Drag Measurement on HPVs
Introduction
History
Although I am a physicist, I must admit that I don't know very much
about aerodynamics. I entered this field after a ride with my BikeE
with panniers: the drag was obviously much higher than without
panniers. At about the same time Angus
Cameron <angusc@nait.ab.ca> posted a proposal for drag
measurements to the HPV-mailinglist [1].
Angus has also written an article about this topic [3]. I had the hardware available (my bicycle
tachograph, which is described on an other web
page) and decided to try it on my HPVs (Human Powered Vehicles).
This attempt resulted in a dedicated hardware and an evaluation
software, both described in these pages.
About the Subject
Let us define the drag to be the force that acts on the HPV if you are
not pedalling. This drag is a combination of several contributions:
- rolling resistance
- aerodynamic drag
- wind
- slope in the path
The first two of these can be attributed to the HPV, the last two are
due to external influences (although the sensibility of the HPV to
these influences is also one of its features). Our goal is to measure
the coefficients of rolling resistance and aerodynamic drag on HPVs.
How these coefficients are defined and how they are obtained from
coastdown data is explained in the section "Theory
of Operation". In a further theory section "Error
Estimate", the effects of wind and slopes in the path are
investigated. The measurement techniques have been applied to two HPVs
in the first place: a BikeE and a Specialized upright. Some time later
the results for a Kingcycle and a Ross Trice have been added. The
diagrams for the speed dependent drag and tables for the coefficients
of drag can be found in the section "Results of
the Measurements". The next two sections describe the specific
hardware ("The Data Logger") and how the
data is evaluated ("The Evaluation Software").
The final section "Future Developments"
outlines some possible improvements of the technique.
Reader's Guide
There are three possible types of readers I can imagine:
- those who are only interested in the results
- those who want to do measurements on their own HPV
- those who are interested in everything
If you are of the first type, the section "Results
of the Measurements" and maybe "Future
Developments" should be sufficient for you.
If you want to start measurements of your own, you should also read
the (rather short) sections about the hardware
and software and maybe download the circuit
diagram and the source code. If you have a bit of mathematical
background you can read the two theory sections "Theory
of Operation" and "Error Estimate", too.
If you are of the third type, I can't stop you from reading it
all....
Disclaimer: These pages will definitely contain some errors
(I'm only human). If you find one, please let me know.
Sections:
| Introduction
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Author: Christian
Starkjohann <cs@hal.kph.tuwien.ac.at>