ost newer bikes (made since the late-1980's) have "vertical" rear dropouts, where the wheel slides upward as you install it. These are a problem when you want to dispense with a derailer, because you need some way to regulate chain tension.
White Industries has come up with a terrific solution to the problem of vertical dropouts and fixed-gear/singlespeed, a bolt-on hub with offset bolts that allow 15 mm of adjustment even in a frame with vertical dropouts!
This hub is available for 28, 32 or 26 spokes, for 126mm, 130mm or 135mm spacing. End caps of different lengths are interchangeable for different spacings.
The current (2014..) model of the ENO hub uses a proprietary splined fixed-gear sprocket, to avoid issues of sprocket movement and cross-threading. The sorocket fits securely in place with White Industry's precision-machined spline system. Use any ISO standard 1.29" x 24 TPI reverse-threaded fixed-gear lockring to secure the sprocket. The freewheel side will take any ISO/British thread freewheel.
The other end of the hub has standard freewheel threading. It has a nutted axle and cannot be converted to use a quick release.
The ENO hub is designed for a 47.5mm chainline, which matches the middle position of a typical mountain-bike triple crank.
A typical "road" double gives about a 46mm chainline with the standard bottom bracket, which is a decent match. Ideally, you would want to adjust the chainline to be perfect, though.
The Sugino RD crank which Harris cyclery features gives 47.5mm chainline when used with a common Shimano 110 mm cartridge bottom bracket, so if you need to buy a crankset anyway, this would be a great combination. The ENO hub uses an oversized aluminum axle with stainless steel ends that fit into the axle. The ENO comes equipped with two sealed cartridge bearings and is available in 32 and 36 hole versions.
The eccentric axle provides a total adjustment of range of 15 mm. Each tooth added or subtracted to your drive train requires that the axle move 1/8" (3 mm) backward or forward.
A 19mm end wrench on the flats at one end of the hub will turn it to adjust chain slack.
Most folks find that the best setup is to use a fixed gear two teeth smaller than the freewheel gear. The idea is to use the fixed gear for general riding, and to keep the lower freewheel gear in reserve for longer rides, steeper terrain, or when you're just plum tuckered out. The lower gear will make the climbs easier, and the freewheeling feature will let you rest on the downhills, but you'll be slower on the flats.
Note that the offset will effectively raise or lower the rear end of the bike a bit, and will move the rim downward or upward with respect to your rear brake, if you use one, so some adjustment may be required, and some frame/brake combinations may not work.
The eccentric axle caps sometimes interfere with use of a freewheel remover to unscrew a single-speed freewheel from the White Industries eccentric hub.
If you have a problem with this, the solution is to remove the axle cap from the hub temporarily. .
The axle cap is held on by a single recessed Allen bolt (4 mm wrench size) that screws down into the centerline of the axle. Once you remove this bolt, the axle cap can be pulled off. The axle cap is usually a snug fit, and takes a fairly strong pull to remove. It is hard to get a good enough grip on it to pull it off, so here's the trick:
Once the small Allen bolt has been removed, use the larger main Allen bolt (the one that holds the hub to the frame dropout) to bolt a convenient-sized box wrench against the axle cap. The wrench will act as a "handle", and a bit of rocking and pulling will get the cap off with no problem.
The ENO hub uses two 6902 cartridge bearings.
If you don't have a shop nearby which can help you meet these challenges, check out: