Airless ti(y)res

I do use the Tannus tires.  That is the brand that was on my bike when you rode it (700x23, 120 psi-equivalent).  I have since gotten a new bike, and it has wider rims.  I now ride Tannus 700x28, 110 psi-eq.  I have been riding airless for about 30 years, and have changed brands as manufacturers have gone out of business.  The Tannus seem to be just fine.  Certainly worth it, to avoid any flats.  I find they last at least as long as normal tires, and I don't have to replace tubes.  Overall, I think the cost of airless vs pneumatic are the same over 5,000 miles.

After initial and continuing success with my first WalMart insert, I have found that getting the right match with tires and rims and bikes is tricky. Have quite a collection of useless gear from attempts to get something else to work.  Once, measuring partner Dave Rogers had to pedal downhill with mountain bike style tires.  It was the same setup where the rig evidently had a "ka-chong" settling moment during break-in and I had to go back and add 9' to a 5K.  

Fortunately, though my original setup is doing fine having increased only from about 18,545 to 18,565 cts / mile over 5 years of use.  Whether that is wearing off rubber or the insert is softening, I do not know.  Fine for a flat 5K but for longer rides and hills, I use a 120 lb pneumatic on a lighter bike.

Oscar

I ride a Hybrid bike with the gnarliest tires I can find. This slows me down, but it gives me confidence - no flats yet in 35 years of measuring. I usually keep them filled with nitrogen, which keeps the pressure from falling as rapidly as with air alone. It's not easy to find a bike shop that is savvy about nitrogen. I buy it online and fill the tires myself. 

Mike Sandford, who did a lot of testing of calibration rides with different tires and different road surfaces, said that he believes the calibration differences due to ambient temperature changes is due more to changes in the tire rubber properties than to changes in the tire pressure.

Based on anecdotal experiences I tend to agree. Early on in my measuring career when I used a wider "beefier" tire I would see big changes in my cal constant when the temperature went up. After I switched to a high pressure, much thinner tire, I did not see as much cal constant change with temperature change.

Also, the theory behind the "effective radius" says that it is dependent on the tire pressure, but it is also very dependent on the friction coefficient between the tire and the road surface. Taken to the extreme, if that friction coefficient is zero, then there will be no change to the "effective radius" of the tire even as it flattens more or less against the road surface due to changes in tire pressure.

Are you suggesting, via corollary, that solid rubber tires also undergo an effective radius change, Mark? If so, do solid tires then offer any potential accuracy advantage?

I measure mostly in urban areas in which broken glass, nails, other debris, potholes, and gravel appear on the roads with some regularity. I realize that these reliable annoyances constitute a good reason to take counter readings at numerous random locations during measuring so that a flat doesn't cause the entire measurement to be tossed. However, it sounds as though my big hybrid tires are subject to greater expansion and contraction than thinner tires of any pneumatic ilk.

Has anyone else noticed, while running a race, that the distance between the ultimate mile mark and the Finish of a standard metric-distance course seems excessively long? If tire expansion occurs during the measurement of this course, it seems this could be part of the difference. In the converse situation during measuring, a big drop in temperature during the measurement could cause a difference in the "wrong" direction. I wondered about this in my last tandem marathon measurement, as we recorded the mile marks. We elected to leave the Finish unchanged after the post-cal numbers were negligibly different, even though the late afternoon/early evening summer air temperature had fallen more than 10°F. We did this because the temperature of the pavement had still changed very little from the day's average. 

I would definitely suggest that just because there is no "pressure" change in a solid tire due to ambient temperature change, it does not mean there would be no change in effective radius, i.e., cal constant due to changing ambient temperature. Tire pressure is not the only thing that changes with changing ambient temperature.

All evidence I have, and I believe all Mike has, regarding differences due to changing rubber properties is anecdotal. But Pete Riegel did have some actual data. Many years ago Pete posted a graph he created from hundreds of pre-cal rides he did over the years. Pete always pumped his tire up to 100psi before doing his pre-cal rides. So if changing tire pressure was the only thing that mattered when the temperature changed, then all of his pre-cal rides in different temperatures should have given the same cal constant (since the pressure was always the same). But they didn't. Something else changed with changing temperature that affected his cal constants.

I'll try to find that post Pete made with his cal ride data.

I also seem to recall that moisture affected solid tires, and differently than tubed tires

Pete also posted data showing that his cal constants went up as his same tire wore away.  My airless tire is showing the same thing.

I prefer wheel radius or "effective wheel radius" to "tire radius."  It is a very complicated situation when you add in spoke and rim expansion and Oscar's own pet radius increaser, damp adhered particulate.  Can't find Sanford article but have wondered if ride on the rough section where there might have been fresh tar was later in the morning .  If stickier tar had picked up rock and increased the effective wheel radius, then that has to be a consideration. I have observed damp morning trails leaving a bunch of grit.  Rides on hot afternoon sun trail sections leave little or none.  Cal numbers match that theory.  I think that is why Pete concluded you are better off calibrating on pavement to measure a trail surface.  

As for intermediate readings, none of those are any good till you recalibrate.  I do the same thing but the major use is when I mess up and have to back up the same day.  Front flat and your day is over.  

I gave up on 65 lb, high air volume, knobby tires after having to reride a section of a 5K course to get course > .0008.  Same thing happened with 90 lb tire on fast heat up day when I rode 8 miles out, flipped bike and retraced 8 miles.  

Must admit I do not understand what wheel friction has to do with "effective wheel radius."  But, there is NO doubt in my mind that hotter, higher volume tires recalibrate to lower constant than either airless or higher pressure / lower volume tires.  Only time I have seen an exception was on trails calibrated damp and recalibrated drier. 

 When part of the tire gets flattened on the road surface, then saying there is a radius of the tire (or wheel) doesn't really make sense since it is not a circle. That's why the term "effective radius" is used. What really matters is the perimeter of the outside of the tire (which is not circular) as it rolls across the road surface. Effective radius is this non-circular perimeter divided by 2pi. It is the radius of an imaginary circular tire that has the same perimeter as the actual non-circular tire perimeter.

When you push a tire against the road surface the arc length of the flattened surface of the tire is shorter than when it was unflattened. But in order to shorten the arc length there has to be a force acting in the direction of the arc length. The only such force is from friction. If there is no friction then there is no force and the flattened section simply spreads out to maintain its original length. If there is no friction then effective radius = original radius. If the friction is high enough that it causes a "stick" condition (once a point on the tire touches the ground it doesn't move or slip) then the effective radius turns out to be the distance from the center of the wheel to the ground. If the friction is somewhere in between, then the effective radius will be somewhere in between.

I assume with your next to last sentence you mean that higher volume, low pressure tires change their cal constant more than low volume, high pressure tires due to temperature changes. Generally I agree, but the difference may be due to other tire characteristics that are almost always associated with higher volume tires, such as thicker rubber and wider ground contact area.

Any thoughts from anyone about what practical applications this data implies?

I gather that solid rubber tires aren't necessarily subject to less expansion/contraction than pneumatic tires of the same size. So, the benefit of solids is limited to flats prevention. Right?

Thinner tires, you have determined - when inflated at the high end of their specified range - hold their size with respect to temperature changes better than larger ones, such as hybrid bike tires. So, is it likely that the greater size stability of thinner tires, pneumatic or solid, constitutes a non-trivial or necessary improvement over wider/knobbier tires?

I take the opinions of the experienced measurers here, particularly the engineers among us, seriously. I will be in the market for a new measuring bike sometime next year - races will come back eventually. I want to make the right choices.

Let's back up a minute: A broken in airless tire varies almost NONE between calibration and recalibration. It approaches random. I have to remember to watch where I put the axle and sit up straight if I do not want to have to add a few inches to a course 20 miles away. And that is even with a 25 F ambient increase during the day. Another bad thing about airless is finding one that fits your bike. Also, DO NOT get a large diameter knobby. The one I tried was mooshy requiring a lot more pedal power.

Oscar

I think there is a consensus that thinner, high pressure tires change cal constant less due to temperature change than thicker or knobbier, low pressure tires. The exact reasons for that are still open to debate.

I have no experience with solid tires. But it seems surprising that cal constants of solid rubber tires would not be affected by temperature since the material properties of rubber are known to vary with temperature. However, I learned in a quick web search that "fillers" added to rubber can dramatically change this effect. In fact, natural rubber gets stiffer with increasing temperature, but if "fillers" are added to the rubber this effect can be reversed, and the rubbers gets softer with increasing temperature! Perhaps the solid tire people have found just the right amount and type of "filler" to add so that the stiffness does not change with temperature.

My guess is that my airless tire has somewhat offsetting effects: temperature increase softens the tire decreasing effective radius but the CTE of the spokes and rim and loss of weight due rider dehydration increases it.

Oscar

You need to stay better hydrated during your measurements, Oscar. 😅

Maybe we need a discussion on strategies for not having to hunt a restroom.

You mean the "Men's Bushes"? 🚹

Just looked at a write-up from Mike Sandford about change of cal constant due to temperature changes. He calls this cal constant sensitivity to temperature the "temperature coefficient," and he makes a couples conclusions about it:

1) Initial pressure in a tire doesn't affect its temperature coefficient. A tire starting its measurement day at 70% of its rated pressure will show the same change between its pre-cal constant (cooler) and post-cal constant (warmer) as it would have if it was pumped to 100% of its rated pressure at the beginning of the day.

2) Narrower tires have a smaller temperature coefficient than wider tires. Or in other words, narrower tires will have a smaller difference between pre and post cals than wider tires.

https://runscore.com/coursemea...rticles/TempSens.pdf

Good, helpful info, Mark. Thank you.

I am concluding from our discussion and from Pete's article that narrow, high pressure bike tires tend to perform better than the hybrid bike tires I now use. My using nitrogen in them may have attenuated some of the expected expansion and contraction due to road surface and ambient temperature. I am thinking that switching to a narrow, 100+ PSI (pneumatic) tire filled with 95+% N2 may give me the best bang for my measuring buck.

"There are several compelling reasons to use pure nitrogen in tires. First is that nitrogen is less likely to migrate through tire rubber than is oxygen, which means that your tire pressures will remain more stable over the long term. Racers figured out pretty quickly that tires filled with nitrogen rather than air also exhibit less pressure change with temperature swings. That means more consistent inflation pressures during a race as the tires heat up." - Popular Mechanics, referring to car tires (only). Yet, I assume that stable pressure advantage also applies to bike tires.

"By reducing the percentage of oxygen, water vapor and other gases in your (bicycle) tires from 22% to 7% or lower, your tires will maintain proper pressure longer than if you use “plain old air.” For example, with 95% nitrogen in your tires, they retain optimal pressure three to four times longer." - A biker blog

I have never tried solid bike tires. I worry that the ride could be more tiring when measuring a marathon or even a marathon. What advice can you offer me about this?