Certain areas of the bike attract certain buzzwords, and the phrase beloved of marketing departments when describing the efficiency of the chainstays on their latest offering is “power transfer”.
Olsen describes the purpose of the chainstay as “locking in place” the rear axle. “With every pedal stroke, the bike is trying to twist the rear axle. If you look from the top of the bike, it’s trying to twist the rear axle anti-clockwise and compress the driveside chainstay.”
The chainstay has an influence on the performance of the bike that belies its simple appearance. Its length and shape will depend much on the purpose of the bike. For a racing bike, says Steward, where the only consideration is performance, the general principal is that shorter is better: a bike with shorter stays will offer good acceleration and efficient climbing.
Bikes intended for other purposes, such as touring bikes, where heel clearance for panniers can be a consideration, may demand a longer stay, he continues. Clearance for wider tyres and mudguards is likely to be a consideration for any designer, except those working on racing thoroughbreds.
Olsen agrees that the greatest influence on the profile of the chainstay is the clearance between tyre and chainring: a gap he estimates between 10mm and 15mm. This constraint forces upon the designer “the exact opposite profile from what you’d want, which is a very wide, flat design.” Lay-up will offer the designer working with carbon a different route to stiffness, but for his counterpart designing metal frames “there’s a limit to how much you want to squash things.”
Like all areas of bicycle geometry, however, the margins are fine. Gribaudo’s Lapierre racing bike, the Xelius EFI, a machine deployed by FDJ.fr in the toughest races in professional cycling, has a 408mm chain stay. Lapierre’s Sensium ‘performance’ bike, one more likely to be selected by sportive riders, has a chainstay just 4mm longer. There are several reasons. As previously discussed, the shorter chainstay will give the racing bike greater acceleration. Conversely, the longer stay will give the ‘sportive’ bike more stability, courtesy of a longer wheelbase. Finally, the longer chainstay will offer greater tyre clearance, offering a further avenue to improving comfort.
Chainstays on carbon frames typically are significantly deeper than those on steel or alloy equivalents. The purpose of this greater depth is stiffness, says Gribaudo: the designer’s response to an area of the bike that, because of the necessity of accommodating the rear tyre, is necessarily thinner than a headtube or downtube. There is another, less obvious reason, too. “The carbon can be deformed many times without any influence on the structure, but the paint cannot,” he explains. “If you can make your structure stiff enough to avoid deformation of the paint layer, you will be able to save a lot of paint crack.”