Wheels MFG Solo-Spline Review + Micro Spline Singlespeed Options

Here we are, well into 2024, and there are still limited options for pulling off a singlespeed setup when using a Micro Spline freehub. This is a bit surprising given that Micro Spline was introduced six years ago and seemed to quickly become a widely adopted standard. When Shimano launched the 12-speed XT and SLX groups, the move to the 23-spline driver made sense. It was a logical evolution from the 9-spline HG freehub, which was well past showing signs of age and had obvious limitations in keeping pace with SRAM’s innovations and expanding drivetrain capabilities. Micro Spline not only enabled the design of wider range cassettes and a lighter hub assembly, it arguably improved force distribution compared to HG. Within just a couple of years, Micro Spline was offered as an option on most mountain bike hubs.

Still, despite its widespread acceptance, singlespeed options for Micro Spline freehubs remain scarce—which I’ll cover shortly. Fortunately, a new solution just hit the market. Wheels Manufacturing has officially released the Solo-Spline singlespeed conversion kit, expanding on their Solo-XD kit for XD freehub bodies. Both systems use in-house machined cogs and offer a complete package that includes everything you need to set it up. I’ve been beta-testing the Solo-Spline ahead of its launch, and you can find my thoughts in this full review.

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Micro Spline Singlespeed Options

Before I dig into the Solo-Spline system, let’s take a look at what options are out there. Oddly, there aren’t many Micro Spline-compatible cogs available. You’d think that one of the singlespeed supply mainstays, like my go-to Endless Bike Co. or Wolf Tooth Components, would have made them by now. Like HG—and unlike SRAM HD—Micro Spline is a simple spline-based driver, so with a reversed machined spline profile, cogs could slide directly onto the Micro Spline freehub body, making the interface pretty simple. However, the only company I know of who’s making direct-mount cogs (and spacers) is Boone Technologies out of Kennesaw, Georgia. They’re nice, but they’re not cheap, and you still need to buy two of their custom-sized spacers. As you can see below, there are only a few options on the market when it comes to conversion kits or solutions, none being ideal and accessible, in my opinion.

• New Freehub + Cog/Spacers ($200+) – The obvious solution is to replace your freehub body with a standard HG model (if that’s an option with your hub), then buy a cog, such as the classic Endless Kick Ass Cog or a Wolf Tooth singlespeed cog; plus, you’ll need a spacer kit
• Boone Technologies Cog/Spacers (~$152) – Boone is one of the only companies making Micro Spline cogs; they’re machined from titanium, about $100 each, and you’ll also need to purchase a pair of their custom Micro Spline spacers to make up the 17.8mm difference on the freehub ($26 each)
• Problem Solvers Zinger ($89-109) – The Zinger conversion kit was one of the first Micro Spline singlespeed conversion kits on the market; with its 6-bolt carrier connection, it’s not quite as elegant as the Solo-Spline, in my opinion; it comes with 18 and 20-tooth cogs, and you can buy 16, 17, 21, or 22-tooth cogs separately for about $20 each
• Reverse Components Conversion Kit ($99) – Reverse components also makes a Micro Spline Conversion Kit, although it appears that it’s only offered with 13 or 14t cogs

By comparison, the Wheels MFG Solo-Spline kit sells for $120 and comes with everything you need to install it and an 18-tooth cog. Replacement cogs are $45, so if you want a different-sized cog, you’re looking at $165, all in. Cogs come in seven sizes and eight anodized colors: black, teal, purple, red, green, blue, and now for a limited time, Colorado Copper. Plus, you can get the entire kit in those colors.

Installing the Wheels MFG Solo-Spline

The centerpiece of the Wheels MFG Solo-Spline kit is a machined 7075 aluminum base sleeve carrier that slides directly onto a Shimano Micro Spline freehub body. This design interfaces directly with the freehub splines to mimic the installation of a standard cassette, locking onto the freehub. From there, you simply slide on the included “End Spacer,” which takes up about 4mm of the carrier and encapsulates the carrier base. Then comes the tricky part: figuring out the chainline and where the cog needs to be placed in the spacer stack. I asked Wheels if they had any sort of measurement guide to mathematically calculate the chainline based on the Micro Spline freehub, but apparently that’s still in the works. I took the manual route and used and angle iron braced on the chainring to estimate where the chainline landed on the carrier, which required no additional spacers inboard of the cog.

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One thing I noticed is that there’s a little rotational play between the carrier and the freehub spline body. However, as Wheels MFG suggested in the install guide, I added grease to the splines inside the carrier and outside, and once fitted with grease, it wasn’t as noticeable. All the included 6061 spacers (1x5mm, 2x2mm, 1x1mm) were positioned outboard of the 21-tooth cog, and once the 7075 lockring was torqued, there was no movement whatsoever.

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As you can see in the photos above and below, the resulting assembly is fairly bulky. However, the only added weight in the Solo-Spline system is the 34-gram carrier. With any other singlespeed setup, you’d still be using a lockring, spacers, and cog, so the carrier is the only supplemental component.

There’s one theoretical benefit to the carrier-based design, from my perspective. Singlespeeding involves a lot of torque, and I’ve seen HG freehubs get marred by cogs as a result. The carrier adds an intermediary layer between the cog and the freehub body, allowing the force to be applied to the carrier and distributed throughout the entire width of the Micro Spline body, thus protecting it from any concentrated force from the cog. That being said, freehub bodies can be replaced, and it’s unclear whether Wheels will offer the carrier as a replacement part. I had no issues with wear on the carrier or the freehub during my test period, however, and I’ll update this post down the road if I do.

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On the trail, the Wheels MFG Solo-Spline kit worked as expected. I didn’t experience any noises, rotational glitches, or any other issues. I wouldn’t think twice about taking it on a long bikepacking trip or any ride into the backcountry. And on that note, I appreciate the fact that the lockring can be tightened or loosened with either a 16-notch 44mm bottom bracket tool or a 38mm spanner. That could come in handy if you need to service it while traveling and don’t have access to a BB tool.

Cogs and Chains

Perhaps my favorite thing about the Solo-Spline system are the cogs. They are 4.5mm thick at the center and have a 9-spline proprietary interface that works with Wheels MFG’s Solo-Spline and Solo-XD carriers. They come in multiple colors and are available with 16, 18, 19, 20, 21, 22, and 23-tooth counts. And I really like that the tooth pattern is generally chain agnostic. The even-numbered cogs have alternating tooth profiles, which Wheels says offer better chain retention, and both the even and odd-tooth count cogs work with 9, 10, 11, and 12-speed chains. The latter includes SRAM Eagle, KMC, SRAM Road and Transmission T-Type flat-top chains. Flat-top chains have recently piqued my interest for use on a singlespeed, believe it or not, particularly for a long-distance endeavor such as the Tour Divide. The flat-top design is purportedly stronger and offers a longer wear life—both compelling arguments for ultra-distance singlespeed endeavors. For what it’s worth, Travis Jolly, the 2024 TNGA winner was running the Wheels MFG Solo XD conversion kit, 34×20 gearing, and a flattop chain, and he reported that he wouldn’t change a thing about his setup.

• Model Tested: Wheels MFG Solo-Spline Singlespeed Conversion Kit
• Actual Weight (with 21-tooth cog and spacers): 111 grams (3.9 oz)
• Place of Manufacture: Colorado, USA
• Price: $120
• Manufacturer’s Details: Wheels MFG

Further Reading

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