Overview of Hubs

Hubs are another of the primary weight bearing component. Not an inch is traveled on a bike without the hubs revolving, and this is an area of the bike where quality improvement can be noticed. It's for this reason that the hub descriptions are so lengthy, our apologies, but over educating has never been found punishable. Hubs are usually made using a one-piece or a three-piece designed hub shell. The one-piece style uses a single aluminum forging, a single piece of machined billet aluminum to form the core of the hub itself. Into the aluminum hub shell a bearing cup race or bearing cartridge is pressed on each side. The three-piece type has a separate center or "core" tube with, what is generally, an aluminum alloy flange that has been pressed onto it (Nuke Proof, and Bullseye for example). The hub flange is the part of the hub that the spokes actually pass through. It is the thin area that rises off the ends of the hub. The Hub flange is referred to as having a "small" (low) or a "large" (high) flange. Small flange hubs have the advantage of being a little lighter, but as the flange gets smaller, your choices of lacing patterns and spoke number drop off, when the spokes and heads start to interfere with adjacent spokes. Some hubsets use a small flange front and high flange rear so that replacement of spokes on the freewheel side can be done without freewheel removal. As a practical matter, having the front and rear hubs use the same flange size makes it possible to use the same length spokes, front and rear, on three sides of your wheels . Remember, to properly "dish" (center the rim over the middle of the axle's over locknut dimension), you will generally require the drive-side spokes to be 1 or 2 millimeters shorter, if you want to maintain an even wheel dish, and exposed length, on each spoke. The "hub flange outer diameter" is the measurement, outer edge to outer edge, at the opposite ends of the same side of the same flange. This dimension is given because the spoke hole might be near the edge of the flange or it could be positioned somewhat inward from the edge with the spoke laying over the extra aluminum. The "spoke center circle diameter" is the measurement, center to center, of two spoke holes at the opposite ends of the same side of the same flange. This dimension is used to determine the proper spoke length, relative to the rim you are using, and some believe that the flange thickness is also an influence in building your wheel. The length of all front axles is 100 millimeters, this measured from outer axle locknut edge to outer axle locknut edge. It is also refered to as the "over lock nut" dimension. The axle locknut is the last piece on each side of the axle, (not the axle itself). Front axles, it fit in standard fork ends, are 9mm in outer diameter. The length of the rear axle varies. The type and age of the frameset, dictate the amount of space available for the freewheel or cassette, and therefore the amount of cogs that will fit comfortably on the rear hub. The rear axle length is measured, like the front axle, from outer axle locknut to outer axle locknut. The possible lengths for the rear axle are, 126mm (which is used for 6 or 7 speed Road Racing bikes), 130mm (which is used for 6 or in a few cases 7 speed Mountain bikes and 8 speed Road Racing hubs), and 135mm (which is the present, common size for 7 speed Mountain Bike hubs), 140-145mm (for tandem bikes). There is sometimes a question about why an aluminum axle is used, in some cases, rather than steel. If the precision sealed bearing cartridges are used the load is distributed over a wider inner race and use of an aluminum axle is acceptable. The sealed bearing cartridges put all the weight on the bearing cartridge in a straight vertical line, so that weight the hub bears is spread over a broader area of hub shell contact. Steel axles are used when the axle bearing assembly is of the cone and cup race type. Since the weight on the hub is placed at an outward 45¡ angle, the stronger, (and heavier) steel axle is required because of it's shear strength. This 45¡ angle also introduces some additional friction to the hub bearings that isn't present in the vertically loaded precision sealed bearing cartridge type of hubs. Larger or "oversized" diameter axles (16-19mm) are used in front and rear suspension hubs to increase rigidity. There is an advantage to having the bearing assemblies and flanges on the rear hub spaced as far apart as possible from each other. As the chain pulls on the right side of the rear hub it introduces a torsional load that makes an attempt, on cone and cup race axle assemblies to bend the axle forward. This continual bending motion on a short spaced rear axle leads to increasing odds of axle breakage, which will occur at the center of the axle. The farther the bearings are spaced toward the locknut, the less of this stress there is trying to shear the axle in the middle. This explains why, in recent years, there has been an increasing migration to "freehub" style rear hubs, when cone and cup bearings are used. The freehub moves the bearings farther toward the axle locknut on the freewheel side. On the freewheel type of hub, the bearings rest, maybe, under the center of the freewheel. On the freehub type, they are under one of the last cogs. The finish work on aluminum hub shells and flanges involves a step where the shell or flange is given a "brushed" finish, with a metal bristle brush in contact with the revolving part. This removes the burrs in machine work and provides a reasonably smooth and even textured surface finish. The next step, in some cases, is tumble polishing. This is done by putting the aluminum parts in an oversized, rock polishing type of machine, (in it's crudest form, a cement mixer may be used), along with a fine, gritty, liquid polish and the parts are "tumbled" for several hours to smooth out the brush work and give an even surface. The last step in hub finish work may be, either anodizing the aluminum or in some cases painting it to avoid future oxidation. The parts of a quick release axle skewer are, (from the lever side), lever (sometimes with drop-out/forkend retaining washer), drop-out/forkend retaining washer, volute spring, another volute spring, and adjusting nut (with a drop-out/forkend retaining washer built into the inner side). Through all of these parts passes the threaded skewer rod. Some rear hub axles are made with internally threaded ends so that bolts can be used to hold the wheel to the bike. The bolt-on style axle allows a large head bolt with greater surface contact to be used to hold the wheel on. Examples of makers who do this are Bullseye, Hershey and T-Gear. For wheel builders, the second edition of the Pedal Pusher/800-Bike-Pro Buyer's Guide will include the all important center to flange measurements it was errantly overlooked in this edition.