Suggestions for My Jetta's Rear End?

[northboundtrain]

What do you all think I could do for this car? I know there is bad turbulence back there because when I've driven it with a little snow on top of the truck, it ends up drifting/piling up at the base of the rear windshield, so there is definitely an eddy/air swirl going on back there.

I'm open to all suggestions from quick and easy -- like a spoiler of some sort -- to the more extreme. Thanks

[MetroMPG]

Tell me you're not driving around with the roof racks on

I would suggest: wheel skirts

Also, you could consider fabricating something like what was suggested in this thread: Improving aerodynamics (of '87 Nissan Bluebird)

[Hockey4mnhs]

yeah agree with metro and i hope to god u dont have the roof racks on

[The Toecutter]

Just for the rear:

-kill roof racks
-install rear wheel skirts
-install side skirts to try to induce some semblence of attached flow so the wheel skirts might work
-tapered roof extension(want 6:1 fineness ratio)
-boattail for the rear bottom



Don't just pay attention to the rear. You have to consider other aspects of the car too. If you have seperated flow early on, aeromods in the back may not do as much good as they otherwise would. I would hazard a guess that you have attached flow until the roof racks by your description of the snow, however.

Since you don't have wind tunnel access, it would be well advised to try tuft testing to see where turbulence are created and try to get an iea of what mods would be worthwhile and what mods would be a lost cause.

[northboundtrain]

The rack is coming off soon now that ski season is just about over (need to loose the snow tires as well).

I think I know what a tapered roof extension is, but what is fineness ratio?

Do side skirts go straight down or do they go out horizontally? I've seen pictures of both.

What is tuft testing?

Thanks for the help.

[lunarhighway]

you might want to try vortex generators, or turbulators on top of the roof to trip the air over the edge. i curently have some on my car,
(they're actually small rubber cones intended to serve as computer card stick on feet... i've stuck them on upside down)
and i suspect they're somewhat effective, but i haven't done any scientiffic testing, althoug my rear window seems to stay clear of raindrops at speed now.
i've experimented with their position some places didn't seem to work or even keep the air of the window.

if you have a clear visual indication such as snow, or water drops, you could actually see if there's any difference.

tuft testing is a way to visualise airflow around an object. basically you stick a lot if little strings to the car so that the can move freely in the airflow... if you have laminar flow tha they should all point in the direction of the flow, if you have a area of turbulent flow they will move all over the place. i haven't tried is as you need someone to follow you in another car and preferably film them. although if you just put them on the window you might be able to see them in the mirror.

[northboundtrain]

Originally Posted by lunarhighway

tuft testing is a way to visualise airflow around an object. basically you stick a lot if little strings to the car so that the can move freely in the airflow... if you have laminar flow tha they should all point in the direction of the flow, if you have a area of turbulent flow they will move all over the place. i haven't tried is as you need someone to follow you in another car and preferably film them. although if you just put them on the window you might be able to see them in the mirror.

That's cool. I'll try taping a piece of string to the roof and see if I can see it in the rear view mirror.

Where are pics of this stuff?

[MetroMPG]

http://autospeed.drive.com.au/cms/A_1044/article.html

[trebuchet03]

Originally Posted by northboundtrain

That's cool. I'll try taping a piece of string to the roof and see if I can see it in the rear view mirror.

Where are pics of this stuff?

On a plane windscreen


If you know the general area where separation occurs - you can attach longer strings in front of this area to get a better picture of where the separation zone is. But, if you put these long strings too far back in a turbulant zone, it really wont work :/

[northboundtrain]

I was in Napa yesterday and was checking out a model of a NASCAR replica on the counter. I studied it for a little while and it was pretty informative. So now I think I understand where to start with some aero mods like a front spoiler, side skirts, and rear wheel well cover. I'm still unsure how to proceed with the rear end though. I don't think I want to try anything real extreme or labor intensive initially. Here's a pic of possible spoilers I drew. I've seen this stuff on other cars so I assume it works. the tapered roof extension seems pretty self explainatory, but I'm not sure how the spoiler on top of the trunk works, though I see it all the time on other cars, and the NASCAR replica had a big one.

Suggestions?