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Old 07-03-2009, 01:59 AM   #1 (permalink)
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Wake of overtaking vehicles?

Something I've noticed out on the road: When a larger vehicle (say a big-rig truck) passes me, there's a moment where it feels like my car decelerates. I had thought that I would get a small boost from the air that the truck is pushing along. While that may be present, I don't really notice it. I do notice (or I have convinced myself that I notice) the deceleration is there.

Has anyone else noticed this?

Are there any theories about why that would occur? The only thing I can think of is that there is a "bow wave" coming off of the truck that is turbulent, and offers more resistance to my car moving through it than the less-turbulent air already in my lane.

Any thoughts?

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Old 07-03-2009, 02:49 AM   #2 (permalink)
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It might be the effect of interference between the two shapes. Without free air around you, it is like an increase in frontal area. Can you use a sealed container of water on your dashboard to observe the effect?
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Old 07-03-2009, 03:35 AM   #3 (permalink)
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I notice this alot on my Ninja. When a bus or 18 wheeler passes me on the left side, i do get a point of deceleration.

It feels (i can feel it since im on a bike) like a pressure change for a moment, until it passes.
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Old 07-03-2009, 07:40 AM   #4 (permalink)
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The effect you arre seeing is due to the displaced air being accellerated as it travels around the sides of the semi, so you are seeing a brief increase in your airspeed (a virtual headwind) as the semi passes you.

As the semi passes, you first get a surf draft (for better mileage) as the air displaced by the bow shock off the front of the semi helps to fill in the low pressure zone trailing your vehicle, followed by a period of increased airspeed (for worse mileage) as the side of the semi passes you, followed by a side draft (for better mileage) as the semi pulls just ahead of you and its trailing low pressure zone helps to pull off some of the air being displaced by the leading edge of your vehicle.

In my aerocivic, the surf draft has no effect since with the boattail I don't have a low pressuse zone trailing my car, at normal highway speeds the accellerated airflow along the sides has little noticable effect on my car, and the side draft has only a very minimal effect on my mileage.
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Old 07-05-2009, 06:54 PM   #5 (permalink)
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Hi All,

Besides Basjoos comment, the air that would be where the truck is, is going over your car then. So, the air is denser, and accellerated sidways. This combines with the ambinet air to make a local windspeed over your car that is higher, and not in-line with the axis of the car.
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Old 07-10-2009, 07:00 PM   #6 (permalink)
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Quote:
Originally Posted by Bicycle Bob View Post
Can you use a sealed container of water on your dashboard to observe the effect?
I don't understand... How would one do this?
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Old 07-10-2009, 07:16 PM   #7 (permalink)
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Oh, leave some air in the container too, so you can see the water move. If you do decelerate, it should move up on the side away from you.
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Old 07-10-2009, 07:26 PM   #8 (permalink)
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OIC.

I was over-thinking it.

It seems a sensitive hind-end would be able to feel the deceleration as easily as one could visually notice the slosh.
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Old 07-10-2009, 08:03 PM   #9 (permalink)
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It does seem so, but unfortunately, the subconscious can really alter kinesthetic information. Just look at the totally useless body english employed by some athletes and most musicians. It isn't hard to find an expert cyclist who declares that he steers at speed only by leaning, unaware of small handlebar motions he is making.
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Old 07-10-2009, 09:23 PM   #10 (permalink)
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I guess you're right. Not everyone is born with a calibrated butt-dyno.

So, do bicycles counter-steer like motorcycles do?

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