How Speed Affects Drag

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How Speed Affects Drag

Postby jonthornham » March 8th, 2012, 6:26 pm

I got a great question on our Facebook page today asking why we posted values NDRV using 30 mph. Well here is the answer and little more info.

The reason we use 30 mph is because the wind tunnel uses 30 mph. That is the standard testing protocol. There are general rules of thumb when looking at time savings, speed and grams of drag. You can say that if you are traveling at the following speeds you will save "x" seconds for every 100 g of drag reduction.

20 mph you save 60 s
25 mph you save 50 s
30 mph you save 40 s

This may sound strange but if you are on the course longer your savings will increase. If you can reduce the drag by 100 grams in all cases you will save more if you travel slower. This is because there is reduction for a longer period of time.

However, as we reduce speed we also reduce the drag reduction. Therefore, 100 g at 30 mph would not be 100 g at 25 mph with the same CDA.. Ultimately, the reduction will not be directly comparable when you compare the same object at different speeds. The drag reduction values would need to calculated for each speed traveled. This way you can could calculate the time savings of each speed.

Let me know if anyone has any more questions about how speed affect drag.
jonthornham
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Re: How Speed Affects Drag

Postby ClydesRock » March 22nd, 2012, 8:52 am

Jon - thanks for this explanation. I would also like to understand how higher speed means lower yaw angle when you have a quick minute.

Thanks!

Rob Zielinski
FLO 90 F&R Clydesdale on a Lynskey T230
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Re: How Speed Affects Drag

Postby Chris Thornham » March 27th, 2012, 1:34 pm

ClydesRock wrote:Jon - thanks for this explanation. I would also like to understand how higher speed means lower yaw angle when you have a quick minute.

Thanks!

Rob Zielinski
FLO 90 F&R Clydesdale on a Lynskey T230


That's a great question. I spent some time putting together a description for you. The concept you are discussing is know as an "apparent wind angle". Let me try to describe it with my diagram below...

Image

Let's start by defining the variables.

V-Bike = The speed you are travelling on your bike. We have a 10mph example and a 18mph example.
V-Wind = The wind speed. The same in both examples.
Angle-W = The angle of the wind hitting the rider. The same in both examples.
V-Effective = The vector addition of V-Bike and V-Wind which is the relative velocity.
Yaw = The resultant yaw angle.

Taking a look at the pictures above you can see that as the rider speed "V-Bike" gets larger, the resultant yaw angle gets smaller.

Does this help? Let me know if you have any additional questions.

Take care,
Chris Thornham
FLO Cycling: http://www.flocycling.com
FLO Blog: http://bit.ly/b3Zlpq | Twitter: http://bit.ly/bOQ08x | Facebook: http://bit.ly/d8KDhI
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Re: How Speed Affects Drag

Postby ClydesRock » April 1st, 2012, 2:02 pm

Perfecto! Thanks.
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