Aerodynamics discussion

[Wonderbar]

Hey guys! So, I'm not an engineer or anything, just a self educated hobbyist and diy'er. I hate to see issues on my cars that could be fixed with a few hours and a hundred bucks.

I also hate to see fake or nonfunctional parts on cars. Our "rear diffuser" drives me nuts and in my opinion, it's a critical piece of both performance and visual on a sports car.

Here is my baseplate/prototype/version 1 of my diffuser for my car. I haven't riveted on the vanes for channels yet. The "R" cutout is to clear my Perrin exhaust, the front tabs bolt in the lower bolts on the rear sway bar, this closes nearly the entire open space in the rear bottom bumper and extends about 2 inches past the rear of the car. It doesn't follow the lines of the fake diffuser on the bumper, instead it creates a clean line on the bottom, where the OEM additional "diffuser" bolts on. Putting the exhaust inside of the air free cavity.

I built a similar panel for my 2006 eclipse using the tissue paper "tabs" and a gopro for testing.

It isn't close to finished, I'm doing a wrinkle coat black on the top and I'm planning on a gritted bed liner or flat black on the road side. Might leave it bare aluminum, haven't decided. This one is all built and bent by hand. Once I'm done tweaking, I'll do a nice laser cut one and anodize or powder coat it.

[sw20kosh]

[plucas]

Update:

Thanks to mobybrz we have coast down test data! I am sifting through this data. I am in the process of putting together a nice report on the aero performance of a stock/stockish brz/frs with cfd data and coast down data for all to access.

[supramkivtt2jz]

Any updates, @plucas?

[Wonderbar]

@plucas, any way you could run data on my rear diffuser?

I'm working on a full aluminum aero kit, to include side skirts, 3 piece front, underbody and fender well pieces.

[plucas]

Originally Posted by supramkivtt2jz

Any updates, @plucas?

Yes and I will post them soon. I have been busy learning a new meshing software that we might upgrade to so I haven't had a chance to update the thread yet. I do have data for our splitter design also (one of the pictures from the analysis is on our facebook page)

Originally Posted by Wonderbar

@plucas, any way you could run data on my rear diffuser?

I'm working on a full aluminum aero kit, to include side skirts, 3 piece front, underbody and fender well pieces.

Most likely yes. Do you have CAD models of the kit?

[Wonderbar]

Originally Posted by plucas

Yes and I will post them soon. I have been busy learning a new meshing software that we might upgrade to so I haven't had a chance to update the thread yet. I do have data for our splitter design also (one of the pictures from the analysis is on our facebook page)



Most likely yes. Do you have CAD models of the kit?

I'm still a while out on the rest of the kit. Ill try to get one for the rear diffuser over to you.

[Panda Mao]

I kno this is an older thread but I am curious about the oem aerodynamics mainly cuz Ive done a few high way driving at beyond 80 mph (nothing crazy) but I felt as if the car stumbles and seems to lose balance when there is a strong gust or air pressure from different angles that it just seems to throw my car off balance a lil bit when I try to stay in the lane.

is it caused by overall light weight car + ok oem aero? or is there other factors? I know for a fact that if I am driving near a semi truck it does throw the car off a bit but thats to be expected from the various wind pressures coming off the 18 wheeler but even if there is no car around me on the high way sometimes with a strong gust of wind blowing (like from the ocean) it kind of stumbles the car a bit.

is it just cuz the car is designed that way by default? or am I really that crappy of a driver? and the car only has your basic mods like intake n stuff no mods done to exterior aero yet.

[diss7]

Great thread. Lots of good info. It does raise more questions at least for me.

I've just bought another 86 for track use only. One thing I want to look into more is the rear diffuser. I find a lot of directly conflicting information on their design and am hoping to get some clarification on which information is accurate.

The diffuser design I was thinking of doing is similar to the photoshop @sw20kosh did a few pages back. Because my subframe and diff will be raised, I can continue the floor paneling under the subframe, and the the diffuser chamber will begin.

1: Diffuser chamber design. Some say to make this as large as possible, curved up. ON this platform, once you're past the subframe, the chamber would start with a near vertical section, that then curved in an arc to become near horizontal at the exit point. Yet, other literature states that diffusers have an ideal angle or around 7-8 degrees, with their effect reducing as you go steeper. Quite wildly different information. Would you take a compromise of both approaches, and start the diffsuer from the end point (at the bumper) angle it down at 8 degrees towards the subframe, and where it approaches the subframe, have either a vertical section (maybe 6 or 8 inches or so) or a very tight arc.

2: Exhaust flow. Is it more benefical to incorporate the exhaust to exit the rear of the vehicle above the diffuser, again as per sw20kosh's photo. In other words, does the exhaust gas promote diffuser function? The alternative being to cut the exhaust through the tunnel so it enters the car at the passenger side (I'm RHD) foot well, and then exit the car from the side, from inside the cabin. (Yes this is a full blown race car) This would then allow the diffuser to be larger. However I am not sure if OEM supercar manufacturers exit the exhaust to the rear because it is benefical, or because they have to. My race class only requires that the exhaust enters behind the rear most cabin opening, ie past the b pillar.

3: Leading to the diffuser. I understand that the smoother this path, the better. However, cannot find a straight answer on whether you would rather aim to lower or raise the volume of air that passes under the vehicle. One side of me thinks that the more air that flows under, the greater the eventual effect of the diffuser. The other part of me says the less air that passes under the car, the more that the entire car acts as a massive spoiler.

3a: Related to question three, is it then beneficial to undo the factory engine bay "exhaust" path (where hot air escapes) put new non vented panels in, and instead redirect air upwards and out a vented bonnet. This would have a two tier effect of reducing underbody airflow (if thats what we're trying to achieve) and expelling air from the bonnet which I understand further increases downforce.

3b: Again related to question 3. A front air dam is going to reduce underbody air flow, good or bad thing for the diffuser / overall ground effect.

4: Seeing that it appears universal that the lower the overall car is, the more compounded these effects are, why then are more people/racers not using a 15" wheel and a 580 slick, rather than the much more common 17 or 18" and a 610 or 640mm slick? Before you say brakes, the AP racing sprint kit for one, fits in a couple of 15" wheels I have.

5: If now had car very low, low airdam, diffuser, the car will then try to seek to suck in air from the sides. So now have a functional reason for side skirts? I think I'm starting to answer my own questions as to whether we're trying to encourage or discourage underbody airflow.

Sorry for the barrage of questions, but all point really do seem related, which is mostly around my understanding of a few critical points of I'm ultimately trying to achieve with the underbody airflow.

[sw20kosh]

Originally Posted by diss7

Great thread. Lots of good info. It does raise more questions at least for me.

I've just bought another 86 for track use only. One thing I want to look into more is the rear diffuser. I find a lot of directly conflicting information on their design and am hoping to get some clarification on which information is accurate.

The diffuser design I was thinking of doing is similar to the photoshop @sw20kosh did a few pages back.

That was not a photoshop.

[Shankenstein]

Originally Posted by diss7

1: Quite wildly different information...

Based on my understanding of aero, there are two main goals.
1) You want fast, laminar/smooth air flow under the car. Fast flow --> lower pressure --> downforce (as it acts on the lower surface of the car). There will be flow separation and turbulence, whenever you have to accommodate a wheel, cooling ducts, or the ground. Your goal is to keep the bulk air flow as laminar as possible. Sir Mix-a-lot would have my head for this, but... "A flat bottom is ideal."

2) Reconnecting the air paths (upper and lower) should be as gradual as possible, and it should minimize the "dead" volume of air behind the license plate. The Kammback/teardrop shape is ideal, but impractical. If you want downforce from a spoiler, you can exaggerate the diffuser angle to take advantage of the low pressure zone that creates.

The optimal angle is usually one that expands the cross-section as rapidly as possible without causing flow separation, turbulence, or instability. Vague, I know. Numbers like 6-10 degrees are often thrown around, but a gentle curve will usually outperform a piece of flat plate.

Originally Posted by diss7

2: Exhaust flow.

Adding air below the car will only disturb the flow and increase the pressure (minimally). Both are bad things. That's why designs like the LFA, Lotus, etc look like this:

Originally Posted by diss7

3: Leading to the diffuser. I understand that the smoother this path, the better.

Yup!

Originally Posted by diss7

3a: Is it then beneficial to undo the factory engine bay "exhaust" path (where hot air escapes) put new non vented panels in, and instead redirect air upwards and out a vented bonnet.

Interesting idea! Heat management and pipe routing would be a challenge. It might be reasonable if you have a turbocharger and turn up from there. There's no room for exhaust... but you said it's a racecar. The NHRA had alot of people doing this, and there's been consideration for banning it, since fluid spray and soot can get be rather tough to see through.

Originally Posted by diss7

3b: Again related to question 3. A front air dam is going to reduce underbody air flow, good or bad thing for the diffuser / overall ground effect?

Air dams are generally a good thing, as long as you're not scraping the ground... and don't care about fuel economy. Frontal cross-section is a major player in drag force (not coefficient).

Originally Posted by diss7

4: Seeing that it appears universal that the lower the overall car is, the more compounded these effects are, why then are more people/racers not using a 15" wheel and a 580 slick?

Smaller wheels/tires will lower the car and improve final drive ratio. Lighter wheels/tires will reduce rotating mass and unsprung weight. If you can get the braking force/cooling and optimal tire width in a 15" package, by all means, go for it. You may sacrifice sidewall stiffness, but the upside is definitely there.

Originally Posted by diss7

5: If now had car very low, low airdam, diffuser, the car will then try to seek to suck in air from the sides?

Yes, but not that much. In the CFD that Hancha Group has done, there didn't seem to be much effect... but similar to air dams, the lower you can go with sides skirts, the better you'll isolate the two air paths.

[cdrazic93]

I can only answer a few to my knowledge (To the best of my abilities)

Originally Posted by diss7

Great thread. Lots of good info. It does raise more questions at least for me.

I've just bought another 86 for track use only. One thing I want to look into more is the rear diffuser. I find a lot of directly conflicting information on their design and am hoping to get some clarification on which information is accurate.

The diffuser design I was thinking of doing is similar to the photoshop @sw20kosh did a few pages back. Because my subframe and diff will be raised, I can continue the floor paneling under the subframe, and the the diffuser chamber will begin.

1: Diffuser chamber design. Some say to make this as large as possible, curved up. ON this platform, once you're past the subframe, the chamber would start with a near vertical section, that then curved in an arc to become near horizontal at the exit point. Yet, other literature states that diffusers have an ideal angle or around 7-8 degrees, with their effect reducing as you go steeper. Quite wildly different information. Would you take a compromise of both approaches, and start the diffsuer from the end point (at the bumper) angle it down at 8 degrees towards the subframe, and where it approaches the subframe, have either a vertical section (maybe 6 or 8 inches or so) or a very tight arc.

2: Exhaust flow. Is it more benefical to incorporate the exhaust to exit the rear of the vehicle above the diffuser, again as per sw20kosh's photo. In other words, does the exhaust gas promote diffuser function? The alternative being to cut the exhaust through the tunnel so it enters the car at the passenger side (I'm RHD) foot well, and then exit the car from the side, from inside the cabin. (Yes this is a full blown race car) This would then allow the diffuser to be larger. However I am not sure if OEM supercar manufacturers exit the exhaust to the rear because it is benefical, or because they have to. My race class only requires that the exhaust enters behind the rear most cabin opening, ie past the b pillar.
from what i understsnd exhaust has to leave near the rear of the vehicle to be street legal, as i know most if not all supercars do that, several like the cobra and the viper have it exit near the front of the rear wheel. From the side as in passanger sideright behind the front wheel may disrupt airflow around that side of the car. I say point it down straight from the header, or angled down next to the passenger side.
3: Leading to the diffuser. I understand that the smoother this path, the better. However, cannot find a straight answer on whether you would rather aim to lower or raise the volume of air that passes under the vehicle. One side of me thinks that the more air that flows under, the greater the eventual effect of the diffuser. The other part of me says the less air that passes under the car, the more that the entire car acts as a massive spoiler.
I know that you want the whole car to use itself as downforce, years ago a racing company named chapparal was making race cars that were being banned, why? They had a giant fan under the car to suck the car to the ground, called the 2J. front aero fins, wings, diffusers, vented hoods, all contour the air around the sides or the top of the car, never have I heard push air underneath.
3a: Related to question three, is it then beneficial to undo the factory engine bay "exhaust" path (where hot air escapes) put new non vented panels in, and instead redirect air upwards and out a vented bonnet. This would have a two tier effect of reducing underbody airflow (if thats what we're trying to achieve) and expelling air from the bonnet which I understand further increases downforce.
Yes, sucking air from the front and expelling it from the top would produce downforce which is good.
3b: Again related to question 3. A front air dam is going to reduce underbody air flow, good or bad thing for the diffuser / overall ground effect.
good thing, also the air that would have traveled under the car is now being forced over it, ro into the radiator and out through the vented hood
4: Seeing that it appears universal that the lower the overall car is, the more compounded these effects are, why then are more people/racers not using a 15" wheel and a 580 slick, rather than the much more common 17 or 18" and a 610 or 640mm slick? Before you say brakes, the AP racing sprint kit for one, fits in a couple of 15" wheels I have.
Yes lowering the car does help reduce air under the car, but at some point it obviously hinders handling otherwise slammed JDM my car is kewl kids would be winning races right and left
5: If now had car very low, low airdam, diffuser, the car will then try to seek to suck in air from the sides. So now have a functional reason for side skirts? I think I'm starting to answer my own questions as to whether we're trying to encourage or discourage underbody airflow.
yes, yes you are
Sorry for the barrage of questions, but all point really do seem related, which is mostly around my understanding of a few critical points of I'm ultimately trying to achieve with the underbody airflow.

[diss7]

Thanks for the answers!

[dradernh]

1: 6-12 degrees - determine most effective angle using flow visualization fluid (e.g., diluted motor oil sprayed onto the roof of the diffuser); strakes no lower than the car's floor bottom; if possible, exhaust over the diffuser roof if running a rear exhaust; if no flat bottom, gently curve the forward edge of the diffuser up into the suspension/diff area of the underbody so that underbody air cannot flow above the diffuser roof; extend it no further than 100mm/4" behind the rear of the car; first (only?) pair of intermediate strakes placed 100-150mm/4-6" inset from outside strakes

2: Over the diffuser roof offers more flexibility in diffuser design/construction; side exhaust is ideal (i.e., lighter, solves diffuser issue, looks cooler, all good); because they have to (i.e., the packaging is what's called for by a road-going car)

3: Lower the air volume (i.e., if flat-bottomed, place it as low as possible without its hitting the pavement)

3a: Shaping air flow to your advantage is the goal - that means controlling all of it that you are able; use a vented bonnet and, to the extent possible, ensure that air venting behind the radiator is made to flow through the vents; use vents that will release air as gently as possible into the stream already flowing over the bonnet

3b: Good

4: If you can use a 15" wheel, use it; however, please do tell us which 15" wheel will accomodate the AP Sprint Kit's 325mm/12.81" rotor/caliper combo

5: Side skirts, horizontal as well as vertical = good; some of the new materials have excellent wear characteristics, enabling you to mount the vertical skirts' bottom edges extremely low (see: NASCAR Sprint Cup cars)

Given the amount of money you're spending, you might consider the aero consulting service offered by Simon McBeath, author of Competition Car Aerodynamics and aero columnist for Racecar Engineering magazine. I did, and it was well worth it, both in terms of first principles applied to my specific car model, and in terms of time and money not wasted trying one thing or another without the resources to necessarily determine whether or not they were working optimally. In the context of the money I was already throwing at the car, the cost was surprisingly acceptable. More here: http://www.sm-designs.ndo.co.uk/smarthome.html. Full disclosure: I have no affiliation with Mr. McBeath other than as a satisfied customer of his service and reader of his works.