Rear Strut Caster?

So I’ve been measuring up the rear of the car as I’m 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

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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Originally Posted by Guy Mayers

Is the chassis level?

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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:

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

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....

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)

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

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

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?

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.

Given the way the toe is adjusted on the rear of a Stratos (Lancia, Hawk & Corse "I") then is is fair to assume that with the symmetrical reverse lower A arm design then with the correct amount of toe in applied you would find that the rear strut is actually vertical?
For every adjustment of the rear toe then the strut will alter in angle but with toe in applied it will pull the outer end of the wishbone forwards which would bring it back into line with the top mount?
The strut top being set forward by 23-25mm relative to the wishbone bracket could make sense in this case?

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Originally Posted by Strat Fan

The strut top being set forward by 23-25mm relative to the wishbone bracket could make sense in this case?

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Yes, and the inner end of the 'A' arm can be positioned differently fore and aft in the pick up point, be it a bush or rose joint type.
It's not forced to be in the centre.

I can see that, but is the pivot point for toe-in not the rear of the tie-bar, so the forward movement of the strut on toe-in will be limited?

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Originally Posted by ChrisCar6

I can see that, but is the pivot point for toe-in not the rear of the tie-bar, so the forward movement of the strut on toe-in will be limited?

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The pivot point for the toe would be the inner wishbone bush where it meets the chassis bracket. The trailing link controls the amount of toe and prevents the wishbone from flailing around.

Good point re toe adjustment moving the upright forward, hadn't really taken that into consideration. Just did a very, very rough sketch (and I've had a couple of beers) and to get the upright to move 25mm forward i think you'd end up with something crazy like 20mm toe-in across the car. So i think even with a reasonable amount of toe in on the rear and the arm spaced as far forward in the pickup point as possible you'd never get a fully vertical strut, not that that is necessarily a bad thing, with a spherical bearing or rose joint i can't see any issue at all, and clearly the rubber bush also has the compliance to cope with the misalignment

Agreed, 1 degree of toe in equates to approx. 5mm of forward movement at the upright end.

The bottom pivot of my old uprights wasn't at 90 degrees to the strut mount, saying that, you'd expect them to be a handed pair but mine weren't, they were both identical, once fitted it was all a bit weird.