Cold Forging at Shimano

Primo Powerbites, and now Hollowbites are advertised as being manufactured using cold forging technology. Effectively, they are smashing a blank piece of metal into the shape they want. Cold forging offers the advantage of precise control over dimensions, no need for heat treating, and aligns the grains along the surface of the part, perpendicular for forging direction. Hot forging, on the other hand, works better at aligning all of the grains of the part (up to a certain thickness, based on material characteristics), but cannot control dimensions as well.

Here is an article on Cozy Beehive about the cold forging at shimano. You'll notice the following graph, which may explain why some of the newer forged sprockets (such as Eastern's Medusa Lite, and Stolen's Mood Ring) are forged from 6061. Kudos to Primo for using a stronger, albeit tougher material to work with:

Sprocket/Crank Interface

In recent years we have seen a shift in sprocket/crank interface paradigm... Ok, that may be a bit exaggerated... At least there has been a change in design.

Traditional, Perpendicular Bolt

My first introduction to crank torque transfer was with the integrated stud on one piece cranks. Although effective, tolorances were often poor, and low-end sprockets were usually thin steel, further compounding any issues.

As BMX progressed, we moved to a bolt to secure the sprocket to the crankset (usually 3/8" or 10mm). Today, virtually all cranks use the bolt-on design. This helped solve some of the tolerance issues and has served as the standard for bmx. From a structural perspective, this design requires either a larger spindle boss or a separate hole cut in the crank arm for a sprocket boss.

A slight hybrid of one and three piece mounting systems, that has resurfaced recently is the use of a headless bolt. Mechanik makes a bolt called the Pin Head. The purpose of this design is not to hold the sprocket to the crank, but only to transfer torque. Using this design allows small spacers to be inserted between the arm and the sprocket to dial in the chainline.

Nano-drive

As Sprockets reduced in size (with flatland at the forefront), a need for less offset in the mounting position. Enter the nano-drive. Most nano-drive bosses use smaller bolts than the standard bosses.

Spline Drive

When faced with the challenge of small sprockets, Tree Bicycle Co. decided to redesign the mounting design. What they came up with was transfering torque through the spindle, rather than the crank arm. With most cranks now using 19mm, 48 spline, Tree's spline drive has a large potential market. Recently Profile decided to release their own version of spline drive (releasing a spline drive sprocket was logical for Profile as they are known for their 48 spline cranks). Unfortunately, Profile decided it was best to use 8 large splines on the adapter, rather than the 9 available on the Tree. I don't know why this happened, but it may be just to avoid any patent issues.

There is one big problem with spline drive: it is limited to 19mm, 48 spline cranks.

Socket Drive/ Nonogram

Flatware's (Odyssey) response to shrinking sprockets was to standardize. Well, sort of. They are the only company making a crank that works with the 9-sided mating surface, but do provide the specs and drawings(PDF) on their site. If this design is adopted, adapters can be made to adapt socket drive to a 48-spline crank, in effect making it another spline drive. The big advantage to this over standard spline drive is it does not limit the spindle design.

BMX-U Sprocket Page