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.
You can put thin spacers between the arm and sprocket with a regular headed bolt; it doesn't require a Pinhead. If memory serves me right, Tree sold sets of spacers for this purpose a few years back.
ReplyDeleteYes, but if they are not the same width as the one below, the sprocket will wobble. And many times people only think about spacing the arms during assembly, not when ordering parts in the first place. Though you could run your bolt a bit out while waiting for the spacers to arrive...
ReplyDeleteHey, a quick question if you still check this blog. I want to get a local machine shop to make me a new BB spindle to fit a 48 spline Profile crank. Do you have any idea what the machining specs are for one of these? Reading through my copy of Machinery's Handbook, it looks as if it's a 64/128 pitch involute spline on a 3/4" shaft. Would this be correct? If you don't know, would you know where I could find out?
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