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.