iBike iAero Generation III, Справочное руководство

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Copyright Velocomp LLP 2008-2010
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HOW TO MEASURE SNAPSHOT CDA WITH THE
IAERO
What is Snapshot CdA Measurement?
Imagine that you’re riding your bike but that you are coasting and your body isn’t moving.
Your feet are stationary, your riding position isn’t changing, and your clothes are the same.
It’s as if someone took a snapshot of you and your bike.
In this fixed position your bike and body present an unchanging (that is, fixed in time)
barrier to wind and friction.
Snapshot CdA is the aerodynamic coefficient measured by the iAero whenever you’re not
pedaling and you’re holding your body fixed. This is the kind of static CdA measurement
performed in wind tunnels.
Snapshot CdA measurement can be performed by the iAero whenever you coast for 10
seconds. While coasting you hold the same position and after 10 seconds the iAero reports
your Snapshot CdA number. If, say, you perform your Snapshot CdA measurements on a
hill, then you can coast down the hill, experimenting with ride positions, equipment, etc.
and the iAero will report your Snapshot CdA measurement on your iAero screen. Since the
hill is the same you can ride back to the top of the hill, change ride positions, coast down
the hill, and see how your CdA changes.
How does Snapshot CdA measurement work? Whenever you stop pedaling the iAero
knows
the following: applied force (zero, because you’re not pedaling), opposing hill slope force
(the iAero measures this), iAero-measured opposing acceleration force, and iAero
calculated opposing frictional force (determined from the frictional drag coefficient
measured during the calibration ride). So, the only remaining opposing force is from the
wind. Since the iAero measures wind speed, it can calculate WHAT aerodynamic drag
coefficient is required to make applied force = opposing force.
Now that you know the theory, here’s the practical how-to:
Measuring Snapshot CdA Drag Coefficients
1) Find a quiet place where the wind isn’t howling, trucks and cars aren’t screaming by
you, and the road surface is like the one where you did your calibration ride.
2) Ideally, this spot is a downhill, so that you can let nature help you coast (gravity
pulls you down and speeds you up), and nature slows you down (opposing wind). If
you don’t have a downhill available that’s OK: just find a place where you can go
fast, then coast for 10 seconds or more.
NOTE: DURING THE CdA COAST DOWN YOU CANNOT APPLY BRAKES, TURN SHARP
CORNERS, OR BE BUFFETED BY WIND GUSTS FROM PASSING VEHICLES. IF ANY OF
THESE HAPPEN YOUR MEASUREMENTS WILL BE INACCURATE.
3) Select the power window on your iAero screen
4) Go to your starting point, speed up to at least 20 mph, then stop pedaling. It’s ok if
downhills make you continue to speed up more; the iAero will figure out the forces
properly.
5) Hold the same riding position while coasting. Do NOT move your pedals while