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  1. #1
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    Default Holden trailing arms.

    Is there a reason HQ - HZ Holdens and Toranas of the same era have different length lower trailing arms?....HQ 560mm, Torana are a 100mm longer at 660mm.
    As far as I know the upper arms are the same lengths.

  2. #2
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    hi
    Differential housing position /location /pinion angle change is effected by trailing arm length [upper and lower ] length of arms and mounting location on body work and diff housing mount positioning .

    If u had both rear ends on a table and raised lowered diff the end result should be pinion angle should be the same or very similar .

    The lower arms should set the wheel base position [include bracket and measure to diff tube c/line ]
    The upper arms should set the diff roll or pinion angle [including bracket on housing in relation to diff tube c/line]

  3. #3
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    There are optimal angles for universal joints to operate within and if you try to make them work outside those angles you’ll flog them out very quickly. The primary geometry is set by the engine and gearbox assembly angle and the diff angle. The travel is limited by the bump stops and max extension through an arc with a length set by the upper and lower control arms.

    Because the Torrie is a shorter car than hq-hz-wb you need different geometry to keep the travel sufficient for suspension inputs while maintaining the driveshaft geometries within +/- 3 degrees which is generally considered an acceptable range to ensure appropriate uj longevity.

    There’s a fun free program on the Dana-spicer page that lets you see all the geometry changes, if you have a play with that it’ll probably make more sense than my attempt at an explanation.

  4. #4
    been here .......too long Smitty2's Avatar
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    Quote Originally Posted by Horaldic View Post
    There are optimal angles for universal joints to operate within and if you try to make them work outside those angles you’ll flog them out very quickly. The primary geometry is set by the engine and gearbox assembly angle and the diff angle. The travel is limited by the bump stops and max extension through an arc with a length set by the upper and lower control arms.

    Because the Torrie is a shorter car than hq-hz-wb you need different geometry to keep the travel sufficient for suspension inputs while maintaining the driveshaft geometries within +/- 3 degrees which is generally considered an acceptable range to ensure appropriate uj longevity.

    There’s a fun free program on the Dana-spicer page that lets you see all the geometry changes, if you have a play with that it’ll probably make more sense than my attempt at an explanation.
    GMH specifically cover this in various early Commodore service manuals.. complete with specs, pics and measuring techniques
    (remember Commodores run 2 piece shafts)

    then some crazy drops a VH commodore on its belly and runs a one piece shaft.. and wonders why the unis only last a week! doh
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  5. #5
    casual poster BasicQ's Avatar
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    As the guys above alluded to the Torana has a shorter wheelbase. Every inch of rear suspension travel creates greater driveshaft angles than if the wheelbase and therefore driveshaft was longer. Trailing arm lengths are altered to maintain pinion angle geometry correct to transmission output angle through the suspension travel. Correct operation must be present in situations from just the driver in the car, no load with an almost empty fuel tank to fully loaded with five adults in the car and full fuel tank.

    When a universal joint is operating at an angle all 4 points on that joint will be moving at a different velocities through one rotation, a variable velocity joint so to speak. The other end of the same shaft must be set up so it’s phased correctly (in-line) and is operating at close to the same angle. If this does not happen forces can build up due to the uni’s 4 joints at each end going through different velocities at the same time. Vibrations and/or broken uni joints are the result. That is what is meant by maintaining correct pinion angle geometry correct to transmission output angle.

    Mystery regular breaking of uni joints or vibrations that won’t go away no matter how much balancing is done often lead back to over zealous lowering without keeping pinion angle in check.

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