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Patent Patrol: SRAM’s Self-Charging Auto-Shifting Rear Derailleur

sram self-charging auto-shifting rear derailleur generator new shifting mechanics
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Will SRAM soon debut a self-charging wireless mountain bike derailleur with a dynamo generator inside? Their patent application, published on November 10th, 2022, details an electronic rear derailleur with the means to re-charge itself whilst in use, negating the need for even a medium-sized removable battery that requires charging off-the-bike. The 28-page document describes the use of a generator coupled to the derailleur cage that can be activated to continually charge the batteries by virtue of the rotation of the upper pulley wheel while pedaling.

An on-board energy-harvesting system would certainly reduce pre-ride faff associated with charging removable batteries. It would also make any future SRAM AXS drivetrain carrying the technology a viable option for ultra long-distance endurance racers who now otherwise have to carry spare batteries. However, one can imagine that the mechanism described would result in some loss of watts from additional drag within the drivetrain, given that it is taking its energy from rotation of the upper pulley wheel during pedaling.

We certainly shan’t be estimating how many watts, but we will give you an overview of some of the novelties described in this rather unusual-looking derailleur that may or may not make it to production. Here’s a closer look at SRAM’s Self-Charging Auto-Shifting Rear Derailleur.

Feature images annotated by Cory Benson

SRAM’s Self-Charging Rear Derailleur Concept

sram self-charging auto-shifting rear derailleur generator new shifting mechanics
SRAM Electronic Self-Charging Rear Derailleur concept; Mounting Portion and Parallelogram (light grey), Clutch and Shift Motor (yellow and green), Generator (dark blue), Generator Gearbox (light blue), AXS Pairing Button (red) and an LED indicator (small green dot)

The patent in question (US 20220355900 A1) details a bicycle derailleur that incorporates an energy-harvesting system. The first few paragraphs mention one of the drawbacks of current electronic derailleurs; the inconvenience of having to recharge the battery, with the inherent risk of it running out of juice whilst out on a long bike ride. Those lucky enough to own an electronic shifting drivetrain will know this all too well; perhaps not so much because they’ve run out of charge mid-ride, but more likely because they’ve forgotten to re-charge the battery before heading out for their next ride. One can imagine the frustration.

SRAM’s patent published last November describes a self-charging derailleur that would eliminate this issue, meaning its owner would no longer have to fret over the charge state of the battery. They could use that mental bandwidth to fret over more important things, like hydration, fueling, and remembering their loved ones’ birthdays, for example. (Ed’s note: We feel like J-M may have recently missed an important birthday!)

The derailleur depicted throughout the document looks to be extremely compact. The electrical generator system, clutch, motor, shifting electronics and batteries are all housed on the derailleur cage assembly (instead of their current location, further aft on the b-knuckle), coupled to the rotation of the upper guide pulley. That makes for a very peculiar-looking derailleur cage indeed, but also (I imagine) a rather expensive one, too, should it ever need to be replaced.

sram self-charging rear derailleur extended larger cogs
SRAM Self-Charging Electronic Derailleur in an extended position; the upper guide pulley (21), driven by the chain, drives an internal pinion gear to activate a generator to recharge a battery or batteries on-the-go, even at slow pedaling speeds

So, how does it work?

Like most patents, this one details a number of different ways in which it might achieve its primary goal, taking care to assure the reader that the mechanisms described within “are not intended to limit the scope” of the invention. The main embodiment describes the use of a generator that is housed on the derailleur’s cage assembly, which can be activated via a number of different mechanisms in order to re-charge a battery or batteries.

The gearing inside the generator of the SRAM Electronic Self-Charging Rear Derailleur

The main mechanism described entails coupling rotation of the cage’s upper pulley wheel (via 35 in Fig. 30), to drive a pinion gear to activate a generator. Within that mechanism, the patent describes use of a spur gear (339), pinion gear (338), belt, and an internal pulley wheel (41) to activate the generator base (46), stating a preferred gear ratio of 33.75:1. So, for every single rotation of the cage’s upper pulley wheel, the pinion gear inside the electrical generator system rotates 33.75 times. That would suggest that the generator is most efficient at very high speeds.

In this scenario an, albeit very small, portion of the rider’s input at the cranks is being used to turn the generator. So, it is reasonable to assume that some of your precious watts will be lost to the electrical generator system in order to charge the derailleur’s battery while pedaling. That said, we imagine that loss to be much less than the losses associated with a larger hub dynamo.

The patent also describes use of a clutch (290) within the electrical generator system that prevents rotation of the gears when the rider back-pedals. Obviously, such a scenario is not so common, but it does occasionally happen to varying degrees on some full suspension mountain bikes during pedal kickback events that result from chain growth.

The patent’s 22 pages detail use of transistors, resistors, and capacitors, the functions of which I shan’t pretend to understand beyond providing a clean flow of electrons to make the battery happy. But, the long and the short of it is this: the on-board generator is, in some way, recharging the derailleur’s batteries so that it, theoretically, should never run out of charge whilst on a ride. This could be a game-changer for the ultra-distance contingent who can’t run the risk of a battery losing charge mid-adventure or endurance race, for fear of having to single-speed it until they arrive at their next refueling spot.

New Shifting Mechanics

sram gx eagle axs wireless rear derailleur shown on a mountain bike while riding
A current SRAM AXS GX Rear Derailleur with a large removable battery housed off the back of the B-Knuckle

Fig. 28 of the SRAM Patent Application reveals completely new shifting mechanics, too. On the current SRAM AXS rear derailleurs, the shifter motor and battery are housed on the aft b-knuckle of the derailleur, in between the rear pivots of the parallelogram linkage that mediate shifts. Here, on the new self-charging electronic derailleur concept, the shifter motor is housed on the cage assembly, sequentially driving pinion gears, spur gears, a screw (81), a nut (91), and eventually a drive pin (97), to rotate the outboard link (6) of the parallelogram in order to execute a shift.

sram self-charging auto-shifting rear derailleur patent energy-harvesting system cage-housed motor shifter mechanism

Sorry, you shan’t be getting any more detail out of me than that. However, should you wish to immerse yourself in the mechanics of it all, I can confirm it is all painstakingly spelled out on pages 8 and 9.

It strikes us that a lot of important (and likely expensive) componentry is positioned on the, arguably, more vulnerable part of the derailleur – the cage. Despite this, the patent document reads, “the motor and/or the electrical generator system may be mounted on and moveable [sic] with the cage. If damaged, the cage, including the motor and electrical generator system, may be quickly and easily replaced without having to replace the other components of the derailleur”.

We’ll call the modular replacement concept a consolation prize.

When in the small cog/big gear, that bigger mechanism on the cage does stick out quite far from the plane where the derailleur hanger bolt contacts your frame (15 below). But as you shift into an easier gear/bigger cog, the bulk of the derailleur does appear to tuck up under the chainstay, staying close to the cassette.

A top-view of the SRAM Self-Charging Rear Derailleur; Fig.3 shows the derailleur positioned to service the smaller cogs of the cassette, while Fig. 5 shows it positioned for the larger cogs, a decidedly NOT parallel movement that better aligns with the angle of the bike’s chain

Finally, an auto-shift function is also described. Diodes and resistors within the electrical generator system are used to sense the speed of the generator, which could then be used to determine the speed of the chain pulley, and therefore the speed of the chain. That information, taken together with information delivered by other sensors on the bike (at the cranks and rear wheel, for example) could be “used to perform an automatic shifting algorithm which keeps the gear ratio at a preferable ratio for the rider”.

Will the SRAM Self-Charging Derailleur Make it to Production?

Who knows. We approached SRAM for comment on the proposed technology. Their reply? “No comment”, of course. We will keep you updated should we learn more in due course.

SRAM XX1 Eagle AXS BlackBox prototype, photo by Michal Červený, detail

The worst-kept secret in the bike industry right now would have to be the imminent arrival of a new SRAM AXS XX drivetrain. We have covered this extensively as various Black Box Prototypes have cropped up on Pros’ Cross-Country Race Bikes. Comparing those photos with the diagrams seen here, it is very unlikely any of the self-charging technology described herein will be a feature of that drivetrain. We reckon it will be several years before we see anything like this rolled out to production, if at all.

SRAM.com

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Greg
Greg
25 days ago

One should take a look at how Mavic did it with their Mektronic rear derailleur. It just used a coin cell battery. The actual derailleur shifting movement was powered by the upper pulley… Which really ought to be how it is. I don’t really care what the UCI says, but they do have a rule against battery powered things, and last time I checked the wording (a very long time ago), derailleur movement would reasonably fall into that.
I believe the road AXS derailleurs have the motor parts in the front p-knuckle area with a wire running along one of the parallelogram plates.

Dinger
Dinger
22 days ago
Reply to  Greg

If the UCI had such a rule, its enforcement has long since expired. Di2, AXS, power meters, etc. There’s a lot of batteries on a bike these days.

I really liked the idea of the Mavic system, though it was ingenious to use the upper jockey wheel and a worm screw to actuate the derailleur.

The setup in the patent drawing looks fairly complicated. Lots of fine-pitch gears. I hope it’s well sealed.

Ves
Ves
19 days ago
Reply to  Greg

Yes, UCI strictly say – no outside energy provided to the bicycle. They are hypocrites. Or, should I say – whores for the money?

mud
mud
25 days ago

I had to look up “faff” being an American

Max
Max
25 days ago

Use the power from braking, that power is lost…

Jaap
Jaap
25 days ago

SRAM might have beaten Shimano to this one!

Shafty
Shafty
24 days ago
Reply to  Jaap

Nope, first-gen Di2, Nexave, used a generator hub to power the shifting mechanism.

Dinger
Dinger
22 days ago
Reply to  Shafty

True, but that was hardly race level equipment and didn’t have the staying power that the current groups have.

WhateverBikes
24 days ago

There a lot that could fail over time with a derailleur like that, and a lot of engineering and parts and manufacturing that all has to be paid for… and again if you are unlucky and hit it hard enough while riding.
I’ll happily keep using my simple, trustworthy, cheap to replace if needed, mechanical derailleur. Sure, it may not shift as crisp as an electronic one, but it works more than well enough for me, so that’s just fine.

Robin
Robin
23 days ago
Reply to  WhateverBikes

That’s true of about anything, but electronic bike parts have proven themselves to be durable. Yep, it’ll be expensive to replace a derailleur like this if it’s damaged in a crash, but the tsunami of failed electric bike parts that people predicted when Di2 first came out just hasn’t materialized.

WhateverBikes
23 days ago
Reply to  Robin

Fair enough, but this specific derailleur has a lot of extra mechanical parts, as shown on one of the illustrations. Very small parts to get this thing to use the power of the moving chain to charge its battery, and to ‘auto shift’.

DynoHub
DynoHub
22 days ago
Reply to  WhateverBikes

You can get shifting without the faff of a fancy mechanical derailleur, Vernon Blake used
a floating chain systems. Why use an expensive rear mech when a bit of wire
and a stick work as well ? 😉

Champs
Champs
24 days ago

And here I was, thinking it could just charge itself kinetically like a Seiko or Rolex Automatic watch.

Dylan
Dylan
23 days ago
Reply to  Champs

For off-road oriented groupsets it’s a no-brainer to use substitute the friction clutch mechanism with one that could meet the recharging function.

Roger Pedacter
Roger Pedacter
24 days ago

No UDH? Total rubbish.

Michael
Michael
24 days ago

I guess “auto-shifting” was just clickbait, then, not part of the actual derailleur? Since it wasn’t mentioned at all in the text.

Billyshoo
Billyshoo
24 days ago
Reply to  Michael

You didn’t read the article in its entirety.

Teodor
Teodor
23 days ago

Creating a problem: you need battery to ride your bike.
Selling the solution: you need an even more expensive derailleur to recharge your battery, so you can ride your bike.

I agree that we need some innovations, because new things are fun, but the only real innovation after the introduction of 1X is Lal bike’s Supre Drive.

Eggs Benedict
Eggs Benedict
23 days ago
Reply to  Teodor

I’m not sure I would refer to 1X as real innovation.

Dinger
Dinger
22 days ago
Reply to  Eggs Benedict

In terms of the technology that was needed to make it compelling and functional, I would consider it an innovation. Were it not for SRAM, MTB’s would still have front derailleurs. Conceiving of a 10-42 or 10-50T cassette probably wasn’t difficult, developing the chains, front chain rings and most importantly, the rear derailleur that managed the chain as well as current systems to was most definitely innovative.

Andrew
Andrew
22 days ago

My guess is that its an engineering exercise to explore the (current) limits of what they could do…. I doubt we’ll ever see 90% of this actually get built.

#theadventurebiker
20 days ago

… or you could just use a derailleur driven by a cable. Problem solved.

Robin
Robin
19 days ago

Golly, why don’t all cyclists have the same tastes? How dare anyone have different tastes and priorities than those of _______!

Kelley
Kelley
20 days ago

Complex and expensive bike componentry? If you don’t like it, it wasn’t designed for you.

HappyMondayz
HappyMondayz
19 days ago
Reply to  Kelley

I agree.

I also think it’s a shame that so much of the bike industry’s engineering resource is focused at the top end of technology and so little goes into alternatives.
I get how it’s about predictable sales then versus resource spend now for SRAM and the rest of the component manufacturers. But there’s a lot of us who aren’t interested in this tech trickling down, we’d rather see development in the simpler, durable, low-tech non-electronic areas. Race spec isn’t what I want or need. A wide ratio 5 or 6 speed system with a hard-wearing chain or a lighter weight 3 speed IGH designed for MTB is.
I’d pay good money for kit like that. As it is I’m spending very little on bike parts and just picking up discounted older gear in sales. I won’t be buying Di2 at Deore/GX level, I simply don’t want it.
The bike industry isn’t making much out of me.. but it could.

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