![]() ![]() In the old days, you could find cranks between 150mm and 185mm in length, often in 2.5cm increments. But as a rule, cranks specifically designed for mountain bike use tend to have higher Q-factors (wider between the pedals) than road-specific cranks. ![]() So, you can set up your bike with a 50 x 34 double chainrings, or 46 x 36 x 24 triple chainrings, or many other combinations not possible with the road-standard 130mm bcd. There is absolutely no reason why you can't use a mountain bike crank on a road bike, and one advantage in doing so is that you can use a middle ring smaller than 38t (which is the smallest you can use with a normal 130mm bcd road crank). Mountain bikes cranks have BCD of either 110 x 74, or 94 x 58. A common chainring combination for such a crank is 50t (outer) x 40t (middle) x 30t (inner) although that is one of many possible combinations. This type of crank will have a BCD specification something like: 130 x 74, indicating it fits outer and middle chainrings with a 130mm BCD, and an inner chainring with a 74mm bcd. Road cranks designed for three chainrings (called "road triples") have a secondary BCD to accommodate a smaller ring for an easier gear. Standards do change over the years, and a manufacturer can build to its unique standard by saying the others should switch, or that theirs is better, or whatever. Eventually a specialty chainring maker will see a market and make a lower-priced odd-BCD chainring, but it's still a mean thing to do in the first place. But manufacturers sometimes have bad manners, and make up their own, unique BCD so if you buy their crank, you'll also have to buy their chainrings, which are generally more expensive. That way, you the rider can change chainrings and have a great selection (from many makers) to choose from. Good manners suggests that manufacturers make their cranks (of a type, say road doubles, or whatever) to the same standard as everybody else. Etc.Ĭampagnolo is the odd duck here, with BCD of 135mm. A 50-tooth chainring designed for such a crank is designated thusly: 130 x 50t. Most road cranks designed for two chainrings have BCDs of 130mm. Here is a list of common BCDs from the past and present, and their respective minimum-sized chainrings: The smaller the BCD, the smaller the chainring that'll fit on it. ![]() If you draw a circle through the centers of all the holes and measure the diameter of that circle, that's the bolt circle diameter (bcd). By length.Ĭhainrings attach to right-side cranks by chainring bolts, so the chainring bolt holes in the chainring have to match up to the corresponding holes in the crank. Aluminum cranks can be machined (carved), forged (smashed), or cast (poured into a mold).Ĭranks are usually differentiated two ways: 1. ![]()
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