SRAM about to revolutionize shifting again? The juggernaut drivetrain manufacturer has just been granted a patent on a chainring-coupled front shift mechanism that could improve aerodynamics of 2x equipped bikes, and allow for wild new frame designs.
SRAM has been granted a patent on an all-new front shift mechanism for a 2x drivetrain that integrates the derailment mechanism into the crankset itself. First covered by myself and Cory Benson at Bikerumor, this chainring-integrated derailleur is wireless and electronic, powered by what looks to be an AXS battery on the chainring itself (194).
The most obvious advantage: it makes front derailleur mounts redundant, housing everything needed to shift the chain between the two chainrings on the chainset.
Now, I am no aerodynamics specialist, but I will float the idea that such an arrangement stands to be far more aero-efficient, taking away a key area on the frame that could be causing undesirable air turbulence, and thus, drag.
This isn’t SRAM’s first foray into a 2x system that doesn’t require a front derailleur – some of you may remember Truvativ Hammerschmidt, a two-speed mountain bike crankset that launched back in 2008. What we have here is conceptually similar, but the execution is very different. While the Hammerschmidt was a sealed system, with only one chainring exposed to the elements, this one is not. The fact that SRAM show it on a full suspension mountain bike is, I think, for a laugh. You’ll understand when you see the number of moving parts on this thing.
Patent Granted: SRAM Chainring-Coupled Wireless Front Derailleur
SRAM filed this patent way back in 2019. Patents are always a bit pie in the sky, and no guarantee that the product is actually even under development, never mind destined to land on the market for you to purchase any time soon. Sometimes, patents are filed for the hell of it, or at least to prevent a competitor taking the idea, or aspects of it, and using it themselves.
What I’m saying is: don’t get too excited.
But also, SRAM have actually been granted this particular patent (as of January 8th), which may or may not mean it is more likely to come to fruition. So, let’s get excited and run away with ourselves for a wee minute.
The Mechanism
I will keep this brief, because bicycle technology can be dry at the best of times, especially when it only exists in the hypothetical realm.
For downshift (big to small), there are two hinging paddles (320) that protrude beyond the chainring teeth. One can imagine that, as the rotation of the crankset guides the top section of chain into the chainring, the shift paddle rotates inboard to push the chain inboard until it falls off the teeth of the big chainring, and onto the teeth of the smaller one – guided by the pegs of the upshift apparatus. For each revolution of the crankset, there are thus two opportunities for shifting the chain down to the smaller ring.
For upshift (small to big), the mechanism is a little more intricate. The upshift apparatus consists of a series of guide pegs that are positioned between the two chainrings. Each consecutive guide peg is positioned radially further away from the center of rotation. To relocate the chain from the small ring to the big one, the upshift apparatus moves into the plane of the smaller ring, such that the pegs slot into the troughs between each tooth of the chainring.
As the chainset pulls the top length of chain through to meet the chainring, instead of engaging with new teeth, the chain is instead lifted upwards by the pegs of the shift apparatus, guiding the chain toward the teeth of the big ring. Per revolution of the crankset, there is only one opportunity provided for the upshift.
Why?
Grand job and all, but why would SRAM go to the effort of creating this intricate solution to a problem that, arguably, doesn’t really exist?
Well, aside from the small improvement in aero-efficiency that this compact shift mechanism may afford, the SRAM patent outlines a few other advantages.
Firstly, not all frame designs actually have a traditional seat tube for the mounting of a traditional front derailleur. Exhibit A: The Rondo Ruut 2.0. This wild-looking gravel bike diverts the seat tube forward such that, instead of meeting the bottom bracket cluster directly, it extends forward to meet the downtube, creating a strut-like frame member that Rondo claim improves compliance, behaving like a leaf-spring. Now is not the time to debate that, but as you can see, you’d not get a traditional front derailleur on that bike. It is 1x only. For now, of course.
The crux of it: integrating the shifting into the chainset takes away that constraint on frame designers, allowing them the freedom to be creative with tube shapes in this area. The Rondo is a fine example of that, as is the Specialized Sirrus.
Another advantage would be the scope for increased tire clearance. An example: the SRAM Force eTap AXS Wide group is a 2x affair, with a traditional, albeit wireless, front derailleur that houses the AXS battery aft of the mechanism, where it is positioned awfully close to the bicycle’s rear tire. Re-positioning the mechanism and battery onto the chainset gets everything well out of the way, allowing more space for the high volume tires that more an more gravel riders (and road riders) are coming to appreciate.
Food for Thought
First, for anyone freaking out about having to replace their front shift mechanism every time they wear out a chainring: stop. The system is modular, so you should theoretically be able to swap a chainring independently of the shifting apparatus. However, it might not be so easy to swap out chainring sizes. It sems that, at the very least, the upshift apparatus (with the guide pegs) would be specific to chainring size, and it’s unclear how SRAM would solve that particular problem.
Second, this system will likely be slower to shift than current drivetrains with a traditional seat-tube mounted front derailleur – especially in an upshift scenario. Upshift is only possible once per revolution of the chainring here, so that’s a possible drawback, or limitation at least.
Thirdly, the fact that this mechanism can never be fully sealed is unsettling to me. The chain will constantly be feeding dirt and debris into the mechanism with each turn of the pedals. For that reason, I can’t see SRAM implementing it for anything other than road cycling – which is where any aerodynamic advantage would be most appreciated.
My penultimate point: this system could, theoretically, improve the accuracy of shifts. With the mechanism integrated with the chainrings, there is reduced scope for misalignments. Current drivetrains rely upon the frame to be a member of the shifting apparatus, as it determines the positioning of the front derailleur.
Finally, I think the most interesting implication of this design is how it removes an important design constraint for frame manufacturers. Engineers no longer need to consider the positioning of a seat tube for the mounting of the front derailleur, leaving them free to experiment with hitherto unseen frame shapes, which could have big implications especially for aero-leaning road bikes, triathlon, and time trial bikes.
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