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 making 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.