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Thread: Rear Strut Caster?

  1. #1
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    Rear Strut Caster?

    So Iíve been measuring up the rear of the car as Iím designing some new uprights and have come across a bit of an anomaly (or not?).


    after measuring up the chassis i noticed in CAD that the centre point of the top strut mount appeared to be 25mm further forward that the centre point of the bottom wishbone chassis pivot point. Assuming i had buggered up a measurement (too many safety beers?) I double checked and the strut definitely leans forward by around 3 deg. So some neg caster to help align under drive? but the issue I see is the bottom arm pivots on a pin, not a ball, so doesn't allow for any rotation of the upright to align with the angle of the strut.


    Obviously there is enough compliance in everything to make it work, but not really ideal? or am i getting worried about nothing? I plan to run a much bigger nylanthane bush on the inner mount which will help out, and spherical bearings and a re-design of the arm setup (adjustable toe link) so should have that degree of freedom.


    Anyone else looked into this? This is the first competition car I've been involved in running struts so a bit of new territory for me!


    cheers
    Attached Thumbnails Attached Thumbnails RearStrutCaster.jpg  

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    SEC Member Guy Mayers's Avatar
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    Re: Rear Strut Caster?

    Is the chassis level? Is the wheel centered in the arch? In 30 years I've never noticed if mine is out of line and I doubt it's an issue. My lower wishbones are rose jointed on the inner end, solid nylon bushings on the outer end. Leading link has a thin rubber bush at the front and rose joint to the wishbone. Early lower wishbones had rubber bushes inner and outer.

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    SEC Member ChrisCar6's Avatar
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    Re: Rear Strut Caster?

    Quote Originally Posted by Guy Mayers View Post
    Is the chassis level?
    I wondered what bit of the chassis to level to when doing my geometry. In the end I used the centre spine. I don't know if this is parallel to the top of the rear frame. I haven't seen a figure for rear caster quoted anywhere. Craig might know.
    This suggests that the two should be in vertical alignment:
    Attached Thumbnails Attached Thumbnails Hawk rear.jpg  
    Last edited by ChrisCar6; 15-10-2019 at 19:34.

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    Re: Rear Strut Caster?

    Hi Guy
    Yes chassis is level and wheel appears to be centered in the arch but at the moment i'm not too worried about overall vehicle setup, just the relationship between the top and bottom mount, from my measurements


    1. Top longitudinal chassis Bar is parallel to bottom longitudinal chassis bar


    2. Both are square to the rear bulk head


    3. The Bottom arm inner pivot axis is parallel to the bottom chassis bar


    4. When I measure from the rear bulk head to the center of the bottom arm pickup point i get 585mm


    5. When I measure from the rear bulk head to the center of the strut top i get 560mm


    Interesting that your car has a rose joint inner end on the wishbone, this would then allow for that degree of freedom, so there would be no issue, mine has the rubber, so an earlier one as you say.


    Hi Chris, yea i found that drawing the other night and came to the same conclusion, which confused me even more!


    thanks
    Colin
    Last edited by ByrneEngineering; 15-10-2019 at 22:17.

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    SEC Member ChrisCar6's Avatar
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    Re: Rear Strut Caster?

    On the diagram above, it looks like rear bulkhead to the center of the bottom arm pickup point should be 432mm? or am I missing something?

    heads to garage with measuring tape....

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    SEC Member ChrisCar6's Avatar
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    Re: Rear Strut Caster?

    i have 560mm from the rear of the bulkhead to the centre of the topmount, and confirm the measurement of 648mm bulkhead to front of rear frame as above.
    i also have 152mm centre of bottom rosejoint to front of rear frame, (as shown in diagram)
    Last edited by ChrisCar6; 15-10-2019 at 22:18.

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    Re: Rear Strut Caster?

    Apologies that should have been 585 not 285, it's very early here! Which lines up with the 23" dimension of that drawing I think

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    Re: Rear Strut Caster?

    Thanks for the confirmation chris so assuming the 23" dimension is accurate on your car the strut top is around 25mm further forward than the centre of the bottom mount

  9. #9
    SEC Member ChrisCar6's Avatar
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    Re: Rear Strut Caster?

    There are no appropriate dimensions on these Lancia diagrams, 'f' & 'g' are the two bits we're interested in..
    With my school set square they look in-line not offset. Maybe best ask Hawk.
    I have yet to work out whether to take rake into account in caster measurements or not. Can. Worms?
    Attached Thumbnails Attached Thumbnails Lancia rear_0002.jpg   Lancia rear_0001.jpg  

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    Site Supporter ProtoTipo's Avatar
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    Re: Rear Strut Caster?

    On Graham Halstead's CAD drawing of his recent Hawk chassis:
    The centre of the strut top is 22.65mm further forward than the centre of the pick up point of the rear 'A' suspension arm.

    On the same drawing, the rear of the rear bulkhead box to the centre of the rear pick up is 592.58mm.
    Less 38.1 from the front of that box.
    Last edited by ProtoTipo; 16-10-2019 at 08:39.

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