Step 1 is the drive ratios. What this refers to is the difference in size between the drive wheel and the driven object — the whorl, which in turn will be moving the spindle (on a spindle wheel such as a charkha or great wheel), the bobbin or flyer (on a single drive wheel), or both bobbin and flyer (on a double drive wheel).
You can measure this most easily by measuring the circumference. Take a piece of string, and wrap it one time around the drive wheel in the same spot where the drive band goes. However long that string is, that’s the circumference for the drive wheel. Now, do the same with the whorl. Let’s suppose that your drive wheel is 15″ around, and the whorl is 5″ around. To calculate the ratio, you divide the number for the drive wheel by the number for the driven object (the whorl). This gets you a 3:1 ratio — the drive wheel is 3 times the size of the whorl. What this means is that for every one time the drive wheel goes all the way around, the whorl is going to make 3 revolutions.
Now, let’s say you have a 20″ drive wheel, and 5″ whorl. That’s a 4:1 ratio, which means every time the drive wheel goes around once, the whorl (and therefore the spindle, flyer, or bobbin) is going to go around 4 times.
You could also get this same effect by keeping your 15″ circumference drive wheel, and going from a 5″ circumference whorl to one that measures 3.75″ in circumference (1/4 of 15″). This is still a ratio of 4:1, though the sizes of the drive wheel and driven object are different from the previous example.
But, let’s say you have that 15″ circumference drive wheel, and what you really want is a 30:1 ratio. At this point, you need a whorl that is half an inch around — very small. What’s wrong with that? The answer is traction — your drive band needs to be able to get a good enough grip on the whorl to cause it to turn, including turning whatever is on the end of it. Mechanically speaking, when your driven object gets smaller, the first thing you notice is it being harder to make it move, and the next thing you notice is that it loses traction and starts to slip — it just can’t get hold of the whorl and make it go.
It’s also harder, from a manufacturing standpoint, to make something really tiny and exact and still durable.
So now let’s say that you move up to a drive wheel that’s 30″ in circumference; now to get a 30:1 ratio, you need a 1″ circumference whorl. There is a lot more room for grip on that! A high ratio setup works more efficiently with larger pulleys (wheels and whorls and whatnot) because of an assortment of traction issues.
There are other factors in play as well. For example, distance between the drive wheel and the whorl changes how much contact the drive band has on the whorl, as does how the drive band is set up. In a double drive system, the drive band is doing double duty, driving both the flyer and the bobbin (just at different rates). There’s more drive band, and more driven objects, and thus more opportunity for slippage and loss of traction — so double drive systems are easier to make work well with larger drive wheels and whorls.
Another factor is that a larger wheel (or a heavier one) will have greater momentum. This means that once you get it going, it’s better able to keep going with less effort, than something smaller and lighter.
So, what does this mean in practice?
1. It’s easier to build a high-ratio wheel with a larger drive wheel.
Therefore, it’s more likely that wheels suited to spinning fast (at high ratios) will have larger drive wheels, while wheels suited to spinning slow (at low ratios) will have smaller ones. A higher speed at the flyer gives you more twist faster; a lower speed gives you less. The thinner your yarn, the more twist it can hold, and the fatter your yarn, the less. So for spinning a very fine yarn, you want high ratios, and for spinning fat yarn, you want lower ones.
2. A wheel with a larger drive wheel will most likely have superior momentum.
This means that once you get it going, it’s going to be easier to keep it going. You’ll get less tired treadling it.
3. A wheel with a larger drive wheel may have limitations when it comes to low ratios.
If you have that 30″ drive wheel and you want a 3:1 ratio, you need a 10″ whorl. Will that fit in your flyer assembly area?
The most common modern wheels are multi-taskers, which do very well for a broad spectrum of types of spinning. They will commonly have drive wheels with diameters from 13″ to 24″, which translates to circumferences between 40″ and 75″ roughly speaking. These wheels can fit in a good range of spaces, and commonly feature multiple ratios between 5:1 and 20:1, making them suited for spinning a wide range of fibers in a variety of ways. Where these wheels fall short is when it comes to being tuned for specific purposes that are at the extremes of the spectrum: super duper thick yarn, or mega ultra fine yarn (or very short stapled fibers that need a lot of twist fast).
Antique “Production” wheels commonly had much larger drive wheels, being suited to producing very large amounts of very fine yarn as fast as a true production spinner could draft. Great wheels, with drive wheels that can be 48″ in diameter (so 4 feet across and over 12 feet all the way around!) are ideal for spinning woolen yarns very very quickly, though the spinner must be able to keep up, of course. Production flyer wheels often have 36″ drive wheels (3 feet across, 9.5 feet all the way around the rim). You’ll also often see such wheels have seemingly small bobbin capacities — they’re for spinning fine yarn for weaving, mainly. You would still pack quite a bit of yardage of fine yarn onto those wheels!
So, when you’re shopping for a wheel, consider what type of yarn you want to spin with it. If you know for a fact you want to spin a lot of fine yarn quickly, you want a big-drive-wheel kind of wheel; if you know you want to churn out bulky low-twist yarn, you want something with a really small drive wheel. If you’re interested in a variety of options that don’t go to such extremes, then a midrange size for a drive wheel is probably a good one for you.
This is also a reason why many veteran spinners have more than one wheel: it is difficult to have only one wheel fill every spinning need. Though many modern wheels can fill most, spinners with work that falls to one end of the spectrum or the other may find that they have a wheel on which they like to do most of their very fine spinning, most of their very bulky spinning, and a wheel that’s good for everything in between.