002
    Raleigh, North Carolina
    Pictures 7-10 show the rotor a little bit further in the cutting process. Pictures 7 and 10 show the outside of the rotor. We've got alot more area that shows as being cut but as you can tell the metal still is not even nor fully cut even after a couple of swipes. Pictures 8 and 9 show the opposite side as being in the same condition.

    Picture 11 shows the rotor almost completely done with only a small section on the left of the picture remaining uncut. The finished product is a nice, even finish on the rotor on both sides. This gives the brake pads an even surface to contact with ensuring that 100% of the pad surface comes in contact with the rotor providing as much stopping ability as possible.
     

    Attached Files:

    841  0

    1999 Mitsubishi Eclipse N/T
    automatic · 2G DSM
    Loading...
  5. Doug99RS

    Doug99RS DSM N/T Wiseman

    1,660
    15
    Joined Nov 10, 2002
    Raleigh, North Carolina
    In conclusion you can see that warped rotors are definitely the source of brake rotor pulsation. Now that I've shown you what you can do to fix warped rotors I'll give you a bit of insight in to what causes warped rotors.

    The number one cause for warped rotors is heat. Heat has the ability to change the shape of metal. Every time the brakes are applied, friction causes the rotor to slow down thus slowing down the rest of the car. In the case of brake pads this friction also causes a side affect of heat. Most brake pads work better once they are heated up but too much heat can cause brake fade and ultimately reduce your ability to stop the car.

    There's a few things we can do to reduce warping caused by heat but no one thing alone is going to prevent it. When you apply the brakes do so in a smooth manor and allow lots of stopping room. While the rotor is exposed to the brake pads for a longer amount of time, the friction is less meaning the rotors don't heat up as much. When you finally come to a stop try not to leave your foot on the brake the whole time. Let the car roll just a bit. This will prevent the hot brake pad from staying in one spot the entire duration of the stop. Also, try to avoid mud puddles, standing water and washing your car when it's just been driven. Water splashing up on the rotors will not only boils and steams up but it also changes the temperature very rapidly and unevenly.

    Another cause for warped rotors is over-tightening or improper tightening methods for wheels. It's very important to torque the lug nuts in a star or alternating pattern. Basically, skip the lugnut closest to the one you just got done tightening. This will prevent the wheel from being snugged up unevenly to the rotor. The rotor is held to the hub by the wheel being tightened down. So if the wheel is unevenly tightened, so is the rotor. The wheel acts as an additional heat sink for the rotors too so make sure that everything is tightened to spec in the proper order.
    Doug
     

    841  0

    1999 Mitsubishi Eclipse N/T
    automatic · 2G DSM
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