When choosing a drop spindle, it helps to know a little about the physics of drop spindles, and how that affects the kind of yarn you can make with a spindle. In this article, we’ll discuss spindle weight, and why it is an important factor in choosing a drop spindle.
It is quite obvious that a light spindle is better for light yarns. If a spindle is too heavy for the yarn you’re spinning, the weight of the spindle pulls the fibres apart, snapping the yarn and dropping the spindle (the old joke is that they’re called drop spindles for a reason!) before you can get enough twist to hold the fibres together. But spindle weight also affects another factor – inertia.
In simple terms, we can think of inertia as a measure of how much an object tends to keep doing what it’s doing, whether that’s staying still, or moving. Objects with higher inertia are more difficult to get moving or to speed up, but once they’ve been set going, it takes more effort to slow them down or stop them, as well. Inertia is directly proportional to mass (if you want the equation, it’s I=mr² where I is the moment of inertia, m is mass and r is radius from the centre of rotation) so simply put, a spindle with more mass has more inertia than a lighter one of the same whorl diameter.
Lower inertia means that light spindles can spin fast – because they are easier to set moving, a spinner can get a fast spin with little effort. Fine yarns and short or fine fibres need to be spun quickly – friction holds the fibres together, and the fewer fibres that are in contact with each other, the less friction there is holding them together. To make a yarn strong enough to use, or even to support the weight of the spindle, means we have to increase the friction by putting in a lot more twist – and that means spinning fast on a drop spindle, or using a supported spindle. On the other hand, lighter spindles are difficult to keep spinning long enough to put any significant amount of twist in heavy yarns.
Spindles are slowed down by three forces – friction from air particles, loss of kinetic energy to sideways movement if the spindle wobbles, and, more significantly, the force exerted by the yarn you’ve just spun trying to unwind itself. The thicker the yarn, the more fibres you are trying to wrap around each other, and the stronger that untwisting force will be; so to spin thicker yarns we need a spindle that can overcome that force. That is, a spindle with higher inertia. Because it takes more effort to slow or stop it, it will be able to spin for longer even with thick yarns.