Bicycle computers sold today can be calibrated using the circumference of the tire with accuracy down to the millimeter. However, most bicycle computers are only designed to read up to speeds of 50 miles per hour. However Sigma has a computer that is accurate up to 180-some odd miles per hour. But, before reading further, I have since stopped using the Sigma computers. With the cost of Garmin GPS units dropping in price, you can get all the features with virtually no work at all. Most outdoor shops even sell super small and light mountain bike brackets that make installing and using a GPS on a motorcycle extremely easy.
However, once the calibrated speedometer is mounted onto a motorcycle, the rider will find it to be very valuable. First, the computer records average speed, total time riding, top speed, a tripometer and an odometer. Not to mention a handy-dandy clock. It is also nice that it is accurate to 1/100th of a mile. Another nice feature of the 800 is that, not only is it only $20 dollars, but it has two different programmable modes, designed so that it can be switched between bikes. The two different modes allow for one to be set for miles and the other to be set for kilometers in case you go across borders often.
Mine has even worked to get out of a speeding ticket. When the cop asked if my speedometer was calibrated I was able to say "As a matter of fact it is! It's accurate to 1/100th of a mile and it said I was doing precisely XXmph. Can I see the documentation that proves your radar gun was calibrated at the beginning of your shift??"
From this image you can see how the computer itself has been mounted onto the front brake master cylinder of the TLS (arrow 1) You can also see the velco that is used to mount the radar gun. (arrow 2) these two tools can be very effective tools to help compete against police officers and radar guns.
Here you can see the VTR and how it mounted nicely on the clutch master clyinder. Both these locations did not require any permanent modifications to the motorcycle. However, both locations allow the rider to easily see the speedometer and access the speedometer without much effort.
To calibrate the Sigma, simply make a mark on the tire and the ground under it, and then roll the tire out once or twice while sitting on the bike, make the second mark on the ground and measure the distance and divide by the number of revolutions. Another way to do this is by making marks on your tire with red chalk then riding the bike a couple of revolutions. Then you take the distance between the marks, divide them by 3.14 and whalla, you should have a number like 1138 (if you have a 120/70R 17 front tire) That number is then plugged into your Sigma and its calibrated. You can then verify the accuracy by riding several miles keeping track of mile markers. If there is a discrepancy you take the actual miles traveled - i.e. 8 mile markers and cross divide that against the registered miles traveled - i.e. 7.5 miles.
The hard part was figuring out what to do about the sensor and magnet. Normally the sensor would be mounted on a small bicycle fork and the magnet would attach to one of the spokes. This is where creativit counts. Finding a place to locate the sensor can be tricky. Make sure that it does not come in contact with anything, like the disk, caliper or get in the way of anything. It needs to have total clearance. Use the smallest zip ties you can find to securely fasten it to the bike. I located mine between the caliper and fork leg.
The magnet that comes with the unit simply would not be usable. Radio Shack offers, what they call, "Rare Earth Magnets". Many Radio Shacks may not have a clue what you are talking about, but be insistent. They do have them and if they don't they should be able to order them. I went to three Radio Shacks before I found mine.
Once the sensor is located in a place that will work, go for a ride (or two) to make sure that it does not shift its location. You want the to sensor to find a place where it will stay put, otherwise, it will move and its alignment with the magnet could be altered. There is a white line on the sensor that will help find the location where the magnets will need to be located.
Click on the image for a full size version and more detail. Arrow number-1 illustrates where the magnet was glued to the disk carrier. Arrow number-2 shows the grey zip ties I used to hold the sensor in place on the fork and arrow number-3 shows the wire traveling down the break line.
Radio Shack Rare Earth magnets are two tiny magnets, about 3/16" in diameter and 1/16" thick. Use a little bit of super glue on the back of the magnet and then stick it to the aluminum disk carrier at the location where it will be in alignment with the sensor. Then any additional magnets needed to close the gap between the sensor can be placed on top of the original magnet that has been glued to the disk carrier. However, be careful when you take the wheel off to have the tire changed that the shop doesn't knock off the magnet. Do not worry about the additional magnets flying off. The rare earth magnets are so powerful that they will not separate and fly off at any speed that can be registered by the bicycle computer. I chose not to glue the whole stack because I'd rather have magnets come off than get caught on something if the sensor ever slipped.
I found that the stock speedometer on my TLS was way off between 0-50 mph. Between 50-80 mph its dead accurate, then it gets progressively less accurate as speeds increase. However, the Sigma always shows an accurate speed. Also, the large digital numbers make it easy to set your speed where you want it and keep it there. Combine this with a good radar detector and you have some good defense against the anti-sport bike cops that'll pull you over to see if you'll confess to speeding. You will also be able to set your speed right at the limit so when you get a signal on your radar detector you will know whether or not scubbing off speed in necessary, to avoid those pesky fines and more irritating insurance hikes that follow.