Vintage DD turntables. Are we living dangerously?

Henry, Come back to me with actual numbers to describe the possible change in mounting distance, given what you perceive to be the problem. My listening room varies in ambient temp by about 4 F degrees, from 68 to 72. According to my calculation, the fractional expansion of an alu bar (which this is not) would be .000052 (4 times 13*10^6, using the inches/F coefficient) over this 4 degree F span of temperatures. Further, the aluminum yoke embraces a stainless steel and brass pod (yes, a heavy and damped pod like yours, except it does not touch the shelf) that in turn embraces the vertical shaft of the tonearm in a clamping collet (not with a flimsy set screw). The alu is constrained from expanding and contracting by the bond between it and that pod plus the fact that it is also surrounded by the concrete-like material that constitutes the plinth itself. In other words, your critique of the L07D is off the mark, at best, and a sidebar to the central question. But let's stop here; I am OK with your belief structure; it has no effect on me whatever, and I am sure your music sounds just fine. This is all about splitting hairs anyway.

Dear Thuchan, I may be wrong in my assessment of the top line MS tonearm mounts; I have only ever seen them in photos. Possibly they are more rigid than they appear to be. If so, my apologies.

Good one Halcro, this debate gets funnier by the minute. Assuming the Kenny was set-up and then played at room temp, there should be virtually no discrepancy in distance. 70°F = 21.1°C, and 79°F = 26.1°C (for those of us...). Is that enough to cause several mm change?

Still, the point is made and it seems valid, at least to me. Why is it better to have arm and platter joined at the hip? Potential for extraneous interactions seems greater, not less.

The linear temperature coefficient of expansion of Aluminium is 0.000023m/m degrees C and is indeed approx. double that of steel.
Using this figure on a LO7D and assuming a 5 degree C delta, we get a change in distance to spindle of around 0.04mm. (approx 1.6 thou inches) Further the arm is lengthening and shortening relative to the ambient temp. This would somewhat mitigate the distance change.

Of course if one goes the pod way, the shelf's thermal expansion, contraction becomes the decider of the dimensional change.

The thermal expansion "problem" has been grossly overstated.

Richardkrebs
The thermal expansion "problem" has been grossly overstated.

So what are we left with,the stylus moving a 4 or 5lb pod?

Time to exit this carousel.

Totem.
I first posted this on another thread.
Here updated for relevance....

To me it is obvious that a pod will move due to Stylus drag. The question is how much.
In order to calculate this I needed a figure for stylus drag. A search on the web proved inconclusive but then ironically the answer came from the original timeline thread. There, one TT is mentioned where specific data is given on the amount of laser pointer movement per revolution and its distance from the centre spindle.
This TT is a beautifully engineered machine with, from memory a 22 kg platter driven by a fractional horse power motor via a thread. Hereafter I will call this TT. "TD"

The specifics were 2 mm movement on a radius of 400 mm, per revolution.
With this information it is possible to calculate the retardation torque and hence the drag. From this it is possible to calculate how much the pod moves.

So assumptions......
A pod similar to Halcro's is used on TD
Platter 22 kg of uniform section
Pod plus tone arm 11.5 kg
Height to record surface above mounting surface 150 mm
Pod feet 100 mm spacing in a equilateral triangle
Pod/ arm CofG, 75 mm above mounting surface
Pod CofG Central inside the mounting feet
Pod feet are not adhered to the mounting surface. ( no penetration of the cones into the shelf )
The same arm and cartridge used on TD is used on the pod.
TDs motor only provides enough torque to maintain original speed before stylus is lowered, after it is lowered.
Stylus is lowered at a radius of 140mm
Platter has a diameter of 320mm

The first answer is the force applied to the platter to cause this retardation torque. This works out at around 0.0031 newtons. Actually a large number under the circumstances but it is slowing a 22 kg platter!
Using this force and applying it at a height of 150 mm to the pod we get a tilting of 8.2 microns towards the platter, more or less.

Observations.
With the stylus at a radius of 140 mm, the two front feet of the pod do not appear to be a right angles to the arm when viewed from above. This will reduce the tilt a little. It is unknown what happens to the magnitude of the stylus drag as the arm tracks towards the centre, so it is possible that the pod will tilt as calculated once the arm is at right angles to the feet, assuming that this happens before the end of the inner grooves.
As an aside the tilting at a radius of 140 mm produces a yawing effect on the pod such that the arm rotates approximately along its axis. This effect is caused by the configuration of the feet. It is tiny and likely insignificant.
The calculations assume that TDs motor does not sense the slow down and produce restorative torque. Since it is a synchronous motor it will act to try and maintain speed. This will put more energy into the system and increase the tilt.
The pods appear to be slightly crescent shaped. If this is the case the CofG will be biased towards the two feet closest to the platter. This will increase the tilt.

The CofG is probably higher than shown here due to the feet and the weight of the arm. If it is the tilt will be greater.

The calculations assume that TDs platter has a uniform section. If its radius of gyration is larger or smaller than this suggests, the tilt will similarly be larger or smaller.

If TDs arm and cartridge was fitted to a pod and used on Halcro's TT, things would be different again. This because the TT-101 does NOT slow down. It is putting even more energy into the system, so the tilt would be larger.

The calcs assume that the motor assy cannot move. In Halcro's case it can and will.

The 8.2 micron figure is an average. The pod will move more or less depending upon the groove modulation.

Actual dimensions and weight of the pod will materially change these numbers.

I do not know if the amount of movement is of any significance, but yeah baby, it moves with the grooves. This compromises one of the three ideals I mentioned featured in the mythical perfect TT. This was my starting point in these discussions.