As cassettes have added teeth and diameter, the forces on the derailleur have multiplied while at the same time we’re asking it to behave ever more precisely. More cogs typically means less space between them, so the movements of the derailleur must be very well defined, and its tolerances tight. The other variable is the frame itself, and its dropouts. Those can vary from brand to brand, model to model, introducing more complexity when trying to design a shifting system independent of the frame. With this in mind, it looks like SRAM is exploring new ways to enhance shifting performance by integrating the rear derailleur directly into the dropout…

sram patent for direct mount rear derailleur that attaches to the thru axle dropout

The premise of the design is a base member (20) that replaces the traditional B-knuckle and sandwiches the frame. It also replaces the dropout and derailleur hanger as we know it by incorporating the thru axle’s female end. Basically, the thru axle threads directly into it, ensuring perfect alignment between the axle, hub, cassette and rear derailleur.

sram patent for direct mount rear derailleur that attaches to the thru axle dropout

The other concern mentioned in their patent is “ghost shifting”, particularly for mountain bikes where a big, fast impact on the rear can shake the derailleur enough to cause it to momentarily shift without rider input. That’s because the parallelograms are obliquely angled, allowing vertical forces to influence them. If you notice this design, the parallelograms are parallel to the ground, so vertical forces can’t impact them.

This is very similar to the current Eagle rear derailleurs, which move in a linear path and use the offset upper pulley to handle the angle of the cassette. But here, the parallelograms themselves sit parallel to the ground, too, rather than angled downward. This could just be for the drawing, though.

how sram direct mount thru axle mounted rear derailleur works

As drawn, the base element (20) doubles as the dropout, which suggests SRAM will need buy in from the frame manufacturers to make this work. And then Shimano would need to follow with a compatible rear derailleur design. The patent mentions that this base element (20), aka “B knuckle”, could be made of metal or composite.

Multiple variations of the design show different bolt systems attaching the base to the frame, but all use some manner of a larger bolt (60/61/62) threading into a nut (66) or possible the base itself. The thru axle would then thread into that bolt. Because of this, it could easily accommodate any thru axle standard simply by switching out the mounting bolt. They mention a quasi-floating design that’s able to accommodate frame intolerances while still keeping the base perfectly aligned with the axle.

sram direct mount rear derailleur patents on bikerumor

The benefits of such a design include:

  • Perfect alignment of the derailleur with the cassette
  • Stiffer derailleurs for more precise shifting
  • Easier installation and setup

Could this mean new mountain bike standards? New features?

sram patent for direct mount rear derailleur and new thru axle standards

The patent spends a fair number of words describing the benefit of a 15mm thru axle. Currently, all bicycle rear thru axles are 12mm in diameter, but this design allows for various diameter axles to be used by swapping the mounting bolts. Theoretically, it mentions how 15mm thru axles are much stronger, which is particularly good for e-MTBs. But, it also mentions that you could have thru axles with various wall thicknesses from a 1mm “SL” version up to 2mm e-bike versions, allowing the same hub to be used on any type of bike, relying on the stronger axle to handle the extra loads.

They also mention that a battery could be tucked inside the base member, which would both protect it and keep the system looking extremely sleek. And they mention eliminating limit screws. To do that, the derailleur could have an element that contacted the largest cog and prevented the derailleur from shifting further inboard (toward the hub), and a limit stop that could contact the base element to prevent overshooting outboard (away from the hub).

It’s important to note that this patent doesn’t guarantee any of this will ever make it to production. And that SRAM mentions that some or all of this design may be used independently of the other aspects. For example, that they may use just one single arm on the base element rather than sandwiching the frame between it. Or that the parallelograms may still be angled downward rather than flat. Since we’re not seeing any of these features on the prototype eTap 12-speed stuff floating around, we’re guessing it’ll be on MTB products first, if ever.

Hydraulically damped derailleur clutches

sram hydraulic rear derailleur clutch could be lighter and smoother than current designs

Another thing rear derailleurs are tasked with is keeping the chain in check over rough terrain. The advent of clutched rear derailleurs has made this job much easier on them, but it brought about it’s own set of issues. Namely, increased force required to shift up the cassette (to a larger cog). Pulling that cable means pulling against the force of the clutch. Make the clutch weaker and you get easier shifting, but less effective chain management. And vice versa. (If you don’t believe this impacts shifting effort, spin around with your Shimano clutch disengaged and shift a bunch. Then engage it and shift a bunch. You should be able to feel the difference.)

Until now, it’s been an all or nothing proposition. Choose between easier shifting and less chain control, or firmer shifts with less chain slap. But SRAM has a proposed solution with this hydraulically damped derailleur clutch.

SRAM rear derailleur patent with hydraulic clutch mechanism
D = Damping direction (chain would get slacker), and T = Tensioning direction (chain returns to being tight).

Traditional derailleur clutches use friction to prevent floppy chain guides. The problem with that is the friction is there all the time, at any speed. This patent changes from physical friction to fluid flow control to change the dynamic. Simply put, it uses a hydraulic valve to control the rate at which the derailleur’s chain guide can rotate in the slackening direction. But it doesn’t restrict return speed.

Functionally, it works both the same and different. The same in that it prevents rapid slackening of the chain caused by fast movements from big hits. Different in that it can be tuned such that it has no impact on slow movements…like a shift.

sram patent for hydraulic rear derailleur clutch

The drawings are very complex, but the concept is simple. Basically a poppet valve (188) restricts flow in one direction (into the return chamber, 182) but allows full flow back the other (damping chamber, 184). A small valve spring (195) keeps the poppet closed against the flow ports (194) in the vane (180). A standard return spring (204) keeps the system tight. Not that you’d probably ever need to, but the force could be tuned by changing the fluid viscosity…if you can get it open. The language suggests it may be designed not to be serviced and could be permanently sealed.

Why use a hydraulic derailleur clutch?

The benefits go beyond improved shift feel. For electronic derailleurs, they’d use less power per shift, so the battery would last longer. The patent also suggests this hydraulic design could be lighter than a friction-based clutch. And the performance is more consistent. Lastly, it’s easier to manufacture and install, which improves efficiencies and profits if not potentially bringing down (slightly) the cost of components.

One concern would be fluid expansion. That’s a tiny bit of fluid to be cycling through a valve rapidly, potentially for hours at a time. Which means it could heat up and expand. So there will be a compensation chamber to allow for fluid expansion. And, that chamber could be pressurized, like a shock’s IFP, which would help prevent cavitation and, in the event of fluid or pressure loss, would ensure the clutch continued to work as designed.

The patent mentions this could be incorporated into any type of chain tensioner, with or without the actual gear changing capability. And, as with so many patents, there’s plenty of random stuff seemingly irrelevant to the technology being described. Like so:

can I use a smartphone or gps cycling computer to shift sram eTap

…and we’ll leave you to noodle on that.

One last note… our sneaky spy shots of production-ready eTap Eagle wireless mountain bike derailleurs being raced at the XC World Cup this season did not use the direct mount hanger idea. But they certainly look to share the exact shape of that electronic hydraulically damped clutch derailleur illustration above. And the prototype derailleurs we saw being raced had, what in hindsight could be a 3D printed cover to disguise that new hydraulic clutch mechanism. Hmm…

46 COMMENTS

  1. Shimano made axle-mounted derailleurs with the first generation of Saint and Hone (which became Zee). It kind of sucked at the time because removing the rear wheel became an ordeal. I’d imagine thru axles would ease some of that.
    Still, hard for me to understand how this satisfies the novelty requirement for patents. Like the author said, no real reason to believe at this stage this will see the light of day, at least in the near future.

    • Yea I remember those well.

      Additionally, If you were to hit this derailleur it would break or bend the actual derailleur instead of the hanger correct?

      • You guys must have different experiences than mine; I don’t think I’ve ever had a hanger break and not had the derailleur also damaged to the point of being unusable.
        I always viewed replaceable hangars as a way to protect the frame (having had a non-replaceable hangar break many years ago). If the derailleur isn’t mounted to the frame, that function wouldn’t be needed.

        Also, I for one, used to break derailleurs and hangars a lot more frequently than I do now. Not sure if that’s just luck or they are better designed now.

          • It is still a replaceable component and that feature will remain till internally geared hubs take over, Even then a tensioner will be needed and likely frame mounted and replaceable. The SRAM design just adds better supper for the derailleur and allows the frame dropout to be made stronger through not needing to accommodate a removable derailleur hanger

        • @i I have experienced and seen countless instances where a hanger was bent and the derailleur was ok because the hanger absorbed the energy and not the derailleur. I have also seen more than a few instances of broken hangers where the derailleur was still use-able. That being said I have worked at bike shops for over a dozen years.

          @NDE How does that appear to be replaceable? Yes the piece could be replaced, BUT it is connected to the linkage of the parallelogram, so you would have to disassemble the top end of the derailleur to replace the Head(hanger). The time a shop would charge to change that price out is way more than what most are willing to spend, let alone the price of the actual part.

          • @Ethan, remember this is a concept for a patent application and will not reveal detail design features. It is very likely that a production unit will have the mounting hanger and upper knuckle removal from the rest of the derailleur for easy replacement and serviceability. Remember that a hydraulic damped clutch will need to be serviced every year as well. There is a lot of possibility with this design concept

  2. So, in case of a crash, you will need to change the derailleur (and possibly the frame) instead of a less costly (and packable) hanger? Or I missed something

    • It does look awfully like a strong enough whack could smash the mech, snap the thru axle and ovalised the dropout all in one go. Maximum cost for minimal gain, classic SRAM.

    • you have missed a lot. This is a patent application not a preproduction design for a finished product. Many features will not be revealed in the drawing. This is standard practice when submitting a patent application. Yo don’t submit detailed design drawings

    • No need to since they only make one gear range. That would be their excuse at least. When the hydraulics fail they’ll whine “…but hydraulics are hard..”.

  3. Yeah great. If i hit the derailleur, while shredding hard, i can just put a new one on the frame. Why go with something like a “cheap” replaceable dropout, when instead i just can buy a new and expensive SRAM Eagle derailleur. Perfect.

  4. Great, with this “advancement” we could guarantee that the derailleur is fubar when bent, because there is no separating it from the parallelogram. Frame derailleur hangers and b-mounts don’t just protect your frame, they help protect the derailleur, too.

    I can’t see this design making it to production. It would basically require that either Shimano and other components manufacturers mimic it, or that frame manufacturers design around SRAMs patented mounting system. Neither is likely.

    • Nobody thought we needed flat mount disc brakes until Shimano came up with that and now everybody thinks they must have it, and bike brands followed suit.

      Sram I’m sure will make their new standard cost free to the frame makers.

  5. Sounds brilliant to me. I’d welcome a stronger and stiffer and more accurately toleranced derailleur to frame interface. And this is coming from working on and building bikes for 30 years.

  6. SRAM derailleurs protrude further from the bike than Shimano ones-which makes it a lot easier to snag/smash/break them. For rocky, narrow trails this is a serious issue.

    Looks like this design won’t do anything to address that problem.

    Also, I’d worry that a small hydraulic system like this would be temperature sensitive….which would also be a bummer.

    • Some people have been saying this for decades. It’s never going to happen. Just like recumbents have “been the future” since the 1970’s. Never going to happen. Some concepts have insurmountable issues and internal gear hubs have quite a few.

    • Nah get that gearbox mounted on the sprung mass not the unsprung….. If only Pinion would go wireless electronic shifting it would be almost perfection!
      And probably the one thing that would make me buy a Zerode even though I’m not allowed to stock them.

  7. There was good reason why Shimano backed away from such a design.
    Different for the sake of being different seldom pans out well in engineering or consumer products for that matter.

  8. What gives first when the derailleur takes a hit? A hanger is a sacrificial part, weak on purpose. There’s nothing sacrificial about that two-sided lattice link. If it bends, you toss the derailleur. If it doesn’t, I fear for the axle and the dropout.

    If they’d opted for a weaker (but still sufficiently stiff) one-sided approach, then engineered it as a replaceable part, they’d end up with… Direct Mount, Shimano’s proprietary design from 2012. And it’d fail just as quickly because frame manufacturers don’t like locking themselves into one company’s product.

  9. wasn’t electronic shifting supposed to do away with periodic readjustment derailleurs? Always absolutely precise shifting? I would certainly take electronic shifting (any of them) over this stuff. Perhaps a better ideal than this (outside of electronic) is to move the derailleur onto the chainstay higher up, and feed the chain into the cassette from the front.

    • Electronic shifting eliminates issues with cables and does nothing to alleviate derailleur alignment problems. Move the derailleur higher on the chainstay? The upper pulley is already right in front of the cogs, feeding the chain to them. The bottom pulley ends up where it does because the single-arm cage is the simplest arrangement yet found for a chain tensioner (which is all the bottom pulley is for).

      • I have seen electric shifting advertised as never needing adjustment many times. Simple, precise shifting “EVERYTIME” is the claim. I would much rather do this, than this hydraulic crapshot.

        As for moving the “derailleur higher on the chainstay”. it’s fundamental shape would have to change, but it should be doable. Feeding chain directly from the front at near 90 Degrees from the current feed location, so that it’s up and out of the way. I’m not a mechanical engineer, but a good mechanical engineer should be able to make it happen. Nothing inherently perfect about the current system. Legacy as much as anything.

        JMHO. Most may disagree.

  10. this all seems pretty half-baked so far. i certainly wouldn’t trust sram to manufacture a non-adjustable, non-rebuildable, minuscule 1-way hydraulic clutch flawlessly on the first go. warranty/tech phone lines will catch fire.

  11. It is only necesary to increase the stifness and accuracy of hte rear derailluer because they canned the front and need more range. the answer is front shifting. Some day everybody else will see this.

  12. Haha someone at Sram thought they could delete the derailluer hanger as a frame part and integrate it into the rear derailleur. They’re going after that sweet, sweet derailleur hanger money!

    Also, Sram has a fantastic history with hydraulics… What could possibly go wrong?

  13. There was good reason why Shimano backed away from such a design.
    Different for the sake of being different seldom pans out well in engineering or consumer products for that matter.

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