Oh, how I rue the time wasted on mere speedometers!
The birthday toy that I specified this year was a Windscribe
Ultrasonic Wind Meter, so at 3 p.m. I carved it out of its
horrible plastic display case and began playing with it.
http://www.davisnet.com/weather/products/weather_product.asp?pnum=276
http://www.ambientweather.com/dainwi.html
After determining that a pompous windbag could produce a 40
mph reading by blowing hard into one end of the tunnel, I
began pushing the buttons hopefully and occasionally looking
at the directions.
About the size of my hand, the Windscribe has a tunnel for
the wind to blow through, various mounts on the back, and
three buttons (On, Wind, Temp) around a large display that's
set at an odd 45 degrees to the wind tunnel.
A cr2032 coin cell is expected to run it for 600 hours, with
an auto-off like a cyclocomputer and a low-battery warning.
A nice touch is that "CR2032" is embossed in large
characters over the battery housing.
Pushing the buttons or holding them cycles through options,
much like a bicycle speedometer. There's even a light option
for owls curious about their night-time speed. It reads in
mph, feet per second, feet per minute, kph, metters per
second, and even knots, which I'm trying to find an excuse
for using.
Accuracy is claimed to be +/-3% for speeds from 0.4 to 150
mph, and +/- 2 degrees F.
The temperature screen, needed for calibration, comes in
only two flavors, F or C, but it does calculate wind chill.
To calibrate, you just put a thumb and finger over either
end of the wind tunnel, push a button, and wait a few
moments.
A padded V-mount screws into the bottom of the Windscribe
and lets you lash it to a handlebar with an included velcro
strap. A screwdriver is all that's needed to adjust the wind
tunnel to point dead ahead.
Like a cyclocomputer, it offers minimum and maximum and so
forth, with the wrinkle that the minimum is -x.x mph wind
speed for tailwinds. (I expect that the rider would shield
it somewhat from tailwinds.)
After strapping it to the left side of my touring bike's
handlebar and zeroing and calibrating everything, I set off
on an exceptionally calm afternoon.
Right away, I noticed that in instant mode the wind meter
zoomed up faster than my Nashbar cyclocomputer, which takes
an average over several seconds. (The Windscribe has a 5
second average mode and and running average mode, but I
chose instant for the first ride and didn't fool with the
buttons while riding.)
This instant reading showed what I had long suspected,
namely that my true speed drops much lower in braking than
the average shown by cyclocomputer, and then rises much more
quickly afterward.
In several corners and short, sharp rises, the Windscribe
showed my speed dropping to 12 mph, while the
cyclocomputer's more flattering average stayed at 14-16 mph.
After each corner, however, the Windscribe climbed back up
to cruising speed much more quickly than the lagging
cyclocomputer, making it look as if I was accelerating much
faster than usual.
It was so calm today that the two meters agreed to within
less than 1 mph on most long sections of the asphalt bicycle
path for the first 6 miles of my ride up the Arkansas River
at an average of just over 20 mph. Long stretches of the
river were smooth, unruffled water, and two fishing ponds
about 5 miles upstream were perfect mirrors of the kind
usually seen in paintings, with fishermen on the far shores
duplicated upside-down.
When I turned ninety degrees and began trudging up the road
to the top of the dam and then the bluffs above that, the
Windscribe began to differ from the cyclocomputer,
suggesting a tiny headwind of under 2 mph.
Out on a wide, deserted road, I naturally experimented by
putting one hand in front of the wind tunnel. The speed
reading promptly plunged and then delighted me by going
negative, confirming Jobst Brandt's comment that the 150 mph
bicycles on the salt flats can enjoy a tailwind because of
the windscreen in front of them, much like the wind blowing
forward behind the deep windshield of a convertible.
At the top of my daily climb, the road begins to flatten out
as it reaches a twenty-foot deep notch through a large
ridge. The Windscribe showed what I had long suspected,
namely that the headwind increases toward the top,
cancelling out some of the effect of the road flattening
out.
Heading back down a 2-mile highway descent, there were two
problems. First, the Windscribe has to be level with the
handlebars, so I couldn't see its display without raising my
head from where I like to tuck.
Second, I wasn't too sure about the reading--it was
generally about 5 mph lower than the cyclocomputer, but I
think that it was not so much a tailwind as a sidewind in an
exposed section of road. I'd been hoping to see much higher
Windscribe readings to explain lower speeds, but much of the
fun of this toy will be figuring out what happens with
quartering and side winds.
Back down in the river bottom in very calm air, I found that
the Windscribe would register passing cars nicely. You may
have to be very close to draft a bicycle, but a pickup truck
doing about 25 mph has a surprisingly long plume of
turbulence--the Windscribe registered what seemed to be a
draft until I fell more than 50 feet behind.
It also noticed cars zipping by in either direction in a 65
mph zone.
Regrettably, I pushed the wrong button at the end of the
ride and lost all the averages (I want to see what the
average wind speed is compared to the average cyclocomputer
speed on a really slow ride). Given the wind around here, I
expect to find out in a few days what the wind is that
knocks me down to 14 mph on one long, level curve.
I like to flip my bike upside down in the garage and was
wondering how hard it would be to remove the Windscribe, but
it turns out that the strap that keeps the Windscribe
pointing into the wind can be twisted around to hang under
the handlebars with a flick of the wrist, so it's no
problem.
It's easily removable and comes with a hand lanyard, so I
may have more fun sticking it out car windows or even
attaching it to an off-road motorcycle. I don't plan to lend
it to a nephew who rows on a college crew team, but his
parents might want to mount one on the prow of the the
shell. Mounting it on various spots on a bicycle might
produce some interesting figures concerning streamlining.
I've already had an inquiry from a skier--in calm air, it's
about as close to a speedometer as a skier can carry. (I've
rejected with scorn a suggestion to mount it on the collar
on the back of a basset hound's neck.)
I note with apprehension that there's absolutely no mention
of how to clean the wind tunnel, so I don't plan to take it
out in cold, wet weather or when the bald eagles and great
blue herons appear to be particularly thick in the sky to
the west.
Again, it's worth pointing out that the readings show only
the speed of the wind through the tunnel, which is not the
same as the speed of a wind at any angle to the bicycle's
path. A small slot allows mounting a yarn tell-tale of the
kind beloved by sailors. (I'd provide a link to Jobst's
wind-tunnel data, but it's his data, so he may enjoy
pointing it out himself.)
Windily,
Carl Fogel