Skip to main content

Reply to "Difference between cal and course surface"

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.

×
×
×
×