OK, first of all, I got the idea from this webpage
http://shell.rmi.net/~frechett/bikecomputer.html
which gives lots of good information, but doesn't cover installation on a
truck.
Why a bicycle computer on a truck?!?
The bicycle computer is calibrated to the circumference of the tire, so
nearly any size tire can be used and you will still have a super-accurate
speedometer/odometer. True, you can change the speedo gear in the
transmission, but this will give you an accurate reading for only a certain
size of tire. If you want to install a tire size in between ranges of
different speedo gears, then a bike computer will give you the most accurate
readings for speedo/odometer. It's also handy if you like to change out
different size tires quite often, such as summer performance tires vs. tall
winter tires, or small-diameter high-performance tires for around town, and
large-diameter touring tires for long highway trips. No guesswork involved
with changing speedo gears, etc.
This is also handy for those of you who like to 'quarter-mile' on Sat.
nights. You can go to a smaller-diameter tire to effectively increase your
gear ratio (I put smaller tires on my truck and it was like going from 3.55'
s to 3.90's). Keep in mind that your factory odometer will rack up miles
faster with the smaller tires.
The bike computer is also a trip computer. It will automatically record
trip distance, time underway (total time vehicle is moving), maximum speed
and average speed. Of course, it also has the odometer (logs up to 99,999
miles) and a clock.
The bike computer will work on any vehicle in which you can mount a magnet
on the wheel and the sensor in close proximity (~5mm). Because of this, I
bought additional harnesses that allow me to use the bike computer on my
bicycle, my motorcycle and my truck. You could even use it to make a
measuring wheel (to measure the length of a field, for instance).
How accurate is it?
Different bike computers may vary, but the model I have (Sigma BC800)
indicates odometer accuracy to within 10m or 1/100 mile. This is easy to
believe once you understand how it works. The Sigma BC800 works up to
183mph - again, different bike computers will vary, so be sure of what you
are getting.
How does it work?
The computer works by using a sensor mounted near the wheel. A separate
magnet is mounted on the wheel. Each time the wheel rotates, the magnet
passes by the sensor and triggers a voltage signal. This signal is relayed
to the computer, which adds and converts the distance traveled (1 revolution
= 1 tire circumference) into miles & mph (or km & kmph, if you prefer). It'
s really quite simple. All you have to do is enter the circumference of the
tire. The computer itself is about the size of a man's large digital
wristwatch (and looks about the same, except with a bigger display. It runs
off of 2 replaceable watch batteries that are designed to last about 2
years.
How expensive is it?
My Sigma BC800 typically runs about $30, but shop around at different bike
shops (local and on the web) as they will vary quite a bit in price. You
can get additional harnesses that will allow you to put the computer on
different vehicles. These harnesses run $10 - $15 each, so again, shop
around.
How is it mounted?
I mounted 3 different harnesses, one on my bicycle, one on my motorcycle,
and one on my truck. Procedures are as follows:
Bicycle: This is a very simple installation (since it was made for this
purpose). Package instructions are straightforward and installation time
takes about 5 minutes.
Motorcycle: (Note: I used an additional harness for the motorcycle. This
is called a "rear wheel kit" by the bicycle people, as it's designed with a
longer wire to mount the sensor on the rear wheel of a bike. This extra
length comes in handy on a motorcycle or other vehicle installation. The
length of wire from sensor to computer is about 4 feet, if you need more you
can simply splice in some extra wire.) A magnet must be attached to the
wheel. If you have spoke wheels, you may be able to use the magnet that
comes with the computer. My motorcycle (an '84 GoldWing) does not have
spoke wheels, so I glued a magnet to the wheel rim using a 2-part epoxy
glue. I got the magnet(s) from Radio Shack - they are "rare earth" magnets
and are much more powerful than a regular magnet. This is important,
because you want lots of power in a small size. The magnets I got were
about 1/8" wide and 1/16" thick and weighed about 0.5g each. Two magnets
come in each packet (around $2 for the packet). I epoxied one magnet to the
rim and then stuck the other magnet on top of that. These are very strong
magnets and will not come apart at (even very fast) highway speeds. For the
sensor, I decided to fabricate a bracket that I could bend to in order to
position the sensor where I wanted it. I made a simple bracket out of some
strip metal (with pre-formed holes) and bolted it onto an existing fender
bolt. The trick is to get the sensor within 5mm of the magnet (this is
according to the bike computer instructions - since I'm using powerful rare
earth magnets, I may be able to increase that distance, but I haven't
tried..). You could come up with a variety of ways to mount the sensor.
Anyway, I ran the wires from the computer to the sensor down the front fork
tube of the bike (inside the fairing) and mounted the computer on the
handlebars (quick rubber-band mount as shown in instructions, just like for
a bicycle). Total installation time about 30 minutes.
Truck: I also used an additional harness for the truck (see Note on
motorcycle installation). The four feet of wire from the computer to the
sensor wasn't quite long enough, so I cut the wire and spliced in some
additional wire. The cut was necessary anyway to easily run the wire
through the firewall under the dash. Again, I used "rare earth" magnets
(see motorcycle installation) and epoxied one to the wheel rim and stuck the
other one on top of it. I mounted the sensor to the brake caliper using
epoxy glue also. This can be a little tricky, since you want the sensor to
line up with the magnet and be within 5mm or so. I lucked out since the
edge of the wheel rim was the same as the edge of the brake caliper, so
location pick was a breeze. I then ran the wiring up above the A-arm and
into the engine compartment, where I spliced it to the wire feeding through
the firewall. On the other side of the firewall (under the dash) I ran the
wire up to the bracket for the bike computer, which I mounted on the
steering wheel tilt lever. It turned out to be a very clean installation
with the computer in full view within the steering wheel. Total
installation time (waiting for epoxy glue to dry, etc.) was about 45
minutes.
Additional notes / tips:
Once everything is installed, you need to calibrate the computer by entering
the circumference of the tire. I found the easiest way to measure
circumference is to put a piece of tape on the tire and line it up with a
piece of tape on the ground (shop floor or driveway). Roll the tire twice
and mark (with another piece of tape) on the ground where the tape on the
tire lines up again. Measure the distance between the 2 pieces of tape on
the ground and divide by two (since you rolled the tire twice for a more
accurate measurement). Whatever you measure in must be converted to mm
since that is what the computer recognizes. Then, divide that measurement
by 1.6 to get mph, or leave as is to get kmph. Enter that number into the
computer (very simple procedure) and you are ready to roll!
I use the same computer on the 3 vehicles mentioned above. I simply enter
the tire circumference of whatever vehicle I'm using and I'm set. The
computer itself pops in and out of the brackets, so is easily moved from
vehicle to vehicle.
Installation procedures can vary quite a bit. The main thing is to have the
sensor within 5mm of the magnet -this is according to the computer
installation instructions; however, since I'm using a much stronger magnet,
I may be able to increase that distance.
I looked at several different bike computers before I decided on the Sigma
BC800. I chose the Sigma because it can handle a simple robust installation
(some computers require several magnets or that a 'special' magnet be lined
up (polarized) with the sensor) and I knew it was good up to 183 mph.
Disadvantages of a bike computer:
There is no backlight, so you can't see the display at night. One option is
to use a small map light or similar light to shine on the readout at night
(I've seen these in auto parts stores). Another option is to just figure
out the error between your accurate bike computer and the factory
speedometer, then correct for the factory speedo at night. For example, if
the factory speedo reads 5mph too fast at 65mph according to the bike
speedo, then simply drive 70mph in a 65mph zone (according to your factory
speedo!).
Also, the bike computer (at least my Sigma BC800) has a very slight lag
during acceleration. This really isn't a problem since I'm mainly
interested in the speed I'm maintaining while cruising.
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