What’s on the market?
There’s something on the move with cranksets as well. The number of options increases from cotterless over Octalink/ISIS, Hollowtech to BB30 with special bottom bracket shell.
Bolt circle and number of bolt circles
The chainrings used for the mounting of former cranksets were mainly 5 bolt patterns at compact measure (58/94 mm) or standard measure (74/110 mm). There is a current trend towards using 4 hole patterns at bolt circle diameters of 64 mm for the small and 104 mm for the big and middle sized chainring. In spite of suitable bolt circles compatibility problems can arise due to the specific form of some chainrings.
Except for the new “BB30“-standard all crank systems are ridable in a customary bottom bracket shell. Even a more than ten year old bike could be upgraded with a current "Hollowtech II"-system with 24 mm hollow spindle provided it is equipped with a BSA-thread and a shell width of 68 or 73 mm. There is 16 years and 239 grams difference between the cotterless Shimano XTR-crankset and the latest "Hollowtech II" model with one-sided rigid crank/spindle connection.
The increasingly used “BB30”-system is composed, as the name suggests, of a hollow spindle enlarged once again to 30 mm and requires a 68 mm wide bottom bracket shell without thread in which the bearings are fitted directly – comparable to the headset.
What’s the point of it?
The measurements of various cranksets show a constant or slightly increased stiffness with less weight. Thus the STW-values have experienced a significant rise.
To consider the torsion of the crank spindle, the stiffness was measured by us at the right and left crank arm. On the right side the rider’s force is transmitted directly through the chainrings to the chain, whereas on the left side the spindle is mounted intermediately and thus susceptible to torsion. Here, the high values of both FSA-systems are absolutely convincing.
The absolute stiffnesses of the right crank arms show only minor differences. One cotterless XTR crankset of 1991 and the latest model are on the same level. Nota bene on the right side, where the torsion of the crank spindle plays no role. The left side, however, showing a plus of 35 percent more stiffness has been improved a lot. The enlarged hollow spindles are less prone to torsion and furthermore lighter in weight. Another advantage is an enlarged support width of the spindle, as with the "Hollowtech" standard the bearings are outside the BB shell. The FSA "MegaExo" crankset that is similar in design tops the list of absolute stiffness values ahead of the "BB30"-crankset of the same company which comes in a close second. The best STW-values are achieved by the "BB30"-crankset.