Aero mods and cooling

[Wimp Lo]

I once read about an aerodynamic study of bricks. It was a school exercise in basic aerodynamics but kind of relevant to the shape of a hatchback.

So the example was a brick which had the front end ground in to a parabolic curve and in a wind tunnel it was marginally better than an unground brick.

Then the ground brick was turned 180° so the parabolic curve was to the rear and then it had significantly less drag than the other two examples (frontal area was identical in all examples).

The moral is that it almost doesn't matter what you do at the front as long as the flow behind is not creating a turbulent wake.

That means be as aggressive as possible with the splitter and making front end down force, the real place to clean it up is around the hatch.

That doesn't take ground effects in to account which is the other place to find improvement.

So just concentrate where air is coming out, not where it is going in.

 

[Siestarider]

Could you take pictures of the pieces you removed?

Had to pull out of shop trash, obvious I freehanded the cuts with a dremel from my creeper. Maybe I will just keep removing until performance gains stop or reverse.

 

[RAAMaudio]

Once you maximize the results you might want to consider using Alumalite in 6mm as very stiff, moderate cost, easy to work with, it will not deflect so would be more accurate and stable in measurements and performance.

 

[Wimp Lo]

Belly pan cut from 1/8" ABS, rough side up.

Aluminum bars riveted on for attachment behind OEM air dam. Nylon zip ties (added another to each pair of holes) ready to zip around Pierce 2 point. Zip holes are set so that tightening them pushes pan forward into bottom lip of air dam.

View from front of installed pan.

View from side installed pan. Yes these are poor pictures accurately reflective of my poor photographic skills.

For reference.

 

[RAAMaudio]

Nice work on the brake duct air inlets

 

[Siestarider]

Measured grille opening at 96 square inches. Radiator face calculates to 376 square inches. Shape of intake diffuser is not that different from ideal, so maybe aeronautical rule of thumb 30-50% of opening in cowl expanding to radiator face area area offers some guidance. Works out to 113-188 square inches of opening. Lends support to idea of opening up grille another row or two, opening up next two rows would be about 150 sq in total intake "nozzle".

Yes, assuming I can get this experimental piece optimized for cooling and coast down time, it will have to be converted to a material that is stiffer but still light. And easy to remove and replace. Oil and filter changes are not feasible with the test piece in place.

More whittling and testing in near future. I would like to at least reach performance of 2" air dam extension on Cd and reach empirical minimum pressure drop across heat exchangers with this mod.

 

[Wimp Lo]

The problem is that these papers you are reading are about optimizing cooling performance vs drag.

Drag reduction is great but not really the problem in racing. To go around a corner faster, down force increases front grip. To go faster down a straight, more power is useful and with Fiesta ST cooling issues, more air through the radiator might support more power.

Once we have down force and cooling, start worrying about drag then.

 

[RAAMaudio]

I thought this over a bit and it seems quite a logically way to look at it, grip, cooling, drag, once grip and cooling are taken care of and the power you wish to have is dialed in then deal with drag if you need to go a bit faster on track.

But let's not forget daily driving for those that want or need to save fuel.

My car likely has some extra drag but my wife just drove it 208 miles and the DIC showed 41.5mpg. Speeds were 58-60 mpg most of the time as following the trailer in CA where speed limit is 55mph for towing, we did climb quite a few hills and a smaller pass, two actually, on the way and the last 20 miles in AZ I was hitting up to 75MPH passing big rigs for fun so she was at those speeds and uphill nearly all the way.

My car has a big turbo, lowered, 225 very sticky tires on 9" wide wheels that do stick out in the wind, fully opened grill, (splitter/undertray off due to damage and have to build a part to put it back on.)

I would love to see the results in the same conditions with the splitter back on the car.

 

[Siestarider]

The problem is that these papers you are reading are about optimizing cooling performance vs drag.

Drag reduction is great but not really the problem in racing. To go around a corner faster, down force increases front grip. To go faster down a straight, more power is useful and with Fiesta ST cooling issues, more air through the radiator might support more power.

Once we have down force and cooling, start worrying about drag then.

Useful collaborator or big-mouthed parasite?

Show us some data.

I want is better cooling performance, hopefully achievable with some aero mods that suit my style. When I want more grip I will buy better tires.

 

[RAAMaudio]

Yes, go for your personal needs first of course and keep sharing the results, very much appreciated

 

[Wimp Lo]

Go to SAE press and order Race Car Vehicle Dynamics by Milliken. That's my reference.

It covers a lot more than aerodynamics but the aerodynamics coverage is good, starting with wings, adjustable wings, vacuum traction, ground effects, etc. It's not super current, probably 25 years out of date but the state of the art was pretty advanced by that point.

Think about an aircraft, are they designed for maximum speed or maximum range? Think about a street car, do you think aerodynamics are chosen for down force or fuel economy?

If you aren't going to race, you can leave a Fiesta ST alone, it won't overheat easily and the aero is already set up for good mileage.

If you want engine cooling to support more power, you need WRC style hood vents.

If you want front end down force with a side of drag reduction, you need a splitter/under tray.

If you want drag reduction, forget about the front of the car and look at the tail end.

One more thing in Race Car Vehicle Dynamics is that for decades race cars had beautifully sculpted front ends that looked incredibly aerodynamic but they didn't understand how to shape the rear, neglected the bottom and generally created cars with high drag and front end lift. That didn't change much until the late 50s.

 

[Siestarider]

Ok, useful highly opinionated collaborator it is. I would not be seeking better cooling if I was not tracking.

I don't want WRC vents unless there is no other way. Your "if this then that" list is pretty crude, maybe your deep appreciation of Milliken enables you to offer suggestions on how an undertray might be shaped to enhance heat exchanger cooling if its the only option on the table at the moment?

 

[Wimp Lo]

That is where it gets tricky, to reduce drag you want to limit air under the car and that means you want to find somewhere else to dump the hot air from the engine compartment.

An effective (extended, stiff and low) splitter will reduce the amount of air underneath which can allow you to send more air from the engine bay under the car but it would still be better to exhaust elsewhere.

If you are dead set at dumping the air there, you could add a downward flap at the trailing edge of the tray to enhance the low pressure zone behind it and suck out engine air but that will undo most or all of the drag reduction and downforce that a splitter can give you.

I was also looking at some of the SFE Aero pieces and they have some rear under tray pieces with small air dams in front of the rear wheels to direct the air away from the tires. That seems like a good idea for drag reduction. It might be a workable concept at the front.

 

[Siestarider]

Boostbumps posted some articles on the Focus RS. The Automobile article has lots of pics. Including from beneath. Looks like RS is losing all the nose cooling air right behind air dam in fore/aft slots. Then NACA ducts in undertray further back are picking air back up to cool something. Then rear corners cut off about like I just did.

So, next mod will be cutting slots right behind air dam. Thanks Ford, for the clues.

Fog light vents may be providing both intake air and brake air via wheel wells. Wish these professionals would take better pics. Lol.

 

[BoostBumps]

Boostbumps posted some articles on the Focus RS. The Automobile article has lots of pics. Including from beneath. Looks like RS is losing all the nose cooling air right behind air dam in fore/aft slots. Then NACA ducts in undertray further back are picking air back up to cool something. Then rear corners cut off about like I just did.

So, next mod will be cutting slots right behind air dam. Thanks Ford, for the clues.

Fog light vents may be providing both intake air and brake air via wheel wells. Wish these professionals would take better pics. Lol.

Are these are the pics your referring to?

 

[Siestarider]

Awesome. Yep, that is them. Wish they were more explicit, but black undertrays are hard to get good detail on. I can barely get reflections of my garage floor, but these guys had car fully exposed at head height.

 

[BoostBumps]

Awesome. Yep, that is them. Wish they were more explicit, but black undertrays are hard to get good detail on. I can barely get reflections of my garage floor, but these guys had car fully exposed at head height.

Not sure how much this helps but enhanced pics to show a bit more detail...

 

[Wimp Lo]

I am not sure those are slots, they look like stiffening ribs, my guess is to resist blowing out at 160+.

The NACA ducts look nice but they don't help pull any air out of the engine compartment.

 

[koozy]

naca ducts look to be used for cooling the gearbox.

 

[Siestarider]

I agree with Koozy, gearbox and something else behind it. Might be air for back side of engine. I have not heard if RS has oil cooler, if so, it must be buried under top engine bay shrouding, so maybe that too. Definitely picking up air under car, maybe extraction there balances cooling air dump behind air dam.

No Wimp, I think that structure behind air dam is dumping heat. But agree, even with enhancement its hard to see exactly how those ribs work, some appear slightly offset vertically, could be only IC heat, too much we cannot see.

BB, thanks for visual enhancement. Love theory, but seeing how factory solves heat loss/aero is a practical leg up.