My opinion (for what it's worth)... I've had a similar experience to Steve on my '03. At correct toe, with the wheels square to the car, and at a relatively low ride height, I can only get about 3.2 on the left and 3.0 on the right. The subframe does not appear to be bent when measured with an Advanced subframe checker and the suspension appears to be straight. Tire temps and wear patterns indicate that, at least on some tracks, the car needs significantly more camber than I can legally get. I might be able to resolve this by swapping parts, but that seems to be a rather expensive and time-consuming gamble.
The rules, as they are currently written, favor cars and suspension parts that are on the high end of the adjustment range, whether from manufacturing tolerances, previous contact, or intentional modification.
Although it's easy to check for slotted control arms, there are other ways to get more camber that would only be measurable with a set of jigs and published dimensions for each part. Very small changes to the upright or upper control arm can make a big difference in rear camber and, if done cleverly, these would be impossible to detect without special tools.
This has a created a situation where legality can not be reasonably verified, at least with the tools currently available to tech. Like it or not, "tech shed legal" is SOP for many guys at the sharp end of any competitive class. Making an item that can not be teched "illegal" simply forces us all into a devil's bargain: intentionally break a rule in the knowledge that you can never be caught, or cede an unfair advantage to less scrupulous competitors.
I love Spec Miata, but one of my main concerns when I came into the class was that it would not be possible to win legally. At the end of my second year, I'm convinced that legal cars can and do win regularly. However, I also think rear camber is an area that is being exploited by some people in ways that aren't being caught in the tech shed. And as mentioned previously, I think the variables involved in racing 15+ year-old production cars are contributing to the problem.
As we did in the front suspension, let's just make it so that every car can be easily adjusted beyond optimal. It doesn't even require purchase of new parts; it just takes a few minutes with a grinder. Level the playing field and let us stop worrying about how that car in front of us got 4 degrees.
Michael Novak, JRHille and Steve Scheifler like this