A4 scale = 70%

Cycle transmission notes page 4

Bottom brackets

'Bottom bracket' is the strange name given to the axle assembly upon which cranks turn. Modern 'sealed' ones have replaced the separate axles, cones and balls of yesterday - they don't need any maintenance and usually give excellent service for many years. There are three main designs and you must select the one which matches your cranks:

Square-taper bottom bracket

There are two versions of this design. Both have 2° tapers but JIS (Japanese) ones are slightly fatter than ISO (European) ones. So a crank goes further onto an ISO axle than a JIS one. They can often be mixed but check that each crank covers at least 2/3 of its axle taper without bottoming on the end.

Shimano 'Octalink' bottom bracket

Shimano have produced 2 versions of this design. They are incompatible with each other and with every other make of crank.

'ISIS drive' bottom bracket

Arguably a better design than both the square-taper and Octalink. The axle is stiff and the drive is positive.

Other bottom brackets

Alternative systems are being offered by some manufacturers, including outboard bearings, splined cranks secured to splined axles with pinch-bolts, and cranks with axles permanently bonded into them. Each design has its merits but none has yet achieved the status of becoming a long-term standard.

Chainline - selecting the right axle length

Whichever system you use, select the axle length which gives the correct 'chainline', placing the mid-point of your chainrings directly in line with the mid-point of your sprockets (within a millimetre or two).

When fitting a new crank, put it on your old bottom bracket first and work out how much longer or shorter the axle should be before buying a new one of the correct length. For example, a 10mm longer axle will increase the chainline by 5mm, assuming that the axle is symmetrical (which ISIS ones always are).

Fitting bottom brackets and cranks

If there isn't a drain hole in the bottom of your frame's bottom bracket shell, drill a 3mm one before installing anything. Otherwise water which will inevitably find its way into your frame will damage the bearings.

The units illustrated above are installed with a 'Shimano' splined tool, which is much easier to use than traditional peg- and pin-spanners. Before installing a bottom-bracket, clean and lightly grease the threads in your frame and the insides of the bottom bracket 'cups'; this aids assembly and ensures that nothing will seize. The left-hand cup has a normal right-hand thread and is installed by turning it clockwise, but the right-hand one has a left-hand thread and must be turned anti-clockwise.

There are two very important points to remember when fitting cranks:

• Always clean and lightly grease the axle tapers or splines before fitting cranks. This contradicts advice which is often given, but anyone suggesting that grease shouldn't be used either doesn't have much mechanical understanding or has a vested interest in shortening the life of your cranks.

• You must tighten the axle bolts adequately. If you have a torque wrench set it to 35 Nm, otherwise use a long spanner or a long Allen key as you won't tighten the bolts enough with short ones. Then re-tighten them after riding 100km or so - loose cranks move under load and damage their sockets beyond repair.

Similarly, clean and lightly grease the pedal threads and tighten the pedals firmly. If you place a thin 'pedal washer' on each thread first, you can prevent steel pedal spindles from damaging aluminium cranks.

© Chris Bell, 2006

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