This morning's questions...

[Tom]

- How much is aerodynamic drag reduced when the air is saturated with water? It seemed like the bike rolled incredibly well.
- Was it the vitamins? I felt like I've been living on dog food for the past couple of weeks, except dog food is probably a better diet. So I ate a vitamin yesterday. I did not use a patch. A 'rest day' ridden entirely by perceived exertion - no HRM, computer on TOD display - had the highest average pace for the year and I pushed the pace perhaps twice on small uphills. The rest of the time I rode relaxed and what I repeated most to myself was "Slow Down".
- Do my neighbors think I'm nuts? I rinsed my bike off using a watering can because the bike was leaning against the wall over three pots of annuals that needed watering and I am fundamentally lazy so I did two things at the same time. I think they think I think my bike needs watering. Actually I did three things. Weird behavior makes them leave me the hell alone. The retired doc next door gets it which is good because I like him and his wife. The rest of them do things like go away for the weekend with the house alarm on and the windows open so every time the breeze comes up a siren goes off over there.

[saab2000]

1. You had a good day. Saturation should increase drag, not decrease it. But the difference would be so small as to be essentially not measurable unless you are NASA.

2. You probably had a good day. I take a multi-vitamin too, but have been told by people who know more than I do that most of the vitamins in them are just water soluble and you pee them out. A proper diet will have vastly more good effect.

3. If you really want to water your bike, get one of those high-pressure washers they use to clean cyclo-cross bikes with in Europe. You can ruin a groupset in just minutes with one of those things! :beer:

[martianbait]

Picking up on saab's advice,

next time you water your bike, use the power washer. In front of your neighbor's open windows. Give them a reason for the alarm to go off. :D

[rpm]

I've read that all the old British time trial records were set right after rains--something about more oxygen in the air. After the next rain, have a go and see whether you can replicate the effect. Or, just accept the good day as a gift, randomly striking you from nowhere, just as the bad days seem to do.

[BdaGhisallo]

Actually humid air is less dense than dry air so you, ceteris paribus, you will go faster in humid air.

I am no physicist but I believe it has something to do with a volume only being allowed to hold a certain number of molecules and when the air is saturated with moisture, the water molecules displace the molecules of gas that make up the air. the molecular weight of water vapor is lower than that of oxygen and nitrogen, hence the air becomes less heavy and therefore less dense.

I think that's how it works, though I look forward to hearing from someone who really knows how it is.

[72gmc]

glad you had a good day, tom. your legs are strong, your plants are watered, your neighbors are repelled, and your pee is full of vitamins. you're outscoring me 4 to 1 (i'm a multivitamin user) but i look forward to tomorrow.

[Too Tall]

Lightning strikes. Every dog has his day. Living right. Cashing dividends. Get the HSG. Book tix to Paris today. yada yada

HEY! Nice to hear stuff like that.

PS - quit putting garbage in your body ok?

[Chief]

Actually humid air is less dense than dry air so you, ceteris paribus, you will go faster in humid air.

I am no physicist but I believe it has something to do with a volume only being allowed to hold a certain number of molecules and when the air is saturated with moisture, the water molecules displace the molecules of gas that make up the air. the molecular weight of water vapor is lower than that of oxygen and nitrogen, hence the air becomes less heavy and therefore less dense.

I think that's how it works, though I look forward to hearing from someone who really knows how it is.


A+ Correct you are! When water vapor content increases, the amount of Oxygen and Nitrogen decreases per unit volume and the density will decrease because the mass is decreasing. Dry air is more dense that humid air! The density of staturated air is about 55% of dry air; therefore, with everything else being equal the aerodynamic drag in saturated air will be 55% of that for dry air.

Ref: http://www.engineeringtoolbox.com/density-air-d_680.html

[Tom]

But, and I think Saab's a pilot so he ought to know, he mentioned drag increasing. I don't know, it makes my head hurt. I do know it all can be solved with heavy drinking so that's what I'm going to do after I finish putting the first coat of paint on my kitchen ceiling.

[Chief]

But, and I think Saab's a pilot so he ought to know, he mentioned drag increasing. I don't know, it makes my head hurt. I do know it all can be solved with heavy drinking so that's what I'm going to do after I finish putting the first coat of paint on my kitchen ceiling.

Even pilots can be mistaken. Trust me, the science says that density of air decreases with increasing humidity and relative to dry air it can be significant. Since aerodynamic drag is proportional to the density, drag in saturated air is nearly 1/2 (45%) less than for dry air and since on the flats aerodynamic drag is the largest resistance, it can have a big impact on your power requirements.

[ada@prorider.or]

well i did a calc
power 400 watts
rider 70 kg bike 6.8 frontal 0.33 drag 0.55 roll 0.0055
air 20 degree c pressure 1012 humidity 40 % so you get air density 1.196
efff drive train 98%

speed is then 52.252 km/h


then i only changed the humidity to 80 %
speed is then 52.307



as you see not that much!!!!!!!!!!


air density 1192 kg/m^3

[Chief]

well i did a calc
power 400 watts
rider 70 kg bike 6.8 frontal 0.33 drag 0.55 roll 0.0055
air 20 degree c pressure 1012 humidity 40 % so you get air density 1.196
efff drive train 98%

speed is then 52.252 km/h


then i only changed the humidity to 80 %
speed is then 52.307



as you see not that much!!!!!!!!!!


air density 1192 kg/m^3

You are corrrect, the difference is indeed small. I must admit that I made a mistake in my computation. The difference between the density of dry air and saturated air is only about 1% not the 45% that I noted. Hence, for the same power output there is less than 0.3% difference in speed as your computations demonstrate.

I also owe Saab an apology, while the drag for saturated air is less than that for dry air, the difference is indeed small. It is difficult to imagine that one would be able to detect the difference without the benefit of sophisticated instrumentation. I hope this eliminates any confusion.