When I ride, I take off my watch because it's annoying under my jacket
and gaunlets. When I'm taking a long ride, I strap it around a piece of
foam on the handle bars. After seeing many people's clock solutions and
reading the thread on installing a bike computer...
I decided that the bike computer would solve my clock problem (for
about as much as a new watch) with night illumination and give me some
nifty features that the stock speedo lacks (including better accuracy).
For the most part, I followed the instructions given on the rit.edu
page below with the following differences: I soldered in a 18"
extension to the wires in order to have more freedom of placement and
added plugs so that the speedo cradle can be unplugged and moved
around; I also used two of the bands included with the speedo to reach
around the fork tube to hold the pick-up (easier to twiddle with than
zip-ties); and I used a Radio Shack rare earth magnet taped to the disk
carier with electrical tape (3M, of course). Then I strapped the speedo
and light on handle bars next to the front break resevoir.
My contribution is in calibration. First I measured the circumference
of the wheel by walking the bike through one revolution of the front
wheel and measuring the distance covered. Then you convert to mm and
divide by 1.61 (since we're doing mph not kph). This gave me about WS =
1143. I found this to be in close agreement with the stock speedo
readings. Today, I took my Garmin eTrex GPS along with me for my ride.
I reset the trip odometer on both the GPS and speedo, then rode the
freeways for a while. 20.3 miles later by the speedo and 19.0 according
to GPS, I stopped and recalibrated using the following formula:
Using this method over more than 50 miles and 3 iterations, I zeroed in
on WS = 1114. This causes the GPS and speedo to agree to within 0.05
miles, and is the best I can do since you have to use an integer WS. ;)
This is bike dependent and you should repeat this for yourself.
My results were that the stock speedo reads about 4 mph fast at 60
(stock reads 60 mph and GPS/BC800 reads 56 mph). This has everything to
do with tire brand/size, tire wear, and tire pressure. I'm running
stock Mez4's at 34/37 psi with 6600 miles of wear. Your Mileage May
Vary. :-D