How much does wind slow a bike?

[ColinJ]

Take it to extremes and the facts are obvious!

Let's say that your average riding speed in wind-free conditions is 40 km/hr. If you are trying to ride into a headwind that is so powerful that you can only manage 0.1 km/hr and you ride for 5 hours then you cover just 0.5 km. When you turn round to come back assume that you get instantly transported by the massive tailwind to your starting point. You have covered 1 km in 5 hours, so your average speed is 0.2 km/hr.

The same logic applies to riding up steep hills. Grovel up some monstrous climb for a few hours at a pitifully slow speed and eventually arrive at the top of a cliff above your starting point. Take the quickest route back by leaping off the cliff with your bike. The drop will be a lot shorter than the way up and the fall will take a few seconds, but assume that you fall the same distance as the climb in zero seconds. That makes your average speed twice what you did on the climb. There is no way that you did that climb at 20 km/hr (half your normal speed on the flat)!

 

[sleuthey]

I never seem to be able to gain as much from a tail wind as i loose from a head wind?

Same here. I think it’s because when I have a tail wind, if I exploited it fully then it would propel me faster than I felt safe so I tend to free wheel more when I have one

 

[Archie_tect]

Unless you live next to railway station and can cycle to the next station with a tailwind and get the train home... observing social distancing.

 

[Twilkes]

Unfortunately on the bike, it's not an even time split. You will spend 75% of the time into the wind and only 25% of the time with the wind, so not enough time to make up.

That's the correct answer - I can't find the reference but a tailwind helps you more per minute than a headwind hinders you per minute, but you generally spend more time in a headwind for obvious reasons!

The fastest conditions for a circular ride is a still day.

 

[Ken Robson]

Know that cycling into a say 20mph wind will slow down a bike by about 10mph but does cycling in a windy day slow you down much? know it slows me down and i dont seem to be able to maintain any where near a good average speed on any loop? Example on a good day i can cycle one 60 mile loop in just over 3 hours this morning it took me over 3 hours and 40 minutes and i was really pushing as hard as i could (windy this morning) ?
I never seem to be able to gain as much from a tail wind as i loose from a head wind?

Just this minute joined this chat line - with modern bikes and equipment it should not be too much of a problem. Back in the 60's riding a basic "racing bike" we rode to Cambridge (40 odd miles )north up the A10 against a Very Very strong gale force wind and rain coming in the opposite direction. On top of that we wore the old fashioned capes which in effect made a sail/parachute and possibly did not achieve anything over 5 mph. However coming back we hardly needed to pedal and must have broken a few speed records .

 

[matticus]

The stronger the wind, the slower your average speed. It's all about the physics.......

Wind resistance is proportionate to the SQUARE of the speed. So imagine you cycle 15 miles out, 15 miles back, at 15 miles an hour on a completely still day. That takes two hours, right? In that time you've suffered 15x15x 2 hours of wind resistance. That's 450 wind resistance units

Now imagine it's a headwind out of 5mph, and you only manage 10mph. That takes 1.5 hours. So to do the whole ride in 2 hours, you need to cycle back at 30mph. Your outbound wind resistance is 15x15x1.5 hours. Your inbound wind resistance is 25x25x0.5 hours. Total is 650 wind resistance units

Of course, you don't manage to deal with the big increase in total wind resistance...so you go slower overall

So does wind resistance have units of Force x time => Newton-Seconds? I'm trying to work out what that means ...

But never mind, I think yours is a pretty good explanation!

 

[HLaB]

https://mywindsock.com/ gives an insight but I think it relies on a constant power, which doesn't often happen. Often I'll go out with a reasonable power levels into a head wind and wear my self out and power levels fall significantly therefore I'll not get the full benefit of the tail wind when blown home

 

[nickyboy]

So does wind resistance have units of Force x time => Newton-Seconds? I'm trying to work out what that means ...

But never mind, I think yours is a pretty good explanation!

The SI of wind resistance is a Grovel. The imperial version is a Pain

Joking aside, that is how the physics works. It's also the explanation for why hilly rides are slower than flat ones, even if you push the same watts for the same time....it's the effect of wind resistance on the fast downhills costing you more than the wind resistance savings on the slow uphills

 

[Dwn]

Yesterday, I rode the 18 miles to my daughter's house, and then returned by the same route in the afternoon. The wind speed was (according to Strava) 14.8 mph, and it was pretty steady. On the way out, largely cycling into the wind, I did an average speed of 12 mph, and on the way back it was 14.7mph. Was actually surprised it wasn't a bigger difference tbh.

 

[Fab Foodie]

In the minutae, I suspect that a cyclist is probably more aerodynamic to a tailwind than a headwind which means wind assistance is slightly lower than wind resistance.
Crosswinds also play a major role too.

 

[dodgy]

There is a decent write up somewhere on the Internet about this. But even if you start and finish from the exact same point and the wind blows (say) 30mph for the entire ride and stays exactly from the same direction, let's say the South, you will always be slower overall than if it was a still day.
You might think the inevitable tail wind will compensate, it does, but not enough.

It's the same reason why pilots carry more fuel on an out and back flight in windy weather.

 

[MichaelW2]

In the minutae, I suspect that a cyclist is probably more aerodynamic to a tailwind than a headwind which means wind assistance is slightly lower than wind resistance.
Crosswinds also play a major role too.

So how can you optimise aerodynamics for a tailwind. Does anyone ride on the tops of the bars or wear flappy parachute jackets to increase air resistance?
My best tailwind experience was on Exmoor. I had to abandon climbing up into a Northerly gale, I was not even going forward. I turned around to ride 27 miles to a railway station. The tailwind boost was incredibly, I cant remember the time but it was fast. My panniers certainly pulled their weight.

 

[wafter]

So does wind resistance have units of Force x time => Newton-Seconds? I'm trying to work out what that means ...

But never mind, I think yours is a pretty good explanation!

Air resistance is a force so is quantified in Newtons. Multiplying this by the distance travelled (metres) gives energy expended (Joules) and dividing this by time (seconds) gives power (Watts).

This link explains more about how drag force itself is calculated

 

[matticus]

... that's why i found multiplying force x time a bit unusual.
force x distance sits more easily with me!

(disclosure: i have a degree in this sort of stuff, even though it's not as fancy as some here have )

 

[Fab Foodie]

So how can you optimise aerodynamics for a tailwind. Does anyone ride on the tops of the bars or wear flappy parachute jackets to increase air resistance?
My best tailwind experience was on Exmoor. I had to abandon climbing up into a Northerly gale, I was not even going forward. I turned around to ride 27 miles to a railway station. The tailwind boost was incredibly, I cant remember the time but it was fast. My panniers certainly pulled their weight.

I tend to sit-up a bit (depending on the wind speed) rather than be bent double over the bars as I would into the wind. I have a spinnaker but rarely deploy it these days....