Out and back - Does the wind cancel itself out

L'Enfer du Nord

Hi,
I did 22km ‘all out’ last night as training for a tri. The road is fairly straight and I did 11 km, turned around and did 11 km back. The wind was behind me on the way out, it was slightly cross but mostly tail, not that strong but constant. I knew it was behind me but certainly didn’t feel I was being blown along. Obvious I was cycling into the wind on the way back, again I certainly felt the wind, but I've felt far worse. My average speed for the way back was nearly 5 km/h slower than on the way out. I know it’s hard to judge but I think on the way out I was only boosted by 1-2km/h on the way back I felt I was slowed by at least 3-4. I was on a hybrid, so aero position probably wasn’t great. I was happy with the effort I put in, reasonably happy with my condition but a bit disappointed with my time.

My question is cycling 10km out and 10 km back with a first tail then a head wind the same as cycle 20 km with no wind or do the ‘costs’ of a head wind outweigh the ‘benefits’ of a tail wind.

The tailwind never speeds you up as much as the headwind slows you down. Add to this the fact that the headwind is slowing you down for longer because it takes you more time to come back against it and you can see why strong wind from any direction is bad news in an out and back time trial.

jwshooter

if its 90% to you but it seldom is.we did 50k yes eve we had a cross wind going down to wexford and a head on the way back another route .hard work .

Lumen

L'Enfer du Nord wrote: »

Out and back - Does the wind cancel itself out

No.

L'Enfer du Nord wrote: »

do the ‘costs’ of a head wind outweigh the ‘benefits’ of a tail wind.

Yes.

The reason is that you spend more time fighting the headwind than you do benefitting from the tailwind.

For the limit case, imagine cycling into a hurricane, where your ground speed was 0.000001kph forwards. You'd take forever to do the first half, and the almost instanteous return would not compensate.

teufelswerk

The wind strength for the last few days is going to be negligible for the overall time that you post. I'd never let myself do a slower average speed and blame the wind if I was doing a loop. The benefits pretty much equal the draw backs with the level of wind we have at the moment for your combined out and back time

cdaly_

Costs outweigh benefits.

IIRC, air drag increases as the cube of velocity so if your relative speed increases by 2%, drag increases by 8%

Also, consider the fact that wind resistance increases as per the cube of the speed. So lets consider a hypothetical case where a cyclist wishes to maintain a speed of 20kph, (20kph =1 arbitrary unit of wind resistance)and consider the effects of a 10kph headwind, and a 10 kph tailwind.

For the headwind:
20kph speed + 10kph wind = 30kph apparent wind.
30/20 = 1.5
1.5*1.5*1.5=3.375
Therefore wind resistance is increased by a factor of 3.375
Therefore cyclist experiences an additional 2.375 A.U.W.R.

For the tailwind:
20kph speed - 10kph wind = 10kph apparent wind
10/20= .5
.5*.5*.5 = .125
Therefore cyclist experiences .875 less A.U.W.R.

Cost of headwind= 2.375
Benefit of tailwind= .875

I am not a physicist and these numbers may well be way off, but still, it gives a rough of idea of the mathematical basis by which headwinds suck balls.

Beasty

Give me nil wind both ways on the daily commute any day (this morning was great) - my return times are always much better on days like today.

To back this up with stats - back on 19 November there were SSW winds of 30-40 kph with much stronger gusts (up to 60 kph). My inward journey was an hour, my return was just over 40 mins - average 50 min per journey. On 9 November it was 10-15 kph SE winds, and my inward time was 45 mins, and return 42 - average 43.5 min - an overall improvement of 13% in my times (both times my average power output was around 230w)

Lumen

cdaly_ wrote: »

Costs outweigh benefits.

IIRC, air drag increases as the cube of velocity so if your relative speed increases by 2%, drag increases by 8%

But that's not the problem. Even if you attempt to keep a constant power output/airspeed, such that you are not wasting extra effort into the wind, the longer time spent into the headwind counts for more.

Imagine you were an airship with a 30kph windspeed doing a 15km x 2 time trial into a 15km headwind.

The first 15km leg (into the wind) would take an hour @15kph groundspeed. The return leg at 45kph groundspeed would take 20 mins. Total time: 1:20.

With no wind the same course would take 1 hour.

seamus

There are some cases where this is the exception, but they need hills. If your journey against the headwind is downhil and your return journey is uphill, then you'll get a better return on your journey than if the wind was dead or the opposite way. This is the general case with my commute this week.
However, steeper slopes negate any effect of a good tailwind (and make a downhill headwind even worse).

L'Enfer du Nord

teufelswerk wrote: »

The wind strength for the last few days is going to be negligible for the overall time that you post. I'd never let myself do a slower average speed and blame the wind if I was doing a loop. The benefits pretty much equal the draw backs with the level of wind we have at the moment for your combined out and back time

Are we both in the same place?, this cycle was along the coast on the west coast.

Lumen

Oh and the airship analogy is possibly weakened by the implication that it is impossible to cycle into a 50kph headwind.

I have no idea whether this is true or not. On those days I stay indoors.

L'Enfer du Nord

seamus wrote: »

There are some cases where this is the exception, but they need hills. If your journey against the headwind is downhil and your return journey is uphill, then you'll get a better return on your journey than if the wind was dead or the opposite way. This is the general case with my commute this week.
However, steeper slopes negate any effect of a good tailwind (and make a downhill headwind even worse).

I actually started and finished at sea level, so I don't think this was a factor

Raam

Lumen wrote: »

Oh and the airship analogy is possibly weakened by the implication that it is impossible to cycle into a 50kph headwind.

I have no idea whether this is true or not. On those days I stay indoors.

Is it really?

edit: only read your first line before posting

L'Enfer du Nord

Lumen wrote: »

Oh and the airship analogy is possibly weakened by the implication that it is impossible to cycle into a 50kph headwind.

Unless the airship is Fabian Canellara

teufelswerk

L'Enfer du Nord wrote: »

I actually started and finished at sea level, so I don't think this was a factor

If you're going to be sarcy, saying that you started and finished at sea level would still prove his point. I.e your outward journey could all be uphill and your return downhill. You've still started and finished at sea level.

Raam

teufelswerk wrote: »

If you're going to be sarcy, saying that you started and finished at sea level would still prove his point. I.e your outward journey could all be uphill and your return downhill. You've still started and finished at sea level.

What if the tide goes out in the mean time?

Beasty

Raam wrote: »

What if the tide goes out in the mean time?

You've got to make sure you don't finish until it comes back in again

L'Enfer du Nord

teufelswerk wrote: »

If you're going to be sarcy, saying that you started and finished at sea level would still prove his point. I.e your outward journey could all be uphill and your return downhill. You've still started and finished at sea level.

I wasn't being sarcy (appologies to Seamus if it seemed that way), just unclear.

I should have said my starting point and turning point were both at sea level, so I think the factors he outlined would even off.

blorg

This physicist reckons a tailwind actually adds more to speed than a headwind takes away- but that out and back will indeed take longer simply due to your spending more time fighting the headwind.

I'm a bit skeptical on the former point- is he right?

EDIT: There is plenty to suggest you should push harder into a headwind so maybe this is right.

Beasty

blorg wrote: »

This physicist reckons a tailwind actually adds more to speed than a headwind takes away- but that out and back will indeed take longer simply due to your spending more time fighting the headwind.

I'm a bit skeptical on the former point- is he right?

EDIT: There is plenty to suggest you should push harder into a headwind so maybe this is right.

Not sure, but I like natural cynic's comment:

You have only elucidated one of the perceptible properties of the cycling universe: There's always going to be more headwinds than tailwinds. This goes along with property #2: there will always be more uphills than downhills.
And as a corollary of the first: when your ground speed matches the tailwind speed you are most likely to pass gas.

Lumen

blorg wrote: »

is he right?

Who the hell knows? Where's Dr Gavin when you need him?

Raam

Anyway, there's no such thing as a tailwind. It's called "going well".

Lumen

blorg wrote: »

There is plenty to suggest you should push harder into a headwind so maybe this is right.

Yes, but not much.

More here.

L'Enfer du Nord

Raam wrote: »

Anyway, there's no such thing as a tailwind. It's called "going well".

Good point! On the way back I contracted a 'mysterious virus'.

Not sure if this had any effect but on the way out I was modeling myself on David Millar when he was with Cofidis on the way back I had an attack of conscious and was trying to emulate his more recent performances.

Highway_To_Hell

Beasty wrote: »

Give me nil wind both ways on the daily commute any day (this morning was great) - my return times are always much better on days like today.

To back this up with stats - back on 19 November there were SSW winds of 30-40 kph with much stronger gusts (up to 60 kph). My inward journey was an hour, my return was just over 40 mins - average 50 min per journey. On 9 November it was 10-15 kph SE winds, and my inward time was 45 mins, and return 42 - average 43.5 min - an overall improvement of 13% in my times (both times my average power output was around 230w)

I think you need to check out "am I a nerd" thread over on the ART Forum, you seem to have a lot of historical data to hand.

serendip

Slightly off topic, but ...

It's similar to going up hill then down hill. It's similar too to driving along a undulating road versus driving along a flat road. It's similar too to driving systematically slow a bit, fast a bit, slow a bit, fast a bit in a car. Could these cases be more fuel efficient?

In all cases, it CANNOT be more efficient. If it were, where would the optimum be? If undulating were more efficient, would we not build undulating roads? If speeding up and slowing down were more efficient, would we not encourage that practice on the basis of fuel efficiency?

As Lumen said, it's easiest to convince yourself if you consider the extreme cases. Very high winds. Very steep hills (vertical!). Extreme variation in speed.

The optimum in all these cases, including cycling, is no wind, no hills, and no speeding up and slowing down.

That said, psychologically, I always think that a short sharp hill followed by a long gentle decent just feels great. And I commute to the south west, so just knowing I'll (usually) have the wind at my back on the way home in the evening is a lift in itself.