For a start, measure your ride height properly.
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You'd be surprised how far off bodywork can be without it being obviously noticable.
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Particularly aftermarket fibreglass parts.
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Measure to a point that's structurally fixed and relevant to the suspension.
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I'd suggest the inboard end of the track control arms.
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It's not as convenient as measuring the arches but it's a hell of a lot less meaningless.
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Secondly make sure you're taking measurements on a proper flat and level surface.
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Ideally this should be a poured concrete surface which has been wet enough to be allowed to settle under gravity and then measured to be flat and level.
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A driveway or car park that
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'looks right' isn't really good enough.
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Finally, forget about the measured ride heights and have it corner weighted instead.
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Done properly it'll compensate for any variation in the chassis anyway.
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Corner weighting is the equivalent of adjusting one leg of a table so that it doesn't wobble.
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The absolute lengths of the table legs is less important than having the weight correctly distributed over them.
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If you've fitted suspension with adjustable ride height then it's really essential to get this done.
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