Does air pressure affect aero efficiency?

pianoman

I'm just asking this question because I've had a nose through my average speeds from the beginning of this year and some of the fastest rides I've done have been on days when either a light rain shower happened or when the wind speed was at its lowest.

Now obviously a windy day will slow you down somewhat but what about air pressure, which I understand drops in inclement weather? Does high air pressure mean denser air to push through hence resulting in a slower time?

Or maybe I'm just not consistent enough :evil:

BeaconRuth

"Aero efficiency" isn't quite the right term but if you changed it to "air resistance" then the answer is yes, most definitely. Ask any seasoned time triallist and they'll tell you that they go fastest on still days when the air pressure is low. Basically, the lower the air pressure, the less dense the air and the less work you have to do to cut through it. If you think of the percentage changes in air pressure you would see on a barometer, then there will be the same percentage change in air resistance, and since air resistance is by far the biggest resistive force you work against when cycling, the difference can be very noticeable.

If you read Michael Hutchinson's book about his attempt on the world.hour record he explains how he really needed a low-pressure day to stand a realistic chance of breaking the record.

Ruth

pianoman

PHEW.

I'm saying that because I was road-testing my new Endura Equipe jersey and my speed was only the eighth-fastest ride I've done all year (I wasn't quite at the races either mind) and I noticed the air pressure in Crosby was 1,030, the highest I've recorded for a ride in 2010. So the new jersey's race-snake cut really IS effective and I haven't wasted £75

BTW this jersey really is the dogs b*******, I find that even small jerseys with non-race-snake cuts fit like parachutes on my puny 34 inch chest so I was planning to use the jersey in time trials. If you fit the same criteria as me (or even if you don't, there's bigger sizes after all) get one now! Oh, and the Equipe Criterium shorts are more comfortable than Giordana - and I never thought I'd say that.

It's good looking, it's aerodynamic, it's British and it's cheaper than Assos. What more could you want?

That's my plug over

White Line

PianoMan wrote:

PHEW.

I'm saying that because I was road-testing my new Endura Equipe jersey and my speed was only the eighth-fastest ride I've done all year (I wasn't quite at the races either mind) and I noticed the air pressure in Crosby was 1,030, the highest I've recorded for a ride in 2010. So the new jersey's race-snake cut really IS effective and I haven't wasted £75

BTW this jersey really is the dogs b*******, I find that even small jerseys with non-race-snake cuts fit like parachutes on my puny 34 inch chest so I was planning to use the jersey in time trials. If you fit the same criteria as me (or even if you don't, there's bigger sizes after all) get one now! Oh, and the Equipe Criterium shorts are more comfortable than Giordana - and I never thought I'd say that.

It's good looking, it's aerodynamic, it's Scottish and it's cheaper than Assos. What more could you want?

That's my plug over

Sounds good. I was think about getting those shorts, and if they're going to be better than my Giordana, then why not!

Also, fixed that for you.

a340driver

"If you half the density you half the drag" and density is directly proportional to pressure (It's called the Ideal gas Law). A huge drop of 30mb below the standard atmospheric pressure (from 1013mb to 983mb) would cause about 3% reduction in drag. You would have the same effect if you were cycling at around 1000' above sea level.

Here's NASA's answer to your question!

http://www.grc.nasa.gov/WWW/K-12/airplane/density.html

My first post!!!

pianoman

it's Scottish

You wouldn't be Andy Murray in disguise by any chance :P

sub55

most testers will tell you that their best rides have been done following a thunderstorm.
so you have reletively warm conditions but with a sudden drop in air pressure.
if we go back to when i started competing many years ago, there was a therory that there was more oxygen in the air, if it was raining. but this therory has been debunked over the years. but many testers still like competing in the rain.

Alex_Simmons/RST

All else being the same, a drop in air pressure from 1020 hPa to 990 hPa will result in a 1% increase in speed.

Simon Notley

As has been pointed out above, it's not actually about air pressure, it's about air density - the less dense the air, the less resistance there will be to your motion. Air pressure is a function of air temperature and air density. Increasing either of these will increase pressure.

The lowest air density will occur when you have an high temperature and low pressure, for example, after a thunderstorm.

If you have recorded both pressure and temperature for all your rides, you can estimate the air density as pressure/temperature. Make sure you convert your units to Pascals and Kelvin respectively though, or your answers will be meaningless (particularly if you use C or F for temperature as you could conceivably end up with a negative air density!)

term1te

I'm sure I read somewhere that in running events you want low pressure (high altitude) for sprinint events, but higher pressure for endurance events. More oxygen about nearer sea level. Maybe the speeds are lower so the aero advantage of low pressure is less? Have to say some of my best times have been in or just after rain. I've always thougth the extra weight of wet clothing and shifting all that water with the tyres would have the opposite effect? You can't beat a still clear early morning after a thunderstorm for the best rides though.

Alex_Simmons/RST

Simon Notley wrote:

As has been pointed out above, it's not actually about air pressure, it's about air density - the less dense the air, the less resistance there will be to your motion. Air pressure is a function of air temperature and air density. Increasing either of these will increase pressure.

Increasing temperature reduces air density.

Air density is also a function of altitude (increasing altitude reduces air density) and to a very minor degree, humidity (more humid air is less dense).

a340driver

"All else being the same, a drop in air pressure from 1020 hPa to 990 hPa will result in a 1% increase in speed."

How did you work that out Alex? Surely the percentage increase in speed will differ depending on the starting speed.

Going from 10mph to 10.1mph requires less effort than going from 20mph to 20.2mph but they're both a 1% increase in speed.

dennisn

You'll go faster at the Mexico City velodrome(8000+ ft.) than you will at one near sea level.

genki

So if I was 1.5% faster this saturday compared to 2 mths ago - same route, same HR etc, and if it was 25C average versus 15C 2 mths ago, is that an improvement, or purely down to the warmer air? Let's assume the air pressure was the same and the wind non-existent.

Alex_Simmons/RST

a340driver wrote:

"All else being the same, a drop in air pressure from 1020 hPa to 990 hPa will result in a 1% increase in speed."

How did you work that out Alex? Surely the percentage increase in speed will differ depending on the starting speed.

Going from 10mph to 10.1mph requires less effort than going from 20mph to 20.2mph but they're both a 1% increase in speed.

Well I probably should have said at reasonable training/racing speeds which is what the OP was doing.

The impact on speed (from different starting speed) is slightly different when talking about changes in air density, rather than changing power required (which is what you are implying).

Using these assumptions:
Temperature 20.0 C
Relative Humidity 50 %
Elevation 50 metres
Mass of bike + rider 92.00 kg
Gradient 0.00%
Wind Velocity 0.00 metres/second
Crr 0.0050
CdA 0.320 m^2

and only changing air pressure from 1020 to 990 hPa

At 30kph you'd get a 0.90% increase in speed
At 40kph you'd get a 0.94% increase in speed

all else being the same.

Alex_Simmons/RST

genki wrote:

So if I was 1.5% faster this saturday compared to 2 mths ago - same route, same HR etc, and if it was 25C average versus 15C 2 mths ago, is that an improvement, or purely down to the warmer air? Let's assume the air pressure was the same and the wind non-existent.

Too many unknown variables to say.

rake

Alex_Simmons/RST wrote:

Simon Notley wrote:

As has been pointed out above, it's not actually about air pressure, it's about air density - the less dense the air, the less resistance there will be to your motion. Air pressure is a function of air temperature and air density. Increasing either of these will increase pressure.

Increasing temperature reduces air density.

Air density is also a function of altitude (increasing altitude reduces air density) and to a very minor degree, humidity (more humid air is less dense).

not so. the real function with altitude is air pressure, its less dense because the air pressure is lower at high altitude. altitude is just a vertical measurement, the air doesnt know or care where it is.
edit. im talking nonsense because the air composition changes at altitude so yes your right.

tri-sexual

agree with some of the above posts
air density is more important than air pressure
warm days = lower air density
cold days = higher air density so require more effort to travel the same distance
higher altitudes have lower air density so its easier to ride quickly ( 1km sprint records attempts are usually done at altitude) but this advantage in endurance attempts is not so important due to the lack of oxygen when compared to that at sea level

a340driver

Alex_Simmons/RST wrote:

a340driver wrote:

"All else being the same, a drop in air pressure from 1020 hPa to 990 hPa will result in a 1% increase in speed."

How did you work that out Alex? Surely the percentage increase in speed will differ depending on the starting speed.

Going from 10mph to 10.1mph requires less effort than going from 20mph to 20.2mph but they're both a 1% increase in speed.

Well I probably should have said at reasonable training/racing speeds which is what the OP was doing.

The impact on speed (from different starting speed) is slightly different when talking about changes in air density, rather than changing power required (which is what you are implying).

Using these assumptions:
Temperature 20.0 C
Relative Humidity 50 %
Elevation 50 metres
Mass of bike + rider 92.00 kg
Gradient 0.00%
Wind Velocity 0.00 metres/second
Crr 0.0050
CdA 0.320 m^2

and only changing air pressure from 1020 to 990 hPa

At 30kph you'd get a 0.90% increase in speed
At 40kph you'd get a 0.94% increase in speed

all else being the same.

That's hilarious I'd love to know how you factored in the relative humidity and an elevation of 50m.
Here you go:
http://www.machinehead-software.co.uk/bike/power/cyclist_drag_coefficient.html

bennett_346

Ofcourse you have to remember that there'll be more oxygen when the air is denser, im not sure how much this would overcome the added drag though.

Alex_Simmons/RST

a340driver wrote:

That's hilarious I'd love to know how you factored in the relative humidity and an elevation of 50m.

Air density is a function of air temperature, barometric pressure, altitude and relative humidity. I created a tool to solve those equations, and the tool is available within some of the files I have posted to the Google Groups wattage forum with excel files set up to assist the analysis of data from power meters in order to assess aerodynamics (and rolling resistance).

Then with that information it's simply a matter of solving for speed from power using the following mathematical model from the paper by Martin et al:



which I have done. It's a cubic equation so a little tricky to solve but do-able in Excel via a couple of methods (Cardano's and Newton's methods).

Pross

Surely the biggest problem is that lower air pressure generally comes with higher wind speeds though? That said I think we've all had that rare float night when we set a PB (and in my case it still hasn't been touched 16 years later!).