Did you ever wonder about the accuracy of your bike computer? At the end of the ride does everyone ask everyone else what they got for mileage, to see if their computers are in near agreement? As you approach the end of your century, does your computer's mileage reading seem to drift further and further from the listed turn point mileages? Well, now you can address this question of accuracy.
A recent call to the county road department revealed the existence of a calibrated mile on a road right here in Alachua county. You will find this calibrated mile in the town of Hague, at the intersection of 25A and CR 237. Immediately after you cross over the railroad tracks that parallel 25A, going north on CR237, about 25 feet past the tracks, you will see markings painted in florescent orange in the middle of the road. These markings are two dashes with a dot between them. Below these markings are the numbers "0+00". Above the dashes and dot are the words "1 MILE" with a squiggly arrow pointing the direction in which to travel; this is the start point for the calibrated mile. As you head north, up CR237, you will see another set of orange markings in the middle of the road that says "« MILE"; continuing you will finally come to the last set of orange markings that says "END" and "1 MILE". Between these words is another set of dashes with a dot between them. The dashes and dot at the beginning and end of the calibrated mile are the exact start and stop points. You may see, at various points along the mile, other markings of other colors, some faded and some readable; just ignore these.
OK, so how do you use these markings?
At the side of the road, line your handle bars up even with the start mark (the first set of dashes and dot). Now zero the trip meter on your computer, so it reads "00.00". Take off and ride as straight as possible up CR 237 to the 1 mile marking. Slow down as you approach the end mark, walk your bike the final feet to the mark if you like, but in the end your handlebars should be even with the dashes and dot at the 1 mile markings. Now look at your computer trip meter. If your computer is very accurate it should read exactly 1 mile, such as "01.00". More than likely it will be greater or less than one mile in its reading. The difference between 01.00 mile and the reading you have is the error, which can be expressed as a percentage. In the case of my computer, my first reading at the end of the calibrated mile was 00.97. This is a 3% error from the actual mileage. This is an error, in practical terms, of 3 miles of error per 100 miles of travel. So for each 100 miles my computer said that I had traveled, I had, in fact, ridden 103 miles. This is not a lot of error and is probably typical of bike computers, but knowing what this error is gives you the information you need to reduce the error, if you so desire.
So how do I reduce the error?
What I did is multiplied the wheel circumference factor (the number that you are required to input to the computer for the size of the wheel) by .03, and then added this product to the original circumference factor. This gives a new circumference factor corrected for the 3% error. When this new factor was put into the computer, I again rode the calibrated mile from one end to the other. This time when I moved up to the end of the calibrated mile my trip meter rolled up to "01.00" mile just 10 feet before the mile mark on the road. Naturally, this still represents some error, because the perfectly calibrated computer will just turn over from reading 00.99 to reading 01.00 mile as you roll the bike that last inch to the dashes and dot. But these computers aren't really made to be all that accurate anyway, and even if you did get it to roll over to read "01.00" exactly at that point, many factors may weigh against higher accuracy than this in real use; in any case, 10 feet of error per mile will only be an error, in practical terms, of about 1 mile of error every 528 miles, or the computer will read 529 miles when I have actually traveled 528 mile. In spite of this unresolved error, it's easy to see that the simple act of correcting the circumference factor by 3%* improved the accuracy of my computer by a factor of about 16, which is a dramatic improvement.
It should be noted too, that substantial errors in your computer's distance accuracy will noticeably affect your speedometer and average speed readings, because this information is derived from the distance traveled with respect to time (the distance being provided by the rotation count of your wheel and the time being provided by the computer) . For instance a 3% error would make a difference of about 1 mile per hour reading if you were doing 30 mph. Not much admittedly, but most people want to be able to claim every mile and mph that they have honestly earned!
There are some things you can do to reduce the error even more, but most of these are a bit
esoteric, and too difficult to explain, for a discussion here. However, you will want to be sure to have your tires inflated to your normal riding pressure and be riding with the bike set up as on a typical ride, this will help provide a reasonably accurate calibration for everyday use. Until they build a GPS** unit for bikes, we will just have to live with a minor degree of error.
* (3% of a mile is 158.4 feet)
** (GPS) Global Positioning Satellites also have some error; ultimately, no measurement is perfect!
Note: And of course, all of the accuracy of this calibration method is ultimately dependent on the accuracy of the road markings, but I'm betting that Alachua's highway engineers are exceptionally careful.