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  • Journal
    Bicycles
    28.11.14

    Desire: Cannondale Hollowgram SiSL2 chainset

    The story behind the latest iteration of Cannondale's game-changing Hollowgram turbine

    Words
    Timothy John
    Photographs
    Jon Denham

Innovation and heritage can make curious bedfellows. Cannondale’s latest Hollowgram SiSL2 chainset, however, has its roots in a project begun without limits of time or cost more than 15 years ago.

Cannondale began to explore the possibility of improving the interface of crank and bottom bracket in 1998. Many other aspects of the bicycle frame and its relationship to supplementary components had advanced: fork steerers had broadened, headsets had taken a great leap forward in the move from threaded to the cartridge units typically used with A-head stems, but Cannondale believed that shell size was applying an unnecessary impediment to bottom bracket development.

The SI (System Integration) bottom bracket was launched to the public in tandem with the CAAD6 frame in 2000, with some 20,000km of testing completed, courtesy of the Saeco-Cannondale team, who had been the first to use a technology whose development had begun two years earlier.

Bicycle crank, black, Cannondale Hollowgram SiSL2 chainsetThe SiSL2 is the latest iteration of Cannondale's ground-breaking Hollowgram chainset. Cannondale's concern for 'backwards compatibility' means that it can be used with frames dating back to the CAAD6. pic: Jon Denham

Chris Dodman, a senior project engineer in Cannondale’s Advanced Products Division, is steeped in the company's history, and was a leading light in the development of its BB30 bottom bracket standard. He joined Cannondale in 1997 after working for three years in the early to mid-1990s with British crank specialists Middleburn on their flagship ‘RS’ mountain bike chainset, moving to Connecticut to share a house with other Cannondale engineers during a golden period for the brand of innovative design.

While the hold on BB development applied by shell size has certainly been released by BB30, the proliferation of standards that has followed in its wake has led many to regard its development as a not entirely unmixed blessing. Dodman defends Cannondale’s position with the significant justification of the firm’s willingness to share the standard it had created; a step taken in 2006, during a period in which technological advance had prompted wholesale changes to the record industry and a culture of ‘sharing’ music.

The threaded BSA standard that BB30 was largely intended to replace survives. Many retain a preference for ‘external' bottom brackets, both for their low cost and ease of replacement. Dodman points out that this wasanother standard in which Cannondale was intimately involved, having marketed under its CODA brand a design pioneered by Magic Motorcycle. The levels of performance offered by BB30, however, both in weight and stiffness, are simply not possible with the traditional standard, he maintains.

bicycle crank, black, Cannondale Hollowgram SiSL2 chainsetCannondale's senior project engineer, Chris Dodman, noted 10 radial spokes on the wheels of high-performance cars and motorcycles, a design that delivers low weight and which can withstand high torque applied in a single direction. pic: Jon Denham

Dodman offers the oversized spindle (its 30mm diameter accounts for the ’30’ in the BB30 nomenclature) as a case in point. To enjoy the weight savings offered by replacing steel with aluminium, it is necessary to increase the size. He draws a parallel with the performance of early, narrow-tubed aluminium frames from Allen and Vitus and contemporary oversized equivalents (he leaves unspoken Cannondale’s reputation as class leader in the development of aluminium frames; the latest CAAD10 provides a timely reminder for any who believed the brand had abandoned the material entirely in favour of carbon).

Both iterations of the Hollowgram chainset offered by Cannondale between 2000 and 2012 had the same 60g aluminium spindle, some 50 per cent lighter than a steel equivalent in the same diameter. It’s longevity might be taken as evidence of how far ahead of the game it was on release at the start of the new millennium. Arriving at the most effective spindle design is a matter of optimising three parameters, Dodman explains: weight, stiffness, and Q-factor (essentially, the distance between the crank arms’ pedal threads, when measured across the bottom bracket). “Seventy per cent of deflection in the left-hand crank comes from the BB spindle twisting."

Bicycle crank, black, Cannondale Hollowgram SiSL2 chainsetThe SiSL2's CNC-machined crank arms are fashioned in two parts from high-grade aluminium and bonded. Manufacturing of all the SiSL2's components takes place in Pennsylvania. pic: Jon Denham

Cannondale claimed significant improvements for the second iteration of its Hollowgram chainset, released in 2007. With 2012, however, came a new development, one named, unsurprisingly, SiSL2. The chainset has many notable features (exquisitely machined crank arms, for example, fashioned in two pieces by CNC machine, then bonded, and which attach to the bottom bracket with tapered splines, like their predecessors) but none is as immediately obvious as the striking 10-arm SpideRing: Dodman’s design, based on an idea that came (as with all the best) while riding his bike to work

Passing various exotic vehicles and a Ducati dealership, Dodman noted that the wheels of high-performance cars and motorcycles each had 10 spokes. Deciding that this was unlikely to be the result of coincidence, and more likely to have resulted from serious consideration by the respective engineers, he began his own investigations.

The result is the beautiful chainset pictured, in which spider and two chainrings are machined from a single aluminium billet, delivered at a claimed 483g, and purged of the flex to which more conventional chainring-crankarm interfaces are subject (the SpideRing chainrings attach to the right-hand crank with a lockring, which, Dodman says, has been “whittled away over 14 years” to the current MK3 iteration).

The SiSL2's SpideRing design relies upon machining of spider and both chainrings from a single billet of aluminium. The striking design is matched by impressive statistics for weight and stiffness. pic: Jon Denham

“We’d done one-piece back in 90s with the Magic Motorcycle stuff,” he explains, “so we knew that it worked. We looked for inspiration in something that could handle high torque in one direction, was very light, and very optimised. I started looking at car wheels and noted a common theme of 10 radial arms. If you look at the McLaren P1, or the wheels on Porsche racing cars, they each have 10 spokes. Also, I pass a Ducati dealership on the way to work, which helped.”

The chainrings of the SiSL2 can be retro-fitted to any Cannondale Hollowgram chainset from the year 2000, and configured for a range of purposes, from an aggressive 53-39 road double to mountain bike triples and even single speed incarnations (increasingly popular in cyclo-cross, aping the trend for the 1x10 and 1x11 drivetrains that have become de rigueur in cross country mountain bike racing). A compact 50-34 is also available. Compatibility is one of the fundamentals of Cannondale’s Hollowgram chainsets.

Bicycle crank, black, Cannondale Hollowgram SiSL2 chainsetThe SiSL2 crank arms are the latest iteration of a design debuted in 2000, and refined in 2007. The arms attach to the bottom bracket with flattened splines and, on the right hand crank, a lock ring. pic: Jon Denham

The Hollowgram SiSL2 further refined the Hollowgram crank arms, and extended the spindle length (increasing it  from 104mm to 109mm) to make it compatible with the revised BB30-73a version of the bottom bracket used on the Synapse, as well as with SRAM’s new Yaw-style front derailleurs. The new design maintains the ethos of ‘backwards compatibility’ – a commitment that Dodman describes as “all important”, and which he says would allow the SiSL2 chainset and bottom bracket to be fitted to a CAAD6 frame. All of its components are made in the same facility in Pennsylvania in which the original Magic Motorcycle cranks were manufactured.

For the SiSL2 and Spidering, Dodman (who was working on the frame design of Cannondale’s Synapse endurance bike in the same period) worked with Dan Connors, a senior engineer based in Cannondale's American R&D office, and who has been involved with Cannondale and its chainsets since the Magic Motorcycle days. The ubiquity of carbon makes it easy to regard aluminium as a material consigned to cycling’s past, for the highest levels of performance, at least. Cannondale’s Hollowgram SiSL2 chainset, however, proves that material is a secondary concern, after the vision of the designer.

Cannondale

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