I’m on my third cyclocomputer since I started riding. It’s a Cateye V2c, and is the best of them all, because it’s wireless and has cadence as well as the usual assortment of current, average and maximum speed readings. It gives me everything I want to know about a ride (Okay, being able to see the percent grades of my favorite local climbs would be nice too) without involving the steep cost of a GPS unit.
However, I do have a minor gripe: accuracy. I’ve noticed that with each computer I’ve owned, I get different distances for the same route.
My benchmark ride is a trip from my house to the Austin Veloway, three laps of the track, and back home. It’s my 15-mile ride. To be precise, it’s 14.9 miles — 2.8 from my house to the Veloway, three laps at 3.1 miles per lap, or 9.3 miles, and 2.8 miles home. But my computer clocks it at 14.7.
Now, you adjust your computer according to the outside circumference of your tire. A 23 mm tire should have a circumference of 2096 mm, and the instruction book that comes with the computer suggests setting it to that number. But tires flex, and since my computer’s sensors are on the rear wheel, which carries the most weight, I thought I would measure by hand.
I positioned the bike on the garage floor so the rear wheel’s valve was perpendicular to the ground. A black Sharpie made a hash mark on the floor. Then I got on the bike and pushed off with my feet for one rotation of the tire. Used the Sharpie to mark the floor again. When I measured the distance between the two marks, and converted from inches to centimeters, then reduced, I got — 2096.
What I thought was odd was that on my previous tires, which were 25 mm, my hand measurements also came out to 2096. So my computer readings were consistent.
I’ve alluded to my finickiness when it comes to record keeping. I had to come up with a way to get accurate readings from the computer. The obvious way is compare it to a known distance.
Several years ago, I zeroed the computer at the entrance to the Veloway. As I mentioned, it’s 3.1 miles around. But a circuit of the track clocked exactly three miles on the Cateye. So if I was a tenth of a mile short for three miles, that meant that I could simply add a tenth for every three miles ridden. If the computer showed I rode 18 miles, I could add six-tenths to that distance for an accurate reading.
This jury rigged system actually worked. When I rode the MS150, the Cateye clocked the second day’s distance at 74 miles. Divide by three = 24.6 — so I could add 2.5 miles (round up the .46), and get 76.5 miles. The official route map showed the distance at 77. Getting closer.
It’s been awhile now, and I’ve been adding my tenths. But I decided last week to check the accuracy of my system again. This time a 3.1 mile lap registered 3.05 miles on the Cateye. Half a tenth off from the previous readings. That meant I could now add a tenth for every six miles ridden, instead of every three.
When I compared a couple of recent rides using this method to maps created on Ride With GPS, the new system came pretty close.
I think you’ve gotta be a certain type of personality to go through the machinations I do just to get what might or might not be an accurate measure of distance traveled. A twisted type. That’s me — accurately.