Originally Posted by smaryka .
But be careful chasing power on the TT bike if you need to alter your position in such a way to get it that you become less aero. Generally speaking, the faster you go, the more aero you want to be, so if it costs you 5% of your watts to be in that position but you're going faster than you would with higher watts and a less aero position, then it's a no brainer. With a powermeter and a decent flat windless testing loop you should be able to use aerolab in Golden Cheetah to test various positions and watts and find the sweetspot.
Also think about your head position, how well your helmet fits your position, how close together you can get your arms, etc. -- all the little wins that will add up to more aeroness. If you're doing a 21-min 10-mile TT then you're going fast enough to be more serious about finding those gains.
However I agree that time spent in the position is what will improve your power there, so once you have a good aero position dialled in, improving your power is just a matter of adjusting to it.
Thanks for this. I do a fair amount of aero testing using my 2011 Cervelo S2 road bike on the same closed course that my club does its monthly TT race series. The course is rectangular with a lap distance of .83 miles and the corners can be taken while pedaling. The wind on this course is usually very calm when I test at 7:00 to 9:00 A.M. and I use the local weather station information to compute the air density given the barometric pressure and temperature. Furthermore, there is very little elevation change (~7-9 feet) and the pavement is excellent. I have been testing mostly on my road bike with different wheels, water bottles (or none) and rider position on the drops, hoods, ect. I have found that even on my road bike that I ride all the time, that when I'm in the most aero position that my perceived effort and heart rate are higher at a given wattage. When I test, I use my SRM and try to keep power relatively constant across most tests (250W +/- 3%) and do a number of laps for each control or test. I always do a control test first with the same bike and equipment with the same rider position and I end a test with this position to make sure very little has changed with the environment over the course of testing.
As an aerodynamicist, I know the limitations of CFD, where one can "tune" the constants used in the turbulence models (one-equation or two-equation) giving the correct overall drag force, while if you test using pressure taps the pressure field around the cyclist or bike they will not match even closely the numbers found from CFD. This is because highly separated flows such as found with bicycle riders is very difficult to model theoretically and the errors associated with them is high. In addition, the environmental level of turbulence will affect the way the air flows around the cyclist and this is very hard to model adequately. This environmental turbulence will affect where and the mode at which the flow separates from the body (i.e. level of turbulence affects the growth rate of the factors involved with the instability that causes certain types of separation for both laminar and turbulent flow).
With this said and some reasons why I test as I do, I really value the data I get from my testing program at Hughes Park in Long Beach, California. Some changes I make to the bike are so minute that you can't see whether it works or not based on the error estimates involved with testing. For example, testing with a water bottle on the down tube verses no bottle or cage is very hard to determine at my usual power levels, which is 250W. However, when I test at 300 or 325W, sometimes I can resolve the data to determine whether it worked or not. However, it is much harder to test at 325W for multiple runs, so I use this type of testing only when I determine that at 250W that I'm having difficulty determining whether the change worked.
I just got my Cannondale Slice RS, so I haven't had much chance to test different positions on it. I've done two TT's of 10 miles at Hughes Park (21:28 at 325W and 21:37 at 327W average) and the only changes I made between the two TT's is for the second one I extended the stem forward one CM and lowered the aero bars 1 CM and adjusted the saddle up to match the road bike's saddle height and changed the saddle angle. I felt comfortable on the bike with the change with the exception that I induced a pressure point in the crotch region that made the ride less than comfortable. The bike came with a Fizik Arione saddle and I'm looking to change it to a TT specific saddle such as a ISM Adamo TT or similar.
Thank you all for your input and I will definitely keep testing and using your ideas to help me improve.