What's the deal with idler turntables and do they have a place in modern HiFi?

I will say this, as far as measurements are concerned. At present, my best tools are the Feickert Pro software and a Timeline, and I’m not seeing any effect (can’t directly measure music with the Feickert, unfortunately). I’ll leave it open to the possibility that at a finer level of resolution, there may be a drag effect, but somehow, I doubt it.

Remember that speed changes occur in varying time domains. At the finest level of granularity, we hear it as distortion. If you’ve never read the IAR review of the Rockport Sirius, the first page of this lengthy tome is a good read for an analog-file. Worst-case, it will cure your insomnia ;-)

http://www.iar-80.com/page12.html

Cheers,
Thom @ Galibier Design

Here´s Teres Audio´s article about http://www.teresaudio.com/manuals/teres_speed_tech.pdf

The "problem" with this hobby is that there are frequent instances where we have phenomenon A associated with physical fact B, and the natural human response to such coincidences is to ascribe A to B.  But we almost never have good scientific experiments to prove or debunk such associations.  Thus we are all floating in a sea of subjective judgements, and there is an industry based on tweaks that thrives upon our ignorance. (Sorry, but I do not think the Timeline is much better than a well designed strobe, like the KAB, for looking at instantaneous deviations from correct platter speed. However, if you DO see a speed deviation with the Timeline, you do have a problem.)

The "problem" with this hobby is that there are frequent instances where we have phenomenon A associated with physical fact B, and the natural human response to such coincidences is to ascribe A to B. But we almost never have good scientific experiments to prove or debunk such associations. Thus we are all floating in a sea of subjective judgements, and there is an industry based on tweaks that thrives upon our ignorance.

Very well said, Lew, IMO. I would add that in audio there are countless variables that can be cited for which it is not readily possible to define a quantitative threshold separating what may potentially be audible in some systems from what is unquestionably insignificant. In the absence of that kind of quantitative perspective there is essentially no limit to what a perceived or claimed sonic effect can be attributed to.

Best regards,
-- Al

Spot on - Lew & Al. 

One false link I alluded to above (my first rim drive experiment) relates to incorrectly ascribing massive torque as the reason for rim drive's attributes.  Actually, it's the architecture itself (rim drive) that is responsible for this. 

This isn't an argument for or against high torque, but rather (as you both stated) to not jump the gun in terms ascribing causality.  This of course goes further down the rabbit hole - that we may well have the equipment, but not know what we really want to measure.

Indeed, a Timeline is no better or worse than a KAB, and at the speed level of granularity responsible for intermodulation distortion, the Feickert is also useless.  At this point, all I can use these tools for is to observe a (likely) loose correlation with what I and my listening panels are hearing.

Cheers,
Thom @ Galibier Design