Over the past few decades, bike drivetrains have undergone many performance-enhancing changes, culminating in very reliable, easy-to-use options that have a wide range. We've seen the widespread use of positive and negative gears, which improve chain retention, clutch transmissions that do the same (and also help keep quiet), and the integration of the 1x system with a wide range of flywheels and climbers, giving them the superpower of "climbing walls".
Then, there came the wireless, electronic shifting, and more recently the introduction of automatic and coasting shifting for the EMTB. We're seeing gearbox technology that's been around for years, combining an electrically assisted motor in a sealed unit that does it all.
While all of these things are impressive, it can be said that they are gradual to some extent. When it comes to drivetrains, we rarely see anything completely out of the ordinary. However, with the release of H**en Mercer's patent for the "Bicycle Automatic Transmission System", it seems that something has changed again.
This is a continuously variable transmission, meaning that the gear ratio varies between the upper and lower limits, there is no fixed number of gears to speak of, and this number is practically infinite. The system is not only fully automatic, but also completely mechanical.
How does it work?
At the heart of the system is the "disc" and rear sprocket's ability to automatically expand and contract in response to different torque inputs. Both consist of a number of pulleys distributed around the center of rotation, each supported on a connecting rod, whose position is determined by the spring and the amount of torque applied in any given moment.
On the chainrings, the mechanism configuration offsets the pulleys to one extended position so that when there is no load on the pedals, they are all in the farthest position from the center of rotation. Here, the chainring is at its maximum effective size. On the sprockets, the pulley can rotate clockwise, but not counterclockwise.
Thus, when torque is applied to the pedals and the chain is pulled through the system, the last engaged pulley locks in the chain, effectively becoming a torque sensing element. The greater the torque applied, the greater the force the spring is forced to compress and, with it, the smaller the effective chainring size.
In fact, when a torque-sensing pulley compresses, all other pulleys compress synchronously. The pulleys are able to rotate clockwise, which allows them to feed the chain in, and as the distance between adjacent pulleys decreases, the chain does not simply pop out. The same is true when the torque is reduced and the effective chainring size increases.
The mechanism is reversed at the rear sprocket. Here, the spring and linkage system is configured so that the sprockets are biased to the smallest possible size, with the pulleys closest to each other.
Thus, in the no-load state, the transmission is in the hardest gear. Stepping on the pedals forces the effective size of the rear sprocket to increase and the effective size of the chainring blades to decrease, allowing the system to move towards a relatively easy gear. As momentum increases, the rider's torque input decreases, the rear sprocket shrinks, and the chainring size increases. The transmission automatically adjusts the transmission to the rider's needs.
Isn't it a built-in chain tensioner?
Theoretically, it is possible to run the system without the use of any chain tensioner. It is not difficult to see that as the chain length requirement at the chainring plate increases, the chain length requirement at the rear sprocket also decreases. While this is the case, the two are not the same.
The system doesn't actually require one chain tensioner, but two. This is because the two ends expand and contract at different rates, which can introduce undesirable chain slack on the upper and lower chain lines. The upper tensioner also doubles as a chain guide, which seems quite necessary considering how violently the chain jumps up and down as it is fed into the chainring.
Who is this transmission for?
H**en Mercer intends to use this transmission in mountain bikes. The current prototype has the problem of the spring being too light and therefore compressing too easily under pedaling load. Perfect spring stiffness may not exist, or at least it won't be the same for every rider or every terrain. H**en plans to redesign it so that the end user can easily replace the springs.
Another issue is the durability of one-way bearings. The invention really eliminated the need to shift gears, allowing users to pedal without having to think about what gear they were in, and leaving it all to the automatic transmission. This can really improve safety, especially in traffic jams. And it's also more friendly for those who are newbies, as they are not very proficient in bike handling. When they don't need to think about when to shift, the more they pay attention to other road safety factors.
Editor: Hamma.