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Old 05-19-2008, 07:24 AM   #1
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Aero aids on an xB.

Having gotten as far as I'm going to get in the near future with driving changes, I'm looking at further ways to up the mpg on Box.

I'd rather not drastically alter the exterior of the vehicle at the moment, plus there isn't much short of a boattail/kammback that is going to help this shape significantly.

So I'm looking at ways to improve the underside. An oddly the most logical place to start looks to be at the back. The rear bumper is pretty low, and looks like a giant airbrake from underneath

In addition to the general unareo shapes under their, that entire lower section of the bumper does absolutely nothing but stick down into the airflow. Optimally I'd just remove it, but the xB looks very odd with it missing. So I'm thinking about using some aluminum or chloroplast to make a underbody panel that goes from just behind the rear suspension and down to meet the rear bumper lower edge.

My big concern is whether the long expanse from where it is attached near the suspension to where it attaches near the bumper will end up flexing or bowing in excessively. Also I'm not sure about water/dirt/etc collecting on top of the underbody panel.

Any suggestions or ideas?


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Old 05-19-2008, 08:33 AM   #2
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For your bumper, what about a "roll pan", which commonly replaces bumpers on lowered pickup trucks?

Obviously there would be custom fabbing involved...

JanGeo is getting damn fine FE out of his xB, might want to see what he's doing...
Originally Posted by JanGeo View Post
I feel sorry for some of you Honda guys getting in the low 30's when I drive a Scion xB getting 40-41 lifetime average - this tank 43mpg (was 44 for a while then some short hops killed it) for a big white box! Here's a sleeper for ya!

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Old 05-19-2008, 09:13 AM   #3
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I think your right about the belly pan, but the back? I wonder...

After looking at some FE sites aimed at big boxy trucks, this looks like one of the simplest approaches. Think of the red lines as 4 plexiglass panels that attach along the edge of the hatch... it does give a bit of a kamm-type drop, but it's main purpose seems to be to separate the low pressure area from the back of the vehicle. On your Scion it looks like it might be pretty easy to implement, and it might look darn cool too.

just a thought.

"If you want to save gas I suggest you permanently remove the drivers seat and steering wheel. That seems to help." -Oscar Halverson

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Old 05-20-2008, 04:17 AM   #4
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How about bumper vents ?
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Old 05-20-2008, 07:37 PM   #5
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Originally Posted by holypaulie View Post
How about bumper vents ?
Something like this? :
1993 Volvo 240 Wagon - 323k miles (awaiting recommissioning)
1999 Audi A6 Avant Quattro - 149k miles(the NEW daily driver)
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Old 08-04-2008, 01:46 PM   #6
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After months of slacking, I finally got around to doing some areo mods to Box.

First up was a new front lip spoiler made from a garage door seal. It is only about 1" tall, but the front is low enough I didn't want more (the bottom of the bumper already has a fair amount of scrapes & gouges).

Next was the big project, as I mentioned at the beginning of this thread, the space under the rear cargo area & bumper is really aerodynamically messy. So armed with some black chloroplast, aluminum flashing & some screws & zip-ties I set about fixing that. I did it over two days, with probably about 4-5 hours in it all total, maybe less. ignore the splashes, I drove through a muddy parking lot between the first & second sessions of working on it.

Overall shot:

From the left side:

From Right side:

The front edge is tucked above the gas tank, and zip-tied in place. I was very careful to not put and new holes in the body of the car where they might rust. So everything is attached by zip-ties, or screws into the plastic bumper.

Detail of Right wheelwell and around muffler:

I pondered enclosing the muffler, but didn't have a good way to do it and not end up worrying about excessive heat buildup.

Detail of Left wheelwell:

From above:

The little bit of flashing is the only part visible from outside. If you kneel or squat down you can just make out some of the edge of the black chloroplast at the edges of the bumper.

I've driven a bit with it on, and aside from one part of the flashing where the tailpipe passes through touching & rattling it works great. Ironically I don't know what it is doing for my MPG as I've loaned my Scangauge to a friend and won't get it back for a day or two.

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Old 08-04-2008, 02:39 PM   #7
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looks good keep us posted on the results
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Old 08-07-2008, 12:20 AM   #8
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Wow! Great work there.

You could put VGs on top/sides, or a spoiler to cut the airflow clean. You could also use the box as mentionned, idea patented by some university (google) for trucks, but might not apply here since the Xb is shorter than a rig.
If you can attach them, air dams for the rear wheels. Grill block in front, maybe even shape the nose angle of the Xb to help the airflow to stay closer to the body above.

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Old 08-23-2008, 08:51 PM   #9
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Improbcat - have you seen any improvements from this mod? I have been thinking about doing the same thing.

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Old 08-24-2008, 05:33 AM   #10
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Nothing dramatic, my most recent tank was just shy of 38mpg. Unfortunately my mpg have been steadily dropping because my O2 sensors are overdue for replacement and I don't have the $250 for them. So this put me back where I was a bit back, but I'm probably not seeing as much as I could have. I will admit to being a bit disappointed that it wasn't more impressive.

My next addition (after the O2 sensors) will be a front belly pan, and blocking off the parts of the lower grills not directly in front of the radiator.


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