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This might be a follow up to the rolling tire characteristics thread, but I'll start a new thread and see where it takes us.

This weekend I travelled to Peoria to participate in a measurement of the Illinois High School Association's state final cross-country course. The course is in a large park, is fairly flat, and is pretty much conducted entirely on grass.

The team measured it a number of different ways- GPS, measuring wheel, steel tape, and calibrated bicycle.

One of the other team members and I set out a 300 meter calibration course on an asphalt street north of the park. I calibrated my bicycle on it, and ended up with a constant of 11.08 counts per meter.

Then I rode back to the park, and calibrated the bicycle on the first 300 meters of the race course, using the flags the taping crew had set out. This time the constant worked out to 10.9833 counts per meter.

The difference in the numbers is 0.827%, or almost 40 meters over three miles. I decided the grass calibration was more representative of the situation and used those numbers to calculate the course distance. My numbers were consistent with those obtained with the steel tape.

The question is why were the numbers so different. I ran into a similar issue early this spring measuring a course that had sections on paved paths and sections on crushed limestone paths. The paths had been frozen and began to thaw during the measurement, and the post-thaw numbers were not at all consistent with the pre-thaw numbers. I managed to get consistent numbers by measuring the course twice after it had thawed, but now I question exactly how accurate that was.

My hypothesis is that the harder surfaces deform the front tire more than the softer ones do, and as a result the constant for the wheel is larger on the softer surface. Any thoughts on this?

I kind of came away from the exercise believing that when measuring a cross-country course, the extra time and effort involved in measuring it with a steel tape might very well be worth it.
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Jay:

You got a 0.83% smaller constant on grass than on asphalt. I recently reported a comparison using my 7 foot portable calibration course - see my web page . In my experiments, where the front wheel was only loaded with the bike weight, I got a 0.16% reduction in calibration constant when on grass. So it was the same sign as what you observed. Perhaps the larger magnitude which you observed is partly due to the rider weight.

Some years ago I introduced a limitation in the amount of off road surface which I would allow for the race course to be issued with a certificate of course accuracy. Up to 10 km the maximum amount of off road surface allowed is 10%. So with your grass surface for a 10k certified race only 1k would be allowed on the grass. If the grass section were to have the 0.86% decrease in constant as you observed, then using the normal road surface calibration you would be laying out the 1000m grass section to be longer than the true distance. If measured by a steel tape it would come out at 1008.6m.

For races over 10k we allow 1k plus 5% of the amount the course distance exceeds 10k, so that is a somewhat smaller fraction allowed to be off road. I figured that long off road sections - usually rather rough - are not welcomed by road racers pursuing PBs.

In the UK races with more than the allowed amount off road are issued with multi-terrain licences and are not issued with certificates of course accuracy.
Mike:

So it would follow that for USATF purposes where the philosophy is "avoid short courses", using a calibrated bicycle calibrated on a hard surface ought to, at the very least, not measure the distance on a soft surface to be longer than it actually is.

That's not scoring high as far as accuracy goes- but at least it's not producing a short course.

This weekend we were trying to determine if changes made to the course were material and, if so, whether or not they made the course longer or shorter. It was more of a validation measurement as opposed to a layout measurement. Being 40 meters off over 3 miles would not have been good at all, even if it found that the course was long.

I think your guidelines are valid- and I will certainly put some extra thought into the next time I am asked to measure over a soft surface.

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