SRAM’s proprietary freehub bodies – the SRAM XD driver and XDR driver – are used in SRAM’s 11- and 12-speed groupsets, built to be compatible with various existing hubs.
The hub of your wheel can be a confusing place.
There are so many different parts that work together to allow the different mechanisms of the wheel to coexist. Furthermore, there are many different compatibility issues and standards to think about when it comes to the hubs.
SRAM’s XD and XDR drivers are a type of freehub standard made by SRAM that is compatible with a range of hubs and cassettes.
But what’s different about the SRAM XD vs XDR drivers? And what cassettes and hubs are they compatible with?
Don’t worry! In this article, we’ll help you to navigate the confusing world of freehub standards and understand which cassettes and hubs you can use SRAM’s proprietary drivers with. We’ll be covering:
- What Is A Freehub Body?
- SRAM XD Driver
- SRAM XDR Driver (SRAM XD Vs XDR)
Let’s get started!
What is a freehub?
During a ride, it’s very unlikely that you’ll be continuously pedaling.
Often, on downhills or on flats, you’ll stop pedaling, but the wheels can still turn – this is often referred to as “coasting” or “freewheeling”.
The reason that you are able to “freewheel” on a bike is the freehub. When you think about it, the pedals are rigidly connected to the chain, which is, again, rigidly connected to the cassette, which, in turn, is connected to the wheel.
So, when you’re not pedaling, how can the wheel be spinning without forcing the pedals into motion?
The freehub is the part responsible for this. It is part of your wheel that connects the cassette to the wheel itself.
The freehub is made up of many parts, but, to simplify, you could say that it is largely made up of two – the “freehub body” and the “freehub system”.
The freehub system is the part that allows you to freewheel.
It is essentially a mechanism that engages when you turn the pedals – forming a rigid connection between the wheel and the cassette, but disengages when you’re not, allowing the wheel to spin independently of the cassette.
The freehub body’s job is twofold – it both houses the freehub system (which allows you to freewheel), and it provides an interface between the cassette and the wheel. This is usually the limiting factor on your bike’s compatibility with different cassettes.
Like almost everything else on a bike, freehub bodies provide an additional degree of confusion when it comes to matching the compatibility of a cassette and a wheel.
Different-shaped freehubs fit with different-shaped cassettes. The “shapes” of these freehub bodies are referred to as “standards” – and generally each brand has a few different ones.
For example, Shimano‘s standards are the HyperGlide and the Micro Spline, Campagnolo‘s are the FW and the N3W, and SRAM’s standards are the XD and XDR.
So, we can say that the SRAM XDR and SRAM XD drivers are the freehub body standards made by SRAM. But what’s the difference between the two?
SRAM XD Driver
First things first, if the SRAM XD driver is a freehub standard, then why is it referred to as a “driver”?
Well, don’t worry, although it’s somewhat confusing, they’re actually the same thing. SRAM just calls their freehub standards “drivers”.
Now that that’s out of the way, what is the SRAM XD driver?
The SRAM XD driver was originally made for 10-speed mountain bikes with the intention of allowing for a high range of cassette sprockets – with its key draw being that it is compatible with a 10-tooth cog.
Although nowadays this isn’t as small as they come (Campagnolo’s Ekar 13-speed gravel groupset comes with a 9-tooth cog), at the time, this was a significant advancement in the tech, and allowed for faster speeds during downhill MTB races, for example.
Another feature of the XD driver is that its spline pattern is different from other common designs such as the HyperGlide: the sprockets essentially have their own threads on the freehub body, so the use of lock discs is no longer required.
Of course, this means that a Shimano HG cassette or individual sprockets cannot be fitted to an XD driver.
The SRAM XD driver was designed to be compatible with many existing hub axles, and the proprietary technology is an open patent which means that many manufacturers have created an XD-type freehub body to allow for compatibility with an XD driver.
In fact, it is now available from more than 250 hub and wheel manufacturers.
It is also compatible with all Shimano-type hub and cassette tools, which is handy since buying a whole new set of tools for one part of a bike is always an expensive ordeal.
Although initially designed for 10-speed mountain bikes, as the technology progressed, it found utility in the 1×11 drivetrain market (for any type of bike).
So, now, if you’re looking for a 1×11 setup on a gravel or mountain bike, you’ll likely see the term XD driver mentioned in the spec.
Again, the key draw here is that, if you don’t have a second chainring, allowing for a 10-tooth sprocket will massively increase your potential gear ratios and increase your gear range on an 11-speed 1x drivetrain setup.
SRAM XDR Driver (SRAM XD Vs XDR)
The XDR (XD Road) driver is essentially the same thing as the XD driver – but made specifically for road bikes.
The reason for the release of this was the jump to 12-speed road bike groups.
The XDR allows for compatibility with 12-speed groups such as their own RED eTap AXS group by adding 1.85 mm of width to accommodate the bigger cassette.
This means that you can fit an XD mountain bike cassette to an XDR freehub – though you’ll have to use a 1.85 mm spacer. And, of course, you won’t be using the full 12-speed capability of the XDR.
This means that it fits properly onto a common Shimano HyperGlide hub on a road bike.
This could prove a useful feature for gravel bikes, for example. If using thinner wheels, such as 32 mm, then you may be using a road bike hub – to which the XDR freehub can be fit.
This means that you can attach an MTB cassette to your gravel wheel to extend the gear range.
Though – most road bike derailleurs have a maximum sprocket size compatibility. If, for example, you used an MTB or gravel-specific derailleur, then you could increase the possible gear range of a given wheel.
But, overall, its primary purpose is the same as that of the XD driver, but for road bikes: to allow the fitting of a 10-tooth cassette sprocket to your freehub.
With the introduction of 12-speed gearing, this is particularly helpful: allowing the expansion of your gear range.