When buying a drop spindle, it helps to know something of the physics of drop spindles, and the spindle’s shape and size affects what kind of yarn you can spin with it. In this article, we’ll discuss spindle weight distribution, and why it is an important factor in choosing a drop spindle.
Some spindle whorls are just flat discs, some are domed to concentrate their weight, and some are weighted at the rim. Why is this?
The weight distribution of a spindle affects its inertia, a measure of the effort it takes to start or stop it. Spindles with less inertia need less effort to set in motion, so are easier to get a fast spin, while dropspindles with more inertia can spin for longer as it takes more force to slow them down.
Rotational inertia at any one point is given by the equation I=mr², where I is inertia, m is mass, and r is radius. Lets put some (extremely simplified) numbers into that equation to demonstrate.
Imagine we have two equally sized spindles, each weighing 50g, and 10cm in diameter (with a radius of 5cm)
One spindle has a centre-weighted whorl. It has 40g of its weight around 1cm from the centre, with the other 10g at 5cm (in real life it’s more complicated as we count every point on the radius).
We can find the total inertia by adding up the inertia of each point on the radius. This means the centre-weighted spindle has an inertia of:
I= (40 x 1²) + (10 X 5²) = 40 + 250 = 290 g/cm²
The other spindle has its weight at the rim, with 40g at 5cm from the centre, and 10g at 1cm from the shaft
I= (10 x 1²) + (40 X 5²) = 10 + 1000 = 1010 g/cm²
That’s over 3 times as much as the centre-weighted spindle. More inertia means a longer spin, so although both spindles are the same size and weight, the rim weighted one will spin for longer.
However, the idea that centre weighted spindles are faster and therefore better for fine yarns is something of a myth. Rim-weighted spindles are just as capable of a fast spin, depending on ergonomics and the skill of the spinner.
The smallest ‘trindle’ – which has almost all its weight at the rim, the traditional whorl being replaced by weights on the end of three slim arms – weighs only a couple of grams and can spin very fine thread- and cobweb-weight yarns, as well as short-staple fibres like cotton, which usually needs a supported spindle.
Some spindles try to strike a balance between centre and rim weight distribution, by removing weight between the centre or the edge, by means of channels or holes. This kind of spindle is fairly good for general purposes, but in fact, rim-weighted whorls are suitable for any yarn type, spinning longer and being more balanced. Total weight is a more important factor.