BIKEPGH MESSAGE BOARD ARCHIVE

My drag coefficient is undoubtedly higher than assumed in the model due to riding a hybrid.

Stinks, (will get a road bike when I can) but it's ingrained the right habit, push pretty hard up the hills, mostly relax elsewhere, and don't fight the wind, you can't win.

My friend, who is a mechanical engineer who worked for a long time in industry, claims air drag is a fourth-order relationship. This model is using second order, which is assuming laminar flow, I think.

Power to overcome aero drag increases with the cube of speed.

Power to overcome rolling resistance increases linearly with speed.

I like the following site. It has nice interactive graphics that allow you to see what dominates in various scenarios. There is some explanation of the math at the bottom of the page too.

http://www.gribble.org/cycling/power_v_speed.html

There are techniques for determining your CdA and Crr from rolldown tests using a bike computer that can accurately record speed vs. time data on a second by second basis. A Garmin 500 with an auxiliary speed sensor will do the trick (just to name one example).

I can offer more info on that if anyone is interested.

4 order with small coefficient and it becomes important around subsonic speed. At speed of 10-20 m/s air could be considered as laminar flow. The density of air plays a big role here. Everything is different in water.

This is all way above my head, but you guys sound like the people I should ask.

I've been telling my friends, everybody rides slower in winter because you are pushing your way through air of increased density (AoID).

Hence, more work, thence more slow. Any legitimacy to this perspective?

I always feel like more clothes makes the ride slower. But you're probably right.

No, I think that's an additional factor, which makes it a second-order function I suppose. That's why I'm twice as slow in the winter.

And then there's all the extra clothes I carry, because I can never figure out the Goldilocks outfit. So I have a pair of heavier gloves, an extra layer, the balaclava, the shoe covers, and that makes me third-order slow.

Vannevar - for sure, colder air is more dense, and yes, this increases your aero drag.

also, I *suspect* that cold weather increases rolling resistance by making the tire casings and rubber stiffer and less compliant.

Winter clothing vs. summer clothing will affect CdA by about .01 to .02. That's for tight fitting clothing. The impact can be much worse if you wear a poofy jacket, which can act like a parachute. Just zipping up that windbreaker can make a big difference.

Lesser effects are the added weight from cold weather clothing. And sometimes such clothing is more restrictive to movement. That can't help either.

There are conflicting studies on the human performance effects of cold weather. Some studies show negative effect. Some show no effect.

sorry, the units of measure for the .01 to .02 change in CdA are m^2.

Thank you, Tom, I feel validated! V.

air resistance increases as the square of surface area as well, not taking into consideration turbulence.

Vannevar the number I've see posted for automobiles is that winter air is roughly 7% more dense then summer air (0 C vs 35C or in that temp range).

I dont know whther density increa

"There are conflicting studies on the human performance effects of cold weather. Some studies show negative effect. Some show no effect."

Interesting. I would have assumed that, just like an internal combustion engine, the human body would benefit measurably from the greater oxygen content in a given volume of air. Any thoughts on why the studies don't bear that out?

Thumbnail guess, the air density differences are real but of minimal effect. I would rate other things higher, such as effects on the body, like inhaling increasingly large amounts of cold air to maintain increasing speed.

air resistance increases as the square of surface area as well, not taking into consideration turbulence.

turbulence, your non-aero dynamic shape, causes a lot of drag.

I had a Zipper fairing on a Mtb which was outfitted for road use.

This looked more like a Harley or police motorcycle, then like a HPV.

What I found was, at about 12 mph air resistance stopped increasing as fast, and I was able to move from 12 to between 14 and 16 mph with no or little increase in energy output.

I also found that the increased surface area of the fairing would 'catch up' with me around 48 mph. I was never able to get the bike above 48, I did hit 48 a few times, but never above. In one instance, Danny Chew and I were 'racing' down the hill into Freeport (on red belt(?)). I hit 48, and soon after Danny was able to pass me, even drafting him I could not get the bike above 48.

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Vannevar the number I've see posted for automobiles is that winter air is roughly 7% more dense then summer air (0 C vs 30C or in that temp range).

I don't know weather density increases are offset by humidity reductions as temps drop significantly below 0 F.

Here are a few studies on the athletic performance effects of cold weather....

http://www.ncbi.nlm.nih.gov/pubmed/22443080

http://www.ncbi.nlm.nih.gov/pubmed/22426577

http://www.ncbi.nlm.nih.gov/pubmed/21563033

The affect of air density on aero drag is pretty well understood - no speculations required.

Just go to that gribble.org site and play with the air density calculator, then use those different values of rho (air density) in the power vs. speed calculator.

So nobody wants to know how to determine your CdA and Crr, huh?

@Tom to be exact air drug should include vortexes and layer sliding along side. But we don't need to count those.

And cold weather and more stiff tires -- less rolling resistance. Rail Road has one of the best (smallest) rolling resistance. but from the comfort point of view and dependency output power on comfort I've read recently an article showing that comfort is important.

@Vannevar yes cold air is more dense. Pilots always have special formulas and/or charts to calculate max load depending on air temperature to estimate take off length.

http://www.engineeringtoolbox.com/air-temperature-pressure-density-d_771.html

http://en.wikipedia.org/wiki/Density_of_air

http://www.denysschen.com/catalogue/density.aspx

http://www.grc.nasa.gov/WWW/k-12/airplane/drageq.html -- look for the Ro (Greece letter). It's a density.

@Been_there_done_that Yes, turbulence is important too. BTW recent studies shows that second person in peloton plays important role also since the second person fights turbulence.

@Tom Do you have an access to a wind tunnel?

Oh, forgot to tell -- Cd depends on speed itself -- this is how power of 4 is appeared in final formula.

http://faculty.wwu.edu/vawter/PhysicsNet/Topics/Dynamics/Forces/DragForce.html -- look for Reynold's Number.

@ Mikhail - no windtunnel. not needed thank goodness, at least not for understanding zero yaw performance. I have been doing testing using the Chung virtual elevation method.

http://rothrockcyrcle.wordpress.com/2012/12/02/the-world-is-my-aerolab-2/

There is a link to Robert Chungs indirect CdA / VE paper in this post.

I have been doing powered tests (using a PowerTap hub), but one can do roll down tests using the same concept, but with power = zero, of course.

Regarding CdA, or more properly Cd - it is a constant for all practical purposes in bike riding situations where aero drag is significant. (i.e. I do believe it is a turbulent flow scenario, not laminar.)

Regarding Crr - I speculate that Crr goes up as temperature goes down just simply based on the idea that pneumatic tire casing deflections require more energy when the casing is stiffer, regardless of temperature. And I tend to think cold temperatures will make the casings stiffer, not softer. I am not sure you can draw an exact parallel with a rail car wheel. One is a pneumatic device, the other is not.

Air density increases about 2% per ten degrees, by the ideal gas law. Icebiker did a study on why we're slower in winter. I find winter tires slow me down around 5-10% right there.

Personally, I would expect the increased amount of oxygen to improve spin efficiency and thus overcome any corresponding increase in wind resistance... thus making a thirty degree drop in temperature negligible. Though, in fairness, I believe it is negligible regardless.

The real difference is in winter clothing selection; everyone knows that you go faster in spandex.