The seat-tube, along with the top-tube, is likely to be the first item on the designer’s drawing pad. The length will be determined to a large, ahem, degree by the angle at which it is set, and the overarching design philosophy: whether the designer has opted for a ‘classic’ geometry, with a straight top-tube, likely to be identical in length to the seat-tube, or a ‘compact’ or even ‘semi-compact’ geometry, where the top-tube slopes downwards towards the seat-tube, and in which the top-tube is likely to be longer than the seat-tube.
Designer Mike Burrows pioneered compact geometry, unleashing it upon the world in the Giant TCR bikes of the mid 1990s. The smaller triangles offer weight-saving benefits, and the ‘smaller’ (lower) frame that results is arguably easier to handle. Lapierre opted for a semi-compact geometry across its entire road range some time ago, Gribaudo says, and explains that the company’s purpose in doing so, he adds, was to offer consumers a greater chance of finding a bike that fits. “When we start the geometry of the frame, we always start with the seat-tube length and top-tube length,” he adds.
Furthermore, the designer is likely to set the seat-tube angle in tandem – and sometimes literally in parallel – with the angle of the headtube. There is a consensus among Steward, Olsen, and Gribaudo that while the headtube angle has a direct influence on performance, the designer’s primary concern in setting seat-tube angle is comfort. “It just comes down to ergonomics really,” says Olsen.
“You’re trying to get the rider balanced on the bike,” he continues. “You don’t want all of their weight supported by their arms, and you don’t want the front end to be too light. Seat position is a big part of that. The lower the front end goes, the steeper your seat angle would need to be; the higher, the more relaxed it needs to be.”
With proper adjustment of seatpost and the positioning of the saddle on its rail, a rider can achieve the optimum position delivered from a 73-degree angle with a seat-tube set at anywhere between 71 and 75 degrees, he believes.