# Straight Arm Mod

From skratchtek

Focus invented the first Technics straight arm mod :

The formula for calculating the ideal length of an underhung straight tonearm is as follows :

Where :

L = The length the tonearm needs to be, from the pivot point to the needle

D = The distance from the tonearm pivot to the spindle

C = The record radius constant, (3.04 for 10 inch records, 3.18 for 12 inch records, see the Radiotron handbook)

C can be found by measuring the outside and inside radius of a typical record's grooves and plugging the values into this formula :

C = (R1^{2} * R2^{2}) / (R1^{2} + R2^{2})

Where :

R1 = Outside radius of record groove

R2 = Inside radius of record groove

More information can be found in the Radiotron Designers Handbook, page 765 : [1]