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Calibration Madness

mikewickiser 2mikewickiser 2Member
I have an odd story to share.
When in Albany measuring the new Freihofer's Run for Women course with Jim Gilmer, I noticed that my constants were off a bit from what I had recalled them lately.
No biggie, I know the Empire Plaza cal course is as solid as it can be, having been measured several times as part of workshops and Validation/Verifications.
I began to doubt my home cal course and decided to give it a check. Thursday I wheeled it and got 999.5 and 998.5 feet. Not too far off but I wasn't satisfied.
Friday I figured I would calibrate on my home course and the ride a couple miles to another cal course I had laid out near the Erie Canal Towpath.
On the first 4 rides (home course) I got poor agreement between the 4 rides. I rode the cal course another 4 times. Poor agreement and the counts were going UP.
OK, I am getting a flat tire. Change the tube and after emptying 2 CO2 bottles I drove back home to get my bike pump.
Air up the tire and all appears to be good to go. Now, back to the home course and calibrate. Good agreement, all 4 rides are within a count of each other. Down to the Towpath cal course and after 2 cal. rides I see a 30+ count difference form my home cal course. the 2 rides are off from one another but I am really concerned about the 30+ difference.
Now,I am getting worried about all the measurements this year on that cal course. Back to the home course and check it with a steel tape. 6 lengths @165' each plus 10 feet and an inch or two. I breath a sigh of relief and stop rubbing my thumb that I smashed pounding nails into the road. The temp was in the 50's so the length was right.
I head back, scratching my head and notice a flat tire on the bike. What the !!? Did I get a bad tube or what?
I went home and took a break. After patching the almost new tube,I let it sit for an hour or so, not loss of pressure.
Back to the cal course, good rides, and away to measure a 5k for Safety forces memorial run. It is in my old home town and I am doing this as a freebie.
As I was riding the course just after 2 miles I notice the bike is handling odd and guess what? The darned front tire is going flat again!!!!. I was able to ride back to the truck before it went completely flat though.
By now I figure I was not meant to measure this course, came home, and got in the hot tub with a beverage.
Saturday morning I get the bike tire and check the tube. It has the tiniest leak in the tread area. The tire looks really good inside but I go out into the sunlight and look really close. In the tread I found the smallest piece of glass buried inside a tiny cut but not showing. Close inspection showed that the glass did cut just through the inner liner of the tire and must have been the source of all my problems. I only saw the cut from the tread side of the tire and never felt the slightest cut or sharpness inside the tire. The tire is repaired now and I will try again after it sits for an hour or so and remains fully inflated.
Sporting a new tube and no glass in the bike tire, I calibrated, measured and recalibrated. The differece in 2 rides of the 5k course was 6 counts. Temp. increased a bit and the post cal average was 1.5 counts less than the pre cal average.
All in all, when I finally got the tire issue resolved, the ride was textbook. WHEW!
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peteriegelpeteriegelRCMBB Founder
I used airless tires of several types for a few years when I started measuring in the early 1980's. I switched to pneumatic after a while. I found that my calibration values did not remain constant as I had expected, and I found the ride to be a bit harsh.

I changed to using pneumatic "Armadillo" tires and pumped them up to 100 psi before every precal. I found the calibration to remain very uniform.

Over the subsequent decades I never had a flat.

Go to the "find" tab on this site and type in "airless." You'll find some discussion there.
AdminAdminMember
I remember that Mike Sandford posted that he believes the change in calibration constant due to temperature change has more to do with the rubber material properties changing than with the pressure change. It would be interesting to do an experiment where you released whatever pressure increase you got due to temperature rise, and see if the cal constant still goes down.

10 years of measuring for me without having a flat during a measurement. But I suppose it only takes that happening once at an inopportune time to make you consider airless. I use a smooth Kevlar tire on my mountain bike that holds 100psi. My cal constant doesn't change much with temperature, probably due to both the high pressure and the thin rubber in the tire.
B bobthurstonMember
Someone recently asked about airless tires so this is timely that the conversation comes back around to it. Like Pete, I used airless tires for a while, maybe 10 years. Eventually I found out that some of my courses, especially on rough surfaces, seemed to have been slightly short, a scary thought.

I switched to pneumatic tires and I gotta say the ride is a lot more comfortable! Also I feel more confident about riding on all kinds of surfaces: pneumatics, when calibrated on smooth surfaces, at least don't seem to lead to short courses.

Although as Mike has pointed out they can certainly lead to other problems!
duanerussellduanerussellMeasurer
Bob, you say some courses "seem to have been slightly short" Based on what?

I have been measuring with airless tires for 10 years, and love them. I also co-measure most marathon and half-marathon routes, and my co-measurer has pneumatic tires. We agree within 3 feet for all of the courses we do together. That, to me, indicates that there is no difference in accuracy between airless and pneumatic. That is a good way to compare, I believe.
B bobthurstonMember
Duane,

The "slightly short" was based on using a pneumatic tire to remeasure certain sections of one of our marathons, and I think a few other courses that were on pretty rough pavement. I found that what I had marked out as a mile, when remeasured with a pneumatic tire, was a few feet short of a mile. I can't remember how short but I think it was dangerously close to wiping out the SCPF.

There could be lots of reasons for this, and I didn't make a careful study of the phenomenon. I regret that now but there it is.

It's encouraging that you get such good agreement with your co-measurer using pneumatics. I have more questions about this but it's probably another thread!
B bobthurstonMember
I'd like to point folks to some great research by Mike Sandford on the topic of pneumatics and solid tires.

A good place to start is surface roughness and tires.

More good articles, by Mike and others, can be found here.

Mike did some extensive research in this area. Most of the old-timers have read it in MN but maybe newer measurers haven't seen it.

I think a good "take away" from this is: Safest thing is use a pneumatic tire, calibrated on a smooth surface. Or use a cal course that very closely matches the surface roughness of the course you are measuring.
duanerussellduanerussellMeasurer
Bob, what psi-equivalent was the GreenTyre you used? I can get them in two different firmness levels.

I would need to see a test like you did, with tires of the same psi. I ride 110 lb psi-eq solid tires, and my co-measurer rides the same in pneumatic. I wonder if he would have the same bounce factor with his hard pneumatics? I need to find a similar surface-comparison stretch like you were fortunate to find.
AdminAdminMember
That's great stuff Bob. Thanks for posting the links.

In the temperature sensitivity study it was determined that pumping a tire up to its maximum pressure, or leaving it at a lower pressure, did not change its temperature sensitivity.

But I don't think it answered the question of whether a tire with a lower pressure rating would have more temperature sensitivity than a tire with a higher pressure rating. For mountain bikes anyway, a lower pressure rating usually means a fatter tire, so their tire width effect would be there. But I have two wheels at home with tires of the same width but significantly different pressure ratings. I'll have to give this a test.
B bobthurstonMember
Duane,
I didn't have a Greentyre, I think that was Mike S. Mine was pretty solid (but did have some give), and it's occurred to me that these newer tires may be a better design both in regard to riding comfort and for measuring accuracy.

Mark, I have been toying with a half-baked (at this point) theory that the thickness of rubber in a tire is a significant variable. I think I read that rubber has a NEGATIVE coefficient of expansion. I hope someone who knows will correct or confirm that.
AdminAdminMember
Pete,
In that 2006 thread you calculate how much the cal constant increases with increasing temperature. You also show that the cal constant increases as temperature rises but the pressure is constant.
Could you put those two together to determine what percentage of the cal constant increase is due to increasing pressure, and what percentage is due to other things?
peteriegelpeteriegelRCMBB Founder
I’ve looked again at the various graphs I developed back in 2006. I found various Excel programs which I used to generate the graphs.

The graphs are:
Constant vs Temperature. This shows that as temperature increases, the constant decreases – the tire diameter increases.
Constant vs date. This shows that as time between measurement dates increases, the constant decreases.
Precal constant vs date. As measurement dates go by, using the same initial pump-up pressure, the constant decreases.
Precal constant vs temperature. This shows that as temperature increases the constant decreases.
Constant vs temperature change. I am not sure what this shows – too much time has gone by since I did the work.

In all of the above it is seen that there is a decline in constant – the tire gets bigger. This is the normal experience of measurement. The postcal is almost always smaller than the precal. My data were derived solely from daytime rides, in which the temperature at postcal was higher than at precal.

There is considerable scatter to the data in all cases, but the trends are clear.

It has been suggested that it is possible to separate the effect of temperature and pressure. I’ve not had much luck with this. The only pressures I recorded were precal pressures, measured using the pressure gauge on my tire pump. I tried a few times to use the pump to record postcal pressures, but a bit of air escaped each time I tried, and I was not satisfied that the readings were solid data.

In answer to Mark's question asking how much calibration changes with temperature, my eyeballing of the graph of precal counts vs temperature at constant pumpup pressure leads to my estimate of a calibration decrease of 2 counts per kilometer for every 10 degrees of temperature increase. With so much scatter in the data a guess is about as good as relying on the linear trend line.

Now I just noticed the graph in my post of 27 January 2006 in which I estimated calibration drop of 5 counts per km for a 10 degree temperature increase. Which guesstimate is right? Reader's choice.

Most importantly, my data are not universally applicable. I used only Armadillo tires, which may or may not behave as other kinds of pneumatic tires.

There are too many variables for my thinker to cope with. What you see is what you get.

I made an attempt to factor in the expansion of the tire rim with temperature, but it led me nowhere.

However – I found that the data was of use to me, as I could use it to predict how my tire would behave under various conditions. Anyone wishing to expand the study will have to begin taking data and seeing where the exercise takes them. The task could become monumental depending on how many tires are investigated.
Last edited by peteriegel

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