Just how close to 100 miles is the century course?

We say our century course is 100.00 miles long. That's like saying a bottle contains 12.00 ounces of beer. It's only reasonable to allow for error - albeit minimal. And we’ve gone above and beyond to minimize it. What’s the big deal? A century is a 100-mile bicycle ride, a non-competition recreational event, an American cycling tradition based on miles. Cyclists, of course, acclaim 100 miles a heroic distance. And sub-four centuries are definitely heroic. Unless they’re 86 miles long. How do you make sure? Outside the US, cyclists haven't established or glorified standard distances like the century and metric century. Footracing has. The marathon, the 10K, and the 5K are universally recognized, and in America, USA Track & Field (USATF) sets the standard for measuring such running distances. Who prescribes procedures for measuring cycling courses? Nobody, as far as we can learn. Ride organizers pretty much catch as catch can. Before the advent of bicycle computers, we measured “Sunrise” courses with automobile odometers. That wasn't as bad as it sounds. We derived correction factors by checking our odometers against mile markers on long stretches of Interstate. While some highway mile markers are erratically spaced, many were established using steel tape and, later, EDMs (electronic distance measurers). We’ve found Interstate markers to be pretty reliable. When bicycle computers came along, we found the distances we'd measured by car matched fairly closely, but we think bike computers are better. But since most can be adjusted by wheel-circumference units no smaller than a tenth of a centimeter, no setting is likely to produce a reading of 1.00 miles smack-dab at the one-mile point of an accurate calibration course. You're more likely to see "1.00" twenty feet before you reach the mile marker and then about 40 feet before you reach Mile 2 - or maybe 35 feet after you pass the first mile marker and then 70 feet past the second. Just as correction factors had proven necessary for automobile odometers, we found we had to derive them for our computers too. We used three different computers to measure last year's courses. We also used three GPS receivers. Finally, we measured the course with Topo USA. For distances of 50 miles or more, GPS readings are quite accurate, but they're not reliable for measuring shorter segments shown on cue sheets. We were amazed that - especially for segments - the software proved to be the most accurate of the four methods. The numbers we got from the different methods agreed fairly closely, but we weren't satisfied. We tout fast finishing times, so it's incumbent on us to make as sure as we can that our century is 100 miles long. From sample points taken with laser rangers, surveyors use software to calculate distances along curved paths. That method might well produce "truer" measurements than the USATF method, but they probably wouldn’t be valid for bicycles. Although the procedures employed by USATF pertain to footracing courses, we think a little modification makes more pertinent to cycling than surveyors’ measurements. And they're generally considered conservative as well as highly accurate. How do they work? USATF assumes footracers run the shortest distances they can get away with. Measuring crews and course certifiers map the shortest path over a course and then measure it with a bicycle-mounted gizmo called a Jones-Oerth-Lacroix counter. Bicycle computers calculate speed and distance by counting revolutions of a bicycle wheel. The JOL counter records some 23 "strikes" per revolution. According to the USATF measurement manual, you lay out an accurate calibration course with steel tape. Then you calibrate the JOL attached to a bike. You do that by riding the calibration course four times in each direction, locking the wheel between runs and recording the count at each turnaround. Calibration runs are conducted immediately before and immediately after each of two measurements of a racecourse itself. Our calibration course happens to be 2000 feet long. That's plenty long enough, although some calibration courses are 1000 meters long. Its length is arbitrary. You just need to know how long it is in order to calculate the number of counts per mile - ultimately for 100 miles. We entered into a correspondence with a representative of the USATF Road Running Technical Council about certifying our course. Initially, he considered entertaining our application. Then we read the USATF measurement manual. That’s when it hit us that a cyclist's path differs a bunch from a footracer’s. While the USATF method assumes runners follow the shortest distance, cyclists conserve speed when rounding turns. And traffic may constrain a cyclist's path more than a runner's. The representative saw our point. Unfortunately, since there was no way to follow USATF guidelines exactly, he admonished us not to apply for certification. So let us be clear: We have no relationship with USATF. Still, we are following its guidelines to the letter - with one exception. We measure the path cyclists are most likely to take. Our measurement rider couldn't safely or practicably follow the shortest possible path. It wouldn't make sense anyway. Instead, he rides alone and follows the most economical path consistent with safety and, more important, consistent with the way cyclists make turns. Cyclists make turns more like cars than runners. And they weave. Who knows? Our measurement cyclist may weave more than average. We doubt he weaves enough to cancel out USATF’s short-course prevention factor of 1.001. We multiply it by the number of counts per mile to ensure that our century course is 100 miles long. USATF believes these procedures limit error to 0.1%. If it were possible to be sure of executing every step perfectly, our course would be 100.10 miles long. What we probably ought to advertise is a course that's at least 100.00 miles long. You can be pretty darn sure you've ridden 100 miles when you cross our finish line - and confident, at the same time, that you haven't ridden much farther. Until the price, size, and accuracy of GPS units makes them practicable for measuring century courses, we can't think of a better method than our modified USATF procedures. Meanwhile, we think you ought to calibrate your bicycle computer against this course - rather than use your computer to challenge our measurements.