If you calibrate on asphalt and measure on grass, which way would your measurement likely skew? My feeling is you will measure long (meaning the course is short) since greater wheel slippage would indicate you've ridden farther than you actually have. Is my thinking right here?
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If you measure on grass, you must calibrate on grass.
This exact situation is being addressed and will likely be finalized at the National Meeting in December. It appears that if a course contains more than an insignificant unpaved section, that section will require bike calibration on that or a similar surface.
Obtaining a reliable measurement on grass is one of the reasons Cross Country courses are not eligible for certification.
Alan I believe you have it backwards. If you had used a larger calibration constant (the one from the asphalt calibration) it would have resulted in a longer course. You would have had to travel further on the course to get to the 1-mile mark on the course if you were using a larger constant. We always use the larger of the pre-cal and post-cal constants because the larger one results in a longer course, right?
I seem to remember that this was the experience of several measurers in the past. That when they calibrated on an unpaved surface they got a smaller cal constant than when they calibrated on pavement. Because of that there wasn't that much concern about calibrating on pavement before measuring an unpaved course because it was expected to result in a longer course.
Jim, the rear wheel might slip on a lower friction surface, because the torque you apply when pedaling might be a stronger than the torque resulting from friction with the ground. But this is very unlikely to happen with the front wheel. It doesn't require much friction force to get the front wheel rolling because the only thing really resisting that rolling motion is angular inertia. And once the wheel is rolling, you don't need any friction force to keep it rolling. So it's unlikely the front wheel would slip rather than roll along the surface.
I believe the real reason different friction values can change the cal constant is because it affects how much the flattened section of the tire changes its length. If there is no friction there is no force to change the length of that flattened section. So even though the tire flattens, it also spreads out, and the effective circumference of the wheel is not changed. On pavement the friction is very high, and I think it results in a "stick" condition, so that flattened section is shortened quite a bit (from an arc to a straight line). Any surface that has lower friction is going to shorten the flattened section a lesser amount, and that means the effective circumference of the wheel will be larger (and thus a smaller cal constant than you would get on a high friction paved surface).
All that being said, I think grass is much more complicated and problematic. As Alan mentions, the grass surface will deform under the wheel. Any theoretical calculation of how that would change the effective circumference would be very complicated (and probably wrong!). The big problem though, is that there is a huge variation in grass surfaces. You could calibrate on one grass section, but then some other section of the course where the grass is longer, or thicker, or a different variety, could have a very different cal constant.
Pete Riegel somewhere wrote that calibrating on pavement and measuring on a trail was most likely to prevent a short course.
I did a direct comparison once: Pneumatic and airless tired bikes calibrated on pavement and then on a trail surface. Then measured a course with both bikes. Then recalibrated both on trail and pavement. Yes, it was a long day. I thought I could see a lot more particulate adherence to the tires in the early morning dampness than in the hot afternoon when the surface dried out. The airless tire recalibrated about the same. The pneumatic did expand but not as much on the trail as on pavement. Normally, the airless tire barely expands during a typical day's 20 F heat up. Based on that, I agree with Pete that a pavement calibration course is much more reliable / repeatable than a trail calibration course. If you don't want the trail or grass course to be long, wait till it is dry in the afternoon to measure.
We have measured several courses that had "not insignificant" grass or dirt segments . For the first few (CT13038JHP, CT13018JHP and CT10040JHP), we measured a 300-Ft cal course on the dirt or grass and determined a dirt or grass constant. The idea being that if the dirt or grass constant was larger than the road constant, we would layout a 1000-ft cal course on the dirt or grass and use it for the "not insignificant" segment. We stopped doing this after 2 or 3 times as the dirt or grass constant was always smaller than the road constant.
Pete Reigel did write that, as Oscar noted, that calibrating on pavement and measuring on dirt or grass would prevent a short course. Also, didn't Mike Sanford do a very complete study of the impact of calibration course surfaces on the constant (therefore the measured course length)?
When measuring these, do you adhere to the CC 30cm standard around turns, or follow the 12cm standard for curbs?
Alan: Calibration courses are always straight, no turns or curves. For measuring the race course, we stick to the "less than 1-ft" from the curb or edge of the road.
That would make sense. At least this metric/Imperial confusion wasn’t on a space mission.
Yup. NASA/Hubble should have used this: Handy Metric/Imperial Conversion Tool
My experience measuring on grass has been congruent with the experiences noted above as it pertains to distance. The historic aversion of RRTC to certifying XC courses has been more about how to recreate the measured route on race day, right?
Wrong. The problem was one that even getting 2 measurements to agree was seldom achievable.
Add to that the lack of any “edge” to the surface or definition for repeatability.
For those reasons RRTC ceased to certify XC courses
All good arguments against certifying cross country courses. However, the argument gets cloudy quickly when the course is a hybrid, road, trail, dirt path, grass or combination. I think the RRTC isn’t helping measures with less than definitive rules. In this case what is “not insignificant”? A 10K with a 1–mi dirt section with defined edges and or repeatable features properly measured will not result in a short course. That used to be our goal - within reason
One of my favorite quotes from Pete Riegel: “Perfect is the enemy of good enough”
Another Riegelism “At least the advertised distance”
"At least the advertised distance" is no longer the rule. It was decided a few years ago that all courses must be certified at whatever distance their measurement came out to. So it is no longer the case that a course that is measured to be 5040 meters gets certified as a 5k. It gets certified as 5040 meters.
You miss my point. A course measured to 5K is at least 5K!
I think we need to eliminate the mindset of "at least as long as the advertised distance." If we don't think we can measure it accurately, then we shouldn't certify it, even if we are sure it's not short.
