Tearing apart the FSA three-flange hub design.
[This is a rant about bicycle physics, perhaps the first in an occasional series. It's written for someone conversant with bike mechanics. Unfamiliar words are likely defined in the Glossary by the late Sheldon Brown. I'm heavily influenced by Jobst Brandt, and in this case I recall he heaped derision on this three-flange hub, but did not explain much about why. He has written an excellent book about the mechanics of the bicycle wheel.]
Strong wheels are wonderful. But modern wheels have traded off as much strength as possible in a quest for lots of gears in the cassette. If excess strength remained, the cassette would get deeper, but instead since the ancient 7-speed cassettes, cassettes have stayed about the same size and the chain side plates have paid the price, getting driven to thinner and more expensive designs.
A conventional rear wheel is a careful balance of forces to form a reasonably strong yet lightweight structure. The difficult kind of strength to provide is lateral: resisting sideways forces on the rim. Spokes act only in tension, so the rim's lateral position is held by tension in the spoke set on each side of the rim, the drive-side set and the non-drive set. The drive set has a small bracing angle, meaning the spokes are almost in a plane parallel to the rim, but they do pull toward the cassette. The total lateral force from the drive set is perfectly balanced by the lateral force of the non-drive set, but the non-drive has a larger bracing angle so the non-drive tension is lower. (This describes a hub with equal counts of spokes on each side, like my Shimano ones - other designs exist but are ultimately comparable.)
A properly built wheel has the following balance: the rim is compressed to the limit of its strength (or is close to it) by the sum of the tension in all the spokes. The spoke tensions are matched within a given set of spokes, but different sets may have different tensions.
In a conventional wheel, the limiting factor in lateral strength is getting enough lateral tension on the drive side with the limited bracing angle. The drive side spoke set also handles the majority of the torque transfer from the cassette by increasing tension in "pulling" spokes while reducing tension in the lagging spokes. These spokes are doing a lot of work!
The FSA design has three flanges: a pair in fairly traditional locations, laced radially, plus a third flange in the rim plane with a large diameter and crossed spokes to transfer torque. Radial spokes cannot transfer torque. Each spoke set has a third of the total spoke count (8 each, rear), and the even spacing of the rim holes tells us the spoke tensions are all similar. The rim design is conventional, so total spoke tension is about the same as a conventional wheel. But since the drive-side spoke set 'lost' a third of its spokes to the center spoke set, the drive side lateral bracing force has been cut by about a third. There's no way around this - the wheel is substantially weaker than a conventional wheel.
Jobst's quick take on this was that this design is so bad the whole company should be avoided by his readers. That's bad news for me, since their chainrings have received good reviews, and they're a player in the crank market which has for the moment abandoned road triples like mine. If FSA were to offer a good triple, I'd like to be able to consider it - but I'm afraid I'd have trouble ignoring Jobst's advice.
If you're interested in more about bicycle wheels, get it from the horse's mouth.
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