Cervelo P2 carbon 56

77-78 degree seattube angles have become more and more common for time trial and triathlon bikes in the past few years. Whereas in the past time trialers often rode at more shallow angles in the 74-75 degree range, the situation is no longer as lopsided. Riders like Zabriskie, Voigt, Julich and many of their Team CSC colleagues now ride at 77-78 degrees, and if it weren’t for the restrictive UCI rules on saddle position, some might even ride steeper. Other teams are starting to catch on to this trend as well, and no wonder if you look at the results Team CSC garners in time trials.

But even if you aren’t a professional cyclist, there are compelling reasons to ride steep on a time trial bike:

1. With lower back flexibility often a limiting factor, a steep seattube angle allows you to attain a flatter back without having to change your body’s “internal” angles (hip angle, knee angle, etc). Basically, a steeper seattube angle rotates the rider forward around the bottom bracket, thereby leaving the internal angles intact while positioning the back more horizontal.
2. A steeper seattube angle can allow you to even open up your hip angle, thereby reducing the strain on your lower back as well as making the pedal motion less restrictive.
3. For triathletes, several studies have shown that running times after riding a bike decrease if the bike was ridden with a steep seattube angle.

That said, a steep seattube angle is not for everybody and it remains a personal preference, so we offer our TT/tri bikes with adjustable geometry, allowing for an effective seattube angle anywhere in the 74-79 degree range.

As you move through the range of effective seattube angles, the effective toptube length also changes.

Note that we say effective seattube angle, as the standard seattube angle is really a very inadequate dimension. It does not describe very well the position of the point that actually matters for fitting, the saddle. A 75 degree seattube angle with a large setback seatpost and the saddle pushed backwards is a lot less steep than the same seattube angle with a zero-offset post and the saddle forward. The variation between these two situations can be as much as 3 degrees, yet both are called 75 degree seattube angles. So to really be able to intelligently compare positions, it is better to use effective seattube angles, which is normalized for a standard seatpost setback and a saddle position in the middle of the range. In the example above, we would call the first situation an effective seattube angle of 74 degrees, and the second situation an effective seattube angle of 77 degrees. In other words, you can achieve the same position for the saddle (and therefore the rider) with different seattube angles, as long as the seatpost and saddle position are adjusted to create the same effective seattube angle.

In the geometry tables below we list the dimensions for 75 and 78 degree effective seattube angles, roughly corresponding to the two positions of the adjustable seatposts if the saddle is clamped in the center of the rails.