Anatomical Considerations in Bike Fit: Fitting the Machine to the Athlete – Part IV

In the last three articles in this series on bike fit (part 1, part 2, part 3) we examined how to set up your ride to provide the most neutral, comfortable all day position. I think of setting up a road position in the same way than an ergonomist would set up an office, that is, with the assumption that you’re going to be putting a lot of time in and that small areas of discomfort can become big areas of discomfort and can limit your long term performance when repeated hour after hour, day after day, year after year. But…

The cyclist differs from the office worker in one significant way — it’s unlikely that the coefficient of drag multiplied by the office chair’s frontal area is going to have a significant effect on job performance. In the office chair — just as when riding a bike in a pack or up a hill — comfort rules. However, when riding solo, sticking your nose into the wind, both comfort and aerodynamics are similarly important.

So, what modifications can we make to our “all day” position to strike the best balance between comfort, power and aerodynamics?

The first and most significant change that we can make is to increase the seat angle of the bike so that, while maintaining the most powerful joint angles of the rider, the cyclist pivots in space to bring the head down, bum up and decrease the frontal area. For an average 6ft tall cyclist, moving from a 73 degree seat angle to an 80 degree seat angle will decrease the frontal area by ~.03m^2, leading to an increase in speed of ~0.8km/hr for the same power output of 200W on the flat. This may not sound like a lot, but over an ironman bike, this equates to a difference of 7 minutes in the bike split for the same power output. In the previous article, I offered some rules of thumb with regard to how bike geometry changes with increasing seat angle. Namely: “For seat angles other than 73 degrees, for every 1 degree, add 1.5cm of drop. So, a 78 degree frame should have the bars set ~7.5cm lower than a 73 degree frame for the same athlete.”

I would suggest using this as a starting point when setting up your TT bike to match your road. Diagonal cockpit will stay the same however horizontal cockpit will obviously be shortened with the increased drop.

It is worth reiterating that this change alone has no bearing on the optimal body position that we have arrived at through calculations in the previous articles. The athlete is simply rotated in space. However, the other changes that typify the time trial or aero position do involve alterations of the cyclist’s body position from the most comfortable position that we’ve arrived at. In this case, the athlete shifts away from the neutral back angle and adopts a crunched back, protracted shoulder position to decrease his/her frontal area even further via the use of aero bars. In doing this, there are a couple of key considerations.

First and foremost, the athlete does not want to significantly alter the most powerful hip position that they have arrived at in setting up the road position, that is, any bending that occurs to get the front end lower should not come from closing off the hip angle at the hip joint, but instead should come from increasing the seat angle (as outlined above) and bending the spine so that it is more convex. In your flexibility tests, you did a measurement for the vertical difference between a straight back and bent back position. This is the maximal amount that you should consider increasing the drop of your bars in an aero position without negatively affecting your hip angle.

For example, if you found that the maximal vertical drop that you could accommodate while bending your back and still being able to look ahead is 15cm, then your elbow position in your TT set up should be no more than 15cm lower than what it is when riding your TT bike on the bull horns. There is a point of diminishing returns in thoracic flexion. While there may be some minor benefit to the CD side of the CDA equation, in making yourself more rounded, once your upper back is the highest point, there is no benefit to your frontal area by bringing the head even lower. If the head is the same height as the upper back, for all intents and purposes, this is as bendy as you want to be.

In addition to increasing seat angle, the greatest improvements to frontal area will come from improving hip flexibility. For our prototypical 6ft guy, an improvement in hip flexibility to take the open hip angle from 115 to the “norm” of 90 degrees will decrease frontal area in the aero position by .05m^2, representing an improvement of ~2km/h or 14 minutes over a flat ironman bike split at 200W. Bring on the yoga!

Hopefully, you’ve gained something from this different perspective on bike fit that focuses on building the bike around the athlete rather than vice versa. As a lifelong student of human movement it is clear to me that there are so many factors other than stature that make us true individuals in the way we move. It seems fitting that we should consider these factors of individuality when trying to best interface athlete and machine

Train Smart.

Categories: Planning

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Alan Couzens

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