Patent Patrol: SRAM Self-Charging Auto Shift Rear Derailleur
Will SRAM soon introduce a self-charging wireless mountain bike derailleur with a dynamo inside? On November 10, 2022, SRAM filed a patent detailing an electronic rear derailleur with on-the-go charging facilities, eliminating the need for even a medium-sized detachable battery that requires off-bike charging. The 28-page document describes the use of an alternator connected to the derailleur cage, which can be activated to continuously charge the batteries by rotating the top pulley while pedaling.
The onboard energy harvesting system will certainly reduce the pre-trip hassle of charging removable batteries. It would also make any future SRAM AXS drivetrain using this technology a viable option for ultra-long-distance riders who now have to carry spare batteries. However, it is conceivable that the mechanism described would result in some loss of power due to additional resistance within the transmission, given that it draws energy from the rotation of the top pulley while pedaling.
We won’t be estimating how many watts, of course, but we’ll give you an overview of some of the innovations featured in this rather unusual switch that may or may not go into production. Here’s a closer look at SRAM’s auto-shifting self-charging rear derailleur.
Main images with commentary by Corey Benson
SRAM self-charging rear derailleur concept
SRAM electronic self-charging rear derailleur concept; Mounting part and parallelogram (light gray), clutch and shift motor (yellow and green), alternator (dark blue), alternator gearbox (light blue), AXS pairing button (red) and LED indicator (small green dot).
The patent in question (US 20220355900 A1) describes a bicycle derailleur that includes an energy harvesting system. The first few paragraphs mention one of the shortcomings of modern electronic switches; the inconvenience of overcharging the battery, with the inherent risk of draining the battery during a long bike ride. Those lucky enough to own an electronic shift transmission know this all too well; perhaps not so much because they ran out of power in the middle of a trip, but because they forgot to recharge the battery before heading out on their next trip. You can imagine the disappointment.
A SRAM patent published last November describes a self-charging switch that will eliminate this problem, meaning that the owner no longer has to worry about the state of charge of the battery. They could use that mental bandwidth to worry about more important things like drinking, refilling, and remembering their loved ones’ birthdays. (Ed’s note: We have a feeling that JM may have missed an important birthday recently!)
The switch shown in the document looks extremely compact. The electrical generator system, clutch, engine, shift electronics and batteries are housed on the shifter cage (instead of their current position, farther back on the rear axle on the steering knuckle) and connected to the rotation of the top idler pulley. This does make the switch cage very idiosyncratic, but also (I suppose) quite expensive if it ever has to be replaced.
self-charging SRAM electronic switch in extended position; the upper idler pulley (21), driven by the chain, drives the inner gear to activate the alternator to recharge the battery or batteries on the go, even at low cadences
So how does it work?
As with most patents, this one details a number of different ways in which it can achieve its primary purpose, trying to assure the reader that the mechanisms it describes are “not intended to limit the scope” of the invention. The basic embodiment describes the use of a generator located on the switch housing that can be activated by a number of different mechanisms to recharge the battery or batteries.
The gear inside the SRAM electronic self-charging rear derailleur alternator.
The described basic mechanism involves the rotation of the clutch of the upper pulley of the cage (through 35 in Fig. 30) to drive the gear that drives the generator. In this mechanism, the patent describes the use of a spur gear (339), pinion gear (338), belt and internal pulley (41) to drive the generator base (46), with a preferred gear ratio of 33.75:1. . Thus, for each individual rotation of the top pulley of the cage, the drive gear inside the electrical generator system rotates 33.75 times. This suggests that the generator is most efficient at very high speeds.
In this scenario, although very small, some of the driver’s effort on the cranks is used to turn the generator. So it’s reasonable to assume that some of your precious watts will be lost to the electrical generator system to charge the derailleur battery while pedaling. However, we believe that these losses are much less than those associated with a larger hub dynamo.
The patent also describes the use of a clutch (290) in an electrical generator system that prevents the gears from turning when the driver pushes the pedals back. Obviously this scenario doesn’t happen all that often, but it does happen to some degree on some full suspension mountain bikes during pedal kickback resulting from chain stretch.
The 22 page patent details the use of transistors, resistors and capacitors, the functions of which I will not pretend to understand other than to provide a clean flow of electrons to keep the battery happy. But, in short, here’s the thing: the on-board alternator somehow recharges the switch’s batteries, so in theory it should never run out of power while driving. This could be a game-changer for the ultra-long-distance contingent who can’t risk running out of battery in the middle of an adventure or endurance race for fear they’ll have to overclock it until they arrive at their next refueling spot.
New switching mechanic
The current SRAM AXS GX rear derailleur with a large removable battery placed on the back of the B-Knuckle.
On fig. 28 of the SRAM patent application also shows a completely new shifting mechanic. In modern SRAM AXS rear derailleurs, the shift motor and battery are located on the rear b-shaped derailleur knuckle, between the rear pivots of the parallelogram linkage that provides gear shifting. Here, in a new self-charging electronic switch concept, the shift motor is housed on the frame assembly, driving the gears, spur gears, screw (81), nut (91) and finally the drive pin (97) in sequence. , rotate the outer link (6) of the parallelogram to perform switching.
Sorry, you won’t hear from me again. However, if you want to dive into the mechanics of it all, I can confirm that it’s all carefully laid out on pages 8 and 9.
What strikes us is that many important (and probably expensive) components are located on the perhaps more vulnerable part of the switch – the frame. Despite this, the patent document states that “the motor and/or electrical generator system may be mounted and movable (sic) with the cage. In the event of damage, the frame, including the motor and electrical generator system, can be quickly and easily replaced without replacing other switch components.”
We will refer to the modular replacement concept as a consolation prize.
In small gear/big gear mode, this larger gear on the cage does protrude quite far from the plane where the derailleur hanger bolt touches your frame (15 below). But when you shift into a lighter gear/bigger gear, it seems like most of the derailleur goes under the chainstays, staying close to the cassette.
SRAM self-loading rear derailleur, top view; On fig. 3 shows a switch positioned to serve the smaller teeth of the cassette, and fig. 5 it is shown to serve large cogs, which is clearly NOT a parallel motion that better matches the angle of the bike’s chain.
Finally, the auto-switch function is also described. Diodes and resistors in an electrical generator system are used to determine the speed of the generator, which can then be used to determine the speed of the chain pulley and therefore the speed of the chain. This information, combined with information received from other sensors on the bike (e.g. cranks and rear wheel), can be “used to execute an automatic shifting algorithm that maintains the gear ratio at the rider’s preferred gear ratio”.
Will SRAM’s self-charging switch go into production?
Who knows. We have reached out to SRAM for comment on the proposed technology, but have yet to receive a response. We will keep you updated as we learn more over time.
The worst kept secret in the bike industry right now has to be the upcoming SRAM AXS XX drivetrain. We’ve covered this in detail as various “black box” prototypes have appeared on professional race bikes. Comparing these photographs to the schematics shown here, it is highly unlikely that any of the self-loading technologies described here will feature on this drivetrain. We believe it will be several years before we see something like this in production, if at all.