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95 suburban vibration

cranecrewcranecrew Member Posts: 27
edited March 2014 in GMC
I have 95 diesel suburban 4x4. From 65mph+ the vehicle vibrates. The faster you go the more it vibrates. Here is what I've replaced trying to stop the vibration. Four new Michelin's, rear shocks(bilstein's),new drive shaft w/u-joints, new rear brake drums, 3 alignments, new steering stabilizer. Vibration seems to be coming from the rear because steering wheel doesn't shake much. Any suggestions!
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Comments

  • dshepherd3dshepherd3 Member Posts: 194
    Have the drive shaft pinion angle checked also the rear trans mount.
  • fleetwoodsimcafleetwoodsimca Member Posts: 1,518
    And certainly recheck the universal joints in the driveline. A worn joint will shake at speed. It can eventually break and drop your shaft on the road! I know that for sure... >:o]
  • cranecrewcranecrew Member Posts: 27
  • fleetwoodsimcafleetwoodsimca Member Posts: 1,518
    That's the included angle between the differential and the drive shaft, with the universal joint at the apex. As the angle is made more acute, the limit of tolerance is approached.
  • alcanalcan Member Posts: 2,550
    Single cardan u-joints don't transfer torque at an even velocity. Output rpm increases, then decreases, by 1% for each 1 degree of joint operating angle, twice per revolution. If the rear u-joint angle matches the front, and the yokes are properly phased, the velocity increase/decrease induced by the front joint is cancelled by an equal velocity decrease/increase at the rear joint. This provides constant velocity of the rear axle's drive pinion. Typical u-joint angularity specs are 1 to 3 degrees, with no more than 1/2 degree difference. Or something like that.
  • fleetwoodsimcafleetwoodsimca Member Posts: 1,518
    Outstanding post! You have surely hit it (let us hope). A friend called me once, distraught over how his 4WD vehicle vibrated on the road ever since he had a new U joint installed. The cure was in line with what I told him concerning the phasing of drive shafts. It turned out that the guy who reassembled his rig got the splines on the slip joint at the rear of the tranny a few teeth out of original position, thus getting the two ends of the shaft out of phase.
  • alcanalcan Member Posts: 2,550
    Yep, that'd do it. With yokes out of phase, the rear joint would be going into the accel phase while the front joint was still in accel, and vice versa. Net result, drive pinion rpm fluctuation and driveline vibration. Don't ask me how I know. LOL Some Volvo and Ford heavy trucks with multi piece drivelines require yokes to be out of phase. Haven't been able to find out why but my guess would be they're trying to cancel an inherent torsional vibration from engine firing pulses.
  • fleetwoodsimcafleetwoodsimca Member Posts: 1,518
    It's MAGIC.
  • cranecrewcranecrew Member Posts: 27
    I agree with you guys about the out of phase u-joint possibility. This is a one piece drive shaft, so unless the drive line shop welded the new yokes in the wrong position I don't think the u-joints are out of phase.
  • cranecrewcranecrew Member Posts: 27
    Could the frame be bent causing the vibration?
  • fleetwoodsimcafleetwoodsimca Member Posts: 1,518
    No splined slip joint in that shaft to allow for shortening and lengthening during suspension movement?
  • cranecrewcranecrew Member Posts: 27
    There is a splined slip joint at the transmission output shaft. The slip joint will only slide on in one posiiton. With the u-joint yokes welded to the drive shaft tube there is only one position that I can install the drive shaft.
  • fleetwoodsimcafleetwoodsimca Member Posts: 1,518
    In years passed, I was aware of splines that could be reassembled in more than one slide-together position, allowing the yokes at each end to thus be in relatively rotated positions from the "proper" position that provided balance in the drive shaft under operational, rotational conditions. When reassembly resulted in the splines mating one or more notches off, vibration ensued until proper reassembly corrected the matter.
    I have briefly looked back over the preceding posts, and I do not see the idea of shaft balance addressed. I have seen weights that were like rectangular patches of metal, that were attached (spot welded?) onto drive shafts. I was advised that these were balance weights. I recall that certain drive-line specialty shops did this variety of shaft balancing. You might pursue that avenue a bit.
    I am plumb out of ideas on your situation! Let us all know what you finally discover.
  • cranecrewcranecrew Member Posts: 27
    I would agree with you about the 'outa phase u-joint' possibilty except for my previous post #10. This driveshaft was made at a specialty driveline shop. I gave them my old driveshaft for measurements to build the new one. Stupid me, but I would assume that they would have balanced the driveshaft. Now that you mention it, there are no 'balance weights' welded to the tube. I assumed that they checked the balance of the driveshaft by spinning it in a lathe or similar method and that it required no 'balance weights'.
  • fleetwoodsimcafleetwoodsimca Member Posts: 1,518
    That might be worth checking.
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