himespau 

Was thinking about this a couple of days ago when a tailwind allowed me to rock home a lot faster than usual and suddenly some dude latched onto my wheel (not sure that's ever happened before). He stayed back there for a mile or so, pulled up said hello and took off more aggressively through a red light than I wanted to follow (there was cross traffic that had the light and I typically don't run reds anyway).

It didn't feel like it was taking more work to maintain my speed, but I had to think the fact that he was back there between me and the wind meant that there was less of it pushing on me. So do drafters in fact steal at least some of the benefit provided by a tailwind?

mmmdonuts

I've never thought about this but that's how it works in sailing. Other boats can steal wind from you.

himespau 

not that I was upset, I was still motivated to go flat out because it was such easy going (and the shock of having someone back there is almost as good as having someone to chase), but it just seemed odd. I guess he had the wind pushing him and me blocking the air resistance up front, so it was doubly good.

I'm a poor judge of when I have a tailwind as I don't often look at the speedometer and just ride at a pace that makes me feel like I'm pushing it so I'm pedaling at a higher gear with the same amount of air blowing past my face (either that or I've never had a really really good tailwind that I couldn't outrun).

ancker

There isn't as much of an issue in cycling because the wind isn't your driving force. The wind is really just a perception of airspeed.
When cycling at 10mph with 0mph wind you have a perceived 10mph headwind. If you're going 10mph into a 10mph headwind, your perceived headwind is 20mph.
When going 10mph with a 10mph tailwind your perceived headwind is 0mph, thus the air resistance is neutral allowing you to move the same speed with less effort. The higher the tailwind, the faster you go at nearly the same level of effort. (Other forces come into play here that I'm not accounting for such as rolling resistance, body weight, etc.)

When someone is drafting you they may block the wind directly hitting you from behind, but the perceived headwind is not changed.

Of course, there is some level of tailwind that actually 'pushes' you. I'm not sure what that number is but my guess would be that it's pretty high. In that case, someone drafting you would likely reduce that effect by a certain percentage.

asgelle

Were you moving slower than the wind? If not, the apparent wind (which is what causes the force on you and the bike) was still coming from the front. The trailing rider was not blocking the wind.

mmmdonuts

Ah, makes sense.

StevenSTL

What's a tailwind? I don't think I've ever seen this.

climber7

interesting timing. take a look at the second question/answer:

https://velonews.competitor.com/2011/...-update_176921

this probably doesn't happen much just from another cyclist behind you, though.

thump55

I disagree.

At any speed, the trailing rider is blocking some of the helpful wind.

asgelle

Think Lagrangian rather than Eulerian.

thump55

Please explain, I'm interested.

EDIT: Oh, and please keep it simple for my redneck mind. All I know is when a combine gets up behind me, it blocks my tailwind, and I go slower.

asgelle

What do you mean? I'm using the terms as they are commonly applied to fluid mechanics.

thump55

I'm sayin' that a person with your obvious wealth of knowledge could explain it in a few simple, easy to understand sentences that even a moron like me can understand.

Phantoj

So you and the combine are both going slower than the speed of the wind?

thump55

I'm gonna say it don't matter.

Yes, there still may a "headwind" if we are going faster than the wind. I think the tailwind still supplies power, and any blocking of that wind reduces the aid to the front rider.

asgelle

Alright. Last try. Think of a canoe drifting downstream in a river. No paddling, it just drifts with the speed of the water. Is there a wake behind it? Is there a bow wave in front?

Now put another canoe a little bit behind it, also not paddling. Does the lead canoe slow down? Is there a wake behind the lead boat? The trailing? Any bow waves?

merlinextraligh

The Hed windspeed calculator confirms this.

https://www.hedcycling.com/aerodynamics.asp

merlinextraligh

There's also a potential advantage to the lead rider from the drafting rider altering the airflow behind the lead rider, but I would wager that effect is deminimis at typical biking speeds.

sced

I've always heard that a race car with a drafter can go faster that if it were alone because drafter is effectively pushing the car in front with a bow wave so to speak.

mmmdonuts

I think it's more about filling the void and reducing turbulence behind the lead car. Even when they are nearly bumper to bumper they are faster together but there is no room for an effective wave or air cushion.

tessartype

A drafted race-car can go faster because there's another car (the drafting car) "sealing" the low-pressure air left behind it. That low-pressure air "sucks" the front car backwards. It is the same low-pressure area that allows the drafting car to go faster.

Now, bikes are far slower, but whenever there's an advantage to drafting (such as in cycling), there should be an advantage to being drafted as well. May not be much, but it's never a downside.


Sailboats are a different matter altogether, because the wind is their propulsion system. With bikes and cars, wind/air-resistance is only a negative force.

Seattle Forrest

No. There's never a tail wind when you're cycling.

topflightpro

Check out Nick's column on Velonews today. It discusses this.

DropDeadFred

one day I'm gonna ride in a tailwind...if it ever stops switching on me

kleinboogie

Tailwind? I don't think it works that way. It's just less air pressure to have to push through. GL