Location and height of front air dam

[eagle]

I have already installed (a relatively small) splitter in my car and am now considering the addition of an air dam



mods are made to reduce drag and not lift. but if I can lower both even better

the problem is I cant seem to get "a right answer" when I search for the air dam's possible location

a lot of racers seem to favor the "air dam above and after" the splitter meaning the splitter extends below the air dam and in front of the car



the wrc car approach is an air dam in front of the splitter



and lastly , I recently saw a bluemotion golf notorious for its "extra aero features" and it seemed to have a small air dam below and after (!) a straight area -lets call it a splitter-



I am considering the "wrc approach" to be honest but a lot of guys that run track days say that

"to get max low pressure all you need is to speed up the flow. Trying to reduce airflow at the same time will have the effect of reducing the effectiveness of a splitter. Most likely it will cause turbulence under the car and increase pressure thereby creating lift"

or

"Placing it under the splitter will have a negative effect.
Will cause the airflow under the car to become turbulent and there for slow it down and create lift.
Yes The function of a splitter is to stop air from hitting the bumper and going under the car (therefore reducing the amount of air going under and more over pushing the car down)
But it also serves the purpose of creating a a flat surface which allows air to cleanly flow underneath which speeds the airflow up reducing lift.
Having the splitter protrude down will create a lip with a low pressure zone directly behind it causing turbulence"


after the air dam I am thinking some front and rear tire spats (I have taken photos again from the bluemotion golf, rather interesting) but that is another topic

what do you guys think? sorry for the long post and many photos but I tried to include everything to be specific

[JulianEdgar]

A proper full length undertray with rear diffuser will reduce drag and lift better than any combination of what you have described.

[eagle]

Originally Posted by JulianEdgar

A proper full length undertray with rear diffuser will reduce drag and lift better than any combination of what you have described.

I am considering it, something like that but since the splitter is lower than the sump I am not sure how it will affect aero and cooling

baby steps, that's why I am going for an air dam first

also, I tried to search what material and thickness I should use
some said marine ply some said pvc , cant find a complete and documented response

[California98Civic]

Originally Posted by eagle

... baby steps, that's why I am going for an air dam first

also, I tried to search what material and thickness I should use
some said marine ply some said pvc , cant find a complete and documented response

So, baby steps is a fine idea. The problem with a full undertray for new people is that it is a considerably larger undertaking and there is still the matter of tuning the diffuser properly. The problem with an airdam is if it is too big, hanging down well below the lowest hanging suspension, engine, exhaust parts of the car, it will increase effective frontal area and therefore increase total drag, or CdA.

The material you use for your airdam should be hard enough to hold shape at speed and not so rigid that it will break when it scrapes something. I use a product that is intended for lawn edging and I installed it at an arc under my bumper cover that gives it support to hold shape at speed while still flexiing when it scrapes the ground in ditches and such. The material is so cheap that I was able to make several versions in succession and settle on what seemed a best design. Its length, height, arc, and placement under the bumper cover all likely affect its drag effects. We don't have wind tunnels, and our testing techniques cannot detect finer changes in drag, but my experience suggests I settled on a good shape--one I modeled on known good shapes. You can find details on my build thread and garage page (both in my sig file below).

In the end, for low drag, the undertray is better, but you can start this way. You can even just do the airdam if you don't like the hassles for working/inspecting under the car that a tray presents.

Hope that helps.

[JulianEdgar]

If you are interested in lowering both drag and lift, looking at what race cars do is a waste of time. Only solar race cars are interested in low drag.

There are two ways of reducing both lift and drag.

The first is to stop air getting under the car. This was widely used on old cars that had very rough undersides. A front air dam will do this (and it actually doesn't need to be sized to not increase frontal area, because Cd will usually drop at a faster rate than the frontal area increases). It should be as far forward as possible and definitely not like the pictured Golf. (That position is causing a small amount of lift).

However, basically no cars of the last decade use the 'stop air getting under' approach. Instead they use smooth floors and so gain high speed flow, that reduces pressure under the car to decrease lift. Even a front undertray alone will partly do this. (If that's your red car in the pics it almost certainly already has a front undertray.)

So if you have a car less than about 10 years old (not all cars, but most), a front air dam is subtracting from the effectiveness of what the manufacturer has already done. Improving the undertrays is, to my way of thinking, far more logical. Do you have any pics taken under the car?

(Oh yes, and I am not a fan of splitters for either reducing lift or drag.)

[Vman455]

I don't think this issue of air dams is nearly as cut and dried as a lot of people here think.

First, as Julian points out above: lots of vehicles, especially full-size trucks, have no underbody paneling and very deep air dams that extend well below the lowest-hanging components. The 2019- Ram even has an extra 2.5-inch air dam that folds down at high speeds, despite the fact that the fixed portion already extends below the lowest-hanging component under the truck:



Second, even production cars with engine undertrays, rear diffusers, and center undertrays still use air dams. I've spent years now looking under nearly every production vehicle at the Chicago Auto Show, and you all would be surprised how many cars have very smooth underbodies and an air dam, both with and without separate front wheel air dams.

Bottom line: I don't think it's prudent to rule out air dams with smooth underbodies, or to adhere to any blanket rules regarding their depth.

For reference, if the OP's car is a Fiesta--here's what the front underside looks like (on American market cars). No undertray, full-width air dam, no separate wheel strakes:

[JulianEdgar]

Originally Posted by Vman455

Second, even production cars with engine undertrays, rear diffusers, and center undertrays still use air dams. I've spent years now looking under nearly every production vehicle at the Chicago Auto Show, and you all would be surprised how many cars have very smooth underbodies and an air dam, both with and without separate front wheel air dams.

Yes, I'd be very surprised. The only car I know of like that is the first Chevrolet Volt, and if you read the SAE tech paper, you can see why it ended up like that. Have you got some pics of cars running standard air dams and smooth undertrays?

[JulianEdgar]

Originally Posted by Vman455

For reference, if the OP's car is a Fiesta--here's what the front underside looks like (on American market cars). No undertray, full-width air dam, no separate wheel strakes:

Wow, if the OP's car looks like that underneath, that's good news - so easy to improve.

[California98Civic]

Originally Posted by Vman455

... I've spent years now looking under nearly every production vehicle at the Chicago Auto Show, and you all would be surprised how many cars have very smooth underbodies and an air dam, both with and without separate front wheel air dams ...

That's very interesting. Why do you think so many have both undertrays and airdams? I would love to see more of the photos. I have long needed to do more under the car, but I dislike the extra access difficulties an undertray creates.

[eagle]

the fiesta posted here as a reference is a stock s or se anyway, mine is a totally different design since a custom turbo kit is retrofitted and many parts are "unique"

I will post photos of the underside/ splitter / etc a.s.a.p but I think a "undetray / tire spats" topic should be made to elaborate on the subject since I am also worried about heat retention / extraction

to continue on topic , I am thinking the same "oem look" air dam but in the front of the car ; my car on the bottom pic, see green line for air dam placement