> the error associated with this measurement is probably going to be, at worst, 5mm for a single revolution. 5mm error for a 2100mm circumfrence wheel comes to 0.23% error. On a ride of 100 miles, this equates to an error of +-0.23 miles. So, if you had this uncertainty in your setting, you can simply ride an extra 1/4 mile and you will know that you have ridden at least a full 100 mile century.
Oh, I do that - did two extra miles based on my computer and 6 or 8 extra based on the actual century distance as shown on my computer. I'd be kind surprised if I would be 5 mm off since every test seemed to be within a half mm. I can see a potential problem, though it may be meaningless. Sitting on a bike on slowly rolling it may cause a greater compaction of the tire than is the case when spinning fast. But I don't know if that is the case.
> First off, when you do your rollout test, unless you have a helper push the bike while you are on it, chances are you will not be in your exact riding position. <
True. I think I minimized this by being against a wall so I could maintain more stability in this area.
> Second, even if you are able to get into position, you are unlikely to ride the same position during you entire ride. <
Also true, though I suspect with fully inflated tires, the difference is pretty small. It would be interesting to test it though. Maybe someday I will.
> Don't forget that the road has bumps which will bounce your tire and cause momentary changes in effective circumfrence. These may or may not cancel out, I don't know. <
Yeah, but as it happens, since I made my careful measurement, I've had a lot of the road surfaces I ride repaved so they are much smoother. If there is any difference, it is too small for me to tell.
> Keep in mind that you may not ride the same line as someone else. <
True, but like I mentioned before, even deliberately weaving far more than any normal rider would, it didn't make the trip more than 2% longer - maybe just 1% (I can't recall the exact result of this test now). Similarly, I think it would take a HUGE difference in how you take a curve to really matter.
> When you stop for a break, you may not stop your bike right on the road and add a few hundreths of a mile riding through a prking lot or two. <
Yes, and this was something that bothered me about the century here. Like I said, it was 4-6 miles off (can't recall for sure), but that included the distance I put in at the various aid stations which usually were not right along the road. It was more like going up a driveway to a station. To me, a century ride should be based on not taking any such detours at all.
> Finally, most bike rides are informal affairs and any published distances by the organisers should be taken with a grain of salt. The organisers probably either used some mapping software to compute their route distances or drove the route in a car. Either case is highly prone to inaccuracy. <
I still hope to get some real info on the map accuracy. Since the figures are so close to my actual measurements, I tend to think my software is pretty accurate, but that could be coincidence. If someone had the Topo USA software and a GPS unit where they could measure a good distance on a fairly straight course, it would give us some useful comparisons. Don't GPS units have an accuracy to a few meters? Over a distance of 10 miles or so, that isn't much even if the error is the maximum and in opposite directions at each end. Of course, if I was doing it (stickler that I am), I'd actually measure the two points several times and go with the average of each!
> There is no magic to the 100 mile mark. Whether you do 98 or 102 miles, you're going to get the same benefit and feel the same soreness afterward. <
Yeah, but I just like to be able to say confidently that I did a century, or a metric century, or a half-century. I'm constantly pushing to reach at least 63 miles just to be absolutely sure I did a metric century. I've only done 3 centuries in four years, but I have done 13 metric centuries or further just since mid-december.