I have been accused of talking to my readers like children.The following sites talk to spinners as if the the spinners are babies and not a spinner complains!
In contrast, I give examples with actual measurements. And, I tell you how to get better results. Try asking Kromski how to spin fines!
Spindizzy invokes the the same differential rotation speed (DRS) that Alden and I talk about. We do the math, and CAJ does it qualitatively. The thing is: DRS is a clockwork mechanism, and to make good clock work mechanisms, one needs to do the math, and get it correct. Then, one needs to fabricate the correct clockwork mechanism. When spinning fines, a difference of 1 mm (1/25 ") in whorl diameters is important. DRS is simply not something that can be done qualitatively.
Joy of Hand spinning extols a high degree of twisting efficiency in DD, just like I do. Except, I use a tachometer and do the math. I tell you how much twist efficiency you can actually expect, and how you can improve your twist efficiency. These details are learned by doing.
Kromskina notes that DD generally spins the finest yarn. I say, "DD will allow spinning fine yarns, much faster, but to use DD to spin the fine yarn quickly, the spinner needs the correct DRS." The DRS that comes standard on Kromski is ~1.6, which is very good for spinning 1,600 ypp singles. Note that Kromski does not supply the DRS for its whorls. By having the correct diameter of whorls to provided the needed DRS, I avoid slippage.
It is terrible for trying to spin 5,600 ypp lace singles. It will do it, but it is no faster or easier than Scotch Tension. The 5,600 ypp lace singles want a DRS of ~1.04, which is very different from a DRS of 1.6. And, the flyer whorl that provides a DRS of 1.04, will NOT allow you to spin the singles for worsted weight 2-ply. As expressed, the Kromski statement is nonsense baby talk, but nobody complains.
If my wheel is set up at a DRS of 1.04, AND I need to spin worsted weight, I either change flier whorls or I run it single drive.
I assure you that one can spin 45,000 ypp singles running single drive, but that you can spin such singles, twice as fast using DD with the correct DRS. And, 3 times as fast using DRS and an accelerator wheel. That is the magic of DRS.
The continuous and limited take up of yarn as controlled by DRS allows the self assembly of the yarn. It is not discussed in the Big Blue Book, but it is this the self assembly of yarn that makes DRS so productive. It is similar to the formation of yarn in the old flyer frames circa 1820. This self assembly of worsted yarn is rather similar to the formation of woolen yarn from a long draw draft. It means that the difference between true worsted and true woolen is simply the fiber preparation. It means that one can spin worsted yarn much, much faster than one can inch worm draft such yarn. I am sure it is why Alden gave the topic so much space in his Big Book of Handspinning.
The single drive setups are like pliers or adjustable wrenches. DD is more like a mechanic's socket drive set. The socket drive set is fast, powerful, protects the nuts and bolt heads, but you need the full set. The pliers and adjustable wrenches are handy, but they will not get into places that a socket drive set will get into, and adjustable wrenches are not as powerful or as fast as a good socket set.
Good mechanics have pliers and adjustable wrenches, but their socket wrench set(s) let them work quickly and do high quality work. I have a socket set for working on machines, and I have a set of DD whorls to provide the proper DRS for working on fiber. They allow me to work quickly and do high quality work.
These days every good mechanic uses a power screw driver/ nut and bolt driver. It improves productivity. Likewise, I use an accelerator wheel to improve my productivity. Just as power bolt drivers work better with sockets than with adjustable devices, the accelerator wheel works much better with DRS than with single drive flyer bobbin assemblies. Spinning worsted with single drive systems requires drafting techniques that cannot be sustained at those speeds, while with DRS and properly prepared fiber, worsted yarn of the correct grist/twist will self assemble a the end of the drafting triangle. A typical commercial spinning wheel can insert twist at about 1,000 rpm. With DRS and an accelerator wheel, spinning at ~3,000 rpm is easy, and spinning at 4,000 rpm is sustainable, and 4.500 rpm is possible when highly motivated. The drafting process for woolen is very similar, but carded rolags are used instead of combed pencil roving.
Some readers have noted that some of my 10s are "twitted", (the term of art for thick and thin yarns resulting from small variations in roving density as a result of storage or transport). It is easily avoided by re-combing (or by spinning ever so slowly and inch worming the draft) . On the other hand, when my hanks are within 5% of the correct grist and the twitting does not affect the final objects, I do worry about it. Sometimes it is nice to be able to use roving out of the bag.
I judge my final objects, not my yarn. I simply make sure that my yarn is within specification to make a good final object. And always I ask, " Did I get value?". Or, Did I go over budget? Or, was my level of effort too high? An object that goes over budget is just as bad as an object that is not functional. In fact, I would say that a small amount of twitting is desirable, as it adds 'home spun" character without detracting from warmth, durability, drape, or hand.