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Thread: shim rotation math

  1. #1
    I feel the need for speed
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    shim rotation math

    has anyone done the math on shim rotation with 2" drop shackles? I just put on the Belltech shackles and went ahead and rotated the shims while I was at it. Truck rides fine, but being the anal bastard that I am, I want to know exactly what shim should be used to restore the factory angle. My rough calculation is that the drive shaft angle changes 2 deg with the longer shackles, so rotating the 2 deg shim (which gives a 4 deg change) seems to be too much. Any words of wisdom out there?
    /Phil
    2003 H-D Screw #141 - 387HP/485TQ

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    Phil, according to all the Lightning owners the 2 degrees or so pinion angle change is a good thing for HP/performance...

    However I do not know how that factors with the 2-piece driveshaft.

    Maybe Aytron can chime in on this one?

    Rob, St. louis MO

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    Thanks for the feedback, Grey. I thought about this some more and came to the following conclusion:

    With the 2 piece shaft (44" long), the shaft angle (front and rear) is reduced by 2 deg with the higher (by 1.5") pumpkin. However, the pinion gets rotated 3 deg down due to the longer shackles (springs are 58" long, shackles are 3" longer than stock). Rotating the 2 deg shim gives a net change of 4 deg, which returns the pinion angle to within 1 deg of the stock angle. Thats close enough for me. On the other hand, leaving the shim alone means that you will be at least 3 deg off, which would concern me even if I didn't have any vibration.

    I will now let this sleeping dog lie unless someone finds a hole in my logic.

    /Phil

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    Well I agree.. i am putting in the shackle/hanger kit tomorow... so I hope aytron will speak up...

    Rob

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    Rob, see these 3 posts for some good reading material. If you are adding 2 inch Shackles and 2 Inch hangers you will have to add a 2 degree shim to the existing one(and do not rotate shim) or just replce the factory shim with a 4 degree. Buy the 10.00 dollar Angle finder from home depot and right down the pinion angle, both driveshaft angles and the transmission angle(measure on the rear transmission seal area)

    See these 3 posts for your answer....

    http://www.f150online.com/forums/sh...threadid=105402
    http://www.nhtoc.com/vbforum/showthre...ht=pinion+shims
    http://www.nhtoc.com/vbforum/showthre...ms&pagenumber=2

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    I read thru this and I have to tell you that in all of the drops I have had in the past, only one has resulted in drive line vibration, and I just rotated the stock shim and solved the problem. I think there has to be some variation in the angle from the factory, or everyone would or would not have a problem. You also have to consider the variances between shackle manufactures, and how this affects the angle that comes on your particular truck. I wonder what if any affect a one-piece shaft has verses a two-piece shaft, and I also have to believe that the factory allows for some variance.

    The bottom line in my opinion is that if you do not have any vibration at any given speed then ride on, but if you do have any minor driveline vibration, and it can be solved by just rotating or adding shims, then you are good to go.

    Hope that helps.:drink:

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    shim rotation

    great info, but...

    answer these questions,
    so that I might understand,

    1. do you have to remove the oem shim,
    to install an aftermarket (another) one?
    2. when installing a shim after I have
    installed 2" drop shackles, does the fat
    end of the shim go tword the rear or front
    of the truck?

    thanks,
    need to get rid of that vibration@20mph.
    mtvoid

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    MTvoid, If you are only installing 2 inch drop shackles, you need to rotate the fat end of the stock 2 degree shim toward the rear of the truck. I had the same type of vibration while braking (slowing down to stop), at 20 to 25 MPH. Good luck! Just rotate the stock shim!!!

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    Thanks Richo,


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