Engineering Design

Design Considerations

Even though many current CVT designs are effective, they suffer from possible slipping or frictional losses. Due to the low torque and low RPM nature of bicycles, designs with slipping cannot be suitable ideas for our design team to implement on bicycles. Since the objective of our bike CVT design is to improve the efficiency and ease of bicycle traveling, any frictional energy loss will undermine our initial purpose. Therefore, to design a CVT that is suitable for bicycle usage, an alternative CVT transmission needs to be proposed and the new CVT design needs to have minimal slipping.

Besides eliminating slipping in the CVT system, maximizing the gear range is also a factor that needs prioritized considerations. Since bikes with variable gear ratios are normally known for their versatility in many different road conditions and in vastly different speed ranges, it is certainly beneficial to have a wide range of gear ratios suitable for different conditions and speeds. However, many CVT designs have very narrow gear ranges. Therefore, in order to have a CVT that has a high performance, our group needs to design a CVT that can shift in a considerably large gear range.

It is also interesting to point out that almost all bicycles will require a constant torque input from the rider in order to maintain a relatively constant riding velocity. However, when thinking more closely at such a universally accepted design, one may start to question whether this is really the best method to maximize power efficiency from the rider of the bicycle. When a rider rides a bicycle, the position of the two pedals on either side of the bike are always transforming between a normal position where the crank is normal to the ground and a parallel position where the crank is parallel to the ground. By common sense, it is easy to tell that when the pedals are in the normal position and the the crank is vertical, it is comparatively difficult for the rider to pedal the bike, whereas when the pedals are in the parallel position and the crank is horizontal, it is comparatively easy for the rider to pedal the bike. Therefore, our CVT needs to implement a more reasonable design that makes the pedaling resistance smaller at normal pedal positions and the pedaling resistance larger at parallel pedal positions.

Final Design

The final design is shown above as a rendered image. Included in this is the CVT, the mounts, the chains, part of the frame and the rear wheel. Like a normal bicycle, the CVT is powered by the operator using pedals. However, instead of the pedal crank being directly connected to the rear wheel, it is connected to the input crank of the CVT. The input crank then moves two connecting rods that are 180 degrees out of phase. The two connecting rods then push two lever arms back and forth. The lever arms are engaged to the output shaft; only during forward motion using a ratcheting mechanism. The 180 degree phase offset means that they are offset so that at least one lever arm is always engaged with the output shaft; this ensures consistent power delivery. The output of the CVT is then connected to the rear wheel using a chain.

Such CVT design translates a rotary input drive to a linear motion through the use of a crank and levers, which is then translated back into rotary motion by connecting the levers to links that pivot on a ratcheting drive. The ratcheting drive, which serves as the output drive of this design, can be varied by changing the point of attachment on the links that the levers have. The point of attachment on the lever arm can be varied through the rotation of the lever screw by using a low-speed 6V DC motor. This change in the point of attachment means that torque and output RPM are varied for the ratcheting drive due to the translation of the linear lever motion into lesser or greater rotation of the ratcheting mechanism.

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