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Technical Disc brake rotor run out

Discussion in 'The Hokey Ass Message Board' started by 36roadster, Mar 10, 2021.

  1. I have a "pulsating" feedback on my pedal, and I reckon it is coming from my front end. I have early Holden (same as Chevy) discs, and I have measured the run-out at 0.004" max. Would this much cause the pulsing problems?
  2. Rotors don't tend to be the source of pulsing back thru the brake pedal. If the rotor has a bit of warpage or run-out, consider that when that area of the disk rotates past the pistons it tends to push the piston on one side of the caliper back into its bore. But it simultaneously allows the piston on the other side of the caliper to be pushed out of its bore a similar amount. In other words you may have a little fluid displaced by one piston that is then taken into the opposing piston as it moves out of its bore.

    As a result, the caliper tends to absorb the pulsation with little or no feedback to the master cylinder and brake pedal. Also consider that .004" is just slightly thicker than a sheet of paper.

    The next thing to check would be out-of-round on the rear drums if the vehicle is so equipped.
  3. While it’s true rotor runout doesn’t cause the pulsation, I wouldn’t say rotors don’t cause it. It’s thickness variation in the rotor that causes it.

    Sent from my iPhone using H.A.M.B.
  4. Boneyard51
    Joined: Dec 10, 2017
    Posts: 5,349


    I had a OT vehile that I had tire balance problems for a while. After two sets of tires my disc brakes started pulsating, shimming, vibrating whatever. Turned the rotors....all my problems when away! Just saying.

    VANDENPLAS, ottoman and 61SuperMonza like this.

  5. 61SuperMonza
    Joined: Nov 16, 2020
    Posts: 489


    I dont know how hard you drive your ride, but it's possible that from hard use the pad friction material can inbed in the rotors causing a pulse. If your rotors can be turned do that. If not get new rotors.
    When bedding the new pads and rotors make sure you do it as recommended from the manufacturer. Not doing this can cause a pulse as well.
    If that doesn't stop the pulse its sure to be the rear drums. Inspect the linings wheel cylinders and have the drums turned or replaced.
    I would be it's in the front brakes.
  6. As above, thickness variation much more likely the problem. If there is enough material left machining the rotors should fix it. Also, make sure the calipers and caliper slides are free.
  7. inthweedz
    Joined: Mar 29, 2011
    Posts: 451


    You say early Holden, are they solid discs (HR Holden) or ventilated (HQ-WB Holden)?? Both types can distort..
    Is the pulse thru the steering wheel?? (front discs), or thru the seat (rear brakes)
    Google has this..
    ACDelco say there should be no greater than 0.1mm runout..
    (0.1mm equals 3.93701'') So your .004 is maximum runout, get them machined if still in specs (thickness), or replace the discs..
    Last edited: Mar 10, 2021
    Crazy Steve likes this.
  8. Mike VV
    Joined: Sep 28, 2010
    Posts: 2,476

    Mike VV
    from SoCal

    Not correct, from experience.
    As the warp in rotor runs through the caliper, the high side(s) WILL push the pistons back into the caliper.
    Been there..!

    From experience, yes, .004" is enough to pound on the pedal. The faster you are going and the harder that you push on the pedal, the harder the pounding WILL be.

    If your rotors are thick enough, just spend the few dollars and have them turned.

    pirate likes this.
  9. brando1956
    Joined: Jun 25, 2017
    Posts: 62


    All good advice above. I've turned a ton of these things and have a word of caution. I'd get them turned but be careful where you take them. I'd stay away from the big box retailers. Their people don't have the training to fully understand what they are doing and many just don't care. You can just throw a rotor on the lathe and hit the switch but you've got to be sure of the proper setup, bit angle, and especially sharpness of the tool. I used Aamco replacement tips and they could get costly. If you were doing a set of rotors that required a lot of material removal you could go through a set of bits on one job. I always kept things sharp enough to ensure a good cut but there are shops that are too cheap to replace their tools when they should and some workers are so clueless they don't know any better. If you go in the door and you hear the lathe making an ear piercing scream you may want to take your business elsewhere. There are few times a properly set up machine is going to make that kind of noise. I have seen rotors and drums done by others that had a "machined" surface that was so full of concentric grooves that they would have made perfect threads.

    Seems like those early chevy rotors were 1.25 thickness so that gives you a lot of meat to work with but remember, rotors don't last forever. If it takes a deep cut that gets it close to the minimum thickness stamped on the rotor I'd replace it. Once you start removing material weird stuff can happen with the heating/cooling cycles they go through and they can end up out of true in a short time.
    61SuperMonza likes this.
  10. brando1956
    Joined: Jun 25, 2017
    Posts: 62


    Had another thought; You might check the wheel bearings, slightly loose bearings could cause. You'll want to check the mfr's torque/adjustment procedure. I.E. rotate wheel while torqueing and back off to the recommended point.
  11. brading
    Joined: Sep 9, 2019
    Posts: 466


    If you have just changed the disc did you make sure the mating surfaces were clean. If the are not clean then the disc can sit at an angle causing run out.
  12. They are standard HJ disks, and the pulsing is felt on the brake pedal.
  13. Actually depends on whether they are fixed or floating/sliding calipers. Fixed then yes runout can cause a pulsation. Floating/sliding then no way the caliper moves with the rotor assuming the caliper actually moves as intended of course.

    I have had rotors with hard spots cause a pulsation.

    From experience of course.

    Sent from my iPhone using H.A.M.B.
  14. patterg2003
    Joined: Sep 21, 2014
    Posts: 728


    Once had some pulsing and thought it was the rotors but it turned out one of the calipers was not free to slide. The calipers had to cleaned up to slide again.
  15. pirate
    Joined: Jun 29, 2006
    Posts: 686

    from Alabama

    Not mentioned yet but over tightening lug nuts can actually distort the brake rotor. Or tightening lug nuts without using a criss cross pattern can distort rotors. Most shops just pick up the impact gun and have at it. I always use a torque wrench when tightening lug nuts. Your mileage may vary and I tend to be a bit anal about my cars.
    ClayMart and dirty old man like this.
  16. squirrel
    Joined: Sep 23, 2004
    Posts: 50,184


    interesting that you're off by a factor of one thousand, and no one mentioned it.

    0.00393701" perhaps?

    but yeah, turn the rotors, see what happens
    Desoto291Hemi likes this.
  17. inthweedz
    Joined: Mar 29, 2011
    Posts: 451


    Thanks Squirrel, my bad.. It's a good thing you are awake.. Lol..
  18. Greg Rogers
    Joined: Oct 11, 2016
    Posts: 487

    Greg Rogers

    Do you have drums in the rear? I worked as a brake mechanic years ago. Once I accidentally dropped a brake drum on the floor when putting everything back together. Took car for a drive and pedal pulsed up and down and terrible vibration. Took dropped drum back off and put in lathe, found it had bent made oval when dropped. Replaced drum and all was fine.
  19. lostone
    Joined: Oct 13, 2013
    Posts: 1,851

    from kansas

    Yes rotors will pulsate a pedal, deal with this at work all the time.

    Also note so will a bad drum. We've had cars that sat along time with drum rears and the pressure against the drum has actually caused problems.

    Not common to see a bad drum but if you have my luck ! Lol

    Turn the rotors if you still have it check the rear drums.
  20. bigdog
    Joined: Oct 30, 2002
    Posts: 691


    My experience has been if the pulsation is coming from the front you'll feel it in the steering wheel also. From the back just in the pedal. Check the rear drums.
    Mimilan and Wrench97 like this.
  21. garyf
    Joined: Aug 11, 2006
    Posts: 247


    If you can find a place to get up to speed and use your emergency brake to slow the car down. This will eliminate the use of the front brakes and narrow your problem area down,of ft. Or rear brake. Driving a hot brake drum or rotor through a deep water puddle can warp it instantly.
  22. Wrench97
    Joined: Jan 29, 2020
    Posts: 503


    ^^ what he said.
  23. 325w
    Joined: Feb 18, 2008
    Posts: 5,677


    Drums and disc are supposed to be stored flat while in their boxes on the store shelves. On edge tHey can warp. Mom an pop auto store owner told me factory rep said that
  24. Mr48chev
    Joined: Dec 28, 2007
    Posts: 30,185


    I've fixed far too my cars with warped rotors and most had a pulsation in the pedal floating or fixed rotor. Fixed does amplify it a bit as it does on my 98 528I

    We have a tire store in town that for years hammered the lug nuts down to what amounted to about 125 to 150 lbs of torque. Enough to break a cross bar lug wrench when I tried to take a wheel that they had installed off to check the runout on the rotor. They would do that and when the car owner went back with pulsation they would sell them a brake job and new rotors. We turned a lot of those rotors in the shop class that I taught. One of the students that did a lot of that is now the head front end/brake mechanic in that tire store. That meaning if you or the tire store hammers the lug nuts on with an air wrench rather than using a torque wrench you stand a good chance of having warped rotors.

    That big puddle you hit after some hard braking can mess them up. Throw in the guys who are in the habit of flying up to a stop sign and then hitting the brakes hard rather than slowing down gradually and rolling up to a gentle stop.
  25. nosford
    Joined: Feb 7, 2011
    Posts: 585


    Yes, rotor runout can and will cause a pulsating vibration felt in the steering wheel and/or the brake pedal. I would say you are at the upper limits on the spec on your best rotor. The manufactures recommend on the car turning of the rotors to true the surface with the hub. See this.

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