This article is from the Misc Bicycles FAQs, by various authors.
Indexing is useful for quantifying ride difficulty. However, one must be aware
of the assumptions above. If a ride consists of relatively steep upgrades
followed by gradual downgrades, one will get too much credit for the hills
because one might conceivably coast on the downgrades and use very little
energy. However, most rides cover a variety of terrain with steep hills and
gradual hills. Too much credit earned on gradual hills will tend to be
balanced out by credit lost on steep hills.
As one's physical condition improves or slackens leading to an increase or
a decrease in the normal flat-land riding speed, it may become necessary to
re-evaluate D, the cyclist's divisor.
To estimate accurately the effect of wind is very difficult. It is generally
accepted that for flat loop rides where there is a constant wind vector, the
larger that vector the more difficult the overall ride becomes. Since most
rides are not ridden under such constant conditions, it is difficult to
determine an appropriate handicap for the index of a windy ride.
For example, a cleverly planned ride might "catch" the tailwind in one directio
with the return trip along a tree-lined road or behind a mountain so that the
headwind is blocked or reduced. In the absence of clever planning, a handicap
of 5-40% may be reasonable depending on wind speed and typical riding speed.
Considering all the assumptions and approximations used in determining a
usable index, we have found consistency in our index average speeds (irp) and
while-moving index average speeds (mirp) between rides with different amounts
of climbing and distance without adding a handicap on windy days. Of course,
we plan rides to avoid headwinds as much as possible.
There was and maybe still is a computerized wheel hub on the market that
calculates power delivered to the rear wheel. This might be the best way to
determine how many Calories are actually delivered to the road. Most everythin
but body efficiency would be taken into account: hills, rolling resistance,
headwind, tailwind, etc.
We welcome readers' comments and suggestions. Please send email to the
address at the top of this article.
The program, bike_power, (by Ken Roberts, firstname.lastname@example.org) with
documentation can be obtained by anonymous ftp from draco.acs.uci.edu
(126.96.36.199) in the directory: pub/rec.bicycles. ftp'ers are requested to
restrict access to 7pm-7am Pacific time.
Copyright 1992, Bill Bushnell and Chris Hull. Feel free to make copies and
distribute this article, but please don't claim it as your own. Thanks.