Buyer's guide: bicycle geometry

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Buyer’s guide: bicycle geometry

A step-by-step guide to the tubes, angles, and profiles of the chassis of your next bike


Seatstays

‘Compliance’ is one of the most frequently used buzzwords in cycling and is attached almost exclusively to seatstays. The tubes that rise from the rear axle to the back of the seat-tube are notorious for transferring road shock to the rider, and designers and marketing departments have focused intently on varying methods to reduce it.

The designer’s goal in using the seatstays to engineer comfort is to “disrupt the path of shock transmission,” Olsen says, but he is skeptical about the effectiveness of curved or s-shaped seat stays, particularly on carbon frames, where the carbon lay-up will have a significant effect on stiffness.

Seatstays are frequently identified as a means of introducing the softening quality of ‘compliance’ to the ride

BMC’s shallow seat stays, where the stay joins the seat-tube someway below the seat clamp, is a design he believes could deliver compliance. For Olsen, greater flex occurs in the seatpost and a compact frame design is an excellent means of harnessing it.

Gribaudo believes that seatstays can “definitely” deliver compliance and with it comfort, and believes the trend for very thin, and even flattened seatstays, can help to filter vibration. For Steward, working predominantly with steel frames, wall thickness has a greater impact than shape on the ability of the seatstays to offer compliance. “Available wall thickness will be the deciding factor in deciding how small in terms of diameter you can go in terms of maintaining a safe threshold,” he says.

Genesis are exploring other methods of achieving the cherished softening quality, he continues, such as the deployment of Shimano’s ‘direct mount’ rear brake, one that attaches to the frame beneath the chain stay and frees the designer of the need to include a brake bridge that unites, and so stiffens, the seatstays. “For us, this would mean being able to flat ovalise the whole length of the seatstays, and you wouldn’t then have to take into consideration the shape of the material for the bridge.”

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