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Originally Posted by 135

The Static Spring Load was calculated at 3600 N (or 810 lbs), i.e. the Static Spring Compression (60mm) multiplied by the Ohlins Spring Rate (60 N/mm). You then used this 3600 N value in the spreadsheet calculation for the Ohlin springs. You then went on to use the same 3600 N value in the spreadsheet calculation for the Swift 65mm 7" 60 N/mm springs.

Since the Static Spring Load was originally calculated based on the compressed spring height of the Ohlins spring (i.e. 60mm), I thought the Static Spring Load would have to be recalculated for the Swift springs because, for example, the Swift spring at 178mm free length may only need to be compressed by 15mm, resulting in a 163mm static spring length at ride height. This 15mm compression would result in a Static Spring Load of 900 N/mm (157.61 lbs/in), which is far from the 3600 N/mm Static Spring Load used.

Also, I'm not sure whether the top mount height should factor into a comparative measurement, i.e. the 26mm difference in top mount height (70mm OE vs 44mm GC plates) could be considered against the Swift springs compressed height of 163mm (in my example) giving a comparative measurement of 137mm (178-15-26mm) against the 140mm OE Top Mount based measurement (200-60mm), meaning a relatively like-for-like comparison.
Again, I'm confused.

Why is the same Static Spring Load used for both springs?
Maybe another question is, why does the Swift 7" spring still need to be compressed by 60mm?

To clarify, the "static compression" is the total spring compression from its free length when the car is sitting statically (not moving) on the ground. That spring is supporting its corresponding corner weight, less the unsprung weight, but it is doing so with a motion ratio which increases the weight supported by the spring relative to the sprung weight supported by that wheel. The static compression includes whatever preload you put into the spring when you installed it - perhaps that is what you are missing in your thinking? Every 60 N/mm spring will have a static compression of 60 mm at a load of 3600 N, regardless of its original free length. The only exception would be if you preloaded the spring to more than 3600 N, in which case it wouldn't move any further until you exceeded that preload.

Originally Posted by 135

Do you think it would be useful to determine the values based on an equal (or equivalent) lower spring perch height, and thus an equal ride height, and calculate adjustments from that point? Perhaps, provided the Top Mount Height is the same.
If measurements are based on using the GC plates, which are 44mm high, and the preferred 326mm ride height requires a 372mm strut length, then doesn't that mean the lower spring perch location must always be 185mm and the static spring length must always be 140mm (plus 3mm for spacers / thrust sheets)? If so, why does your calculation for the Swift spring result in the static spring length being 118+3mm?
Does your 3mm thrust sheet measurement cater for thrust sheets both above and below the spring?

A 178 mm (7") long spring (Z65-178-060) will compress 60 mm to a static length of 118 mm (+3 mm if you include thrust sheets). Swift thrust sheets are 1.35 mm thick each, so actually 2.7 mm if top and bottom.

Originally Posted by 135

Not that it makes too much difference but were there multiple fractions of a millimetre that were rounded for the OE top mount in the diagram above resulting in a 1mm shortfall between the component measurements (70+143+158) and the strut length (372)?

As you can see above 3 mm is actually 2.7. I figure accuracy to 1 mm is sufficient for these calculations as they really are approximate to get your spring perch location in the right ball park.

Originally Posted by 135

I can imagine placing a 10mm spacer between the chassis' strut tower mounting point and the top of the camber plate would simply result in raising the vehicle - but by how much?

This effectively increases the strut length by 10 mm. The strut has a motion ratio relative to the wheel because of its angle relative to vertical. I figure it is about 0.95. Therefore a 10 mm spacer on top of the strut top mount will raise the car about 10/.95 = 10.5 mm.

Something I omitted in my original approach was recognition that the OE top mount compresses under load. I have posted elsewhere that under static load it compresses about 5 mm. So if you are measuring things on the bench hoping to figure out your new ride height, you need to take that into account.

Originally Posted by 135

One observation is that, while on-vehicle, it may be difficult to use the top of the steering knuckle as the strut datum, therefore, could the lowest strut thread be consistently used as the strut datum?

Granted, it might not be suitable in your case since you have added a 10mm spacer below this point but I suspect for practically everyone else this would be a suitable datum point.

At least, that's what I've been using. Specifically, since my focus is on the position of the lower spring perch, all my measurements are from the lowest strut thread to the underside surface of the lower spring perch.

The Ohlins instructions locate the top of the lower spring perch from the very bottom of the strut - not particularly useful once installed.

They also reference the bottom of the check nut from the last thread - which is what you are suggesting.

In my case I wanted an exact (within 0.1 mm) repeatable datum point that I could use to preset the spring perches before assembling the struts in the steering knuckles. So I actually measure from the small lug on the side of the strut, not from the top of the steering knuckle (although they are theoretically in the same place).

After the struts are assembled, I am no longer interested in that measurement. Any change I would make from there would be based on ride height and corner weight measurements. That original measurement is just a starting point.

As a point of interest though, my initial front spring perch locations were accurate enough that I did not need to make any adjustment at the front. I did make some rear adjustments to achieve corner balance though.