Universal Derailleur Hanger - UDH

In the past, frame maker made their own rear derailleur hangers. And often with a new frame design, they also designed a new hanger, one that fit just a little better, looked a little more integrated, etc. As a result, there are thousands of rear derailleur hangers in the world.

As you may have experienced, this is a nightmare. Whenever you break your hanger, the odds are slim that your local bike shop has exactly that hanger you need (which is why we ship a spare hanger with every frame, to make sure you're never stranded without).

Years ago, SRAM introduced the UDH (Universal Derailleur Hanger). This was a hanger that could function like any normal derailleur hanger, with the added bonus that it would rotate under big impacts so it would move your derailleur away from danger if you hit something.  To use the UDH, all manufacturers had to do was build a specific hole shape into their frame, and then the UDH could bolt into that. 

This was great for manufacturers, stores and consumers as they could use one derailleur for every frame (instead of all the styles that existed before) and this UDH hanger is compatible with everything. SRAM, Shimano, Campagnolo, doesn't matter. Plus it did a good job of protecting the RD in an impact. 

UDH was originally a mountain bike product, and there was one more incentive SRAM used to convince frame makers. In the future, they might make product that would only work with "the UDH".  That made it an easy choice for frame makers. UDH was compatible with all current drivetrains and some mystery future drivetrain, which is more than they could say of any other hanger including all the ones they had designed themselves in the past. So virtually all the MTB frames changed to UDH. 

This means that SRAM controls a piece of real estate on all those MTB frames. Not only the shape and dimensions of the hole where you insert the UDH, but also the area around, as SRAM defined stay-out zones (areas where the drivetrain maker will tell frame makers to leave free, as the cassette, chain or derailleur might need to occupy that space).

This was a brilliant variation on the Trojan horse story, but in this case the Trojans (frame makers) don't suffer, it is the rival empires (other drivetrain makers) interested in trading with the Trojans that are outmaneuvered. 

When MTB frames were overwhelmingly equipped with UDH hangers, SRAM made its next move. The mystery drivetrain did not technically only work with UDH, it only worked with the hole in the frame that was needed for UDH. It called this Transmission. There has been some confusion on the terminology, so just for clarity:

• UDH is the hanger that SRAM makes available to all frame maker and which accepts all standard drivetrains
• UDH-compatible are frames with a hole and stay-out zone in the dropout that follows the specifications set by SRAM, and thus fit the UDH hanger
• Transmission is the SRAM drivetrain design that doesn't use a UDH hanger but instead it uses the hole and stay-out zone in the dropout that otherwise the UDH hanger would use. So instead of attaching the derailleur to a hanger and the hanger to the frame, the Transmission derailleur mounts directly to the frame. This is why it is also called a hangerless or direct-mount design.

Today, SRAM introduces a Transmission groupset also for gravel but only at the highest level (Red XPLR). So while all other groupsets (Force, Rival, Apex) keep the standard rear derailleur design (compatible with any frame), Red XPLR gets Transmission and thus needs a UDH-compatible frame. 

This is why as part of OPEN 12.5, all our new frames will be UDH compatible. Meaning you can build them up hangerless with Red XPLR Transmission or with any other drivetrain (SRAM, Shimano or Campagnolo) using the UDH hanger. It doesn't really change anything for the current UP or WIDE as they remain compatible with all the drivetrains we currently use (Force/Eagle, Rival, Ekar, GRX) but the new UDH-compatible frames will give people who build up their bikes from scratch the option to also spec Red XPLR.

I also wrote up a more in-depth look at Red XPLR