Aero position in a tailwind

That is correct.

Right.
Old cycling coach told me once to use a good tail wind as an opportunity to sit up & stretch a bit without loosing much speed.


Edited by Oldteen 2010-09-22 1:41 PM

yes, you can get a *push* by sitting up in a very fast tailwind... however, it can be misleading, as "generally" speaking, outside the wind tunnel the wind is (almost) never exclusively coming at you from one direction.

Also... a constant 25+ mph wind is going to be some pretty serious weather.

20+ mph wind with gusts significantly higher are almost the norm in southern New Mexico. My girlfriend jokes that she's no longer comfortable riding if there's no wind; it's too uncomfortable. High wind is expected to the extent that we have jokes about it.

One of the routes I ride often has a tail wind going uphill, which becomes (of course) a headwind on the downhill return. There's no chance to coast.

It may help intitially, but I have found, that even with the EASIEST of spinning with a 20mph tail wind, I can hit 25-30.  So then it's a 5-10 mph "headwind" and better to be in earo.

But if you want to coast?  Then sit up.

Edited by Kido 2010-09-22 2:00 PM

Assuming you plan to keep pedaling (and don't run out of gears), for all intents and purposes, the sail effect is a myth.  I believe the breakeven point between aero and sitting up (assuming a poor aero position and huge sitting up position) is around 100km/h.

Shane 

Same here man....I love having miles and miles of Pacific Coast Highway right outside my door, but I'm guaran-damn-teed to have to deal with a PITA headwind on at least one half of the ride. 

Shane is right again... keep in mind that the vast majority of the power needed is used to overcome the drag of the rider. Sitting up even in a very strong tailwind won't generate enough force to overcome the drag your body generates.

It's actually very easy to test this... if you can ever find a straight tailwind (hard to find as wind is pretty much always multi-directional), sit up and coast... if you can keep going without slowing down, then yes, the wind force is sufficient to push you forward and overcome the drag... if you slow down (which I can pretty much guarantee you will), the force is not sufficient and you better stop being lazy and pedal.

Ps. Yes, that was a joke. Ds.

Its all about the net velocity.    if your going 20 mph, and you have a 15mph tail wind, it may feel like you have something pushing you because your power is only overcoming a 5mph headwind.  If the tailwind was truely faster than how fast you were traveling, then you should be able to maintain X speed without pedaling on a flat road.

But honestly, that never happens.  Just stay tucked in because even a strong 25mph tailwind gust  doesn't last more than a few seconds.

Are you saying the laws of physics is a myth?  Not sure I understand where you pull 100 km/hr out of?

~Mike

Shane's assumption was that you will keep pedaling and you don't run out of gears.  Which for most people means that if you really do have a STEADY and STRAIGHT 25 mph tailwind...which almost never happens....you will likely be going much faster than 25 mph.  Thus you still have a net headwind. 

If you want to rest and coast/soft pedal at a slower speed than the wind, then I assume the sail effect would take effect.  But if you want to maintain the same power output, you're better off staying aero.

FWIW, I do power training in an area that has a pretty steady 20 mph tailwind with gusts to 25-30 mph. 

When I try to push my FTP with the tailwind (240 watts), I'm going about 28-30 mph, so there is still a net 8-10 mph headwind.

When I do easy recoveries with the tailwind (110-120 watts), I'm still going 21-22 mph, so there is still a net 1-2 mph headwind.  I suppose I could pedal at 80 watts and 19 mph with a 20 mph tailwind to create a sail effect...but it would significantly hurt my overall time.

What is this aero position of which you all speak?  Do you actually hold on to the windbreaking pointy gear/shift things?

Seriously, I can only stay in aero for about 5-10 miles at a time.  I tend to hold onto the hoods when:
a) I have a sustained uphill
b) I have a serious tailwind*
c) My back/hips are killing me

*Less impact than into a headwind. 

It sounds as if you bike balls-to-the-wall the entire ride.  I find that I can do that for no more than about an hour, if that.  So in my world, I'm looking for opportunities to rest and/or conserve energy.  It just seems intuitive to use the periods of direct tailwinds to sail a little.

I tend to bike about 17 mph and only hit 28 - 30 going downhill, never on the flats...even with a tailwind.  

I think your point about still having a net 8 mph headwind with the wind at your back is missing the point.  If the reason you are able to go 30 mph is becuase of the tailwind, then it makes no sense getting aero to reduce the 'net' headwind because doing so negates the benefit of the wind at your back.  Unless you are saying you can bike 30 mph when there is no head or tail wind?

~Mike



 

I'm not going balls to the wall.  It's with a 20+ mph tailwind.  At a max effort, I've crested 39 mph on the flats with that same tailwind (at about 900 watts).  With no wind, or during an out and back, I average about 22 mph for a 40K TT.  The reason I can go 28-30 using my 40k wattage is because of the tailwind...or because the net headwind is only 8 mph and not 28 mph may be a better way to put it.  Either way, there is still a net headwind, and sitting up will slow me down.  I'm not sure why that's so hard to understand.

You say you ride at 17 mph?  Is that with or without a 20 mph tailwind?  If you actually ride at 17 mph with a 20 mph tailwind, then by all means, sit up and enjoy the sail.  Again...the theory shane proposed is that you are continuing to pedal.  If your strategy for making the best possible time is to soft pedal or coast during the tailwind portion of a race on the flats, then you likely have a very bad race strategy.  But your theory of creating a sail is correct.

Either that, or your idea of a 20 mph wind is not the same as mine.  Again...we are talking about steady wind...not gusts which last a few seconds.

This user's post has been ignored.

OK, let me try this, the reason you are going fast is becuase the wind is pushing you faster.  You are exerting the same effort but getting more speed becuase the wind is additive to your speed.  Why is it additive?  Becuase it is pushing you based on your projected cross-sectional area in contact with the force of the wind!  If you reduce that cross-sectional area to minimize the effect of the headwind, you get a corresponding reduction in the force that is pushing you to the higher speed.  Why is that so hard to understand? 

OK, I've stated my belief in the laws of physics and forces and I've seen nothing to refute the reasoning in my OP.  Obvisouly you feel you've made a case but I don't see it.  So, you do what makes sense to you and I'll do what makes sense in my mind and all will be right with the world.  :-)

~Mike

Edited by Rogillio 2010-09-23 6:41 AM

Reread what I wrote (especially the italics).  I did not say that the laws of physics are a myth; in fact the model that I built up for this situation is purely based upon the physics involved and it shows that (if you keep pedaling and don't run out of gears) that sitting up becomes beneficial when ground speed is around 100km/h.

ETA - I have included the spreadsheet if you want to play with the numbers; it is just a trial and error sheet as when I built it I wasn't having any luck with the cubic solutions in Excel.  The assumptions are:  87kg (bike and rider), .005 (Crr), 1.22 (rho), .4 Cda (aero) and 1.2 CdA (sitting).  Note that a CdA of .4 is not a great aero position and 1.2 is a hug sitting position (if you use better values, the convergence point moves to a much higher tailwind.

Also, many people significantly overestimate the wind speed at the road level; the reported wind speeds are measure above the ground (I think it might be 10m but can't remember for sure).  The Beaufort Scale is useful for estimating wind at ground/sea level.

http://www.slushpupie.com/media/images/beaufort_scale.jpg

Shane

Edited by gsmacleod 2010-09-23 7:18 AM




Attachments
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Aero vs Sitting.xls (15KB - 20 downloads)

I looked at your file but don't agree with your conclusions.  The force of the wind on the rider is directly propotional to the crosssectional area in contact with the air.  This is a fact and beyond contention.  If a rider continues to exert the same effort in the presence of a 30 mph tailwind as he was exerting in a 20 mph tailwind, he will go faster.  It has nothing to do with the weight of the rider, coefficients of drag or anthing else.  It's simple relativity.  If the medium in which you are traveling is moving faster than you, the force on your will be maximized when your crosssectional area is maximized.  Again, the is irrefutable.  The only thing that is not completely obvious is what speed the wind will be going faster than you becuase the faster the wind blows the faster you will go.  But all things being equal, the 'cross over' point is exactly the speed you would be biking at given the power exerted that equals the velocity of the wind.  That sentence is awkard, let me try again.   The cross over point for a given power output is when the speed the wind is pushing you is equal to the speed you would be going in the absence of the wind.

~Mike


ETA, neat little calculator:

http://www.exploratorium.edu/cycling/aerodynamics1.html

This shows that with a tailwind (negative input on the speed of the wind), the power required to maintain your speed goes down.  No big revelation here I guess as that is obvious.  What should be obvious is if you hold the power constant, your speed will incread propotional to the force of the wind and that force is directly proportional to your crosssectional area.



Edited by Rogillio 2010-09-23 8:23 AM

The force the wind exerts on the rider is not directly proportional; rather it scales with the square of the wind velocity:

F = 1/2 * rho * CdA * v_wind^2



True



Not true; the power to overcome rolling resistance scales with the mass and ground speed.



Completely untrue; CdA is crucial when it comes to determine the force a fluid exerts on an object.



No.



True, however that doesn't mean that you can't go faster than you would just by sitting up and coasting.



This is incorrect; you can see from the file what happens to speed with a tailwind (in aero) when power is held constant.  You can compare that to the power required to go the same speed when sitting.

Shane

It's somewhat kindergarden math whether or not you're going faster than the wind and it's easy in theory to quantify. However, if there is a benefit to sitting up, like stretching or resting or being able to output more power with less strain, then it's still a good idea to sit up even if the effective headwind is 5-10mph.

The reason is that, as we all know, the resistance the wind offers is not linearly correlated with the speed. The difference in power needed to overcome air resistance going from 4 to 8mph is different than going from 24 to 28mph. So much like riding up a steep hill at 10mph one could argue "get in aero because you have 10mph wind resistance", the benefits (which are different ones from being on the flats with a tailwind) of sitting upright outweigh the benefits of being aero.

In conclusion, depending on your comfort level, length of bikeride, and other factors, you may be better off sitting up with a heavy tailwind; not because you're functioning like a sail (it's still an effective headwind), but because there are other benefits to getting out of aero for a while.

And the force is a fuction of cross sectional area and velocity.

Fwind = r cw A vwind2 /2	where	r :	density of air in kg/m3
		cw :	coefficient of wind resistance, dimensionless
		A :	frontal area in m2
		vwind :	wind velocity in m/s

~Mike



Edited by Rogillio 2010-09-23 8:50 AM

A nice vector diagram but not really germane to the discussion.

The force is a function of the cross sectional area and relative velocity of the wind as given previously:

Force = 1/2 * rho * CdA * v_wind^2

Shane

I think I know why there is confusion on this topic. Shane is assuming a constant power output from a rider (in which case Shane is correct), whereas Rogillio is assuming a constant speed at reduced power (where Rogillio is correct).

Lets assume we have a rider capable of a putting out a sustained 250 wats of power, hes on a smooth and level road and there is a 72 km/hr (20m/s) tail wind. He's 75kg, has standard racing tires (Crr = 0.005) and a choice of Aerobars (CdA = 0.28 m^2) and sitting up on the Bullhorns (CdA = 0.40 m^2)

IF the rider is planning to ride at or slightly above his normal racing speed - say around 36km/hr, he's better off sitting up to catch the wind. His power output to ride 36km/hr will be quite low (around 40 w) and identical whether he's in the Aerobars or upright on the bullhorns. Going 36km/h in a 72km tailwind means lots of energy saved for the run.

BUT if we assume he will crank out 250watts in a 72 km/hr tailwind, then things become interesting... At 250 watts he will be able to hit about 50km/hr sitting upright and 52.3km/hr in the aerobars. So yes, Shane is correct in this scenario - though personally, at that speed I'd rather give up the 4% in speed and be upright just to be closer to my brakes 

To model this properly requires solving partial differntial equations - I'm not going to do that here. I did do a simplified model using excel and performing a 'goal seek' to numerically get my results. I've attached the spreadsheet.

Mark Galanter
(Hopefully a better engineer than triathelete)

EDIT: for some reason BT renames the file extension when downloading it... just rename it to an .xls extension and it'll work...





Edited by mgalanter 2010-09-23 9:01 AM




Attachments
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Bike Power.xls (27KB - 22 downloads)