There are about three types of transmissions in bicycles, with different principles: automatic transmission, manual transmission and manual stepless transmission.
the following is an introduction from the high-end bicycle internal transmission. Among the bicycle internal transmissions (internal variable speed hub), the truly high, large and high-speed ones are the German Rohloff 14-speed internal transmission and the Sothis 14-speed internal transmission introduced by Hong Kong Fuhui Precision. These two types of transmissions are about 1.7 kg in weight, but the gear ratio width of Sothis is slightly higher than that of Rohloff, and the former is 539.7%. The latter is 52.6%, which is more than the traditional 27-speed external change, followed by Shimano's 11-speed (the ratio of speed change is 4.9%, and the flexibility and durability of the structure are different from those of the former two). These three types of speed changes are gear-shifting, which works by using the principle of planetary gear train transmission, and there are multiple groups of compound planetary gear trains inside the transmission, so that the master-slave relationship among sun gear, planetary gears and internal gear rings in the transmission can be changed by external forces to achieve the purpose of speed change. Principle of planetary gear train speed change: The planetary gear mechanism has three moving parts that can rotate relative to each other: sun gear, planet carrier and gear ring. A group of planetary gear trains can realize four different combinations of gears:
① Low gear sun gear is active, the planetary carrier is passive and the gear ring is not moving.
(2) sun gear in the middle gear is not moving, the planet carrier is passive, and the gear ring is active.
③ The sun gear in the high gear (overdrive gear) is stationary, the planet carrier is active and the ring gear is passive.
(4) sun gear is active in reverse gear, the planet carrier is not moving, and the ring gear is passive.
when all moving parts are unconstrained, the transmission is in neutral. ?
Planetary gear transmission is usually composed of two or three groups of planetary gear mechanisms, and multiple clutches are used to control the combination of the above moving parts to realize different gears.
Besides, the current representative of the continuously variable transmission "nuvinci N36" uses eight steel balls to rotate around the main shaft, and the two sides of the steel balls friction to drive the power. The shift lever moves in series in the main shaft to drive the eight steel balls to move between the driving side and the driven side, and the contact friction surface between the steel balls and the two sides will change, thus achieving the purpose of speed change, so the speed change has no polarity, but this principle leads to its low transmission efficiency, short service life and a speed change ratio width of 36.
finally, the automatic transmission is a slightly lower-end internal transmission device with only three or four gears. The principle is mainly based on the principle of centrifugal force, in which a plurality of centrifugal steel balls are evenly distributed around the main shaft, and the outer sides of the steel balls are pressed by springs. The central raceway of the steel balls is on a conical sleeve that can be moved in series. One end of the conical sleeve is subjected to the spring force and the pressure of the centrifugal steel balls to achieve position balance. When the wheel rotates, the pressure of the steel balls on the conical sleeve is changed by centrifugal force. The stress balance of the cone sleeve is broken, and the position of the cone sleeve will change when the balance force is broken to a certain extent, so as to push the internal gear to shift gears and achieve the purpose of speed change, so that the speed change ratio width of the transmission is not high and the structure is simple.