The wind factor

[Biker Joe]

Every one of us knows the role wind plays in our cycling activities, ( and I’m not talking about the baked bean variety here), but I wonder how many of us actually understand the ins and outs of cycling aerodynamics.

I will try here to explain what is going on and what is involved when riding into the wind.
This is probably common knowledge to the more experienced riders which is why I have posted it on the Beginners Forum.

As we are all aware, when we cycle into a head wind we encounter various degrees of wind resistance. Even on a calm day our movement through the air will produce this resistance.
We have to push our way through the mass of air in front of us. The faster we go the more resistance we encounter and we have to use more energy to overcome this increase in resistance.
What we are experiencing is known as aerodynamic DRAG. As we move through the air, when cycling, high pressure builds in front of us and low pressure forms behind us. This produces drag. You could say that we are being sucked backwards. The faster we go or the stronger the wind is blowing the more drag we experience.

The unit of force generally used to measure drag is the Newton. If you’re not into physics that probably doesn’t mean a lot to you.
Take your average kitchen scales. If you push gently down on the scales until it reads 100gms you are exerting a force of around 1 Newton. If you press a little harder until it reads 500gms (or about 1 pound), you are using about 5 Newtons of force.

Let me give you some examples of how this all works in practice:- (The numbers I am going to use from now on are generalized but will give you enough information to understand what is going on. It doesn’t take into account body position or gradient).
We’ll take a rider who weighs 165lb.There’s no wind, we’re doing 5mph. The drag is 0.8N and we need just 1.78 watts of power to keep us going. (Easy peasy)
Now we up the speed to 10 mph. (Still no wind). The drag is now 3.06N and he requires 13.69 watts.
At 20 mph and no wind the drag rises to 12.25N which is a power requirement of 109.27 watts. (There’s over 2lb pulling us backward)

The wind is now blowing at 10 mph.
At 5 mph road speed that gives us 6.8N, 46 watts.
10 mph, 12.25N, 109.27 watts
15 mph, 27.56N, 369 watts.

The wind now rises to 15 mph.
At 5 mph road speed that’s 12.25N, 109.27 watts.
10 mph, 19.14N, 213.9 watts
20 mph, 37.5 N, 587 watts.(Hard work)
Remember, these are figures based on a pan flat, tarmac road. If you’re also faced with a gradient as well, then things look somewhat different.

As an example let’s take a rider trundling along at 5 mph in a 10mph head wind. On the flat he’s experiencing 6.8N drag and using 46 watts of power. Now the road rises to just a 3% climb. With the added effects of gravity this equates to 28.9 N, and requires 193.7 watts of power. A significant increase.
If the same rider were to increase the speed to 10 mph instead of 5 mph he/she would experience a drag of 34.35N and require 307 watts of power. So even a slight gradient has a significant impact.You have to accept that, when the wind is blowing against you, your speed will drop, your journey will take longer and you will use more power.

The three main things you can do when the wind starts to blow is to present a lower profile to the wind,( This is easier done on a bike with drops), keep in a lower gear and keep up your cadence. This will make it easier to adapt and accept the loss of speed and time.
Riding in a group will help in the wind. If there are just two of you riding the second rider can shelter behind the first. Riding close behind the first rider also helps them because your close proximity behind them breaks the low pressure area forming behind them and they subsequently experience less drag.

I hope this has given you some understanding as to how and why the wind affects you as a cyclist and thereby help you cope with the wicked wind.
Happy riding in the wind.

 

[simmi]

Very interesting Biker Joe

Do you happen to know about cross winds?

At what point do they start to help or hinder, is it as simple as either side of 90 degrees or is there more to it than that?

 

[wisdom]

Food for thought there,Its almost a daily occurance on the Fylde coast.I'll put some of your findings to the test.

 

[Biker Joe]

Very interesting Biker Joe

Do you happen to know about cross winds?

At what point do they start to help or hinder, is it as simple as either side of 90 degrees or is there more to it than that?

A cross wind gives rise to a much higher drag. Why is that?
In cross winds the relative frontal area is increased dramatically as air passes around the cyclist. The air becomes turbulent and disorganised and in cross winds the turbulence is not only behind the rider but also off to one side. This has the effect of making the rider appear ‘larger’ to the wind.
A ten degree cross wind is very common. At this angle things get interesting. Air jumps across the arms, and across the open chest area exiting around the outside of the arm and the side of the body. The ‘exiting’ air becomes disrupted and drags air from the upper back into its vortex and the resulting drag is significantly more than if the air is coming directly from the front.
In cycling terms, crosswind angles are often refered to as ‘YAW’.
Unfortunately, I do not have any numbers to crunch as the math is quite involved.
The bottom line is that, at certain wind angles, typically around 10 degs, drag is increased significantly over wind coming from directly in front.
I hope this is of some help.

 

[simmi]

A cross wind gives rise to a much higher drag. Why is that?
In cross winds the relative frontal area is increased dramatically as air passes around the cyclist. The air becomes turbulent and disorganised and in cross winds the turbulence is not only behind the rider but also off to one side. This has the effect of making the rider appear ‘larger’ to the wind.
A ten degree cross wind is very common. At this angle things get interesting. Air jumps across the arms, and across the open chest area exiting around the outside of the arm and the side of the body. The ‘exiting’ air becomes disrupted and drags air from the upper back into its vortex and the resulting drag is significantly more than if the air is coming directly from the front.
Aerodynamically, crosswind angles are refered to as ‘YAW’
Unfortunately, I do not have any numbers to crunch as the math is quite involved.
The bottom line is that, at certain wind angles, typically around 10 degs, drag is increased significantly over wind coming from directly in front.
I hope this is of some help.

Yeah cheers Biker Joe, it is very flat and open around me and have noticed that winds from near 90 degrees up to straight head winds make a huge difference to my speed, so at 90 degrees you are still creating a lot of drag with your increased profile.

 

[Crackle]

http://www.kreuzotter.de/english/espeed.htm

 

[rich p]

I have noticed my hair gets mussed up in a sidewind and my trousers get wet when I pee into a headwind,

 

[black'n'yellow]

Aerodynamically, crosswind angles are refered to as ‘YAW’.

Aerodynamically, 'yaw' describes the lateral rotation of an aircraft (or other body) in relation/reaction to the wind direction - yaw does not describe the cross wind itself.

 

[Sittingduck]

Stick to shorts

 

[Ern1e]

Good post very intresting subject.

 

[Biker Joe]

Aerodynamically, 'yaw' describes the lateral rotation of an aircraft (or other body) in relation/reaction to the wind direction - yaw does not describe the cross wind itself.

That's correct. However, in cycling discussions, the angle of the cross wind is often described as yaw.(in overly simplified conceptual terms)

 

[black'n'yellow]

That's correct. However, in cycling discussions, the angle of the cross wind is often described as yaw.(in overly simplified conceptual terms)

You may be right, I don't know. But in your previous post you mentioned 'yaw' in relation to aerodynamics, not cycling.

 

[Pedal pusher]

Thanks biker joe very interesting post

 

[Mr Haematocrit]

Cycling yaw calculator

http://www.cyclingpowerlab.com/Yaw.aspx

 

[HovR]

Really informative post, a great addition to the forum!

Riding close behind the first rider also helps them because your close proximity behind them breaks the low pressure area forming behind them and they subsequently experience less drag.

Out of interest, how significant are the effects of this to the rider in front? If the calculations aren't too much of a chore having some figures to this would be great! (And it would also be a great excuse to take an extra long turn at back of the pack )

Thanks!