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Making a better Thorens TD150. stage 3: upgrading parts
1)
above: the standard TD150 frame
2)
Above: the proposed subchassis frame design. If the ballast hanger idea
works, weight distribution will be equally distributed between the three springs
and center of gravity of the oscillating mass would be lowered.
3)
above: real parts less springs, less any counter weights. Taking
weight measurements at each spring location.
4)
5)
Total weight loads: ( 1 + 2 + 3) 190 ozs. or 11 .875 lbs
Average weight load is 63.3 oz each spring before counter weights are added.
In a preliminary test, the original Thorens spring will not quite carry the above load. The
spring nearest the tonearm coil binds while the other two are still within
a working range of their compression. No point in attempting to balance
the load until a workable spring rate is found.
Data for the standard Thorens spring:
6)
click on thumbnail for full size image
 Type: conical compression, ground ends, open 
 wire alloy: unknown but it is steel and isn't rusty after 30 years of use. 
 No. of working coils: 11 
 mean diameter at small end: .602 inches 
 mean diameter at large end: .906 inches 
 avg mean diameter: .754 
 wire diameter: .059 inches 
 free length: 1.935 
 spring index: 13,,,? 
 static deflection : .5 to .7 inches in the standard setup (see image # 1)
(varies from spring to spring) 
 spring rate: ? 
 natural frequency: estimated 4 to 6 hz 
More notes:
The three springs carry the tonearm, which has a natural resonance between
its suspended cantilever at the cartridge (carries the stylus) and the
effective tonearm mass of around 8 to 12 hz, This is an industry standard
but may vary outside this range if arm and cartridge are not well matched.
Footfall resonance is thought to be approximately 3 hz but will not be an
issue as the turntable will be mounted from the wall.
Even more notes about springs:
Spring stiffness is expressed in terms of how much force is needed to
compress a spring one inch. If a spring is rated at 4 lbs, this means that
it takes 4 lbs of pressure to deflect the spring one inch.
Static deflection is the amount of distance the spring is compressed when
under a normal load and at rest.
Natural frequency is the number of oscillations a spring will go through
under a given load. Some formulas for calculating natural frequency:
 NF = natural frequency in hz 
 K = spring stiffness 
 DST = Static Deflection 
 hz = cycles per second 
NF = 3.13 SQRT K/load
or
NF = 3.13 / sqrt DST (in inches)
or
NF = 5 sqrt 1/DST (in cm)
The natural frequency of the sprung suspension should exist between foot
fall and the cart/arm natural frequency. If foot fall is
approximately 3 hz and the low end of cart/arm frequency is 8 hz, then the
target for spring frequency is 4  6 hz.
