Fuelly Forums - TJ: Bug Shield testing

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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

Look at differences between runs... you'd need more samples to conclude anything that small.

^^ 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 :/

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Can you experiment with different attack angles? Or is it a molded one-shape only type of thing?

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.

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.

Quote:

Originally Posted by DRW (Post 73350)

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.

Man, that's a steep angle of attack - I'll bet it has to be to change the velocity of flow above it :p

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 :p

It pretty much sends anything small to the moon :D

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.

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.