With the introduction of the new SLX group, Shimano formally announced their new Direct Mount rear derailleur system. We say “system” because it’s not really a new “standard” in the moan-and-groan sense of the word that means you’ll have one more thing to figure out when upgrading parts.
Simply put, the new Direct Mount rear derailleurs will work with either standard dropouts or new direct mount dropouts. SLX is the first group to be announced with it, but it’ll be a running change for XT an XTR. The changed parts have been made at the factory, and they’ll start popping up in a few months. They’re completely backwards compatible, meaning you can use one of the new derailleurs in place of an old derailleur that mounts to a standard dropout.
Visually and physically, the key differences are that the mounting bolt between the B-Link and B-Knuckle goes from a M7 to M10 bolt, same as the bolt used to mount the derailleur to your dropout. The link is still used with a standard derailleur mount. Just remove the link to install on a direct mount frame by bolting the B-Knuckle directly to the DM dropout. It sheds a bit of weight, adds rigidity and it simplifies the system overall.
The pic above shows where the DM bolt goes versus the B-Link mount. The derailleur ends up in the same place. The blue link shown is from a Yeti. Rumor has it Giant, Trek, Rocky Mountain and others will be launching bikes with this standard this year. It’ll work with both thru axles and quick release wheels.
The silver parts here are some early prototype mounts.
Old derailleurs won’t be forward compatible because the spacing in the B-Knuckle gets a bit wider to accommodate the thicker dropout tab and the bolts are bigger. That’s not to say some bike brand or enterprising aftermarket manufacturer couldn’t offer DM dropouts that would work with older derailleurs. It shouldn’t be that hard, it’s only a matter of unbolting the B-link, you’d just need a thinner DM dropout with a hole for an M7 bolt.
We talked to one major bike brand’s engineer about the new design and he said the reason Shimano went with a larger diameter bolt rather than just keep the M7 bolt already used between the link and knuckle was to improve serviceability. Many bike shops already have the facing and other tools to tap or thread a derailleur hanger for M10 bolts, so using the same size on the DM dropout means shops will be better able to fix or service them without having to invest in new tools. That benefit applies to bike manufacturers, too, since they’re already designing their dropouts with M10 holes and threads.
Above, you can see how thick the DM hanger is. It’s wider than the current B-Link by a tiny bit. Below is an older XTR (left) compared to a new DM-ready XT derailleur (right). It’s hard to tell the difference unless you’re actually measuring it:
Besides increased stiffness, it also makes wheel changes easier because it opens up the space around the axle – a lot. Even better, any bike with a replaceable dropout can easily be upgraded to the DM if or when the frame manufacturer offers a DM dropout. Or, any new DM frame could easily offer a traditional dropout for lower-spec models or as an option on framesets.
You can see the difference in bolt diameters on the old XTR versus new XT.
If you’re confused, check this out. The photo above is of a current, standard mount XTR Shadow Plus rear derailleur. Basically, all that the new Direct Mount derailleur will entail is relocating the mounting point of the derailleur from the old bolt (red arrow) and now attaching it at the B-Knuckle (blue arrow). The old B-link in between, with the red X, will not be used at all with the new mount, so essentially the Direct Mount system is just removing a redundant bolt and chunk of aluminum.
So the system will be more rigid, but I thought derailleur hangers were supposed to bend to protect the derailleur? That’s a valid question. Realistically, replaceable derailleur hangers have been getting stiffer for some time now. As the rest of the parts improved, the flexible derailleur hanger soon became the weak link. In order to improve shifting, especially when shifting under load, having a rigid derailleur hanger has become even more important.
Won’t that mean if I hit something with my derailleur, my pricey XTR unit will explode? Well, maybe, but then you might be better off. More often then not, in the shop when I saw a derailleur damaged due to impact, it wasn’t from the impact itself, but from getting caught in the spokes. This also meant that the hanger and spokes were damaged and possibly the rim (by nipples pulling out) and chain as well. What usually happened was the customer would crash and bend the hanger just enough so that they didn’t notice, yet the first time they shifted into their lowest gear, the cage would catch a spoke and everything would explode. However, newer derailleurs are surprisingly durable as many of mine have the gouges and scratches to prove it. It takes a pretty massive impact to render a derailleur useless, but then hopefully you can still singlespeed your bike and get home.
The beauty to all of this is if you don’t want a DM derailleur on your bike, you don’t have to use it. All of the new derailleurs will ship with the standard B-link and standard mounting bolt, and the B-knuckle should be a bit more stiff on the new derailleurs thanks to the larger hardware and wider tab. So if you’re looking for the best shifting, lightest weight, and easiest wheel changes the new DM derailleur has you covered. If you’re happy with your current frame but need a new derailleur, the new derailleurs have you covered there too.
*Additional Update: I just confirmed with Shimano that since the new Direct Mount Hanger derailleur mount is threaded identically to current derailleur hanger as Tyler mentioned, all previously used derailleur alignment tools will still work. So Direct Mount derailleur alignment, if bent, will still be corrected in the same manner using a tool such as the Park DAG-2.
Takeaways – One: DM rear derailleurs good – stiffer, lighter and backwards compatible. Two: By “running change”, we’re betting we’ll see a few other changes to at least the XT group when the new ones hit, likely late spring or summer if we had to guess. Three: That white frame in the pics? Check back April 1 for the scoop.