Bicycle Bottom Bracket Evolution

Bottom Bracket Evolution: In pursuit of stiffness and light weight…

Remember the Magic Motorcycle bicycle crankset of the early 1990s? If not, it may be more familiar as the design marketed under the Coda brand by Cannondale from 1993 until the end of the decade – and the millennium – when the Connecticut firm introduced the BB30 oversized bottom bracket shell and axle assembly still used today with its System Integrated (Si) Hollowgram crankset…

BB30 bottom bracket (press fit in this instance) and
Hollowgram cranks on an aluminium Cannondale

While Si’s hollow aluminium cranks still follow the construction pattern established by their predecessors, BB30 differs in one significant respect from the Coda bottom bracket design, which was arguably the most prescient in recent cycling history.

Conceived by Magic Motorcycle Co.’s Alex Pong, the Coda design placed the bearings outboard of the bottom bracket shell in order to make for room for an oversized aluminium axle. It did so because Pong’s aim – and that of the designers of alternative bottom bracket formats who followed – was to surmount the limitations imposed by a bottom bracket shell diameter chosen some 100 years earlier. This, the original “standard” on which more recent designs attempt to

Square taper axle crank interface

improve, is either 34.9mm or 36mm, depending on whether it is BSC or Italian threaded. 68mm wide for road bikes, the conventional bracket shell is big enough to house a solid steel axle rigid enough for racing, turning in acceptably durable ball bearings. Even with a small hole bored through the middle, the axle is more than adequate to the task; until very recently, top track sprinters still favoured the anvil reliability of the old-school square taper steel axle when subjected to their enormous power and explosive delivery. Used as part of a machine that was relatively flexible and heavy everywhere, the traditional steel axle played its part well. But, as bicycle engineers began with the advent of new materials to apply fresh thinking to the racing bicycle, the whole bottom bracket area became something of a stumbling block to progress.

Put simply, the standard layout limited designers’ scope to play with technical developments that might improve performance. Self-evidently, high-performance bikes need to be light, but they also need to be stiff to ensure that as little as possible of the rider’s effort is lost to flex. A nearly flex-free transmission is critical for efficient power transfer, and the bottom bracket system – axle, bearings and shell or housing – is at the heart of the transmission. It affects performance for reasons related to axle stiffness, to bearing life, to the security of the crank-to-axle interface and to the extent to which the bracket shell can contribute to overall frame stiffness.

KCNC’s bearings and axle assembly

The typical pedalling downstroke applies force to the crank from various directions as the pedal travels through its arc. While all cyclists to some extent apply radial force, which bows and either extends or compresses the crank, the force that actually turns it is applied at a tangent to the pedal circle.

This tangential force deforms the crank by twisting and bending along the axis between pedal and bottom bracket axles and applies a torque or turning force to the crank.

When it is applied to the right hand crank, its attachment to the frame need only constrain the crank to rotate in a precise arc around the axis. Given that the right-hand crank is generally integral with the spider, it is in practice rigidly attached to the axle.

The axle transmits the force applied to the left-hand crank to the crank spider and through it to the chainring. For efficient power transfer the axle itself must be torsionally stiff enough to transmit the left-hand crank’s torque without twisting excessively along its length.

Fat axle, thin splines: Hollowtech II

It must also be properly supported by the bottom bracket bearings, which should be placed as widely as possible to support the axle against bending; axle overhang is undesirable as it permits distortion and implies the existence of excess material.

The bearings in turn rely on the rigidity of the frame to locate the bottom bracket shell so that it cannot move sideways under pedalling loads.

While not strictly part of the design of any of the various bottom bracket standards, the interface between crank and axle has nevertheless influenced their development. Before there was any point in lightening the near-solid steel axle, something had to be done about the traditional “square-taper” interface.

Introduced over half a century ago to replace the then-archaic cottered crank, square-taper is simple and provides a secure connection in all planes between crank and axle. However, it is, of necessity, of sturdy build. The wedge action of the taper generates severe tensile stresses in the crank head, as do the flats of the axle as it tries to rotate inside the crank. Add that the square orifice incorporates sharp corners susceptible to crack initiation and this part of the crank itself must be of substantial build to ensure durability.

The answer is some sort of splined interface, which can be designed to avoid the wedging effect and which, because it transfers torque without generating excessive radial stresses, requires less material to be used in the crank head.

Because deeper splines are better in terms of their durability, both Shimano’s Octalink concept and the 10-spline “open standard” ISIS Drive® design it inspired had axles of larger diameter than seen with square-taper. The more recent Hollowtech II system revolutionised crankset design by making the oversized axle a permanent, rigidly-attached part of the right-hand crank assembly, removing the need for a separable, heavy and potentially flexible interface. The left-hand crank slides over 38 fine splines, two of them double-width to ensure crank alignment, on a non-tapered axle. The large number is need to compensate for the fragility of the shallow splines dictated by the thin axle walls.

Once snug against the bearing face, it is secured using bolts that pinch the crank around the axle. Alternative approaches using the same fixed axle layout include Truvativ’s GXP. Campagnolo’s Ultra-Torque system, which employs a half-axle embedded in each crank, may be

BB30 and KCNC axles side by side

considered a variation on the theme. The problem with cartridge bearings is that, in order to accommodate a fatter axle, smaller balls must be used, resulting in reduced load-carrying ability and durability. The 7700 Dura-Ace group’s Octalink bottom bracket assembly addressed this by having both ball and, to take the big radial loads, roller bearing elements. While reasonably light, it is complex, tricky to install and expensive to manufacture.

Simple all-in-one cartridge-type Octalink and ISIS Drive® BB assemblies with small balls have proven less durable than similar products with square-taper axles. All cartridge type assemblies suffer from having their bearings located inside the cartridge and a long way from the crank interface.

By placing the bearings outboard of the shell, the Coda design put them as far apart and as close to the cranks as realistically possible, reducing bending loads on the crank to virtually nil. This was the arrangement adopted by Shimano for the original 10-speed Dura-Ace groupset of 2003. At its heart was a novel crankset turning in bearings placed in housings that screwed into the bottom bracket shell.

Named Hollowtech II, the system uses 6805 cartridge ball bearings with an internal diameter of 25mm. A 24mm axle needs a 0.5mm sleeve. This bearing size is now the de facto standard for “outboard” bearings in external housings that thread into the bottom bracket shell. It is also used where a manufacturer offers a frame designed to take a crankset with 24mm diameter axle but fitted with net-moulded housings for 6805 bearings.

BB30 bearing assembly

Back in 1992, Magic Motorcycle’s key design innovation had been to locate the bearings outside the bracket shell, allowing them to be of larger external and, critically, internal diameter. The bearing size chosen, 6806, has a 30mm bore, which is big enough to house a durable aluminium axle. The problem for the Coda design was that the outboard aluminium housing was wider than ideal, increasing “tread”, or Q-factor, and reducing heel-to-crank clearance.

When it came to the development of the BB30 system, Cannondale decided to place the bearings closer together to improve heel clearance and, by shortening the axle, reduce torsional flex. To do this, Cannondale’s engineers had to place the bearings inside the bracket shell, which meant making it significantly bigger than the traditional threaded shell. BB30’s 42mm OD bearings are a press fit in the shell, which must be accurately machined.

Outboard cartridge bottom brackets

“A 30mm diameter aluminium

axle of the same weight is

about 40 percent stiffer than

a 24mm steel axle…

The BB30 axle is about 20

percent shorter – and will be

lighter and stiffer by the

same amount.”

It is inevitably larger than that of a regular threaded shell, which means the surface area available for the attachment of frame spars is increased, allowing the frame designer scope to increase frame stiffness around the critical bottom bracket area. In the case of BB30 and its PressFit variant, this scope is limited by the width of the shell, which stays at the standard 68mm. More recent formats have attempted to improve on BB30 by increasing the width of the bottom bracket shell to give even greater surface area, but won’t work with BB30 cranksets since their axle limits them to a 68mm-wide shell.

Sitting at the very heart of the bicycle, the bottom bracket axle was, until very recently, made in high-tensile steel. Strong and rigid, steel is also heavy. Early attempts to replace it with something lighter ended abruptly with Laurent Fignon’s fall near the end of the 1982 edition of Blois-Chaville, as the Paris-Tours classic was then known. Riding solo with some 12km to go and a realistic hope of victory, Fignon was left sprawling on the road when his titanium Campagnolo Super Record axle snapped. Faced with a PR disaster, Campagnolo immediately dropped the titanium axle and went back to reliable old steel.

Given that aluminium alloys are particularly unforgiving of poor design and inappropriate application, Magic Motorcycle’s decision to ignore conventional wisdom and machine their bottom bracket axle from the lightweight material was a significant move. With a diameter of 30mm, an aluminium axle can be made stiffer, stronger and lighter than a conventional 17mm thick steel axle. The size increase boosts bending and torsional stiffness to the point that fatigue failure, a major problem with aluminium, is virtually obviated.

Shimano’s 24mm axle is offered in steel and aluminium, the steel version being substantially lighter. It is marginally heavier and less stiff than the aluminium BB30 axle but fits inside the smaller, lighter 6805 bearing. Also 24mm in diameter and made in steel, the Truvativ GXP axle may be expected to offer similar performance.

How much twist? A rough calculation says that a 20kg force applied at a tangent to the left-hand pedal twists a conventional 17mm steel axle by a little more than one quarter of one degree. Working with axle length between crank attachments and assuming 1mm wall

Short, wide, green and stiff: a BB30 axle

thickness, a steel 24mm axle (as used by Shimano and Truvativ) is about 16 percent stiffer and roughly the same weight. A 30mm diameter aluminium axle of the same weight  is about 40 percent stiffer than a 24mm steel axle of the same length – in this case 90mm. The BB30 axle is about 20 percent shorter – and will be lighter and stiffer by the same amount.

Words by Richard Hallett