Everyone wants to talk about whether the latest mountain bike drivetrains have one rather than multiple chainrings and the range of the cassettes, but not much gets said about how the cassette interface dictates those choices. When SRAM introduced their innovative 1×11 mtb, their concept hinged on having an ultra-wide range cog cluster in the back of the drivetrain to replace the need to have multiple chainrings on the crank up front. The XX1’s cogset is like a wide-range 9sp cassette bookended by a 10T and 42T cog, both of which required some re-engineering of the cog/hub interface. The result is what SRAM calls an “XD cassette driver”, and it is both a great advance in design and an impediment to market acceptance.
In the last thirty years or so, there have only been seven other significant cassette designs. Shimano’s Uniglide hubs integrated the bearing and ratchet mechanism of the freewheel directly into the rear hub; the cogs slid onto splines except the 1st-position cog that threaded on to the cassette body to hold rest on. The later Hyperglide replaced the thread-on 1st position cog with a lockring that fits the female-threaded body, and the splines now included a narrow spline to align all the cogs relative to each other. This would ensure that the shift ramps of adjacent cogs would be properly positioned from end to end of the cassette. Other than one little abortive idea at the beginning of the 10sp-era, the only thing Shimano has done since is make the Hyperglide body wider to accept more cogs (6/7sp to 8/9/10sp to the recent 11sp width). Suntour’s cassette spline pattern died with the company; Mavic wisely abandoned their own proprietary spline pattern to make their hubs and later wheel systems compatible with products from people who actually know how to make drivetrains.
Campagnolo created two spline patterns in the 1990s. Their 8sp hubs looked vaguely like Hyperglide without the one narrow spline to align the cassette. They abandoned that at beginning of the 9sp-era for a design with much deeper splines, presumably so that the larger spline interface of the cogs would prevent the cogs from gouging into the soft alloy cassette bodies Campagnolo wanted to use for weight reduction. This is an important point, because to make the spline interface larger, Campagnolo had to increase the depth of the splines towards the axle rather than make the tops of the spline taller. Why? Because an 11T cog, at least as an option, is absolute requirement for racing and consumer sales. Taller splines would have been too big for an 11T cog to fit on the cassette body. However, the deeper splines limit size of the bearings supporting cassette body, arguably to the detriment of longevity. Many aftermarket and OEM hub manufacturers had difficulty redesigning their Hyperglide-compatible bodies/axles to fit Campagnolo. Chris King’s “RingDrive” cassette hubs could not be adapted to the Campagnolo splines, and it would be 15 years until their recently introduced R45 road hub, with its slightly downsized RingDrive driveshell, could fit a Campagnolo cassette.
Shimano had attempted to sidestep the bearing size issue with their early 10sp alloy cassette bodies by having taller splines for the larger cogs and shorter splines for the shorter splines for the smaller cogs which didn’t need as much support anyways. That way, the depth of the splines had not changed from regular Hyperglide (without going on a tangent, let me say that Shimano’s own hub designs preclude downsizing the bearings in the cassette body since they also support the hub shell). An additional advantage was Shimano’s 10sp cassettes could fit on older Shimano hubs with non-aluminium cassette bodies. However, in the end Shimano abandoned the stepped Hyperglide splines and alloy cassette bodies, and their 10sp mtb cassettes and new 11sp cassettes in both road and mtb revert original Hyperglide splines. It has to be said that many, many other hub manufacturers have made alloy cassette bodies for Hyperglide, but often the individual cogs bite into the soft metal like a bulldog on a chew toy. This happens less when the larger cogs are riveted onto an aluminium carrier/spider, as featured on higher end cassettes.
But what if you wanted to run a cog smaller than 11T? Would there be room at all for a bearing? This is the big design issue with SRAM’s XX1 10-42T cassette, since a 10T cog would give the desired top-end gear to a single chainring that is still small enough to yield the necessary low gear from the 42T cog. Shimano’s new M9000 drivetrain also offers a 1×11 option, but without that 10T cog as an option, you have to give up something significant on the high or low end, depending on what chainring you choose. Since Shimano became the industry heavy in the 1980s by way of technical innovations and OEM dominance, they’ve not been prone to taking cues from others. By choosing to stick with their Hyperglide cassette interface, they are wagering that momentum of the existing design will win out over SRAM. But the gearing of their 1×11 is just not as versatile as SRAM’s; nothing can change that.
Complicating the issue is the market trend for oversize 142 x 12mm thru-axle rear hubs, meaning that there would be even less room for bearings between the 10T cog and the axle. SRAM solution was to pare back the cassette body to an abbreviated spline interface next to the hub flange. From their existing high-end road and 2×10 mtb groups, SRAM adapted their “Powerdome” cassette, in which almost all of the cogs are machined from a single piece. This unified cone of cogs is riveted onto the enormous 42T last position cog, which actually provides the interface with the splines. Rather than having a lockring, the XX1 cassette has more like a thin-walled “lock tube” that barely slips through the tiny 10T first position cog and reaches to a male-threaded portion of the XD body just outboard of the splines.
Besides the lilliputian 10T cog, the other, less obvious technical obstacle to the XX1 cassette is the titanic 42T eleventh position cog, the biggest ever on cassettes. The problem is that such a big cog puts a lot of torque on the hub, especially if used with a really small front chainring, and the cog teeth are so radially distant from the axle that any slop in the bearings or runout is magnified. This issue may be what ultimately limits or at least slows SRAM 1×11 market penetration, because any XD cassette driver will require a higher level of precision and durability. I’m curious to see how Shimano’s 11sp drivetrains do in the long run, because they have made 11-40T cassettes that fit existing Hyperglide bodies to accompany triple cranks with 22T granny rings…… and there are legions of companies making shitty OEM cassette hubs that may not be up to the torque loads that ring/cog combo facilitates.
Going back to Chris King, their original RingDrive “driveshell” could not be adapted to Campagnolo cassette, to the chagrin of many a roadie. But King managed to thrive by selling the most coveted mtb hub on the market. Talk to most shop monkeys, and we get that far away look in our eyes when the subject of King hubs comes up. Even those of us who hated the irritating King signature ratchet whine on road bikes, love the instant hookup the King hub gives off road. Then SRAM rolled out XX1, and King didn’t have an immediate response for XD-compatibility requests. But then again, King never has an instant response. Despite offering components in more anodized colours than other leading over-the-counter brands, King rarely brings a product to market without relentless engineering and testing. As legions of would-be 1×11 riders cried out, King demurred when asked directly about their plans. Then less than a month ago, they announced that XD driveshells would be available for shipping on May 1 as an option on all new mtb hubs as well as an aftermarket upgrade. And true to their word, my new XD driveshell was right on time.
Holding the new driveshell in hand, it is easy to see the magnificence of the CNC machining, just amazing. The heat treating used to strengthen the steel gives the driveshell a colour that varies from bronze to purple. I bought the conversion kit ($177 retail) which is the driveshell plus the alloy driveshell seal. If you go that route to convert the hub, you’ll need the special King hub tools to remove the bearings out of your current driveshell and reinstall them in the XD replacement. For $207, you get the XD driveshell with bearings pre-installed, for which you basically only need a 2.5mm allen key to pull the hub axle out and wiggle the new driveshell in. In retrospect, I regret not getting the driveshell with bearings. I have the King hub tools at the shop, but now that the old driveshell has no bearings it’s not very usefull unless I make it into a salt shaker or something (holy crap! that just came to me now…i gotta make that happen). I’m not likely to ever go through the hassle of swapping the bearings from one body to the other again.
So now my Chris King hubs laced with Sapim CX-Ray spokes to Enve rims can acccept an XX1 cassette…..bike porn.