What are direct mount rear derailleur hangers?

At derailleurhanger.com we offer several “direct mount” rear derailleur hangers. In this blog post we will describe what are the differences between standard and direct mount rear derailleurs, and based on that, how direct mount rear derailleur hangers differ from their standard counterparts.

To understand why direct mount rear derailleur hangers are needed, we first need to explain (a) what is a direct mount rear derailleur, and (b) the “B2 link” (or “b-link”) that was originally used to connect the direct mount rear derailleur with a standard rear derailleur hanger before the advent of direct mount derailleur hangers.

Direct Mount Rear Derailleur

Shimano first introduced direct mount rear derailleurs with the release of their Shadow rear derailleur line in 2012. In the Shadow product line, the pivot point on the rear derailleur was moved further to the rear and the rear derailleur itself positioned more directly under the rear cassette. As a result, the Shadow rear derailleur has a lower profile than standard rear derailleurs, which obviously has benefits in the more varied terrain encountered with a mountain bike. This geometry also provides other capabilities compared to standard rear derailleurs:

  • Creates a stronger linkage between the frame and rear derailleur, resulting in more precise shifting;
  • Provides more room to remove and replace the rear wheel, enabling faster and easier rear wheel changes;
  • Enables the frame to be designed such that the rear dropout can accommodate thru-axle wheel mount;
    • Note:  Thru-axle wheel mounts are desirable for wheels with disc brakes because the disc brake places more off-center force on the rear axle than do rim brakes.

Please go to this link on the Shimano website to see diagrams contrasting the positioning of the standard versus direct mount rear derailleur. Also, here is an article by pinkbike describing Shimano’s announcement of its new direct mount rear derailleur during the 2012 Sea Otter Classic, including some excellent photos contrasting the standard versus direct mount.

Aside #1:  Please note the bit of irony here. The direct mount rear derailleur provides more room to make it easier and quicker to replace the rear wheel. However, with the rise of disc brakes, more bike frames are being designed with a thru-axle configuration, which makes it more difficult, and takes more time, to replace the rear wheel regardless of whether a direct mount or standard rear derailleur is used on the bike.

The “B2 Link” (or “b-link”)

When Shimano first released the Shadow direct mount rear derailleur, Shimano included the “B2 link” or “b-link” to connect the rear derailleur to a standard derailleur hanger. As described above, the B2 link was required because the pivot point of the Shadow rear derailleur was moved further to the rear. Bike manufacturers soon began making rear derailleur hangers that mounted directly to the new direct mount rear derailleurs (rather than rely on the B2 link). These new rear derailleur hangers that mounted directly to the direct mount rear derailleur were termed “direct mount hangers.”

Aside #2:  The second bit of irony is that both the original direct mount rear derailleurs, and direct mount rear derailleurs of today, generally ship with the B2 link. Hence the “direct mount rear derailleur” actually added a secondary link (the B2 link) between the standard rear derailleur hanger and the direct mount rear derailleur. In other words, the “direct mount rear derailleur” was not truly “direct” until the advent of the direct mount rear derailleur hangers.

Direct Mount Hangers

As noted above, direct mount rear derailleur hangers were developed by bike manufacturers so that the B2 link (or “b-link”) would not be required, and the direct mount rear derailleurs could truly be attached directly to the direct mount rear derailleur hanger. The direct mount rear derailleur hangers have two common geometrical characteristics compared to standard rear derailleur hangers…

  • When compared to the standard rear derailleur hanger for a given bike model, the direct mount rear derailleur hanger is longer to span the extra length once occupied by the B2 link;
  • When both the standard and direct mount rear derailleur hangers are oriented vertically, the B-screw tab on the standard derailleur hanger is oriented in the same vertical plane, whereas the B-screw tab on the direct mount derailleur hanger is at an angle off the vertical axis. That is because the direct mount hanger is meant to be oriented at a rearward angle to accommodate the more rearward placement of the direct mount rear derailleur. See the images below of our #513 hanger (standard mount) versus our #508 hanger (direct mount) that are used on the same BMC bike models, all viewed from the wheel-side of the hanger.
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Standard versus Direct Mount Hangers

Summary

Direct mount rear derailleurs provide a few advantages compared to standard mount rear derailleurs, including more precise shifting (especially when a direct mount rear derailleur hanger is used, rather than a standard rear derailleur hanger with a B2 link), and enabling bike manufacturers to use thru-axle rear wheel mounts to accommodate the extra forces created with disc brakes. However, only a few bike manufacturers have frames that will currently accommodate a direct mount rear derailleur. For example, while approximately one-third of the 22 teams in the 2019 Tour de France used bikes with direct mount rear derailleurs, most of the teams had to have the direct mount hangers custom built for their bikes – something that is clearly not affordable or doable for the average cyclist. Therefore, for most riders the standard mount rear derailleur and rear derailleur hanger will work just fine, at least until more bike manufacturers start using direct mount rear derailleurs on the majority of their bike models.  At derailleurhanger.com we will provide direct mount rear derailleur hangers for bike models that have enough volume in the marketplace to justify manufacturing the direct mount hangers.

For your reading enjoyment, here is a fun blog post by Killa’s Garage regarding direct mount rear derailleurs.

If you are in need of a replaceable rear derailleur hanger, you can use our “1-2-3” search method and filter mechanisms available on derailleurhanger.com to quickly find the hanger required on your bike or your customer’s bike. If needed, you can also click on the “Contact Us” link on our website and we will respond quickly to provide personalized assistance. When using our “Contact Us” form please be sure to include photos of the derailleur hanger from your bike, as that will significantly speed up our ability to help you.

At derailleurhanger.com we work very hard to provide the best quality replaceable derailleur hangers, carry more hanger types than any other provider, and to provide excellent personalized customer service. Please let us at derailleurhanger.com know if we can assist you in any way. Happy riding!