Originally Posted by x43x

I thought the offset seatpost wasn't for fit adjustment, but for comfort and positioning of the legs when pedaling. Most off-the-rack bikes are sold with offset seatposts aren't they? No setback seatposts are for those wanting a more aggressive and forward position over the pedals, resulting in more power. That is why time trial bikes have no setback.

Originally Posted by wikipedia

Road and triathlon bicycles

A road racing bicycle is designed for efficient power transfer at minimum weight and drag. Broadly speaking, the road bicycle geometry is categorized as either a traditional geometry with a horizontal top tube, or a compact geometry with a sloping top tube.

Traditional geometry road frames are often associated with more comfort and greater stability, and tend to have a longer wheelbase which contributes to these two aspects. Compact geometry road frames have a lower center of gravity and tend to have a shorter wheelbase and smaller rear triangle, which give the bike quicker handling. Compact geometry also allows the top of the head tube to be above the top of the seat tube, decreasing standover height, and thus increasing standover clearance and lowering the center of gravity. Opinion is divided on the riding merits of the compact frame, but several manufacturers claim that a reduced range of sizes can fit most riders, and that it is easier to build a frame without a perfectly level top tube.

Road bicycles for racing tend to have a steeper seat tube angle, measured from the horizontal plane. This positions the rider aerodynamically and arguably in a stronger stroking position. The trade-off is comfort. Touring and comfort bicycles tend to have more slack seat tube angle traditionally. This positions the rider more on the sit bones and takes weight off the wrists, arms and neck, and, for men, improves circulation to the urinary and reproductive areas. With a slacker angle, designers lengthen the chainstay so that the center of gravity (that would otherwise be farther to the back over the wheel) is more ideally repositioned over the middle of the bike frame. The longer wheelbase contributes to effective shock absorption. In modern mass-manufactured touring and comfort bikes, the seat-tube angle is negligibly slacker, perhaps in order to reduce manufacturing costs by avoiding the need to reset welding jigs in automated processes, and thus do not provide the comfort of traditionally made or custom-made frames which do have noticeably slacker seat-tube angles.

Road racing bicycles that are used in UCI-sanctioned races are governed by UCI regulations, which state among other things that the frame must consist of two triangles. Hence designs that lack a seat tube or top tube are not allowed.
A cyclist riding a time-trial bicycle with aerodynamic wheels and aero bars

Triathlon- or time-trial-specific frames rotate the rider forward around the axis of the bottom bracket of the bicycle as compared to the standard road bicycle frame. This is in order to put the rider in an even lower, more aerodynamic position. While handling and stability is reduced, these bicycles are designed to be ridden in environments with less group riding aspects. These frames tend to have steep seat-tube angles and low head tubes, and shorter wheelbase for the correct reach from the saddle to the handlebar. Additionally, since they are not goverened by the UCI, some triathlon bicycles, such as the Zipp 2001, Cheetah and Softride, have non-traditional frame layouts, which can produce better aerodynamics.