Do headwinds affect your speed more than tailwinds?

[Osiris]

When riding in windy conditions, whether on a recumbent or a diamond frame, my perception is that tailwinds help a little, but that headwinds have a much bigger impact on speed.

These are just numbers I'm making up for purposes of illustration, but it seems that at a rolling speed of say 20 mph in windless conditions, a tailwind of 10 mph can add 1 mph to your speed at the same power output. Under the same conditions however, a 10 mph headwind can reduce your speed by 5 mph at the same power output.

Is this actually the case, or is it just an illusion?

 

[Charles.Plager]

It's almost certainly true.

Aerodynamic drag adds a force proportional to speed squared. So increasing the wind you hit by 2 mph is going to have a larger effect than decreasing it by 2 mph.

 

[tiltmaniac]

Definitely the case-- headwind saps energy to the square of it's relative speed.

Tailwind allows higher speed of about the tailwind speed (there is still rolling resistance, but aero dominates, so it is mostly about the wind plus the wind you create by moving through the air).

10mph headwind:
If you're going forward into a headwind at 10 mph, with a 10 mph headwind, you'll feel drag proportional to (10+10)^2 (so proportional to 400), where if you had no headwind you'd feel drag proportional to (10)^2 (100).
So, you got an extra 300 drag.

10mph Tailwind:
Same mathbut this time we subtract: (10-10)^2, which is zero.
Since your drag without any wind at 10mph is proportional to 100, and your drag with a tailwind of 10mph becomes zero, the delta drag is -100.

300 is bigger that 100..
So, a headwind will cause more energy loss than a same sspeed tailwind can help

 

[Osiris]

Definitely the case-- headwind saps energy to the square of it's relative speed.

Tailwind allows higher speed of about the tailwind speed...

I have a question about this. What you're saying here sounds intuitively right, but my own experiments don't bear this out. On flat roads, I can easily reach a top speed of 30 mph on any of my recumbents on windless days, so if I'm interpreting you correctly, with a 10 mph tailwind, I should be able to hit 40 mph., right? The thing is, I've traveled many times on a 1 mile long stretch of flat road aided by a tailwind of at least 10 mph, yet the fastest I've ever managed to go was 34 mph. Last week I had a tailwind of 17 mph on that very road, based on my observation that at 17 mph, I could feel zero wind resistance. The flags at a nearby apartment complex confirmed that I indeed had a strong tailwind, and that it was traveling in exactly the direction I was headed. Nevertheless, I still couldn't get anywhere near 40 mph. Why is that? Something else must be slowing me down, but what?

 

[tiltmaniac]

I have a question about this. What you're saying here sounds intuitively right, but my own experiments don't bear this out. On flat roads, I can easily reach a top speed of 30 mph on any of my recumbents on windless days, so if I'm interpreting you correctly, with a 10 mph tailwind, I should be able to hit 40 mph., right? The thing is, I've traveled many times on a 1 mile long stretch of flat road aided by a tailwind of at least 10 mph, yet the fastest I've ever managed to go was 34 mph. Last week I had a tailwind of 17 mph on that very road, based on my observation that at 17 mph, I could feel zero wind resistance. The flags at a nearby apartment complex confirmed that I indeed had a strong tailwind, and that it was traveling in exactly the direction I was headed. Nevertheless, I still couldn't get anywhere near 40 mph. Why is that? Something else must be slowing me down, but what?

There are other losses in the system, not just aero: drivetrain, ground friction, etc. not to mention body dynamics.

Aero dominates at higher speeds.

 

[DavidJL]

The total power consumed by wind resistance is proportional to the windspeed squared times the ground speed. This results in gaining about half the speed of a tailwind and losing about half the speed of a headwind. Consider that you could still ride in a 40 mph headwind, albeit very slowly, but could never make 40 mph in still air. With a tailwind, you have lesser air drag, but you are taking it at a higher ground speed, so aero is still important even with tailwind.

 

[Balor]

Remember that rolling resistance is linear and always present.
Otherwise, in *some* cases, *side* winds (with aero/disk wheels and fairings) can actually propel your forward due to sail effect. Not reliable though.

 

[cranky cyclist]

I would love a sail propelling me forward, especially uphill!

 

[billyk]

The total power consumed by wind resistance is proportional to the windspeed squared times the ground speed.

Where does this come from? Please explain.

It seems to me that - if we're only talking about wind resistance - that the aerodynamic drag is due only to the relative wind. Riding 20mph in still air produces the same relative wind, thus the same drag, as riding 10mph against a 10mph wind. Or, again considering only wind, as riding 30mph with a 10mph tailwind.

Of course other factors come into play in real life (rolling resistance, drivetrain losses), but for aerodynamic drag only the relative wind should matter.

I think the formula should be:
Power consumed by wind drag is proportional to (headwind+groundspeed)^2

What am I missing? Please enlighten us.

 

[tiltmaniac]

Where does this come from? Please explain.

It seems to me that - if we're only talking about wind resistance - that the aerodynamic drag is due only to the relative wind. Riding 20mph in still air produces the same relative wind, thus the same drag, as riding 10mph against a 10mph wind. Or, again considering only wind, as riding 30mph with a 10mph tailwind.

Of course other factors come into play in real life (rolling resistance, drivetrain losses), but for aerodynamic drag only the relative wind should matter.

I think the formula should be:
Power consumed by wind drag is proportional to (headwind+groundspeed)^2

What am I missing? Please enlighten us.

That looks correct to me.

In gory detail:
D = Cd * A * .5 * r * V^2

Cd == coefficient of drag
A == frontal area
R == fluid (air) density
V == air velocity

Note that Cd there includes, to quote NASA docs, "form drag, skin friction drag, wave drag, and induced drag components".

 

[ed72]

just use this

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

 

[Balor]

just use this

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

Nice, didn't see that one before.

 

[ed72]

At 30 mph in calm conditions, it take a hypotheotical rider 265 watts to go 30 mph with 55 watts due to linear effects (tires and drivetrain) and 210 watts for aero.

With a 10 mph tailwind, it takes 154 watts total of which the linear part stays the same.

With a 10 mph headwind, it takes a whopping 445 watts overall to maintain 30 mph groundspeed with the resistive component staying the same at 55 watts but aero losses now total 390 watts.

Time to distance freaks aka TT specialists on out and back courses hope for a tailwind out and the headwind back because it is better to work harder into the wind from a time savings stand point and who the hell wants to suffer on the first half. Similar arguments can be make for when to break or bridge.

 

[Osiris]

I just ran the number through Kreutzotter (http://www.kreuzotter.de/english/espeed.htm), and these are the results I obtained using the highracer recumbent option:

In still air at sea level, a steady output of 300 watts gets me 26.3 mph. I haven't done any testing at 300 watts, but this result seems very believable based on my other speed/power tests with the V20.

With a 20 mph tailwind, my speed at 300 watts is predicted to be 39.7 mph; a gain of 13.4 mph.
With a 20 mph headwind, my speed is predicted to be 15.7 mph; a loss of 10.7 mph.

I'd want to test the accuracy of these numbers, but it does back up my perception that tailwinds don't help you nearly as much as headwinds hurt you.

 

[Balor]

I'd want to test the accuracy of these numbers, but it does back up my perception that tailwinds don't help you nearly as much as headwinds hurt you.

Basically, even if you disregard all other things and bluntly assume linear gain and loss, the very formula for calculating AVERAGE speed will insure that it *will* suffer.

An old trick question:

Assume a cyclist climbs a very steep mountain at speed of 5 mph. Shouldn't it be possible to double your average speed by riding down as fast as it is possible?
Try giving an intuitive answer before googling up the correct one .

 

[ed72]

yes. tailwinds are easier. headwinds are suckier.

increasing power at slower speeds reduces time to distance much more than increasing power at higher speeds.

For instance, 45 mph downwind and turn around upwind at 15 mph does not equate to 30 mph average, obviously. But. 4o mph downwind and turn around and return at 20 mph also does not equate to a 30 mph average.

Basically, even if you disregard all other things and bluntly assume linear gain and loss, the very formula for calculating AVERAGE speed will insure that it *will* suffer.

An old trick question:

Assume a cyclist climbs a very steep mountain at speed of 5 mph. Shouldn't it be possible to double your average speed by riding down as fast as it is possible?
Try giving an intuitive answer before googling up the correct one .

Yes, but Scotty tells me the dilithium crystals won't handle it much longer.

 

[McWheels]

Time to distance freaks aka TT specialists on out and back courses hope for a tailwind out and the headwind back because it is better to work harder into the wind from a time savings stand point and who the hell wants to suffer on the first half. Similar arguments can be make for when to break or bridge.

I'm going to struggle with this one. The general adage for hills is that it's better to spend any extra energy on the uphill, since you get closer to a linear benefit while at lower [wind] speeds. Therefore wouldn't it make more sense to give it the beans with the tailwind when a greater proportion is rolling resistance and mechanical drag instead of into wind when you're already fighting an exponential problem?

 

[ed72]

I'm going to struggle with this one. The general adage for hills is that it's better to spend any extra energy on the uphill, since you get closer to a linear benefit while at lower [wind] speeds. Therefore wouldn't it make more sense to give it the beans with the tailwind when a greater proportion is rolling resistance and mechanical drag instead of into wind when you're already fighting an exponential problem?

Always try hardest the slower you go and the final average speed will be higher. If you play with the calculator that I linked, it might eventually make intuitive sense. A real world example that applies to recumbents would be how to handle rolling terrain. You want get up to speed coming down and then not work very "hard" until the road turns up and then apply much higher power than when descending. The actual power levels depend on the individual, terrain, and aerodynamics of the bike/rider system. Obviously, there is a limit to exceeding FTP but there is a reserve (W' or HIE) that is always best spent the slower one goes. On a long out and back, it is a risky strategy. Bestbikesplits and XERT (or knowing one's PD curve and being good with math) would help solve the optimal pacing strategy but anyways.....probably more controversial than waxing vs. wet lube or clinchers vs. tubies.

The math is not all that hard but my head hurts to explain....here's a link to help.

https://www.bikeradar.com/us/road/g...ts-better-to-ride-hard-into-a-headwind-46946/

 

[paco1961]

But in race scenario you also need to consider the psychology of the head to head battle. my old cross country coach used to have us run hill drills all afternoon where we’d climb at moderately hard pace and then blast the first 10-15 strides off the crest of the hill and into the descent. His philosophy - which has served me well for more than 3 decades of racing - was that the run off the top of the hill drives a psychological nail in your competitors coffin. So moderately increased effort on the climb, short red line off the top then recover a tad on the way back down.

 

[Balor]

Yea. Basically, our intuitive ideas *mostly* center around conservation of energy and effort for a given mileage. Makes total sense, if you are starving hunter in Africa... and not much in modern setting: mechanical efficiency (MPG, so to speak) be damned, you want maximum average speed.... but somewhere deep down same 'modules' are ticking away and suggesting same course of action as in savannah.
This is indeed counter-intuitive, and unless you train very hard at this - maintaining even same *power* into head wind is much, much harder. Having a powermeter helps a lot, if you just stare at power number and disregard everything else. I'm piss-poor time trialler, but I know a bit about psychology.