Metro aerodynamics - Page 19 - Fuelly Forums

Android Users - Coming Soon! - Migrating from aCar 4.8 to 5.0

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Old 09-10-2006, 07:52 PM   #181
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Can't really comment on the coasting.
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Old 09-10-2006, 07:57 PM   #182
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No pics yet. I'll get some though.
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Old 09-11-2006, 05:48 AM   #183
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Can't really comment on the coasting. I'd like to say it coasts longer, but that may be the placebo talking I don't have a regular route with "benchmarks" I can compare it to.

I may get a chance to do the A-B before I go. I don't think I'm leaving until the end of the week. I figure it will take me around 10 minutes to remove the 30 or so screws and take the tray apart/off at the side of the road.

The only thing I'm wondering is whether I should do (1) one A&B bi-dir run for a range of different speeds, (2) or stay at one speed, and do a number of A's and B's.

I'd prefer to know the difference at several speeds, but (2) generates more reliable results.

It takes a long time to do these runs.
Just test 2 speeds and interpolate say 30 and 60 MPH and then maybe a coast down for your drag reduction but than again "you should be working"
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Old 09-12-2006, 12:23 PM   #184
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Excellent weather today (little wind), so took a couple of hours off and did 6 bi-directional runs @ 90 km/h (56 mph).
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Old 09-12-2006, 12:38 PM   #185
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Yup, you certainly win the for the closest estimate. What's your source, BTW? I'd like to know how they calculated the figure and what's taking into account.
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Old 09-12-2006, 12:53 PM   #186
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Just noticed something:
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Old 09-12-2006, 01:10 PM   #187
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Excellent weather today (little wind), so took a couple of hours off and did 6 bi-directional runs @ 90 km/h (56 mph).

3 with, 3 without the undertray .

57.78 mpg (US) - with undertray
56.55 mpg (US) - without undertray

1.23 mpg difference, or 2.2% increase with tray

Not really happy with the results. It was a lot of work for such a small gain. But them's the numbers.

A question for the aero enthusiasts: my car rides quite high off the ground - 6 inches or maybe slightly more. Would an undertray help more on a lower vehicle (like the A4, which probably has 3 or 4 inches of clearance)?
Well that's better than nothin. I thought it would be higher to. I think your onto something with the ride hight. You may already have seen this.

http://www.wisil.recumbents.com/wisi...ero_review.htm
"With increasing ground clearance, the airflow in the nozzle-type space between the vehicle underbody and the ground produces low pressures causing overall lift to be lowered to negative values and then to rise again as ground clearance continues to increase. The increase in overall lift is due to the fact that the flow velocity under the car decreases as ground clearance increases, thus reducing the low pressure level. Air drag increases as a function of ground clearance though this increase is smaller by one order of magnitude than that of the lift forces. ...lift variations occur mainly in the nose of the car."
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Old 09-12-2006, 01:11 PM   #188
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I think there's something wonky in that math. I don't believe 10% Cd reduction = 2% FE increase. Maybe someone can work it out with better numbers before I get to it.

To add insult to injury, doing these runs dragged my tank average from 81.x to 79.x . AAARG! I would really like one 80+ tank this year!

I had to pulse and glide all the way home to try to reverse some of the damage - it was lower than that when I left the test road after the last run.
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Old 09-12-2006, 01:22 PM   #189
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Thanks for that link, zpiloto. I haven't put much effort into understanding effects of ride height on drag yet, aside from knowing the basics: that lower is better than higher, and 3 inches is about optimum.
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Old 09-12-2006, 04:21 PM   #190
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Quote:
Originally Posted by MetroMPG
1.23 mpg difference, or 2.2% increase with tray

Not really happy with the results. It was a lot of work for such a small gain. But them's the numbers.

A question for the aero enthusiasts: my car rides quite high off the ground - 6 inches or maybe slightly more. Would an undertray help more on a lower vehicle (like the A4, which probably has 3 or 4 inches of clearance)?
That's the pattern I see more and more...you should EXPECT 2-3% increases with most mods? Those 10 percenters are few and far between?

Also...you could lower your car by using smaller wheels/tires and not pay a penalty due to your trans change? I'm guessing that you went past the sweet spot as far as your final drive ratio. Just a guess though.
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