Postcalibration is required!
Message from a certifier:
“I have someone who thinks post calibration with solid tires isn't necessary. I see in the Measurement Manual on page 33 that it isn't yet permitted. I remember someone respected in the RRTC community, in the past, finding much to their surprise, that the post cal actually resulted in a larger count with solid tires when the temperature increased. I wonder if it is because of the tire material softening and resulting in a decreased radius?
I have been unsuccessful in finding that publication. Do you remember or have reference to that article?
Complicating the issue was a response on Pete Riegel's forum by someone, saying that he didn't think a post cal for solid tires was necessary. I believe that this measurer jumped on that response as gospel."
The text in question was a letter from Chuck Hinde in Measurement News #87, January 1998. In it he mentioned that his postcal, using a solid tire, was always larger than his precal. Pete Riegel confirmed that this was usual.
The measurer may have been confused by a post from Neville stating that under certain conditions postcalibration was unnecessary. These conditions are not the usual ones existing in a standard measurement.
The fact that for solid rubber tires the calibration constant increases with increasing temperature is not surprising for anyone who is familiar with the properties of rubber when it is both stretched (the rubber tire is stretched as it is mounted on the rim of the wheel) and heated.
Most (but not all) materials expand when heated. There are some materials that do not. One of the best known examples of such materials is water which contracts when it is heated from 0°C to 4°C. Rubber behaves in a similar manner to most other materials when it is heated in an unstretched state - it expands. When rubber is stretched, its behavior when heated is just the opposite - it contracts. The reason for this effect, in simplifed terms, is due to the following:
- Rubber molecules in their relaxed (unstretched) state prefer to be in a coil.
- As rubber is stretched, the molecules straighten out, like unraveling a ball of string. The molecules are prevented from slipping past one another by links between them.
- When stretched rubber is heated, the molecules then have sufficient energy to overcome the force of stretching so that they can return to their preferred coiled state. As a result of this action, the rubber is seen to contract - the tire gets smaller as the daytime temperature increases.
There are some easy ways (for the skeptics or the curious) to demonstrate this behavior of rubber. Hang a weight on a rubber band to stretch it. Heat the rubber band with a hair dryer. The rubber band will get shorter.This message has been edited. Last edited by: Matthew Studholme,
When you change the air pressure in a tire the true radius of the tire changes very little. However the amount the tire flattens under the weight of the rider changes significantly, and this changes the effective riding radius of the tire.
If heating the tire changes the material properties of a solid rubber tire, wouldn't the change in the flattening behavior still be much more important than any change in the true tire radius?
I have to confess I only have experience with pneumatic tires, but it evident that there have been recent advantages with non-pneumatic tires, so that experience from eight years ago is not necessary applicable today. The report that solid tires show an increase in calibration factor with increase in temperature is at variance with the report of Duane Russell (May 8th) that his current airless tire decreases its factor by 0.17% in going from 20 to 90 degrees.
In 2000 Mike Sandford stated the following: “A flat tire will be obvious and will invalidate all measurements made since the last calibration check, because it will be impossible to say with complete certainty that slow leak has not preceded the flat. This is the principal reason why a post measurement calibration is absolutely vital." Of course with a non-pneumatic tire this principal reason disappears. A minor reason for a postcalibration is that it may correct for temperature effect. If the temperature on the race course happens to be midway between that of the calibrations, then it probably works quite well. This might happen if the calibration course is sited beside the race course, but in most cases postcalibration is counterproductive and degrades the accuracy of the measurement, so that it is best not done with non-pneumatic tires.
If a measurer can demonstrate that his non-pneumatic has a calibration factor that consistently decreases with increase in temperature and he measures the race course at higher temperature than that of precalibration, it is not logical to require him to do a postcalibration. The certifier should realize that he has the last word on what constitutes a valid method, and he is empowered to waive anything he sees in the "gospel".This message has been edited. Last edited by: Neville,
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