2008 Scion xB help (dirty rear window)
 

I have a few things I am planning on doing to my 2008 Scion xB to improve the aerodynamics but I have one problem that I need help with. The rear of the 2nd generation Scion xB's tend to collect dirt like a magnet. My guess is that the air flow over, under, or from the sides is being sucked into the low pressure area behind the car. My guess is most of the dirt is coming off the road. The xB's with the added rear spoiler seem to gather the same amount of dirt and also hurt gas mileage. I thought someone here may have had a similar problem with a vehicle and found a cure.
Any thoughts?

Al

xB in the aero forum is kinda a contradiction of terms....
Wouldn't know where to start there except trade it in on something with a better shape.

You missed the point of my question all together. Actually the 2008 / 2009 xB's have a Cd of 0.32 which isn't bad for a car I can actually put stuff in like 5 full size adults and gear. I live in the mountains of Northern Arizona and go to Phoenix about once a week, 90 miles away. With the Mountain driving and two moves hauling everything I could fit into the car my average MPG is about 29.2 with over 30,000 miles on the car. rkcarguy, if you have any useful information I would be pleased to here it. However, comments like, "trade it in on something with a better shape" was one of the most worthless replies I have seen anywhere. I just finished reading some of your other posts here and I know you can do better than that.

Al

I understand, but the thing is you'd have to add a big ugly boat tail or something and that's just not something that 99.9% of new car owners are willing to do which is why I posted what I did.
The chopped off end of the "box" just isn't going to be improved with a simple spoiler, you'd have to make the transition more gradual somehow..

Getting poor shapes to flow better is the goal of our aerodynamics. What you start with doesn't matter as much as what you end up with.

The dirt building up on the back is a common problem with hatchbacks, minivans, basically vehicles that only "appear" more aerodynamic than the xB. The problem comes from the wake of the underbelly as shown in this topic about an European Swift. They've already discussed the back-middle spoiler keeping the dirty undertray air from building up deposits on the rear window.

The Metro has a CoEf of .36 and the '94 Mustangs have a CoEf of .34. A boat tail is ideal for the most slip but "boat tailing" the rear quarter can still give good results. If your only response is to buy a new vehicle or move within walking distance, you're not contributing anything useful.

Hucho's book has a chapter just about vehicle soiling. Unfortunately the commonly seen examples are trade offs between increasing drag and "cleanliness" for lack of a better word.

Some people claim vortex generators help with soiling on the back of vans/hatchbacks. The airtabs style don't seem to affect MPG one way or the other, but perhaps they may help with dust.

I will try and find a copy of Hucho's book to read. It's a little expensive but seems to be the book of choice if you want to learn about aerodynamics. I was just thinking back to the 60's and 70's. Some station wagons had a curved piece of metal that look liked it would take the clean air from the roof and force it down the back side of the car thus keeping the dirty air from coming in contact with the rear of the car. Anyone ever had a close look at one of these? I have only seen them at a distance.

Al

I have a 2006 xB, I would be nice to make in more efficent. Any thing we can do for underneath the car??

As I'm missing the front underbody panels, I made my own.

The next big project for me is to fill in (most of) the space between the rear axle and bumper. That's the worst part of the underside of the car.

I also can vouch for blocking off one of the lower grilles-- it doesn't seem to matter which one.

xB
 

The xB is a "full-wake" vehicle.There is virtually no body taper underneath,along the sides,or at the roofline to the rear of the body.Consequently,the wake of the xB is as large as it's frontal area.Had the xB been shaped like Klemperer's "aero-brick" of 1922 (with Cd 0.16),the xB would see a 25% improvement in HWY mpg.The xB would probably benefit from any boat-tailing you could tolerate.A friend has a Toyota Highlander,getting 28-mpg HWY.I am to fabricate a 30-inch long tail for his car when the time avails itself.We are reasonably confident that the tail will deliver mpg comparable to his wife's Camry (34 mpg HWY).He's trying to wrap his brain around the length issue,and it will be receiver-hitch mounted for easy on-off and perhaps dedicated only for road trips.The difficulty of significant drag reduction with van-type rear vehicles is reflected in the comment about trading for a different type of vehicle.A full aero trailer could net you a 30% increase,but then you'd be pulling a trailer.Not very practical.

Let me try this again. I am NOT seeking help with trying to get better gas mileage. I have already decided what mods I can live with based on my driving needs. The only question I am asking for help with is trying to reduce the amount of dirt that collects to the rear of my 2008 Scion xB. I don't want to redesign the complete rear of the car adding a 4 foot tail or similar type of contraption. I am just trying to find an EASY way to try and keep the dirt off the rear by maybe deflecting the air away from the low pressure area behind the car. Years ago many station wagons has a deflector on the rear of the roof that I thought might be there for that reason.

Al

You're right - that's what they were for. Add one and you will: (A) increase fuel consumption, and (B) have a cleaner back window.

al74dart,

I'm with MetroMPG on this
- if the dirt bothers you that much, you have to take the hit and add the deflector
I'm sure you can find a bolt out there that someone says helps with dust

depending on the driving you do there may be ways to mitigate the issue by removing it on high speed highway driving - because there is less dirt

you could attempt side skirts and aft wheel fairings to lessen the dirt you kick up

Or... If you have a high pain threshold you could develop your own spoiler like this:
Green Car Congress: Novel Spoiler Design Reduces Fuel Consumption for Minivans, SUVs
but with out a wind tunnel (or tremendous patience) this would be hard


but if all you want is the dust gone - buy a big scooping spoiler
it is the simplest path to clean(er)

Finally, thank you.
Not practical was the point I was trying to get across, not come across as an ass. It seemed to me this was going to be a question no one answered because they were trying to be nice. You wanted facts so I provided them, but did not realize it was only to keep dirt off the rear of the vehicle, not to increase mpg via aero. Some kind of boat tail that slipped into a hitch would probably be the best bet, but will render your "barn doors" in-accessable.
As mentioned before, probably the best bet is installing one of those turn down spoilers found on the back of some earlier suburbans and wagons. My 1990 Suburban had one and while not perfect it did force the air(and rain) to wash down the back window and keep things cleaner.

Thanks guys, the back of my car is impossible to keep clean so it is worth the little in MPG I will loose. I hope to gain enough from my other mods to more than offset the loss. I am planning on blocking the upper grill completely once I get a Scangauge so I can monitor the temp before and after. I am also redoing the underside of the car with sheet aluminum where I can and adding deflectors in front of all four wheels to move the air away from the tires. I should have new tires soon that will also help. I usually bump the TP up by 15% - 20% over the recommended pressure but always under the Max pressure rating for the tires. Living in Arizona the tire pressure can get really high when it's 115F in the shade. I am also working on a windshield wiper deflector I can live with. I am not sure how much it will help. I may be better off trying to build a bug deflector that will push the air up enough so I won't need to build a windshield wiper deflector.

Al

back-soiling
 

In addition to the air deflector at the top,a full-width "shelf" at the bottom of the rear windshield projecting straight back,perhaps no further than the end of the rear bumper,might keep some of the road debris from being vacuumed into this space from below.Some kinda flexible material that wouldn't poke you or a passerby.

skip the wiper shield
 

al74dart,

sounds like a good plan
but I would skip the windshield wiper shield - unless you want it for aesthetics

On an Xb there is not enough drag there to bother with

if you still want it low drag looking (I did :o)
Just adjust the blades to the lower edge of the windshield
and tuck them down the best you can and still good travel

you already have low profile blades stock right?

Yes the blades are stock and I pretty much agree with you on the blades. I don't think I would gain that much.

Al

We have a 2005 xB, maybe I can give you the information you're looking for. Our year model came with (probably an optional) rear "wing" or "spoiler". It directs the wind from the top of the vehicle straight down across the back window. We hardly get any dirt build up on our back window. Ours looks like this...and is OEM from Toyota/Scion
http://scionpartspeople.com/images/S...%20spoiler.jpg

I don't know if this "wing" is an option for your year model or not, but there are aftermarket "wings" very similar to the one we have. Hope that helps.

On the MPG discussion, our 05 xB with an auto transmission regularly gets 34-35 MPG. It is completely stock with no mods at all. My wife is the primary driver and most of her driving is down a 45 mph highway with very few redlights. She is a very good driver and has always gotten great mileage in all of our vehiclels. Though the vehicle may not look very aero, it definately performs at the pump and the room inside is phenominal. Don't knock the xB til ya try it, it's a great little vehicle.:cool:

The wing in that article looks slightly similar to this one from ScionPro.
Welcome to SCIONPRO.com!
http://img06.picoodle.com/img/img06/...gm_ce4e89b.jpg
To make more aerodynamic you would need to lower it slightly and put a wedge in between it and the car to tilt it backward below the roof line. Its shape will also give you little bit of a cam back.