2008 Jetta: An indepth guide to underbody aero (56K)

[charlesB]

I had the privilege of having a new Jetta in my driveway today, and since the weather was decent, I decided to take some pics for you guys.

The new Jetta has extensive underbody aero treatments, and makes a very good guide for those of us with lesser vehicles.

A- Front wheel/wheel well spoiler. This piece curves in toward the center of the vehicle. It spoils the air headed toward the tire, and diverts air away from the wheel well and front suspension.

B- Rear wheel well spoiler. This small, wide spoiler helps the air 'skip' over the rear wheel well and suspension.

C- This is the bottom of the front bumper. It is completely flush and tucks all the way back to the front belly pan.

D- Rear wheel spoiler. Could be much larger, but is still probably effective.

E- Dimpled panels. These panels run nearly the length of the wheel base, and cover everything except the exhaust piping.

F- Rear bumper tucks in completely flush to rear undercarriage.

G- Again more dimpled panels. Only fitted to one side due to the muffler.

[Hateful]

I'm surprised any car company bothered with the underside at all.It seems my Cavalier was designed more for the assembly line than the road.

[Mighty Mira]

Originally Posted by charlesB

The new Jetta has extensive underbody aero treatments, and makes a very good guide for those of us with lesser vehicles.

The pictures are interesting.

I'm not so sure that this makes a great guide for us though, compared to a full undertray for example. I suspect that it may have just been a way to get a slightly better drag count without compromising too many other things (for example, worst case worries about exhaust heat building up, oil leaks, manufacturing costs, bad word of mouth from mechanics having to remove them to inspect/fix things, it being only necessary to match other competitors' offerings, etc). The Cd of the Jetta is 0.31, which is quite good, but not awe inspiring. Other things that make me think this are that 2/3 of the undercarriage still looks like a series of barn doors (no different than most other cars), and that the deflectors are tiny and unobtrusive, hidden out of sight lest a consumer see them and go into convulsions thinking that they don't look very streamlined.

In any case, I'd want to see some A-B-A comparisons to a full undertray before I'd take this as being the pinnacle of automotive aerodynamic design. But it is interesting. Thanks for posting and annotating.

[Erdrick]

I am assuming you took it for a few test drives. Was it the TDI? If so it would interest me greatly. Otherwise the aero stuff is nice, but gassers just don't get my attention.

[ZugyNA]

The aero stuff wouldn't be there unless it reduced CD. Good info. Looks like dimples work.

"Vehicle: 2006.5 Jetta TDI DSG
Vehicle Mileage: 1750 Miles
Speed: 72.4 +/- 0.1 mph (GPS)
Temp: 60 degrees F
Wind: Out of the N.N.E.

With VG's Under Front Bumper:

Run 1 North: 46.6 mpg
Run 1 South: 47.2 mpg
Run 2 North: 45.0 mpg
Run 2 South: 49.2 mpg
Average: 47.0 mpg

Difference from Baseline: +2.6 mpg"

Life and car design are based on compromise....you take what you get. How many cars other than a Ferrari or whatever are going to have full underbody fairings...a mechanics nightmare...let alone the issues of heat buildup and expense.

[trebuchet03]

Originally Posted by ZugyNA

The aero stuff wouldn't be there unless it reduced CD. Good info. Looks like dimples work.

Could be for noise reduction too I believe BMW or Audi (which would make sense why it's on a VW) advertised dimpled panels... They had a commercial awhile back with a car in a wind tunnel - turned upside down.

Originally Posted by ZugyNA

How many cars other than a Ferrari or whatever are going to have full underbody fairings...a mechanics nightmare...let alone the issues of heat buildup and expense.

Noble and Lotus too But especially in the case of the Noble, and likely the Ferrari - it adds aero, but more importantly - stiffness When it's designed for stiffness, it will be a nightmare and 5/2. I posted a picture awhile back of a Noble's underside - with full under body - riveted and glued on. The shop guys didn't really care - it was just an extra billable hour drilling rivets

[DrivenByNothing]

Is there a minimum speed for the dimples to be helpful (something above zero)?

[ZugyNA]

Originally Posted by trebuchet03

Noble and Lotus too But especially in the case of the Noble, and likely the Ferrari - it adds aero, but more importantly - stiffness When it's designed for stiffness, it will be a nightmare and 5/2. I posted a picture awhile back of a Noble's underside - with full under body - riveted and glued on. The shop guys didn't really care - it was just an extra billable hour drilling rivets

I think almost all serious sports cars have most of the drivetrain tucked up above the frame? Most don't have the openings covered though.

[trebuchet03]

Originally Posted by ZugyNA

I think almost all serious sports cars have most of the drivetrain tucked up above the frame? Most don't have the openings covered though.

Yes, that's true -- I posted a thread with the underside of a Noble M12 GTO. But openings make it less of a mechanics nightmare, unless you need to repair a weld (which is the reason I was able to get the photos I did). But having the drive train above the frame (not sure if that makes sense, but I think I know what you're saying) doesn't change that you're adding a LOT of stiffness VIA geometry by rigidly fastening an under tray (recall the recent Ford ads for "closed box frame rails").

Glue Screwed and Riveted







And then, there's the Elise.