Basically, figuring drive ratios for single drive bobbin lead wheels and for single drive flyer lead wheels has been quite easy. Things become a bit more complex with the double drive wheels because the drive system on these wheels uses both a flyer whorl and a bobbin whorl.

The first step to figuring double drive ratios is to measure the circumference of the drive wheel and the circumference of both whorls. Use some inelastic cording to do this and measure the circumference down inside of the grooves. The cording used for measuring should be of the same diameter as your drive band. Carefully cut the cords to the exact size and then measure the length of the cords and divide by 3.14 to get the diameter of your drive wheel and whorls.

Working through the problem:
Let’s say that the drive wheel has a circumference of 79 inches, the flyer whorl circumference is 7.5 inches and the bobbin whorl is 5.5 inches.

The diameter of the drive wheel is 25.16 inches (70 divided by 3.14)
The diameter of the flyer whorl is 2.38 inches (7.5 divided by 3.14)
The diameter of the bobbin whorl is 1.75 inches (5.5 divided by 3.14)

Now divide the diameter of the drive wheel by the diameter of the flyer whorl: 25.16 divided by 2.38. This gives a ratio of 10.57.

Then divide the diameter of the drive wheel by the diameter of the bobbin whorl: 25.16 divided by 1.75. This gives a ratio of 14.38.

Next, add these two ratios together: 10.57 plus 14.38 for a ratio of 24.95. Then divided the total of the two ratios by 2: 24.95 divided by 2. This gives us an average ratio of 12.48.

Almost done – there is just one more small adjustment. Double drive wheels work because the whorls slip under the drive band so we need to consider slippage to arrive at the effective ratio of the wheel. 6 percent is considered to be the standard adjustment factor for slippage (Alden Amos: The Alden Amos Big Book of Handspinning). To figure slippage, take the average ratio and multiply that figure by .06. An average ratio of 12.48 x .06 gives us a factor of .7488. Subtract .7488 from the 12.48 and it gives an effective ratio of 11.73. So the wheel in question would have an effective ratio or 11.73:1. The flyer would make 11.73 rotations every time the wheel was treadled once.

Accelerating wheels are those wheels that operate by using systems of multiple drive wheels and pulleys. Examples of these wheels include the Timbertops Chair wheel, Bosworth charkha and Journey wheels, and great wheels with Minor’s head attachments.

To figure these ratios, start by calculating the diameter of the drive wheels and the pulleys.

Once you have the diameters calculated, divide the main drive wheel diameter by its pulley diameter. This is the prime ratio.

Then divide the diameter of the second drive wheel by the diameter of its pulley. This is the secondary ratio.

Then multiply the prime ratio by the secondary ratio. The result is the spinning ratio of that wheel.

Here is the calculation for a great wheel with a Minor’s head. The drive wheel has a diameter of 43 inches, its pulley is 1.25 inches in diameter, which gives a prime ratio of 34.4. The secondary drive wheel has a diameter of 4.5 inches and its pulley diameter is just .625 inches, giving a secondary ratio of 7.2.

Multiply the prime ratio of 34.4 by the secondary ratio of 7.2, which gives a spinning ratio of 247.68:1.

For double drive accelerating wheels, take the diameter of the secondary drive wheel and use this to figure the double drive ratio. This figure will be the secondary ratio. Then figure the ratio of the main drive wheel to its pulley - this is the prime ratio. Multiply the prime ratio by the secondary ratio. The result is the effective ratio for that wheel.