Dealing with underside airflow & the front bumper (transition radius to undertray)
 

It seems to be warming up, and that means I can actually work outside more (this year's cabin fever has been horrible). So, I'm looking at a bellypan for the Paseo and have been thinking about how I want to make the transition between the front bumper and the pan. In the picture below you can see the car currently has a small air deflector on the bottom (all current stock equipment is gray). I'm thinking the best route to go with a bellypan would be to drop down to that level from the front of the bumper (see the red dots in my pic). Any ideas or comments?

Here are my less than great pictures:

http://www.tercelreference.com/downloads/panels_5.jpg

http://www.tercelreference.com/downloads/paseo.jpg

http://www.tercelreference.com/downloads/front.jpg

Sounds reasonable - you definitely want a radius between the bumper bottom and the pan, if possible. See the EV-1 for example:

http://a332.g.akamai.net/f/332/936/1...v1.f34.500.jpg

There was a pic out there of Brigham Young U's EV1, shot from underneath the car on a lift that better shows the bumper/front pan transition. But I can't find it at the moment.

What does Hucho say on forward facing transition radii? 2 inches?

Thanks for the pic. That is pretty darn close to what I was thinking of doing with my bumper actually. I was thinking of adding a vent for the radiator in the front radiused section of the underbody panel. In any case, I think I'll use the EV-1 as a template.

I'll have to look at Hucho's book. I haven't been reading it much lately and am only on chapter 4. But, I believe he said something like 2-4" when he was describing the VW bus. I remember the section, just not the exact number.

Daox -

I like the shape you were thinking of. An alternative I have been thinking about is to find an old bumper of my car from a junk yard and "double-deck" it :

http://img80.imageshack.us/img80/389...mperhf7.th.jpg

I think this would be harder for you than for me because it's easier for me to find a Saturn bumper.

I would call this the "Biplane Mod".

CarloSW2

What about doing something a bit more drastic? Reshaping the front a bit more like such? It would be much more of a bland look, but I would think a bit more aerodynamic.

http://www.tercelreference.com/downloads/front1.jpg

You'd be lowering the stagnation point, and sending more flow over the car than under (as well as increasing front downforce).

But will that reduce drag? Not sure. But I'd guess "yes", since you're diverting more air away from the dirty underside.

According to Hucho's book, the last proposed design would be better.

If I undertand you guy correctly, what you are saying is that the bottom edge of the bumper should have a rounded edge ( 2" ) instead of just a sharp cutoff ?
( When there is a bellypan installed . )

Why is this ? Does it add more / less velocity to the airflow travelling under the car ?

It helps keep the flow attached. In the book it references A pillars more so than underbody airflow. However, the principle is still there.

Raises an interesting question though: if there's no belly pan for the dam to transition to, is a lower radius strictly necessary?

As we seem to be rebuilding the nose in addition to adding underpaneling, this is what I would do:

Front-End Radius
http://i264.photobucket.com/albums/i.../Front-end.jpg

The red dots show the front clip/underbody panel while the green dots show the front-wheel splitter.

Raise the stagnation point a bit and place it where it will be most useful (i.e. radiator opening). Front wheel splitters will lower drag due to the yaw-angle of air flowing under the car. The same idea can be seen on the UFE III and Loremo LS

UFE III Rounded Front + Wheel Splitter
http://i264.photobucket.com/albums/i...oaS/ufeIII.jpg

Loremo LS Wheel Splitter
http://i264.photobucket.com/albums/i...oaS/loremo.jpg

You'll see the biggest effect if you add a diffuser on the rear, build sideskirts, and make things out of fiberglass so they can be smooth. I think I'm getting a bit over the top, but aerodynamics is a package deal...:D

- LostCause

I would probably say no. If there is no bellpan you want to direct air over the car versus under it. A larger radius would direct more air under the car.




To answer LostCause, I am planning on making wheel splitters after the bellypan is on the vehicle. Not exactly sure how I'll be doing that all just yet, but thanks for bringing it up.

The stagnation point is very interesting. I guess I hadn't thought about it that way. If its brought up to the the opening for the radiator, the opening can be smaller and thus improve overall aerodynamics.

I guess I really need to read more Hucho. I haven't gotten to the cooling system section yet. :o

ciao

I think, they, a radius front-end, is the best way for a frontal pressure reduction.
But, if the underbody, isn't smoother, the rougnes, cause a dramatic air velocity decrease under the car with a large amount of drag.
Without a smoother bellypan,it is preferible to add an front airdam for a reduction of airflow under the car.

Daox, even without a smooth undertray, Hucho shows your nose diagram (post #5) as the best of 5 variants. Fig 4.31 in 4th edition.

Observation
 

I've noticed alot of newer cars have a small under the chin spoiler (usually black rubber/plastic) that sticks beneath the bumper about 1.5"-2". It's usually set back a bit from the leading edge. My '05 Civic has one, and everytime I look at it, I wonder about just extending it down a bit and making a small airdam out of it.

However, looking at these designs, I question the validity in doing so as I may just introduce more frontal surface without return on investment. Then, I look at what AndrewJ has done with his 5th Gen Civic, and he's obviously made some beneficial changes. Would it be safe to say that unless you can provide a transition to smooth underside pan, or initiate the airdam from the leading edge, that extending the chin spoiler as in my Civic would do little to improve aerodynamics?

If I did go as far to extend it, I think I'd try for some integrated front wheel spoliers as well, but everything would be setback, under the bumper a bit. Coming from an aviation background I see un-necessary drag (parasite, interference, lift) as my enemy. Once you start working in the steep part of the drag curve, a little parasite drag gets big quick. I have to believe smooth transitions at normal driving speeds are hard to beat.

Haha, thats about 20 pages ahead of where I am in the book. Ah well, I'm headed out to the wife's grandmother's house this weekend for Easter. Nothing to do there and plenty of time to read. :)

unfortunately , I do not have this book, but that one you report, make to think me.

One of drag sources, it is, the consequence of acceleration of the air flow under the car.
The maximun acceleration airflow, is created from underbody roughness.

it is possible, that the text must be interpreted, and to know, which level of roughness Hucho referred.

sorry, I do not have all pics with me

http://img89.imageshack.us/img89/5035/spoilerye1.jpg

bellypan transition
 

make a template for a ten-degree wedge out of cardboard and cram it into the front of the Paseo's front tires.Anything that falls below that angled line projecting forward and upward will be scrubbed off the car when going up or down driveway ramps.Its called the approach,ramp,and breakover angle,and is standard practice for all automotive design.Your transition piece cannot fall below that imaginary line or its toast!

I come from engines cultures.
In this area (thematic), it is very important to understand the importance of orifice and the value of orifice coefficients.

For similitude, the central part of a car, it is paragonable to half tube:

http://img341.imageshack.us/img341/3807/profilezv1.jpg

below the effusion of inlet and outlet for an tube

http://img147.imageshack.us/img147/7941/imbocchixp5.jpg

look the lowest coefficients of intake similar at horn.

applying the same one in ours cars, we can drop drag coefficient.

For the rearend of car the thing is more complex why the interference with other air flow.
but, this is a whay to consider

I would see about the purpose first, then if at all possible deleting that piece of crap hanging down getting in the way... So the end product would look more like: