Copyright Velocomp LLP 2005-2014
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more accurately than GPS or barometric pressure methods.
HOW ACCURATE IS THE HILL SLOPE MEASUREMENT?
Your Newton uses an accelerometer that measures hill slope and bike acceleration
simultaneously. When your bike is at rest or you ride at a constant speed (in either case
there is no bike acceleration) the hill slope displayed on your screen is very accurate
(within 0.1% of the actual slope). When you are accelerating on your bike (going faster,
coasting on a downhill, or braking) the hill slope may jump around a bit because the
accelerometer not only senses hill slope but bike acceleration as well. In this circumstance
the hill slope displayed on the screen will be only approximately correct (generally within
1% of actual slope). NOTE, HOWEVER, THAT WATTS ARE CALCULATED WITH HIGH
PRECISION UNDER THESE RIDING CONDITIONS,
including those where the hill slope
displayed is “off”.
WHAT ARE TYPICAL VALUES OF “CDA” AND “CRR”?
CdA (coefficient of aerodynamic drag) is dependent mostly on ride position, rider height,
and rider weight. Lower CdA means less effort is required to overcome opposing wind.
The time trial position results in the lowest values of CdA: numbers between 0.22 and 0.28
are typical. The specific value will depend considerably on the details of bike type, rider
position, and rider size.
The “drops” position is not as aerodynamically efficient. Values between 0.32 and 0.38 are
typical.
The “hoods” position (hands grasping the brake levers) is the most typical riding position.
CdA values of 0.34 to 0.43 are typical.
The CdA of a cyclist riding a comfort bike can be 0.45 or higher.
Crr (coefficient of rolling resistance) measures the amount of opposing friction caused by
road surface, tire type, and bearing friction. A low value of Crr means low rolling
resistance.
On a very smooth surface, such as those found at an indoor track, Crr can be 0.003.
Asphalt roads have a typical Crr of 0.0055, and dirt roads can have Crr values of 0.008 or
higher.
Values of CdA and Crr are accurately estimated by ISAAC Dial-in Wizards, or they can be
measured by the Coast Down procedure.
WHAT DOES “HEADWIND” AND “TAILWIND” MEAN?
The wind speed displayed by your Newton is the magnitude of ambient wind blowing
directly in your face,
net of the wind caused by your pedaling.
A few examples make this
easy to understand:
•
You ride at 15mph directly into a 4 mph natural headwind. The Newton will read +4
mph headwind.
•
You’re going 20mph and there is a tailwind of 7 mph. Your Newton reads
-7mph (it knows you’re getting a bit of a free ride from mother nature!)
•
You’re riding in the peloton at 20mph and the natural winds are calm. However,
because the riders ahead of you are pushing the wind, your Newton will likely read
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