TT torso angle and power?

Discussion in 'Power Training' started by morning wood, Jul 7, 2007.

  1. morning wood

    morning wood New Member

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    Some recent side on shots on my TT bike revealed what I thought that I need to get lower. Getting a reference horizontal line and projecting a straight line gave a torso angle of 16 degrees from the horizontal. As I had no spacers to remove from the front end I’ve fitted an inverted mtb 70mm riser stem to get the bars lower this has resulted in a torso angle of 11 degrees. I’ve ridden in this position and it doesn’t seem too bad but not raced in it yet. I guess I could lower still with a longer stem but what’s an acceptable angle? I guess torso parallel to the ground would be good!!!

    In the absence of a wind tunnel etc is there anyway of knowing what the reduced torso angle/frontal area is worth with respect to TT times/power?

    Cheers.
     
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  2. jbvcoaching

    jbvcoaching New Member

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    You could take frontal shots and calculate FA, then plug in the changes in FA (assuming fixed Cd) here:
    http://analyticcycling.com/ForcesSpeed_Page.html

    I suppose you could also assume a certain FA right now, then project how much less it will be as you lower your front end by certain amounts.

    As far as power goes, most people tend to produce the same power as they drop lower and lower, until they hit a certain point...beyond which it tends to fall precipitously. That point is individual for everyone, and my experience is that you can gradually get lower over time with specific training in lower positions. It's easy enough to keep dropping your front end and testing to find that point.

    The trick, of course, it to find the fastest combination of power & drag. I've generally approached it from the perspective of finding the lowest FA, confirming the drag of that position with field testing, then tweaking things (and of course, training in that position) to maximize power in that position. I really only lose at most 5% compared with similar all out test efforts on the road bike, and that's worst case scenario.

    When it's all said and done, I've spent lots of hours on this over the last few years. If you value your time at all, or you don't find the process interesting, a wind tunnel can be much more cost and time effective, assuming sufficient expertise in the tunnel staff.
     
  3. AndROOb

    AndROOb New Member

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    Totally agree with jbvcoaching as this has been my experiment this season. What I did find equally interesting was that when I dropped my bars to a level where I lost power, I still went faster! IOW, the aerodynamics outweighed the power output.

    Another interesting point I would make is that as I got lower at the front, the easier and more comfortable I found it was to have the arm-rests set closer together. I put this down to there being less pressure from the upper arms on the rib-cage, which made breathing less restrictive. This is a problem I had when my torso was not so horizontal, and consequently, had my arm-rests set wider apart.
     
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