Single drive versus double drive wheels
Ask different spinners and you will get different opinions on which is best, so let's start with the difference. A single drive wheel has the drive band going once around the wheel and the flyer and has a separate brake on the bobbin or flyer. This is also known as the Scotch tension braking system. A double drive wheel uses a drive band that is longer, as it goes around the wheel, to the bobbin, back around the wheel again then around a whorl on the flyer. Some wheels come one way, others the other way, sometimes you can get both (kind of...). Sometimes you get to choose (Ashford Traveller and Traditional). So what is the big deal?
Most beginners will find the Scotch tension system easier to understand and master. Both the brake and the drive band can be adjusted separately so control of twist and take-up are easier. The double drive system provides a firmer, more consistent yarn and with normally higher ratio options. If the wheel you like is a double drive wheel, see if it comes with a Scotch tension single drive option; this gives you freedom to use both.
The Technical Information - Single vs. Double drive

Single drive wheels have the drive band going around the wheel one time. The drive band will then go around the drive pulley on the flyer (some manufacturers have the band drive the bobbin, not the flyer; Ashford, Lendrum, Majacraft and Kromski Prelude drive the flyer, Louet drives the bobbin). The rotation of the wheel causes the flyer to rotate and depending on the difference between the diameter of the wheel and the diameter of the flyer pulleys, you develop a ratio between the two. A brake band (sometimes called Scotch tension) with a spring attached, is placed over a pulley built into the bobbin (this brake is on the flyer for Louet wheels) and the tension is adjusted. This information applies to nearly all single drive spinning wheels built today, regardless of brand.

Double drive wheels also use one drive band but it goes around the wheel twice (you make a double loop of the drive band to do this). At the flyer, the two loops of the drive band do two different things: one of the loops will go around the whorl (the whorl will have one or two grooves of different diameters - your ratios are established here with the whorl. Whorls are located at the rear of the flyer and are removable so you can change the ratio of the drive; the whorl must be removed to change the bobbin); the other loop will go around a pulley located at the end of the bobbin. This information applies to nearly all double drive spinning wheels built today, regardless of brand.
How they work
Spinning takes place in the following manner: the main wheel drives (rotates) the flyer/bobbin by means of the drive band. Tension on the drive band is adjustable and you set it to make sure there is no slip at the flyer/bobbin. You treadle, the wheel turns, the flyer and bobbin rotates) and twist is imparted to the fibers in your hand. In order for the yarn to get on the bobbin automatically, there must be, from time to time, a change in the speed of the bobbin. It must: slow down (be braked) on single drive wheels; speed up on double drive wheels - so the flyer can facilitate wrapping the yarn around the bobbin. This is where the two methods diverge.
A single drive wheel has a separate dedicated band, a brake band (Scotch tension), around the bobbin or flyer . In order to facilitate the accumulation of yarn on the bobbin, the bobbin or flyer is "braked," or retarded, when: 1 - you have the proper tension on the brake band; 2 - you allow the twisted yarn in your hand to be pulled into the orifice of the flyer (the "take up"). On a single drive wheel, you have a way to adjust tension on the drive band (not really needed if using an elastic band) and a separate adjustment for the brake band/take-up control; adding tension to the brake increases the pulling force of the yarn from your hand. Ideally your want a gentle pull that quickly gets the yarn on the bobbin.
On a double drive wheel, there is no separate brake band but rather one of the loops of the drive band will cause the bobbin to rotate while also slipping during spinning while the other loop drives the flyer. The shape of the whorl grooves (V-shaped) results in positive drive of the flyer all the time while a different shape of the groove on the bobbin pulley (U-shaped) allows "slippage" of the bobbin while you add twist to your fiber. While spinning, the bobbin slips on the drive band to allow the bobbin to rotate in sync with the flyer. When you allow the yarn to be pulled into the orifice you are letting the bobbin gain speed (less sliipping) in relationship to the flyer arms causing yarn to be wrapped on the bobbin. This all works if: 1- your tension is set correctly; 2 - you allow the twisted yarn in your hand to be pulled into the orifice of the flyer. The tension adjustment affects both the drive of the flyer and the slipping of the bobbin, so one adjustment must result in good performance of drive and take-up. This is not difficult to do. Also note that for a double drive flyer to effect take-up the pulley on the bobbin must be smaller than the whorl. Never match a small whorl with a bobbin pulley that is larger.
Many double drive wheels allow you to adjust the set-up to mimic a single drive operation (a brake band (Scotch tension) on the bobbin, drive band on the whorl) and this works very well, but it is still a double drive wheel because the flyer, bobbin and whorl are manufactured that way. It is easy to go back and forth with these different set-ups on a double drive wheel.
Most wheels are made either single drive or double drive. Only the Ashford Traditional and Traveller are available as true single drive wheels or true double drive wheels (based on the manner of construction). For these Ashford wheels we always recommend the single drive set-up.
Hybrid Double Drive Set-up
If you have a double drive spinning wheel, you can make some adjustments that might make tensioning of the drive bands and control of the take-up easier and more like a single drive wheel. Use two different types of drive bands as single loops. For the drive of the flyer, use an elastic drive band. An elastic band will provide positive drive with minimal tension (easier treadling). Use a traditional string drive band (cotton, hemp, etc.) from the wheel to the bobbin pulley. Remember, for this set-up you want the flyer to rotate without slippage, but for the take-up of yarn, you need braking at the bobbin. This set-up gives you both. To control take-up, move your mother-of-all in the usual manner to achieve the feel you want. Since the flyer drive is using an elastic band, any movement of the mother-of-all (to control take-up) has virtually no effect on the positive rotation of the flyer. Now, in a sense, you only have to adjust the take-up; the flyer tension takes care of itself.
How to set the drive band tension on a single drive wheel that has an elastic band.
In order to take full advantage of an elastic drive band on a single drive wheel, you want to set the tension correctly at the beginning to avoid most other adjustments as you spin. We suggest you set the tension using the smallest whorl on the flyer. Increase the tension to the point that slippage ceases and you have positive drive. Normally you can hear when a drive band slips (more likely to occur on the down stroke of the treadle), so just increase till the sound goes away. Now, when you need a slower speed, just change the band to a larger whorl and most likely you will not need to make any adjustment to the tension setting.
But won't the drive band be tighter and don't you say elsewhere that this causes harder treadling? Tighter tension on a small whorl can be harder, but as you change to a larger whorl, you gain mechanical advantage and it will feel easier to treadle even though the band is now tighter. That is why you want to adjust and set the tension using the smallest whorl. Do this first. Optimize tension with the smaller whorl and the larger whorls will always be easier. Of course you can loosen the tension on those larger whorls and it will, indeed, decrease the force needed to treadle even more.
Question: Why is it harder to treadle when I change to a smaller whorl?
One generally changes whorls on a spinning wheel in order to achieve a desired output speed at the flyer while maintaining a given comfortable treadling rate. The rate of treadling (cadence) is kept fairly constant, but different foot forces are required when using different size whorls. In particular, it generally takes twice the force to treadle smoothly when using a whorl that is half the diameter of a larger whorl (law of energy conservation). This does not mean it is difficult to treadle with a small whorl, only that you will most likely feel a difference. This is normal.
There are several reasons why it takes more force to treadle when using a small whorl, but the primary cause is something inherent to all transmissions: it takes more energy to make something spin faster. It requires more work (energy) to put additional twist into yarn from a spinning flyer. This is the "feel" you have as you treadle.
There are two fundamental ways to make the flyer spin at a higher speed. The first way is to use the same foot force and simply treadle faster. This works, but is not a good idea since the high cadence quickly becomes uncomfortable. Ideally you want a regular, comfortable cadence for all speeds and allow the mechanics of the wheel (ratios from different whorl sizes) to create different flyer speeds for you. This brings us to the best way to increase speed.
This second (and best) method for achieving a higher speed is to change the transmission ratio by using a smaller whorl at the flyer (this is how all treadle wheels work). This allows one to maintain the same comfortable cadence, but does requires greater foot force in order to produce the required energy to rotate the flyer at a higher speed.
A secondary cause of higher treadling forces when using smaller whorls is friction. When one changes to a smaller whorl, it is generally necessary to increase the tension in the drive band. This is because the smaller whorl has less surface area along which the drive band must maintain traction in order to not slip. To regain a sufficient traction force, the drive band tension must be increased. It is this greater tension that results in higher friction at the bearing surfaces that support the flyer. To compensate for these friction losses, one must treadle slightly harder. Using elastic drive bands (generally not recommended for double drive wheels (but some elastic bands do work well on double drive wheels provided the specifications of the band is optimized), but see hybrid note in previous section), instead of string, can help mitigate this problem because the rubbery surface doesn't slip as easily, therefore requiring less friction-inducing tension in the band.
There are, of course, other reasons for this force question (dynamic, inertial, and friction effects due to the fact that the wheel - and therefore the flyer as well - is not always rotating at a constant speed) but we can ignore these for this discussion.
How about foot placement on the treadle? - yes, this can be important as it can greatly affect the "feel" of the wheel. You want to have your foot (feet) in the most advantageous location that results in comfort for you, ease of treadling and, with some wheels, the ability to not only use your toes but also your heel(s) to generate "force." Place the foot too far one way or the other and negative factors develop. Good foot placement is greatly helped by your being positioned correctly at the wheel and sitting at the proper height. This will be determined by your physical size and the design of the wheel. Once you "discover" the best location on the treadle(s), you will recognize the feel and will, almost automatically, place your foot/feet there each time you address your wheel. Tip: take your shoes off. You will feel the forces at work better and you can move your feet to optimize your work.
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