The only test ride so far was on the rail/trail in front of my house. No honking up steep hills or skidding, but the cog seems to be staying centered. As the rains here in Oregon ease up, more miles will test the viability of this conversion.
Surly New Disc Fixed Hub Conversion
I wanted to try a bolt-on cog hub on my Steamroller. Seemed like a smart concept that does away with the specialized tools needed to install and remove a threaded cog. Wanting to stay away from the expense of a Level hub, and not being a machinist, I needed a true bolt-on conversion of a front disc hub.
Surly’s New Hubs use the same size bearing front and rear, so it seemed a perfect candidate to experiment. No custom axles or sleeves would be needed to convert a front hub to a rear, just stock hardware and some spacers.
There’s only one cog on the hub, so no flip/flop version. But then, I’ve only flipped my wheel once in 3 years. If a bail-out gear is still desired, a spare cog can be carried, and swapped on the road. No chain whip or lock ring tool required. But, what I ended up with is a clean looking hub with no empty threads on the non-drive side, and no special tools to swap cogs.
I scrounged 5mm screws and washers from the parts bin, but will probably buy a set of disc rotor bolts, too.
To begin the conversion, use 15mm and 17mm cone wrenches to loosen a locknut and remove it, along with the sleeve nut. Leave the dust covers in place. Extract the axle from the other side.
Insert the new axle and spin on the sleeve nuts, adjusting the axle so the same length protrudes from each side of the hub, measured from the flanges, not the sleeve nut.
The 10mm rear locknuts are also about 1mm longer than the 9mm fronts, so that makes the O.L.D. 102mm without any spacers. Another 18mm to meet the Steamroller’s 120 spacing ends up with 4mm on the drive side, and 14mm on the non-drive side. With my cheapie caliper, this appears to get the flanges centered within 0.5mm. Close enough. Chain line measured to the center of the cog is 46mm.
I’ve got a Sugino RD crankset with 48t ring fastened on the outside, mounted on a 103mm Sugino BB. Sheldon says this combo should give a 42mm chain line, but on my Steamroller, I measure it at 47mm.
I could have used a splined cog and drilled it myself, but like I said, I'm no machinist. Boone titanium cogs are an option, but the last time I checked, an 18t was $50 shipped. Kogswell makes 16t and 18t kogs. A 15t or 14t home-brewed cog may have issues with the chain hitting the bolts, so there are limits to the gearing choices.
Departing slightly from the true bolt-on nature of this conversion, I spent five minutes with a dremel to grind the splines on the kog to clear the bearing housing, and it pops right on. If the whole conversion meets my expectations, a Boone cog is on my birthday wishlist. A little blue Loctite on the 5mm screws, and I figured the 6.2 Nm (55 in-lbs) torque spec of some disc rotors would be sufficient. There is some slop due to the size of the bolt holes, so centering of the cog is necessary for even chain tension.
Building the wheel was also a bolt-on proposition. Flange to flange distance is 54.5mm on the disc hub, and 60mm on the original rear hub. Flange diameter is 58mm on the disc hub, and 54mm on the rear. Using spocalc, the spoke length was 1mm shorter for the disc hub. I disassembled the Steamroller’s wheel, and since there’s only a couple of hundred miles on it, decided for this proof of concept to re-use the spokes on the MA3 rim. I may relace it with shorter spokes if it appears to be a problem.