Shimano wide-ratio sprockets for road bikes are listed here, including compatibility by step number.
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SRAM’s XDR and XD are freebody standards
XDR and XD are SRAM’s proprietary freebody standards.
XD was originally developed as a 10-speed freebody for MTB when it was first released in 2015. Later, when the freebody height had to be increased to accommodate 11 speeds, it was necessary to increase the height of the freebody, but because it was designed for MTBs, it was made 11-speed by taking advantage of the high number of teeth on the low cog of the sprocket and extending it out toward the spokes. The same is also done by Shimano.
On the other hand, sprockets for road bikes at that time had a low maximum of 25T or 28T and could not be physically pushed out to the spoke side, so the XDR was developed to accommodate 11-speed sprockets for road use.
Therefore, XD and XDR have basically the same structure, but there is a difference in the height of the free body: sprockets for XD can be used with XDR free, but not vice versa.
Features and Advantages of XD and XDR
The XD and XDR are characterized by the fact that they do not have splines throughout the entire conventional freebody, but only in the low several steps, and that they are threaded to connect the sprockets on top of them. Since the free body and sprockets are attached to each other by their own threads, conventional lock rings are no longer needed. As a result, the free body itself can be lowered, making the free body lighter.
Naturally, Shimano’s HG (Hyper Glide) standard sprockets, which are standard in Japan, cannot be installed.
As for the XDR, the biggest advantage is that it allows the selection of a minimum top gear of 10T. Shimano’s road-use components still have a minimum top gear of 11T even after the introduction of 12-speed, but SRAM’s road-use components are compatible with 10T.
Disadvantages of XD and XDR
The disadvantage of XD/XDR would be its low availability.
SRAM components are not that popular in Japan, and SRAM has always had many Shimano-compatible products, so they are not well known, but Mavic, DT Swiss, Fulcrum, and others sell XDR-free wheels or replacement free bodies.
However, in terms of distribution in Japan, the disadvantage is that very few products are actually sold and availability is low.
Points to keep in mind when buying XD/XDR wheels
When buying XD/XDR wheels or a complete bike equipped with XD/XDR wheels, it is important to note that the wheels will be “fully SRAM assembled”.
SRAM 11-speed sprockets cannot be used in combination with Shimano 11-speed components because SRAM 11-speed and above are not compatible with Shimano. If the sprockets are not compatible with Shimano components, all other components around the drivetrain will also have to be SRAM.
SRAM itself is a manufacturer that produces excellent components, but the disadvantage is that they are not readily available in Japan. There are only a few of them available on the used market as well as new ones, and it is difficult to find the right parts.
In addition, XD and XDR wheels do not have high resale potential because they are not in very high demand. They can often be used by replacing the freebody with Shimano’s HG standard, but there are few compatible freebodies in circulation, and if the freebody costs more than $100, it would be realistic to trade them in for a sparrow’s eye price.
This is fine if you want to build a road bike with SRAM, but if you buy XDR standard wheels without knowing what you are getting into, you will be in for a world of pain.