They have already become established in mountain bikes, but road cyclists and gravel bikers have not yet had to deal with them. But in the form of the brand new SRAM Red AXS XPLR, the large UDH rear derailleurs are venturing into new territory. So how does the new standard work? And what does "UDH" actually mean?
The abbreviation stands for "Universal Derailleur Hanger". The universal derailleur hanger, but that is only part of the story. Anyone who has ever had to replace the derailleur hanger on an older frame will understand why a uniform shape of this component is practical: There are currently dozens, perhaps even hundreds of versions - just look at schaltauge.de where, with a little luck, those searching will find the right spare part.
Universal Derailleur Hanger: no more different derailleur hangers!
The UDH puts an end to this: all frames that meet the standard developed by SRAM are equipped with the same derailleur hanger. The "interface" of the frame to which the UDH derailleur hanger is mounted does not always have to look the same - there are five different variants that give the frame manufacturer some freedom in terms of design. But the derailleur hanger developed by SRAM is, as already mentioned, always the same. It is made of plastic and aluminum, is inserted into the frame from the inside and then fastened with a hollow screw that has a left-hand thread. The screw is equipped with an internal thread into which the thru axle is screwed. The UDH derailleur hanger is designed so that the chain cannot get jammed between the sprocket set and the frame, which is a big advantage, especially with carbon frames.
A conventional derailleur is attached to this derailleur hanger as usual - for example, as shown, a Shimano GRX on the Canyon e-gravel bike. But even a 60-year-old racing bike derailleur can be installed, because the derailleur hanger has hardly changed since the early 1950s. On better steel frames, it was an integral part of the frame, on simple frames it was a screwed-on plate - if it was bent, it could be straightened or, in the latter case, replaced. Aluminum derailleur hangers are mounted on aluminum and carbon frames, which serve as a predetermined breaking point if forces act on the derailleur in the event of a fall, etc. This means that the derailleur hanger cannot be too stable; on the other hand, however, this also means that a flexible derailleur hanger can impair the precision of shifting. And the more sprockets that need to be operated at the back, the more of a problem this becomes.
Next step: the “Hangerless Interface”
And this is where the real advantage of the Universal Derailleur Hanger comes into play: It is just an intermediate step on the way to the "hangerless interface", i.e. a rear wheel mount that does not require a derailleur hanger at all. A special derailleur is mounted directly to the frame, and with the new SRAM Red AXS XPLR, such a derailleur is now available for the first time for drop bar bikes.
Mountain bikers are already familiar with the system, whose advantages are its great robustness and high gear shifting precision. But how exactly does the technology work? It's actually very simple: the actual UDH derailleur hanger is removed; a large hole then becomes visible in the frame. The reduction sleeve is pressed into this, then the derailleur is pushed over it and screwed tight. The fastening bolt is hollow; the thread for the thru axle is on the inside. The derailleur can only be properly attached once the chain is mounted and the rear wheel is inserted; this presses the derailleur onto the frame and thereby fixes it.
SRAM Red AXS XPLR: A rear derailleur that doesn't need to be adjusted
The direct mount rear derailleur has no adjustment screws as its position in relation to the cassette is precisely defined. The adjustment takes place as soon as the chain is mounted and differs significantly from the usual procedure; anyone who has done it once swears that adjusting the gears is now much easier. Determining the chain length is also easier - this now only depends on the length of the chainstays.
The extreme stability of the new rear derailleur ensures maximum precision in gear changes; the electronics of the SRAM AXS detects any force applied to the rear derailleur and allows it to swing inwards in a flash. This is intended to prevent any damage and also allows individual parts of the expensive change to be replaced.
The only catch is that the direct mount rear derailleur cannot be attached to a conventional frame. Conversely, the UDH derailleur hanger is compatible with any normal rear derailleur. The number of frames equipped with it on gravel bikes is still manageable, and of course there is only one corresponding rear derailleur so far, namely the aforementioned SRAM Red AXS XPLR. The MTB scene shows that this will change - let's see how long it takes for the new standard to be established on drop bar bikes.