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an article on driveline vibration

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by MudNurI, Dec 29, 2002.

  1. MudNurI

    MudNurI 1/2 ton status

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    I found this on randy's ring and pinion site- I have seen a lot of questions lately about vibrations etc... think this explains it a lot. I myself had a really bad vibration- remember the jake brake post!....fixed it with help of you all- but this is for those still bothered by it.

    the article

    Driveline Angles & Vibration
    November, 1998

    I have had more than a few customers come back to our service shop after a gear ratio change to complain that we caused a new vibration in their vehicle. In actuality, we inadvertently caused the vibration by increasing the driveline speed.

    When changing to a lower gear ratio (numerically higher) the driveline speed increases proportionally to the change in the ratio. For example, when changing the ratio from 3.08 to 4.56 there is a 48% change in the ratio and a 48% increase in driveline speed. This means that a vibration that would have been noticeable at about 90 mph is now noticeable at 60 mph. This higher driveline speed makes the vibration more noticeable because while it previously existed at 90 mph, it may have gone unnoticed due to the amount of vehicle vibrations it had to overcome in order to be noticed. If the customer installed taller tires and raised the vehicle before changing the gear ratio, the vibration is probably caused by the lift. This vibration caused by the lift and wrong driveline angles would not be noticeable until the new ratio was installed, since the tall tires and stock ratio kept the driveline speeds too slow for the vibrations to be felt. Vibrations due to wrong angles are usually easy to identify because they are harmonic or cyclic. This means that the vibration varies in pitch or intensity even when traveling at a steady speed. The results is a "whir ... whir ... whir" type of noise.

    The solution to wrong angles is easy in theory, but not always easy in practice. There have been many good technical papers written on this subject, so I won't go into detail now. I will however cover a few of the basics. In order for a two u-joint driveline to be free of vibration it must be in balance, and the angle of the front u-joint must be the same as, or very close to, the angle of the rear u-joint. If there is a noticeable difference in the angles there will be a vibration.

    If it is not possible to match the front and rear angles, a constant-velocity joint can be used at the transfer case end with no angle in the u-joint on the differential end. Although it is expensive, and only works if the pinion is not offset side to side from the output shaft, this is the only cure in many lift situations. I have seen vehicles use a constant-velocity joint at each end of the driveline. This is more expensive but will fix any angle problem. Another solutions is to rotate the differential pinion angle up or down to match the front angle. This is usually not easy and can be accomplished with wedges, by moving the spring pads, or by rotating the axles tubes in relation to the differential center section. For some vehicles there are aftermarket kits available for lowering the transfer case or installing adjustable length control arms. Although these may seem like extreme measures, they are sometimes the only way to eliminate or lessen vibrations.

    Note: If the vehicle has a differential pinion offset to the side from the center of the output shaft on the transmission or transfer case, the angles due to this offset will not cause vibration. This is because the pinion shaft and the output shaft are still parallel with respect to these side to side angles.

    I have seen some lifted vehicles that vibrate no matter how much care is taken to balance the driveline and match the angles. It is one of the things that we sometimes have to live with in order to have a lifted vehicle.

    Brandy
     
  2. Batmanjr

    Batmanjr 1/2 ton status

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    Neat, Thanks for posting, I'm sure a lot of people could use this info, So I'm bringing it back to the Top! /forums/images/graemlins/grin.gif
     
  3. MudNurI

    MudNurI 1/2 ton status

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    it's not a pad its a bump

    Brandy
     
  4. J.Lance

    J.Lance 1/2 ton status

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    Thanks for the info. As I demonstrated with my Jimmy, the same holds true for stock height and geared vehicles. I ended up with a T-case lowering kit and degree shims in the back. I don't like the idea of my t-case being lower so I'm going to break out the angle finder and see if I can improve on this setup a little.
     
  5. pilgrim

    pilgrim 1/2 ton status

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    I've had vibes for awhile now, and i'am still UNable to figure them out...
    1.tranny is rebuilt (465)
    2.transfer case was disasembled and checked out fine, same deal with the rear end.
    4. new driveshafts

    so i checked and found the angle at the t-case to be 5 degrees but the angle at the diff was 1.. so bam i removed the 4 degree shims and now it sits perfect at minus 1 degree down... but now it vibes still at a more constent roar, noticeable at 2000 in 4th gear...

    so this really isn't a ?, but rather my story of the /forums/images/graemlins/angryfire.gif time i'am haveing will vibes /forums/images/graemlins/angryfire.gif

    but i still love my truck /forums/images/graemlins/grin.gif /forums/images/graemlins/burb.gif
     
  6. MudNurI

    MudNurI 1/2 ton status

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    I'm going to sound totally dumb here- but did you check where the driveshaft goes into the rear axle? I had the same noise, new u-joints etc. so I knew that wasn't it, my angles were okay- but for some reason, and it must have been when we swapped axles- that bolt- wasn't tightened- I replaced the gasket and tightened up the bolt...nada noise since.

    forgive the lack of "technical names"

    Brandy
     

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