Twists Per Inch
The ratio between the drive wheel and the flyer determines the speed of the wheel, and how many twists per inch (tpi) goes into the yarn. For instance, if you want to spin a medium yarn with 10 tpi, place the drive band in the groove of the whorl size closest to 10:1. What this means is that one revolution of the drive wheel will put 10 twists in one inch of the yarn. Therefore, if you draft one inch of fiber, hold it for one revolution of the wheel before drafting another inch of fiber and letting the yarn wind onto the bobbin, you will consistently spin a 10 tpi yarn. If you wanted 5 tpi you would draft 2 inches of fiber to one wheel revolution. But if you treadle 2 revolutions in 2 inches, you will still be spinning a 10 tpi yarn. This is the way you control the amount of twist in the yarn.
Understanding drive ratios will help you to determine what type of yarn a wheel will spin. It is important to know the ratios when buying or selecting a wheel. A wheel with higher ratios like 21:1, is best suited for spinning a fine yarn that requires a lot of twist. Whereas, a wheel with lower ratios like 3:1 is best for spinning bulky yarns that require less twist.
TIP: If you buy a new wheel, the ratios will be included with the documentation that comes with it. If your wheel is used and you do not know the drive ratios, here is an easy way to determine them. Tie a piece of yarn onto the flyer, and position the crankshaft on the drive wheel to the highest position. Place the drive band on one of the whorls and turn the drive wheel very slowly by hand, one revolution, and count how many times the flyer turns for one revolution. This number is the ratio.