My prius-owning, fellow-HPV-riding, efficiency-conscious, chair-of-the-local-university-physics-department friend has explained to me the increased wind resistance as temperature drops and it is very significant and far more so than rolling resistance even at moderate bicycle speeds.
I wonder how much I'd have to slow down in the winter to completely offset the additional drag due to the colder air? I will ask my physics friend and see what he says.
Everyone seems very certain that the air resistance is a much more important factor than rolling resistance. But nobody cites any evidence. Brian, how does your experience biking in the winter allow you to differentiate between extra wind resistance and extra rolling resistance? Paul, your physicist friend can easily calculate the increase in wind resistance, but there's no fundamental formula that tells you how the characterisitics of rubber change with temperature. So ask him how he knows how that side of the equation changes with temperature.
After digging for a long time, I did find some data on rubber characterisitics and, and I also figured out how to do a quick and dirty experiment that would be able to show whether there was a significant change in those characteristics with my bike tires. That was for a discussion on a winter-bicycling list. I'll try to find that data to share with you, but in the meantime I'd be interested to hear what basis you have for your confidence that air resistance is more important.
I will certainly be quizzing my physics friend for all the dirt, but in the meantime (he is on vacacation this week),
is it your allegation that for some reason rubber tires have more rolling resistance when it gets colder out? Now there is some data I would like to see, since my practical experience with rubber is that it is less flexible and "grippy" when it gets colder out.
Anyway, at anything over 20 mph or so (and it increases dramatically with your speed), rolling resistance is a joke compared to wind resistance. Certainly your web searches must have shown that.
The effects of aerodynamic drag as a function of drag coefficient, air density, and velocity, are probably reasonably well documented for automobiles and even conventional bicycles; and the information is probably accessible via the internet.
In contrast, quantifying the factors that affect rolling resitance of tires (such as viscoelastic properties, road surface roughness, and tire deformation) would involve considerably more reseach IMHO.
My assumptions, posted just about a year ago on this thread, were based on qualitative observations. In other words, I don't have quantitative data.
While the rolling resistance analysis might be an interesting subject for a graduate thesis, my simplistic answer would be that while bicycling at temperatures below 20 deg. F, I'm more concerned about how to maintain circulation in my extremities than about the rolling resitance of my tires.
-Brian
__________________
'gonEfishnt
2001 5S, "Sputnik"; Various Modifications; 76.6 LMPG at 75K miles
Anyway, at anything over 20 mph or so (and it increases dramatically with your speed), rolling resistance is a joke compared to wind resistance. Certainly your web searches must have shown that.
IIRC from what I've read in various technical papers on this subject 40 MPH is the "magic" number when the other losses of friction and combustion efficiency are included.
is it your allegation that for some reason rubber tires have more rolling resistance when it gets colder out? Now there is some data I would like to see, since my practical experience with rubber is that it is less flexible and "grippy" when it gets colder out.
Tire traction/grippiness is not the same as rolling-resistance. Tires designed to be gripper tend to have more RR, but the traction itself is not what causes the RR. Rolling resistance is primarily from the tire rubber deforming. The fact that the rubber is more flexible at higher temperature means that less energy is needed to "bend" the tire sidewall and tread components as it rotates.
This is also why higher tire pressure leads to a drop in rolling resistance (tire holds a "rounder" shape). A studded or treaded tire will have much more RR than a slick tire as the bits of tread each deform.
The reason there are no specific data for tire rolling resistance in part has to do with the fact that rolling resistance coefficient values need to be determined experimentally and cannot easily be predicted. Also, rolling resistance coefficient also varies based on the driving surface, as deformation of the road surface (ie dirt roads) or evacuation of water on a wet road, all require energy, and will not affect all tires equally. Without some sort of systematic test for all tires similar to the EPA driving cycle, rolling resistance between tires cannot be easily compared, and even then, those values will vary dramatically based on the bearings used on the car, alignment characteristics, and many other factors.
I located the data I had found on rolling resistance vs. temperature.
On the Bayer Rubber web site, I found a curve of "tan delta" vs temperature for two different kinds of tire rubber. This was back in 2004; I couldn't find it again on their web site (it appears they sold some of their rubber business to a German company), but I did save a pdf of that page. PM me with your email address if you'd like me to email a copy.
Tan delta is a measure of the energy lost to hysteresis as the tire is flexed. It's a relative measure, that also depends on stiffness--it's the fraction of the energy needed to flex the tire that is lost to hysteresis as the tire. Thus, the rolling resistance should be proportional to the product of the stiffness and tan delta.
(If that didn't any sense to you, and you care to dig into the details, I recommend this explanation: http://darwin.nap.edu/books/0309094216/html/42.html
from "Tires and Passenger Vehicle Fuel Economy: Informing Consumers, Improving Performance -- Special Report 286", (2006) Board on Energy and Environmental Systems (BEES), National Academies Press, p. 42)
The plot of tan delta for "s-sbr" rubber with silica instead of carbon black is at about 0.2 at 20 C. It drops to 0.15 at 30 C and goes up to 0.25 at 10 C. Then below 10 C it rockets up reaching 0.5 at 0 C and 0.7 at -10 C.
That means that even without factoring in stiffness, the rolling reistance can increase dramatically at lower temperatures.
More later on my own measurements, and on the relative importance of rolling resistance and aero drag in the insight in general.
This IS some high tech discussion of tire rolling resistance. Thanks for consending all the technicals to a "plain" english sentence Charlie.
And this has also been one of the undefined unknowns of our commonly expereinced cold MPG losses. Some of the other more obvious are combustion effifiency losses during warm-up and with a "colder" engine.
As such I'd hate to see this discussion become lost in the clutter of a hot air intake thread so I'm splitting it out into its own thread.
Please let the group know what you discover as your work progresses.
Thanks for the thread split--I was thinking that might be a good idea.
Before I found that Bayer Rubber data, I did some crude experiments. I was mostly interested in bike tires at the time, so these are using bike tires and inner tubes:
1) Bounce test. I dropped a wheel with a semi-knobby MTB/hybrid tire from three feet and measured the bounced height, and assumed that most of the loss in height was energy absorbed by the rubber in deforming and springing back. 50 psi.
2) Spring test. I hung a weight (cement block) using an old inner tube. I expected to bounce it up and down and count bounces, but it damped out too quickly, so instead I set it wobbling in a way that alternately stretched different pairs of the four segments of tube it was hanging from (I'm not sure that's clear but the details aren't too important).
Results:
50 F garage:
1) Bounce: loss of 8 inches out of 36 inches drop height (i.e., it came back up to 28 inches).
2) Spring test: about 10 cycles of joggling before it damped out "pretty much completely".
After perhaps 15 minutes at 5 F outdoors:
1) Bounce: Loss of 14 inches (ie. came back to 22 inches).
2) Spring test: 2.5 cycles.
The next morning, after sitting outside overnight at between 0 and 5 F:
1) Bounce: Loss of 16 inches (ie came back to 20 inches). With air added to bring the pressure to 52 psi, loss of 14 inches (ie came back to 22 inches).
2) Spring test 2.5 cycles.
Conclusion: Very roughly, rubber hysteresis losses appear to increase by 2 to 5 x going from 50 F to 5 F.
Note that what can often happen is that if the tires are very lossy at low temperatures, the loss generates heat in them, and thus to some extent solves the problem. In a big, heavy vehicle, the losses are big enough to heat the tires pretty effectively. On a bicycle, that doesn't tend to happen much, so the high rolling resistance in the cold is a bigger problem. I imagine the Insight is somewhat intermediate in that regard--significant self-heating on the tires, but not as much as in a big car. And even with the self heating, the tires are still colder than they would be in a warmer environment, since the self heating happens then too (although to a lesser extent).
Interesting test results, and I concur that it is a good experimental way to quantify the difference.
Intuitively I would expect the real world losses due to temp to be much less on a tire that is run at 45-55 PSI vs 35-40 psi, the deflection is less.
The tire heating in winter is somewhat offset as the tire hits the cold road ahead, I have one of those non contact IR temp probes, and will do some tire temp rise experiments when it gets cold.
Air density changes between winter and summer is a big factor as well. Any pilot will tell you that the take off speed in cold air is much shorter than when it is hot.
Getting real numbers is the problem.
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