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I've noticed that chainrings 50T and larger all use five bolt holes instead of four, invariant of BCD. How come? Is this simply due to tradition or is there a mechanical stress reasoning?

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There probably is a slight tradeoff between the number of bolts and the "weight" of the ring. But mostly it's a PITA to have lots of different standards (not that Shimano et al seem to let that stop them in other areas). –  Daniel R Hicks Oct 2 '13 at 23:59
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TL;DR: It's just marketing.

In the 1930's, René Herse used three-arm, 70mm BCD cranks, and you can buy 50T chainrings today that fit that specification. In 1949, Stronglight introduced the Model 49, that bolted the big chainring to the crank with a tiny set of five bolts near the spindle, and then bolted a secondary chainring directly to the first. In 1958, Campagnolo introduced big 5-arm cranks (151mm BCD!).

Mostly everyone adopted and refined variations of the five-arm design for the next four decades (TA tried six bolts for a while), until Shimano introduced the 4-arm XTR crank (112mm BCD) in 1996, followed up by more 4-arm cranks (104mm BCD) for LX and XT in 1997. The marketing reason was that they were lighter (slightly), but it also meant that Shimano was the only company selling chainrings to fit the nascent standard.

Shimano was the 800lb gorilla of the 90's MTB marketplace, and where they went others followed. Over the next decade, 4-bolt cranks became common in mountain biking, where smaller chainrings are the norm. Hence the lack of large chainrings in four-bolt patterns.

Attempting to make the same leap again, the 2013 Shimano Dura Ace 9000 road crankset uses a non-symmetrical 4-arm crank with a 110mm BCD and up to 53 teeth. Once again, the stated marketing reason is weight - apparently this crankset saves 12 grams over the 5-arm Dura Ace 7900 crankset.

Much reviewers ink has been spilled over the stiffness of the new four-arm design and it's underlying strength, but it's not really clear that the cranksets of today are dramatically superior to those 3-bolt cranksets of the 30's, which were used to power tandems to the top of the Poly Chanteloup - and modern reproductions of those cranks are still lighter than the modern Dura Ace cranks.

More reading: Jan Heine's History of Aluminum Cranks, Stronglight 5-arm crankset timeline, Museum of Mountain Bike Art & Technology Shimano timeline, Cycling News Dura Ace 9000 review

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