"You are banging your head against the problem that's been known for years. It's NOT easy to measure Caster on a kart. We COULD do it but it would be expensive. The problem is that you need a reference 'plane' to start with and you need accurate measuring tools."
The problems is not that difficult.
You need a flat plate that will sit across the chassis. (I have used a bit of granite counter top, its accurate enought, though I dont recommend just using your floortray.) This forms your datum reference
Then buy an inclinometer (Wixey digital angle guage, £23) from Machine-dro.com at Hoddesdon which will measure to 0.1 degress (6 seconds)
place the Wixey on the plate so it is measuring the angle fore to aft. set it to zero.
Now set with the steering straight ahead, set it in the same alignment on the side of the stub axle. This will tell you what the stub caster is in relation to plate. Do the same thing to the other stub.
Both readings should be the same, assuming you have identical stubs. Otherwise you can use a ground rod (Ringwood Precision engineers, 330mm long) and put that through the kingpin holes and measure its angle to the vertical.
It doesn't matter too much if you come up with an answer of 20 degrees when the 'proper datum' would give you 15, because you can track changes made against your datum, and compare one side with another. You need to be able to get the same readings repeatedly.
I agree Itpro is right if you were designing the chassis from the start and wanted to set it up to exactly x degrees (within 0.0001 of a degree) to an external datum, that can be more complicated, but here you are looking at predetermined settings. It's when you cannot make them match that you have to start checking other things.
If you want to get more accurate, set your datum plate flat using an engineers level and your kart in relation to the datum with a laser.
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