Unfortunately, there's almost no information out there on the Middleburn XBB cranks - especially regarding running the XBBs as single speed cranks. The UNO rings that typically ship with the Middleburn square taper or ISIS cranks have an offset. They don't offer the XBB cranks in single speed so you have to wonder if something weird was going on. Middleburn doesn't exactly flood you with technical information so I was on my own.
Dan (ISAR) at HBC was really helpful and after a few measurements on the cranks we were pretty sure we had it dialed. It turns out we did and the conversion is easy. I'll post a quick step by step so the next guy doesn't have to re-invent the wheel. Apologies for the low-rent photos. Dim light + phone camera = ghetto blog posts.
Here's where you're starting... bone stock. Bone stock is no good, right? Right!
Next, you'll remove the drive side arm. Just as a friendly note, the non drive arm is not removable. Yes, the bolt on that side is just there to mock you. You can remove the axial play in your cranks with the threaded lock ring just inside the non drive arm. I digress... back to pimping your ride. Remove the drive side arm with an 8mm hex key. The self extracting bolts are a little on the soft side so use good tools and take your time.
You should see something that looks like this. The lock nut holds the spider on the drive arm assembly.
You'll want a Shimano TL-UN96 tool to remove the lock ring. I got it from my favorite local shop for $20 (Harrell's Bicycle World). Support your local bike shop, please.
After removing the lock ring and spider assembly you should have something like this.
The spider assembly weighs 117g using a 41g 33t chainring. This isn't a weight weenie build but its nice to know your before and after. The HBC ring and spacer weighs 90g.
Next up, put the new toys on. Put the spacer ring on the spline first (1.2mm thick). This is needed to compensate for the offset Middleburn designs into their system. HBC makes the ring and the spacer separately so you can flip the ring after its worn. You can't do that with Middleburn's rings since the offset is integrated into the chain ring.
The chain ring goes on next. You can align it with the crank arm a few ways. I prefer splitting the radial arms in the ring with the crank arm. There isn't a wrong answer so do what makes you smile. The chain ring surface should be a touch higher than the face of the spline on the drive side arm. If its not, something is wrong and you won't get the chainring tight.
Tighten the lock ring back down.
Reinstall the cranks & set your chain tension. Wheelie time!
One of the things I love about the Black Sheep is their HACS system for adjusting chain tension. (HACS stands for horizontally adjustable chain stays) The chain stays telescope in and out and are locked down with a single set screw on either side. To adjust the tension I loosen them so they're hand tight, push down on the seat to set the chain tension, then tighten the set screws. Dead simple & reliable. I've had EBB & sliding dropout systems in the past. Black Sheep's HACS system is superior to both.
Here's the money shot...
