How does a headwind affect speed and power output?

Commuting, Day rides, Audax, Incidents, etc.
Jdsk
Posts: 24478
Joined: 5 Mar 2019, 5:42pm

Re: How does a headwind affect speed and power output?

Post by Jdsk »

Chris Jeggo wrote: 13 Nov 2022, 10:30pm Jonathan, please quote the source of your two screenshot equations.
Wikipedia. And it's exactly the same equation for drag in every textbook on aerodynamics.

Jonathan
User avatar
Chris Jeggo
Posts: 571
Joined: 3 Jul 2010, 9:44am
Location: Woking, Surrey

Re: How does a headwind affect speed and power output?

Post by Chris Jeggo »

Suppose a tricyclist is just sitting, relaxing, on a trike which is nailed to the road in a headwind. Where does your power come from?
User avatar
Chris Jeggo
Posts: 571
Joined: 3 Jul 2010, 9:44am
Location: Woking, Surrey

Re: How does a headwind affect speed and power output?

Post by Chris Jeggo »

Jdsk wrote: 13 Nov 2022, 10:32pm
Chris Jeggo wrote: 13 Nov 2022, 10:30pm Jonathan, please quote the source of your two screenshot equations.
Wikipedia. And it's exactly the same equation for drag in every textbook on aerodynamics.

Jonathan
Save me some time please. Wikipedia URL?
Jdsk
Posts: 24478
Joined: 5 Mar 2019, 5:42pm

Re: How does a headwind affect speed and power output?

Post by Jdsk »

Chris Jeggo wrote: 13 Nov 2022, 10:35pm Suppose a tricyclist is just sitting, relaxing, on a trike which is nailed to the road in a headwind. Where does your power come from?
The power to push the the air past an anchored object comes from the wind. (You can gather it with a turbine and generate electricity.)

Same drag equation as pushing the object through the air.

Jonathan
Last edited by Jdsk on 13 Nov 2022, 10:43pm, edited 2 times in total.
Jdsk
Posts: 24478
Joined: 5 Mar 2019, 5:42pm

Re: How does a headwind affect speed and power output?

Post by Jdsk »

Chris Jeggo wrote: 13 Nov 2022, 10:38pm
Jdsk wrote: 13 Nov 2022, 10:32pm
Chris Jeggo wrote: 13 Nov 2022, 10:30pm Jonathan, please quote the source of your two screenshot equations.
Wikipedia. And it's exactly the same equation for drag in every textbook on aerodynamics.
Save me some time please. Wikipedia URL?
Drag (physics)
https://en.wikipedia.org/wiki/Drag_(physics)

Jonathan
User avatar
Chris Jeggo
Posts: 571
Joined: 3 Jul 2010, 9:44am
Location: Woking, Surrey

Re: How does a headwind affect speed and power output?

Post by Chris Jeggo »

Screenshot from 2022-11-13 22-46-10.png
It is the second equation that applies here.
Nearholmer
Posts: 3898
Joined: 26 Mar 2022, 7:13am

Re: How does a headwind affect speed and power output?

Post by Nearholmer »

Case (2): Stationary trike in headwind - cyclist stops trike rolling backwards by pressing down on forward pedal. Cyclist does no work because pedal is not moving downwards - no work, no power.
I don’t think so.

We all know that it takes effort, consumes energy, to remain stationary in a decently strong wind, but we aren’t moving, so no work, right?

Wrong. We are making multiple tiny movements, bracing ourselves against the buffeting, constantly slightly over compensating and moving forward, relaxing slightly and moving backward, and repeating that cycle. Each time, force is exerted over distance, a quite large force but over a tiny distance.

The better braced we are, the less each little power-blip will be, but unless we lash ourselves to the mast and relax, I’m sure we’ll output power, use energy, and get tired.

Trees and flag poles don’t I think expend energy standing up in a gale, they receive it from the wind, store it by bending elastically, and when the wind abates for a moment give up most of it by pinging back to their normal position, displacing air in so doing. They presumably warm-up a bit in the process. If their elastic limit is exceeded they snap.
Last edited by Nearholmer on 13 Nov 2022, 11:10pm, edited 2 times in total.
User avatar
Chris Jeggo
Posts: 571
Joined: 3 Jul 2010, 9:44am
Location: Woking, Surrey

Re: How does a headwind affect speed and power output?

Post by Chris Jeggo »

Nearholmer wrote: 13 Nov 2022, 11:01pm
Case (2): Stationary trike in headwind - cyclist stops trike rolling backwards by pressing down on forward pedal. Cyclist does no work because pedal is not moving downwards - no work, no power.
I don’t think so.

We all know that it takes effort, consumes energy, to remain stationary in a decently strong wind, but we aren’t moving, so no work, right?

Wrong. We are making multiple tiny movements, bracing ourselves against the buffeting, constantly slightly over compensating and moving forward, relaxing slightly and moving backward, and repeating that cycle.

The better braced we are, the less each little power-blip will be, but unless we lash ourselves to the mast and relax, I’m sure we’ll output power, use energy, and get tired.
Yes, sure, we can lash ourselves to the mast, or nail our trike to the road. We agree.
Jdsk
Posts: 24478
Joined: 5 Mar 2019, 5:42pm

Re: How does a headwind affect speed and power output?

Post by Jdsk »

Nearholmer wrote: 13 Nov 2022, 11:01pm
Case (2): Stationary trike in headwind - cyclist stops trike rolling backwards by pressing down on forward pedal. Cyclist does no work because pedal is not moving downwards - no work, no power.
I don’t think so.

We all know that it takes effort, consumes energy, to remain stationary in a decently strong wind, but we aren’t moving, so no work, right?

Wrong. We are making multiple tiny movements, bracing ourselves against the buffeting, constantly slightly over compensating and moving forward, relaxing slightly and moving backward, and repeating that cycle.

The better braced we are, the less each little power-blip will be, but unless we lash ourselves to the mast and relax, I’m sure we’ll output power, use energy, and get tired.
That can be the case. But consider the case where a stake anchors the trike to the ground. Just like a building. No power source in either. Then the power to push the air past the object comes from the wind.

Jonathan
Last edited by Jdsk on 13 Nov 2022, 11:45pm, edited 1 time in total.
Nearholmer
Posts: 3898
Joined: 26 Mar 2022, 7:13am

Re: How does a headwind affect speed and power output?

Post by Nearholmer »

Yes, sure, we can lash ourselves to the mast, or nail our trike to the road. We agree.
Just remaining seated on the trike, even if it’s nailed to the road, we will be going through the same micro-cycles of force and distance holding our bodies upright against the wind. To do no work, not to go through those bracing cycles, we have to relax, in which case we will instantly get blown off the trike.

If it’s windy where you are, go outside and try standing stock still. It’s tiring, more tiring than standing still when there is no wind, and it’s tiring because you are doing extra work, not by making gross movement like walking, but through millions of tiny movements.

The power to resist thevwind comes from inside us (which sounds a bit like eastern mysticism).
Last edited by Nearholmer on 13 Nov 2022, 11:20pm, edited 1 time in total.
User avatar
Chris Jeggo
Posts: 571
Joined: 3 Jul 2010, 9:44am
Location: Woking, Surrey

Re: How does a headwind affect speed and power output?

Post by Chris Jeggo »

Nearholmer wrote: 13 Nov 2022, 11:16pm
Yes, sure, we can lash ourselves to the mast, or nail our trike to the road. We agree.
Just remaining seated on the trike, even if it’s nailed to the road, we will be going through the same micro-cycles of force and distance holding our bodies upright against the wind. To do no work, not to go through those bracing cycles, we have to relax, in which case we will instantly get blown off the trike.

If it’s windy where you are, go outside and try standing stock still. It’s tiring, more tiring than standing still when there is no wind, and it’s tiring because you are doing extra work, not by making gross movement like walking, but through millions of tiny movements.
If it's a recumbent trike you could fall asleep on it.
This thread has been about mechanics, dynamics, not physiology.
Nearholmer
Posts: 3898
Joined: 26 Mar 2022, 7:13am

Re: How does a headwind affect speed and power output?

Post by Nearholmer »

Mechanics and dynamics don’t stop just because the machine is made of flesh and blood. The ‘motor’ in most of this thread has been a very physiological cyclist, for instance.

Yes, on a recumbent you could relax and have a Kip in a gale, in which case (assuming the brakes are good), you and the bike become like the tree or the flag pole.
Last edited by Nearholmer on 13 Nov 2022, 11:28pm, edited 1 time in total.
Jdsk
Posts: 24478
Joined: 5 Mar 2019, 5:42pm

Re: How does a headwind affect speed and power output?

Post by Jdsk »

Nearholmer wrote: 13 Nov 2022, 11:01pm ...
Trees and flag poles don’t I think expend energy standing up in a gale, they receive it from the wind, store it by bending elastically, and when the wind abates for a moment give up most of it by pinging back to their normal position, displacing air in so doing. They presumably warm-up a bit in the process. If their elastic limit is exceeded they snap.
I missed this bit. With or without the elastic cycle the energy to push the air past the object eventually comes from the wind, as you say. It doesn't require a power source in the trike or tree or building.

Jonathan
Jdsk
Posts: 24478
Joined: 5 Mar 2019, 5:42pm

Re: How does a headwind affect speed and power output?

Post by Jdsk »

Nearholmer wrote: 13 Nov 2022, 11:22pmYes, on a recumbent you could relax and have a Kip in a gale, in which case (assuming the brakes are good), you and the bike become like the tree or the flag pole.
Yes, another type of anchored object in a moving fluid.

Jonathan
Nearholmer
Posts: 3898
Joined: 26 Mar 2022, 7:13am

Re: How does a headwind affect speed and power output?

Post by Nearholmer »

Yes. So, what is the point under discussion?

Put another way, what is it that is currently not agreed?
Post Reply