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Spotted: FSA’s new K-Force Light crankset

AJACCIO, France (VN) — FSA’s new K-Force Light crankset is a sizable departure from the previous version, featuring a proprietary five-arm spider and special chainrings designed to decrease weight and improve stiffness and shift quality.

The new crankset, spotted on Bianchi’s 2014 line and elsewhere but, up until now, always without any firm details, is the first from FSA to step away from traditional bolt circle diameters (BCD) and towards a proprietary chainring and spider design.

Weight is down to an impressive (but unconfirmed) 589 grams without its BB386 bottom bracket, and all the usual tooth options will be available: 53/39, 50/34, 52/36, and 46/36. 170mm, 172.5mm, and 175mm crankarm lengths will be on offer.

The proprietary rings are CNC aluminum, and mount up to hollow, carbon crankarms with Torx T-30 alloy bolts.

Why the proprietary ring design, which will prevent consumers from picking up third party ring options? FSA says this headache is in the name of science: The loading on a crank is not consistent throughout the pedal stroke, and crank design should reflect that. Load spikes as each foot begins to press downwards, and ebbs near the top and bottom of the stroke. So it makes little sense, FSA says, to use a symmetrical spider.

FSA is certainly not the first brand to make the move towards such a system, either. Shimano’s distinctive 9000-series crank is quite similar.

The new K-Force Light sticks with five bolts, but moves away from the 130bcd and 110bcd standards and into a completely proprietary system, just as Shimano’s latest 11-speed cranksets have done. FSA calls it Asymmetric Bolt Spacing, or ABS, and claims that the use of offset spider arms, with one bolt hidden behind the crank arm, allows for both a lighter and stiffer crankset. The company claims that FEA (Finite Element Analysis) shows that its asymmetrical design results in improved stress distributions relative to a standard four- or five-bolt design. More uniform stress distributions allow FSA to use less material while maintaining or improving strength and stiffness.

Like Shimano, FSA has built some of the “spider” into the chainrings themselves, using wide, hollow mounting points. This makes the chainrings much stiffer, and allows FSA to build them around a much smaller spider — small enough for compact rings — and run every chainring combination off that same spider. With traditional, flat rings, running a standard-sized ring (a 53t, for example) on a tiny compact spider can result in excessive flex and poor shifting. Big, fat chainrings solve that. Again, the design is very similar to Shimano’s latest.

What is not similar to Shimano, however, is the arm placement. Apparently the FEA from each company spit out different results, because FSA’s arm design is rotated about 90 degrees from Shimano’s. The latter has a wide gap between arms directly opposite the crankarm, while the former places two arms close together at that point. There’s more than one way to skin a cat, as they say.

Availability is set for this fall, and pricing is not yet decided.

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