What is a Continuously Variable Transmission?
A Continuously Variable Transmission (CVT) is a transmission that can change steplessly through an infinite number of effective gear ratios between maximum and minimum values. This contrasts with other mechanical transmissions that only allow a few different distinct gear ratios to be selected. There are a number of different types of CVTs that are used across a range of automotive and industrial applications. The most common type of CVT in use today is known as a Variable Diameter Pulley Continously Variable Transmission (VDP-CVT).
In a traditional VDP-CVT, there are two V-belt pulleys that are split perpendicular to their axes of rotation, with a V-belt running between them. The gear ratio is changed by moving the two conical sections of one pulley closer together and the two conical sections of the other pulley farther apart. Due to the V-shaped cross section of the belt, this causes the belt to ride higher on one pulley and lower on the other. Doing this changes the effective diameters of the pulleys, which changes the overall gear ratio.
How does Treadlie Engineering’s CVT work?
The base functionality of the Company’s VDP-CVT is identical to that of a conventional VDP-CVT. A key difference is the makeup of the pulley sides which are segmented into separate ring sectors rather than being made of a single continuous surface element.
The working principle of our segmented bicycle VDP-CVT is based on a series of equal width pulley ring sectors moving into contact with the V-belt from a retracted position such that only the pulley diameters needed for a chosen gear ratio are engaged, while other diameters are out of the way of the belt when it is moved between gear settings.
This achieves the following:
- A comparably narrow and fixed pulley width that can be incorporated into a bicycle crank set without exceeding well established limits of acceptable Q-factors (human stance width applied to a bicycle); and
- The system gear ratio is only limited by how large a sensible largest diameter of drive and driven pulley can be incorporated into given space constraints because pulley width remains constant at any diameter.
An electric motor located in the bicycle crank changes the diameter of the front (drive) pulley through a planetary gearing system. The electric motor is controlled wirelessly through a switch located on the handlebars.
Importantly, the gear ratio setting of the Company’s VDP-CVTs can be achieved by manipulating the active drive pulley diameter only, the passive driven pulley diameter simply reacts by automatically adjusting its diameter. This avoids the irritating, confusing, and inefficient annoyance of a conventional chain and derailleur transmission, where the cyclist has to select the front crank chain ring and the rear wheel sprocket separately and independently from each other to arrive at a desired gear ratio.
What is the effect of the Company’s CVT?
The effect of the Company’s VDP-CVT is that the narrow operational width dimension of both the front and rear variable diameter pulleys is fixed and does not vary with changes in the gear ratio of the VDP-CVT. The fixed and narrow pulley width permits the Company’s VDP-CVT to be used in bicycle and EPAC electric bicycle applications.
The Company’s VDP-CVT also affects the rideability and feel of the bicycle. The Company’s VDP-CVT provides an infinite number of gear ratios between a maximum and minimum ratio, giving the rider a choice for setting a truly suitable gear ratio depending on the speed of the bicycle, terrain or rider ability rather than being limited to discrete gear ratios.
The VDP-CVT does not require the drive train to be in motion to effect gear changes in the way derailleur drivetrains do. This allows for changing into smaller gear ratios when stationary at traffic lights for example. Critically for riders conscious of energy efficiency, changing the gearing ratio through the Company’s VDP-CVT does not result in loss of power transmission during the gearing change, which is otherwise the case for all discrete gearing systems. Using our CVT is therefore much less tiring when riding in hilly or mountainous terrain than using conventional discrete gearing systems.
The narrow width of the Company’s VDP-CVT is also advantageous in other technologies and applications outside the bicycle and EPAC industries. This will be explored as a potential avenue for commercialisation through Treadlie Engineering’s parent company Advanced Transmission Systems Holdings Ltd.
