There is a strong relationship between the fittings on the spinning wheel and the type of yarn easily produced. This explains why many spinners have more than one wheel.
The working unit on all modern spinning wheels is the combination of the flyer (the 'U' shaped piece) and the bobbin on to which the yarn is wound as well as the whorl. The bobbin is mounted on the flyer shaft and rotates independently of the flyer. This winds on the yarn as it is spun. The difference in the rates of rotation of the flyer and bobbin, dictates the amount of twist imparted to a length of the yarn. To give control of the twist, a range of whorls is generally available. The whorl ratio given is the diameter of the drive wheel compared to the diameter of the whorl. The smaller ratios are used to give the low twist for soft yarns from longer stapled fibres. The high ratio used for imparting a high twist in short fibres like cotton and cashmere.
The bobbin should hold at least 100gm of yarn to limit the number of bobbin changes when spinning.
The drive ratio, measured by the ratio of the diameter of the driving wheel to the whorl, largely determines the type of yarn which can be spun easily. For a range of yarns, a spinning wheel with a wide set of whorl diameters is helpful.
Most softer woollen knitting and weaving yarns can be spun on all the commonly available wheels which are supplied with drive ratios in the range 8 to 12:1. Finer yarns require higher twist and a drive ratio of 14 to 20:1 or more. Bulky yarns and soft spun yarns of over 1cm diameter require a ratio in the region of 3 to 8:1.
The size of the flyer orifice can limit yarn plying. It is important to check this when choosing your spinning wheel. The flyer is best with an orifice of around 1cm. A larger orifice gives a tug on the yarn for each turn of the wheel. This makes fine yarn spinning more difficult. Bulky and complex plyed yarns need an orifice in the flyer of 1.5 cm or more as well as larger bobbin and flyer hooks.