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11 replies

Post measurement calibration variation

paulhronjakpaulhronjakRRTC Vice Chair (east)
I have been having a problem with a higher finish constant and it has been driving me nuits, since there is no sign of a leaking tire (said because the next week the working constant will be the same as the working constant this week without adding air). The temperature has always been higher after the measurement and this is a pneumatic tire.

I finally figured it out! I keep the bike in the garage and take it out just before going to the calibration course, two miles away. With the cold weather, it is probably 10 - 20 degrees warmer in the garage than the outside temperature and the tire does not have a chance to cool down to the outside temperature prior to my calibration rides.

Just another bit of information to consider. BTW, I have a pressure guage now and will be trying out Nevilles system.
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peteriegelpeteriegelRCMBB Founder
It's easy to check whether the warm garage is the problem. If the tire is indeed still cooling down, do two rides and wait 5 minutes. If the tire has not yet come to equilibrium temperature the next two rides should give a few more counts. If it doesn't change the tire ought to have reached final temperature.

A two-mile ride should be plenty to allow the tire to come to final temperature. I've had some winter rides, and also keep my bike in the garage. I've not seen this problem.

How much bigger is the finish constant than the starting constant?
laurentlacroixlaurentlacroixMember
It usually takes a good fifteen minutes from the time the bike has been set up, the warm-up ride completed and the last calibration ride ended. I'll add another two calibration rides if there's a significant change in counts from the first to the fourth ride.

Transporting the bike on a car-mounted rack would allow even more time for the front wheel to come to the ambient temperature. I guess that with the pressure monitoring it would be easier to determine when the tire has reached equilibrium.
peteriegelpeteriegelRCMBB Founder
I was once concerned about warmup, but now believe that it does not take long. When I transport my bike in a closed car I am usually calibrating within a few minutes of unloading, with only about 150 meters of travel to the cal course. I have not yet seen a gradual change in counts on the cal course over four rides, as would happen if the tire was changing size.
nevillenevilleRRTC Validations Chair
When a measurer arrives at a race course distant from his calibration course, he will not be checking for equilibrium by repeatedly measuring over a calibration course before measuring the actual race course.

I do not believe an actual "warm-up" ride is necessary, but I would recommend waiting 15 min before starting measurements if the bike is taken from a car on a warm and sunny day.

Last Saturday I did a calibration early in the morning at 600 kPa, but when I removed the bike from the car in the afternoon after a two-hour drive, pressure had risen to 660 kPa. It took about 15 min in the shade for pressure to settle down to 615 kPa.
Last edited by neville
peteriegelpeteriegelRCMBB Founder
It's not necessary to allow the tire to come to equilibrium when taking it from a hot car before measuring the race course. The tire will equilibrate as you ride over the first km or so, and the error involved, over an entire race course, is tiny.

With calibration, of course, it could make a difference. By all means, be sure your tire has come to final temperature before you calibrate.
nevillenevilleRRTC Validations Chair
To get an idea of the magnitude of not waiting for equibrium before measuring a course, one can take my experience of last weekend. If I had taken the bike out of the car and immediately measured the 5-km course by riding at 12 mph without pressure monitoring , this would have added an extra 9 feet. Granted most measurers would not be this quick off the mark, but on the other hand cars can get a lot hotter than mine was last Saturday. Note that pressure monitoring would largely compensate for the error.

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