Aero position in a tailwind (Page 2)

mgalanter - 2010-09-23 8:57 AM

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...

mgalanter - 2010-09-23 10:57 AM

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 

pga_mike - 2010-09-22 10:00 PM

Seriously, I can only stay in aero for about 5-10 miles at a time.   

JoshKaptur - 2010-09-23 9:57 AM


Sounds like your bike fit is WAY off.

gsmacleod - 2010-09-23 10:27 AM  There is a problem with these results; based upon the fact that (if the rider can continue to apply power) they will be able to ride faster than the tailwind, I get the following using your data:
Shane

mgalanter - 2010-09-23 10:13 PM

You're right Shane - the calcs were not completly right. When I got home today I took a close look at this and fixed it up. The key problem with the existing equations is that they only work for relative headwinds. When you have a relative tailwind, you need to take the road velocity as negative for the power calcualtion because the force is pushing in the oppostite direction of travel. Also, the standard Coefficient of Drag no longer applies at a tail wind. You now need a coefficient of drag for the back-side of the bike, since the fluid flows over the rider from back to front. Anyways, I worked my way through it and published what I hope is a definitive explanation on my website: http://tri-it-and-you-may-like-it.webs.com/

for those not interested in the physics and math, I'll just list my conclusions here:

 So what are my conclusions?

1. As long as you are travelling slower than the tailwind then go ahead and sit up!

2. A strong tailwind will allow you to go faster. At some point you will 'outrace the wind' and start having a relative headwind. At that point get into your aerobars.

3. The stronger the relative headwind the more important it is to be in aerobars. When riding into a strong headwind - even if the bike is moving slowly on the road - GET INTO YOUR AEROBARS!

4. If you can produce 200watts when riding in a tailwind... the sail effect won't take place untill you are getting close to 200 km/hr... in other words, Headwid, Tailwind, it does not matter... When pushing high wattage stay aero!

5. Tail winds are the best and headwinds suck!

Cheers,
Mark

mgalanter - 2010-09-24 10:51 AM Lets assume that the course is perfectly straight out-and back 40km course (20km north and 20km south).  The wind is 20mph (32km/hr) from the south. The rider is puting out a constant 200wats and riding in aero position. The total time to cover this distance: 82.8 minutes.

The same rider on the same course in perfectly still air will cover the 40km in 66.9 minutes.

You are right - power is proportional to the square of wind velocity and the cube of road veloity. So what you lose on the upwind is never fully regained on the downwind.

Calculations are below...

mgalanter - 2010-09-24 8:51 AM Lets assume that the course is perfectly straight out-and back 40km course (20km north and 20km south).  The wind is 20mph (32km/hr) from the south. The rider is puting out a constant 200wats and riding in aero position. The total time to cover this distance: 82.8 minutes.

The same rider on the same course in perfectly still air will cover the 40km in 66.9 minutes.

You are right - power is proportional to the square of wind velocity and the cube of road veloity. So what you lose on the upwind is never fully regained on the downwind.

Calculations are below...

 

mgalanter - 2010-09-24 12:35 PM The power to overcome rolling resistance is less for the headwind because you are riding slower into a headwind. Power to overcome rolling resistance is equal to the coefficient of rolling resistance x mass x accel. of gravity x Velocity.  The only component of that equation that is variable is the Velocity.  

Rogillio - 2010-09-26 12:49 PM

Looking back at my OP, I believe your proved my assertion to be correct and that the sail effect is not a myth at all but sound physics.  Like all things, there are limits and if you allow the wind to push you to the point where you are no longer going faster than the wind, the benefit is gone.

gsmacleod - 2010-09-26 5:56 PM

The problem with you assertion is that there is no point where, assuming you are still able to put power to the pedals, that you aren't facing a headwind.  If you aren't going to pedal when you have a tailwind, then by all means sit up to make yourself a sail.  However, if you are planning to put even a small amount of power to the pedals, then staying aero is still the better option.

In the attached file you can see that when air speed and ground speed are equal (and in the same direction) your position doesn't matter as you have zero air resistance due to the effective windspeed being equal to zero which only leaves the power to overcome rolling resistance (in this case, ~35W).

However, if the rider is still putting a small amount of power to the pedals, it will be easy to travel faster than the speed of the tailwind.  The second column shows what happens in the same situation if the rider applies enough power to travel only 10km/h faster than the tailwind (so 40km/h ground speed); the aero rider requires 61W while the rider sitting up as a sail, requires 67W.  So, in this case the aero position saves ~10% of the wattage required to travel 40km/h.

Shane

Rogillio - 2010-09-26 8:29 PM

If the air is moving 10 mph faster than you are biking, what does getting 'aero' do for you?  The air out in front of you is moving away from you faster than you are moving

My assertion is 100% correct.  You are changing the problem to fit your argument.  Read my OP again and tell me if it is factually correct or not.  What I wrote, is "if the wind is traveling faster than you, it behooves you to sit up....".   You are changing the problem by saying if you continue to pedal, you will NOT be faster than the wind.  OK, if that is the case, read the rest of my OP, "if you are traveling faster than the wind, stay aero..".

Rogillio - 2010-09-26 7:29 PM

If the air is moving 10 mph faster than you are biking, what does getting 'aero' do for you?  The air out in front of you is moving away from you faster than you are moving.  

My assertion is 100% correct.  You are changing the problem to fit your argument.  Read my OP again and tell me if it is factually correct or not.  What I wrote, is "if the wind is traveling faster than you, it behooves you to sit up....".   You are changing the problem by saying if you continue to pedal, you will NOT be faster than the wind.  OK, if that is the case, read the rest of my OP, "if you are traveling faster than the wind, stay aero..".

~Mike

jsiegs - 2010-09-27 1:26 PM

Well, obiviously it all depends if the air flow is laminar, and if you're dimpled.  Anyone know your Reynolds number?

Obviously, if the air is pushing you from behind, you will be best served by catching as much of it as possible, that's just common sense and sound physics.  The point gsmacleod and many other are trying to make is that that scenerio will virtually never play out in real world traning/racing situations.  For all intents and purposes (to us triathletes), the "sail effect" is a myth.  For what I would call impractical purposes, i.e. 200+mph tailwind, not pedaling, or pedaling @ < 35 W, it is very true.  So maybe if this were mythbusters, I might call it "plausible". 

Rogillio - 2010-09-27 2:48 PM

In the real world most of us are not going to be biking 30+ mph...even with a 20 mph tailwind.   You also need to consider how push you still have left on say 53/23 gears at 30 mph.  So on paper you might be able to continue to pedal and gain speed indefinately but in practice, on flat ground there is a very real limit and you would be spinning like a hampster put not delivering much (if any) power to the drive train.

Rogillio - 2010-09-27 3:48 PM


In the real world most of us are not going to be biking 30+ mph...even with a 20 mph tailwind.   You also need to consider how push you still have left on say 53/23 gears at 30 mph.  So on paper you might be able to continue to pedal and gain speed indefinately but in practice, on flat ground there is a very real limit and you would be spinning like a hampster put not delivering much (if any) power to the drive train.

So you, et al can call it a myth all you want and I'll just sail merrily on.

~Mike