Here is real data from an 8 km layout measurement.

What happened?

What should be done?

Precal Postcal
6:00 AM 8:25 AM
24F 29F
3307 3285
3306 3285
3306 3284
3308 3285

Calibration course 300 meters

First Measurement 88268
Second Measurement 88258

Clarification: this is not a made-up problem. The data are real, and I do not have any more information than the above data. What's the most likely explanation?
Last edited {1}
Original Post

Precal-3306.75 avg. per ride = 11022.499 per km, 11033.522 (11034 WC) once SCPF is added.
Postcal-3284.75 avg. per ride = 10949.166 per km, 10960.115 (10961 WC) once SCPF is added.

WITHOUT SCPF:
Ride 1=88268/11023 = 8.0076204km
Ride 2=88258/11023 = 8.0067132km
Difference = 0.0009072km/8.0076204km = .00011329

WITH SCPF:
Ride 1=88268/11034 = 7.9996374km
Ride 2=88258/11034 = 7.9987311km
Difference = 0.0009063km/7.996374km = .00011329

Sounds like the measurer forgot to add the SCPF.

OR:

The measurer had a low air pressure and inflated to a higher pressure between the pre-calibration ride and the first measurement ride.

OR:

A combination of the two.

SOLUTION/S:
Ask measurer whether SCPF was taken into consideration. If not:
1. Have measurer move start/finish/turn-around in order to bring measurement up to full 8km distance.
2. Instruct measurer on SCPF calculation.
3. Encourage measurer to check tire pressure before calibration rides.
4. Instruct measurer on need to recalibrate wheel after re-inflating tire due to low pressure or flat tire.
Last edited by michaelbowen
I agree with Michael on one of his scenarios - tire was inflated after initial Cal ride.

SCPF has no bearing on Cal rides, yet Pos-Cal had considerably fewer clicks. Increased tire size (due to inflation) is most likely. Even wobble wouldn't have been that consistant in Pre ride.

I would encourage the measurer to re-measure, as that is a large difference in Cal rides. I don't recall ever having that much variance, even when I did Pre at 20° and Post at 50°.
In 1985 Allan Steinfeld (then RRTC Chairman) sent me to Rotterdam to participate in the first-ever international course validation. Carlos Lopes had set a potential world record at Rotterdam, and it was to be checked. I met at the venue with Jos Hermens, race director, and measurers Helge Ibert of Germany and Lennart Julin of Sweden. We used a 100 meter steel tape to check out a calibration course on a straight piece of bike path, and calibrated our bikes.

At the end of the measurement something was wrong. It took some head-scratching, but we finally found that there were two nails at one end of the calibration course, and we had made one set of calibration rides using the wrong nail. We measured the difference, and things came out OK.

Use of the wrong end point on the calibration course is another possibility for the calibration variation. As I’ve not yet heard back from my source, I don’t know whether this is what happened, or the more likely pump-up between calibrations. The two course rides were virtually identical, and gave no sign of increasing or decreasing constant, so I’m not sure which horse to back.
"...so I’m not sure which horse to back."

Which is why I don't bet on the horses!
It was waaaaaaay too cold to be out measuring a course. That was the problem.
I'm with the tire-inflation guys on this one. When it's that cold the air inside the tire shrinks (put half a bottle of water in the car overnight with the lid on tight and you'll see it). The tire was underinflated for the pre-cal ride, and up to temp on the post-cal.
I have noticed that I ride a much better cal ride when there is good light, after the sun comes up. If I return late and do post cal rides as it starts to get dark the rides are not as good, i.e. in the gloom it takes for more counts.

But due to the consistency of the cal rides I would guess the bike wheel got larger. That may be adding air but maybe it just warmed up. Stuff expands when it gets warm.

Some years ago I was reading the UK measurement manual and they had some adjustments for temp variation.

I find that I have to ride my bike fast for a couple of miles to warm it up, before the counts on my cal rides are as close as I want them.

On the course I re-warm it up by doing a trial run around the course, often putting down the mile markers on that run, before I do the rides needed for certification.

I also notice that if it rains and cools off the road surface and my bike tiers the measurement rides will change noticeably, even though I am riding on low profile, high pressure tires.
Too much change to be accounted for by simple warm up of the tyre from 24F to 29F.

Can only speculate on possibilities WITHOUT additional data.

Standard procedure with this set of data would be to go out and repeat the whole calibration and measurement and recalibration sequence.

Once one had obtained self consistent data one could then have another go at narrowing down the range of possibilities.

Pete you have the report so why dont you or someone else go to the course and remeasure it yourself and tell us what you get. So we can see which calibration (if any) is consistent with the two rides.

I worry about sub zero riding since ice on road may cause problems, but it is not very obvious what has happened here. Perhaps cal course was icy for pre cal, but by the time of the post cal the rise from 24F to 29F had taken place and the ice had melted (due to being mixed with some salt or the passage of a road gritter or traffic or whatever.
I got the data from a certifier who wanted an opinion. I know no more than what I posted. If the certifier gets back to me with more info I'll post it. Until then, possess yourselves with patience.

Tire warm-up rides before calibration are often recommended but I have found that they make very little difference.

Speculation is all we have for now, and it looks like we must choose between a mid-cal tire pumpup or a misreading of the proper end of the cal course.

Also possible is a long chain of erroneously read data, but this possibility isn't credible to me.
I'll put a round of beer on the tire pump-up after the pre-cal ride. But I still think they forgot to add SCPF

But I can't help but agree with Mark over the (painfully) obvious fact it was too cold to be doing anything even resembling a measurement job.

Sorry, there's not enough beer money in the till that would make me get out there in 20-something-degree weather.
Most likely explanation? There aren't any. Info begs a redo.