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Technogram: Crankset and bottom bracket

James Spender
24 May 2016

To find out what connects to what in a bottom bracket and crankset, we decided to pull one apart.

If the inventor of the first two wheeler, Baron Von Drais, could see us now he’d not be pedalling in his grave, because his dandy horse riders rode astride their pedal-less machines while pushing off the floor with alternate feet for propulsion. But ever since the invention of the chain-driven safety bicycle, we’ve gone from scooterists to cyclists, and now have the pleasure of orbiting our legs at a fiendish pace about a ball-raced bottom bracket and chainset, such as this ensemble from FSA.

This particular configuration of the K-Force Light double chainset comprises 53 and 39-tooth alloy chainrings (10, 9), which attach with chainring bolts (8) onto the four-arm spider (13) of the right-hand crank (14). Integrated into the crank is a spindle (12), supported by two cartridge-type bottom bracket bearings (6). In this particular bottom bracket version the bearings are press-fitted into two alloy cups (4), allowing the BB386 Evo bottom bracket to be fitted to bicycles with standard English threaded BB shells. 

Protecting the bearings and spindle from moisture and dirt are two rubber seals (3) and a sleeve (5), while a wavy washer (7) is inserted between the right-hand crank and the right-hand bearing to help preload the bearings correctly. Finally, the left-hand crank arm (1) mates with the corresponding splines on the end of the spindle (11), and is held in place by its tolerance fit and secured with a crank bolt (2).

Technogram: Front light

Spin to win

There are a few things to be aware of if you're swapping chainrings. They are often designed to work together as a system – with pick up ramps on the back to aid optimal shifting – meaning orientation and matched brands are therefore essential. Don’t forget to check the BCD (bolt circle diameter) and number of bolts too.

Periodically check chainring bolts for tightness. Correct torque is imperative in order to spread the pedalling force across the bolts and chainset evenly, such that a loose bolt could result in a warped chainring.

Follow the manufacturer’s instructions for correct chainring orientation. Usually a small tab protrudes from the outside of the large chainring, which should be lined up with the inside
of the right-hand crank arm to prevent a dropped chain from jamming between the arm and large chainring.

The number of different bottom brackets standards available has created a minefield. However, just because your desired bottom bracket doesn’t directly match your bike’s BB shell type, don’t despair. Your LBS can guide you through the plethora of conversion kits now on offer.

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