SRA change from 140g to 180g


Assuming there is 1mm difference in thickness betw 140g and 180g record and I started out with a 140g record with SRA at 90 degrees. When I put on a 180g record, SRA changed by 0.25 degree. (Provided I mantain the same VTF and using 9inch arm.

Is this correct? sin-1 (1mm/ (25.4mm*9))

I am amazed at how much I need to adjust VTA to maintain the same bubble reading on the Phantom 2.

I ultimately set by ear and ignore the bubble.
128x128glai
I would add the Graham is more easily upset by floor bounce and suboptimal azimuth adjustment. I think this may be due to the unipivot design which lends the highly focused sound. I can see and hear the Graham wobbles for very short time when it is initially placed into the groove ( not a drop). The minus K platform also improve the Graham more than the gimbal bearing designs ( triplanar, davinci, TW 10.5). I cannot confirm whether this is purely due to design choice as I have only had Grahams unipivots. I have not had audiocraft or talea.

I also find the Graham extraordinarily sensitive to VTA adjustment. If my math is correct, 1mm record thickeness difference would yield 0.19 degree change in SRA on 12 inch arm vs 0.25 degree change on 9inch graham. 0.06 difference would be inconsequential.

I do set by ear. However, I like to gain geometric understanding to help me appreciate the designs.

Anyone can confirm the calculations? I am very far from my high school days.
Glai, thanks for the comments-appreciate it. Mid last year you posted several times regarding the Spiral Groove Centroid arm. Did you ever try that or explore this option?
You r welcome. I have not tried the centroid but feel that it should be at least competitive with the graham. I am waiting to see how the SG2 perform first.