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Old 09-19-2007, 08:19 PM   #1
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TJ: Bug Shield testing

For starters, while I know adding a bug shield should be a given MPG penalty on any kind of vehicle, but considering the aerodynamics of a TJ (two completely vertical/flat surfaces; the grill and the windshield) I figured it might just help. So I had a bug shield I had bought a while back and I figured why not.

I used the route from my house to school as my test route. Tests were performed over 3 days. Run A is going to school, B is heading back, a trip distance of 12.3 miles. Engine was cold for all runs. I go in around 10, leave around 7. Run A temps were 64F-66F, Run B temps were around 66F-68F Cruise control was set at 65 mph.

Run A
Shield on: 19.6
Shield off: 20.3
Shield on: 19.8

Run B
Shield on: 17.9
Shield off: 16.2
Shield on: 17.3

Now my question is that, going in, there appears to be a small hit in FE but it could be almost neglibile, but going back there is a descent gain in FE with it on, why is that? I'd have figured a penalty on both trips
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Old 09-19-2007, 08:44 PM   #2
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Look at differences between runs... you'd need more samples to conclude anything that small.
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Old 09-19-2007, 09:19 PM   #3
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^^ ditto

Your environment factors are too large to easily see a result

Do you have consistent wind conditions - that is, do all run B tests have a headwind (explaining lower overall results and potentially increasing an mpg/aero change).

If you're going to collect data over time... Collect a lot more, but also record all of the temp/wind conditions for every test. That's what sucks about long term testing - it's more difficult to control variables :/

-----
Can you experiment with different attack angles? Or is it a molded one-shape only type of thing?
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Old 09-19-2007, 10:06 PM   #4
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Kewl, I'll keep on going.

treb, I'll double check the wind info I have, don't recall the direction. B is worse, FE wise since its mostly uphill, while A is mostly down hill.

It is a molded one piece thing, I was hoping that the collected data would be bad enough to warrent me chopping it up some. I'll post up a picture of it tomorrow.
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Old 09-21-2007, 09:19 PM   #5
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I wonder if the bug shield could be mounted at the base of the windshield? I was thinking of doing something similar to my car. It should reduce the high pressure area that builds up at the hood/ windshield junction.
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Old 09-22-2007, 10:16 AM   #6
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Quote:
Originally Posted by DRW View Post
I wonder if the bug shield could be mounted at the base of the windshield? I was thinking of doing something similar to my car. It should reduce the high pressure area that builds up at the hood/ windshield junction.
I?ve seen cowl covers for sale in JC Whitney before for trucks that would basically do the same thing you are looking for I believe.
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Old 09-22-2007, 06:12 PM   #7
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So some pics of what it looks like





And some simulated "air flow" testing with a hose.







I've got next Friday off so I am going to try another set of runs inside a smaller time frame.
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Old 09-22-2007, 06:36 PM   #8
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Man, that's a steep angle of attack - I'll bet it has to be to change the velocity of flow above it

The best question as far as overall performance of it is... Does it actually work? If it doesn't do a decent job - and decreases mileage... Sell it on fleaBay
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Old 09-22-2007, 07:47 PM   #9
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It pretty much sends anything small to the moon

Honestly when I ran it way back when I didn't notice much of a change. Tried selling it then, no takers, figured I'd try to quantify what it exactly did before I try to sell it again.
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Old 09-22-2007, 11:01 PM   #10
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If you have a hill without much traffic see if you can come up with some sort of coast down test. Start at a spot on the hill at a fixed speed and let it coast in N as far as it will roll, to a terminal velocity where it stays at a constant speed down the hill, or to a spot at the bottom of the hill where you can see it's speed at that point. Usually I have good luck with coasting tests and they are pretty consistent as long as the air temp and wind temp are the same and the car is at a consistent warm up state. Here the wind always blows the same direction in each place so wind is almost never an issue.

I have never been able to see a mileage difference with the small rear spoiler on my car but a coast down test shows a tiny improvement. So I leave it on there on the premise that every little bit helps. One hill on my commute if I put the car in N at a road sign at the top of the hill at 60mph it will always cross the 55mph mark on the speedo at a certain sign at the bottom of the hill. Without the spoiler it crosses the 55mph mark about a car length before the sign. I typically use this hill on my way home to make sure there isn't anything wrong with the car's rolling ability. If I pass a large truck I don't get a good coast though so traffic has to also be consistent.

Finding a good hill and practice coasting on it on your normal commute till you figure out your variables and get it consistent. It is very handy to be able to do a mod and figure out over the next few commutes whether it helps your aero or not.
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