Optimising CdA

[largeheadsmallbrain]

Has anyone experienced an increase in CdA while getting lower and flatter backed in the TT position?

I had been about 2in higher at the front, along with a slight upslope of the Aerobars. But dropping the front end to achieve a flat back and aerobars horizontal, the iAero suggests I was loosing time and had a higher CdA. I wonder if this is a true reading. It is perfectly possible that I am less aerodynamic overall depending on how the wind swirls round me.

The rides were on different days on out/back courses. Despite meticulous wind calibration there was still an inaccuracy in the average wind ( about 2.5mph and 3mph for both rides)

I think I'll have to do all tests on one day

[Steve_Davidson]

Has anyone experienced an increase in CdA while getting lower and flatter backed in the TT position?

I had been about 2in higher at the front, along with a slight upslope of the Aerobars. But dropping the front end to achieve a flat back and aerobars horizontal, the iAero suggests I was loosing time and had a higher CdA. I wonder if this is a true reading. It is perfectly possible that I am less aerodynamic overall depending on how the wind swirls round me.

The rides were on different days on out/back courses. Despite meticulous wind calibration there was still an inaccuracy in the average wind ( about 2.5mph and 3mph for both rides)

I think I'll have to do all tests on one day

Lower is not always better as from an aero perspective it sometimes puts your head in a less optimal position when wearing an aero helmet and can affect your shoulder shape / position. Going too low also brings in other restrictions and often results in a power shortfall.

It is always ideal to do your testing within a tight time frame on the same day to get conditions as close as possible.

cheers

Steve

[coachboyd]

In my experience I have found that making yourself more narrow (bringing in your arms and elbows), makes a much bigger difference than being lower. If you get lower, but have to open your arms to breathe properly then your cda could actually go up.

The iBike is a great tool for getting faster on the bike, but it can take a good day of aero testing to really find your optimal position. It's a good thing you don't have to pay 600 dollars an hour for a wind tunnel.