Help me understand how these work (or don't)

I've noticed that a lot of newer cars include small plastic tabs in front of the wheels, almost mini-airdams. These are on my BMW 128i:

I'm assuming these are designed to improve aero efficiency by steering air away from the spinning tires, but exactly what dimensions they should have, or any other effects they might impart, are a mystery to me. So, lacking any real knowledge, I decided to fab up some for my Volvo V50, which is not exactly the most efficient highway car and could use some help. Whether I've actually helped is why I'm here.

So this is what I've done. Small tabs in front of all four wheels, plus a small airdam at approximately the same height (the gaps between the two are to allow air to flow to the factory brake cooling channels; I'm not wedded to keeping them open if it makes a huge aero difference, as this is not going to see lots of heavy braking). They are made of ABS plastic and are quite rigid.

Front:

Rear:

The tabs extend up the wheel arches due to 1) the curvature of the body, and 2) doing so allowed me to use factory screws to attach them. I can, of course, trim them to conform to the curvature, but I figured let me ask before I do something irreversible.

So what have I done? Should I expect this to make any difference in aerodynamic efficiency at all, good or bad? Anything I should change in the design? Or just rip them off?

Doing a little digging and I found this thesis that includes some good information. The math is way beyond me, but there are clear descriptions and diagrams that make sense of at least the basics.

Edit: I'm going to link to any other papers and articles I find in this post.

Assessment of conventional and air-jet wheel deflectors for drag reduction of the DrivAer model: suggests that conventional wheel deflectors may be of limited utility, especially on aerodynamically efficient vehicles

Drag reduction by application of aerodynamic devices in a race car: not necessarily applicable here, but I figured it might be of interest to some

I'm interested in your results. 

Reading the linked paper more closely, it seems that on the model in question, after testing five heights for front deflectors (rears were not tested in this paper) between 20 and 60mm, while larger deflectors had lower Cd locally, the overall Cd for the vehicle was lowest with the 40mm deflectors due to changes in pressure at the rear of the car. The deflectors I installed measure 35mm front and 45mm rear at the shortest point (fully inboard), with both 150mm wide. For reference, my 128i (lower and smaller than the V50) measures 35mm front and 25mm rear, while my parents' Audi Q3 (taller, but also more similar in shape to the V50) measures 25mm front and 45mm rear. This suggests that I'm at least in the ballpark for height.

What I can't find any information on is how far toward the outer edge of the wheel the deflectors should extend. The paper linked shows them starting at the inboard edge and extending roughly 2/3rds of the distance across the wheel, ending before the bodywork turns upward. This is also the case on the other vehicles I looked at, and from other images found online. What I don't know is if further extension toward the outer edge is aerodynamically detrimental, or if this is an aesthetic choice, or some combination of the two. Trimming mine back to achieve something more consistent with what I'm seeing on production vehicles is easy enough, but I'd rather not do it until I have a better sense of the aerodynamic impact.

Edit: I found one discussion (not peer-reviewed) suggesting ~100mm inboard and ~100mm above the ground as fairly standard position for fixed straight deflectors. Mine are certainly further out than this; I would have to take about 40mm off each to get them 100mm inboard. The fronts are 125mm from the ground, and the rears 115mm, so a bit more than is apparently typical.

In reply to FlordPerformance :

Not unless driving in reverse - these are on the front

Likewise, I'm also interested.

I'll have to dig into those papers after work. (Not like I'll understand most of it, but I'll at least give it a good college try.)

Very slow (read: zero) progress on this lately, but I did trim the tabs down to eliminate the overhanging outer portions, leaving something that looks more like what I've seen in factory setups. I found a photo of the factory aero version of the V50, known as the V50 DRIVe, which shows similar tabs in the front (further inboard than mine), along with a small spoiler under the chin - zoom in to see them more easily. This post on ecomodder quotes some information on the changes to these cars, including the aero mods. Unfortunately, the way I rigged them initially didn't leave me much to play with, so I'll probably order some new sheets of ABS to push the tabs inboard to better replicate the factory pieces. As you can see, there's also a grill block covering ~60% of the upper grill area. I'm going to fab one of those up and see what it does.

looks like this popped up while I was on a hiatus.  I'll take a look at the papers you linked to, and then see if there's anything I can bring to the discussion

02Pilot said:

I've noticed that a lot of newer cars include small plastic tabs in front of the wheels, almost mini-airdams.

[...]

I'm assuming these are designed to improve aero efficiency by steering air away from the spinning tires.

Yes, basically you've got that correct.

02Pilot said:

Doing a little digging and I found this thesis that includes some good information. The math is way beyond me, but there are clear descriptions and diagrams that make sense of at least the basics.

I wouldn't worry about the math.  It's mostly there so that other people coming along can perform the same tests and see if they get similar results (there's also a bit of "I know what I'm talking about" in there.).  I think this article is probably the most useful, although it still has some... we'll call them "limitations"

as you note, they indicate a limit of ~40mm height to the airdams, and have the following CFD images:

the report attributes the drag increase for the 50mm "front spat" (as I've seen them called) due to changes in the drag coefficient at the back of the car.  I think though, that it has more to do with how the flow is getting changed at the back edge of the front wheel, where you see that there's an increase area of flow getting "bubbled" out from the front wheel (whereas the 30mm front spat has a more 'elliptical' shape to that bubble of air velocity around the front wheel.

there's also some interesting visualization from the following ground plane image:

one of the things to point out is how the velocity at the ground gets pretty low just behind the spat, and the velocity profile makes a shape fairly similar to what you'd see with aircraft landing gear fairings.  I suspect this is why these spats (from OEMs) tend to end short of the outside of the tire... from their perspective, it just needs to be wide enough to force a flow of air to sweep around the outside edge of the tire.

you also might notice that the lack of spat in front of the rear tire is creating 'unhelpful' flow velocities.

a final thing to keep in mind with these spats, that I'm not seeing in the modeling of any of these three papers, is that there's a high pressure on the front face of them (although it looks like there's even higher pressure behind them, due to their presence and the flow that's getting pulled around by the tire); and there's relatively low pressure on either side (because of the higher velocity 'clean' flow either side)... though these things around going to spin off some vortices because of these pressure differentials... and especially if the spats were angled/bent backwards (instead of being only perpendicular), but also with the front spats that tend to reduce in height as they extend toward the middle of the car.

02Pilot said:

So this is what I've done. Small tabs in front of all four wheels, plus a small airdam at approximately the same height (the gaps between the two are to allow air to flow to the factory brake cooling channels; I'm not wedded to keeping them open if it makes a huge aero difference, as this is not going to see lots of heavy braking). They are made of ABS plastic and are quite rigid.

Front:

Rear:

The tabs extend up the wheel arches due to 1) the curvature of the body, and 2) doing so allowed me to use factory screws to attach them. I can, of course, trim them to conform to the curvature, but I figured let me ask before I do something irreversible.

So what have I done? Should I expect this to make any difference in aerodynamic efficiency at all, good or bad? Anything I should change in the design? Or just rip them off?

so, the front spats seem fine.  although, I'd encourage you to try and make them 'bend backward' by scoring the backside along the bumper line and down to the bottom.  That has two benefits:  1) it makes the part that's reaching up to grab the bolt in the fender liner 'less vertical' (which is going to force a fair amount of air outside the bumper to go up); 2) it makes the outboard part of the front spat more "aircraft wheel pant-like" and encourage the outboard part of the flow to wrap around the tire and reattach on the side of the car (like you see with the velocities in the 'mid-wheel flows Case 3 & Case 5 CFD images above).

another thing I notice, though, is that it loose like you 'center airdam' is jutting forward?  If so, I'd recommend that you see if you can mount it in a way that it'll be vertical or bend slightly back.  If not, I'd suggest you just remove it, and try instead to make similar underpanels to the "V50 DRIVe".

(V50 DRIVe underbody panels image from the ecomodder thread you linked above)

speaking of, I suspect the upper grill block will have more of an impact that the spats.  although, aerodynamics is all about trying to "stack" a bunch of beneficial things all together.  so, I wouldn't toss the spats, and I'd encourage you to add them ahead of the rear wheels too.

clear as mud?  or snow?

In reply to sleepyhead the buffalo :

Very helpful, thanks. I'm a complete novice in this sort of thing, so your explanations help a lot, particularly in interpreting the imagery.

I need to update the photos, as I have made some changes, and will probably make more to take your analysis into account. The portions of the spats outside the vertical portions of the bumper and quarter panel have been removed, leaving just the sections under the car. Because of the way I fabbed the originals, the front spats are now very narrow; I intend to get more ABS and make new ones that extend further inboard. The rears will probably stay as they are, but I'll post photos - if you see anything horribly awry, I can always make adjustments.

The front airdam is relatively perpendicular to the ground I think, but I'll double check it. I can see about bending it if need be, but do you think it's worth it, or should I just take if off? Alternatively, I could shorten the height and relocate it to the undertray, similar to the DRIVe configuration (I noticed that my ND has something similar).

Underbody panels may get done eventually, but that will take some time. The grill block is easy - I may even get to it this week.

I had a bug in my head about the grill block, so I threw a quick proof-of-concept together.

This mimics the DRIVe grill arrangement. It's fairly thin ABS plastic, but I've got enough zipties in there that it should be fairly solid; I can always do it again with heavier material if need be. It should also help the engine warm up more quickly, which will be nice, especially since cold start/warm-up cycles really seem to kill fuel economy on this thing.

Edit: I assume those cephalic horns are closed off for optimal hydrodynamic performance in open water. I will guess that next to the ground, they should be opened up a little with a 1" coiled gap on the outside curved coil edge? I will report back when I find out how those horns work.

In reply to VolvoHeretic :

I think you're presuming that I do a considerable portion of my driving underwater, which, oddly enough, is not the case. Perhaps you need to suggest to the powers that be that we start a Hydrodynamics subforum.

02Pilot said:

In reply to sleepyhead the buffalo :

The front airdam is relatively perpendicular to the ground I think, but I'll double check it. I can see about bending it if need be, but do you think it's worth it, or should I just take if off? Alternatively, I could shorten the height and relocate it to the undertray, similar to the DRIVe configuration (I noticed that my ND has something similar).

Underbody panels may get done eventually, but that will take some time. The grill block is easy - I may even get to it this week.

Yeah, I get it on the underbody panels.  most likely the grill block will have the most significant impact on mileage/drag.

re: airdam
it wasn't real clear if you'd seen much mpg improvement from the spats & airdam?

I could see the benefit of keeping/removing of it going either way, although my "gut" instinct is to pull it off.  Downforce isn't a priority, and this car has an undertray(?)... so the bumper is trying to smooth the flow around the corner and onto the undertray.

Also, I'm not sure it's really acting like a "dam" in the sense we're used to about them 'limiting' how much air is getting under the car.  It might be acting more like a splitter, in that it's causing the flow under the bumper to 'accelerate' more... just less efficiently than a splitter would.  If that's the case, the airdam might-could-be helping with evacuation out of the radiator via: vortex generation / separation bubble pressure zone / other... if there's some space for its exhaust to flow out of the undertray?

In reply to sleepyhead the buffalo :

I've been quite lax about charting mileage improvements, but suffice it to say that, if they have occurred, they are not dramatic.

I'll get some photos of the undertray and spats when the snow melts and I can get under the car comfortably. There are some ducts in the tray, and it ends at the front subframe, so it should be able to vent out the back easily enough.

Pulling the airdam off is easy enough. I might yank it and the existing spats to try to get a baseline with just the grill block, then make up new spats based on what we've talked about and the DRIVe example and see what those do. I made sure that everything is 100% reversible and easy to remove, so it won't take much.

Very quick update on one thing. A relatively brief drive in today's 26F weather demonstrated that the grill block definitely helps with warm-up time. That alone should improve fuel economy a little bit.

02Pilot said:

Very quick update on one thing. A relatively brief drive in today's 26F weather demonstrated that the grill block definitely helps with warm-up time. That alone should improve fuel economy a little bit.

yeah, should be... iirc (and check with our more knowledgeable ecu gurus)... most engines are running "open loop" until the engine warms up.

Finally got time to pull the airdam off and take a few pictures and measurements. As things stand right now, the modifications are the grill block shown earlier and the wheel tabs/spats, shown below.

Front, 35mm high x 50mm wide:

Rear, 50mm high x 100mm wide:

And, for reference, the factory front bumper and undertray arrangement:

Starting with the next fill-up, I'll start monitoring fuel economy and see where things stand before I do anything else.