Standard Mount vs. Direct Mount Derailleur Hangers

A history lesson

To understand the differences between a standard mount derailleur (and hanger) and a direct mount, let's first review a few key features of what we now call a standard mount derailleur. We should all be familiar with how a rear derailleur attaches to the bike. Hopefully it uses a replaceable derailleur hanger, which has an M10x1 threaded hole where you attach the derailleur. Without going into too much detail, this M10x1 hole location must be positioned within a small window relative to the rear hub axle center. All major derailleur manufacturers specify exact dimensions for their derailleur position relative to the rear wheel, and are all within a few millimeters of each other.

If you look at all our standard mount hangers, they all are relatively similar when looking at the relationship between derailleur mounting hole, the B-Screw Tab and the hub axle socket. Standard mount derailleurs pivot around the bolt that attaches it to the frame (called the "b-knuckle"). When you are removing a wheel from a frame, you usually need to pull the derailleur back, rotating it around the M10 bolt. The b-knuckle of a rear derailleur stands off from the hanger quite a bit, and contributes to a rather crowded area of hub, skewer, chain, cassette and cable housing.

In 2012, Shimano released their Shadow MTB rear derailleurs which use a direct mount system. These derailleurs moved the main pivot location further back and below the cassette compared to standard mount derailleurs. By relocating the upper pivot, this allowed for easier wheel removal/installation. It also positions the body of the derailleur more underneath the cassette so that it is less exposed to potential impacts. Shimano was also looking towards the future with this design in allowing frame manufacturers to build dropouts that could handle thru axles better. By moving the entire derailleur further away from the hub and frame dropout, this gave manufacturers more room for design improvements.

In 2017, Shimano started making road rear derailleurs with the direct mount system, and most likely will become the norm for all their rear derailleurs. In 2018, Campagnolo also started making direct mount rear derailleurs, albeit with a more traditional b-knuckle upper pivot.

How are they compatible with each other?

Fortunately, Shimano and Campy direct mount derailleurs are backwards compatible with standard mount hangers. This is done with the "b-link", which connects the derailleur to a standard hanger, but positions the upper pivot in the correct location as if it was attached to a direct mount hanger. It didn't take long before frame manufacturers started making hangers that allow a direct mount derailleur to bolt directly to the hanger without the need for the b-link. This is what we call a direct mount hanger.

Standard Mount Derailleur	Direct Mount Derailleur On direct mount hanger (notice no b-link)	Direct Mount Derailleur On standard mount hanger with b-link

As with many of the bicycle "standards" we now have to understand, this can cause confusion when trying to find the correct hanger for a bike. Not only do we now need to match the hanger to the frame, we also need to match the derailleur to the hanger! We currently offer a number of hangers in both a standard mount and direct mount version. For example, DROPOUT-266 and DROPOUT-350 shown below. They both have the same shape and fasteners where they attach to the frame. Therefore, both hangers fit the same frames, and are interchangeable in that regard. However, they position the M10 hole for the derailleur in much different locations.

Standard Mount for GT DROPOUT-266	Shimano Direct Mount for GT DROPOUT-350

What works? What doesn't?

• If you have a standard mount rear derailleur, you can only use it with a standard mount hanger.
• If you have a direct mount hanger on the frame, you can only use with a direct mount derailleur with the b-link removed.
• If you have a direct mount rear derailleur, you can use it with a standard deralleur hanger (with b-link), or use it on a direct mount hanger (without b-link)

Currently, SRAM does not offer any direct mount rear derailleurs, and always need to be mounted to a standard derailleur hanger. If your frame has a direct mount hanger, and you are switching from Shimano direct mount to SRAM, you will need to replace your hanger with a standard mount version of your hanger.