undertray for our cars?

[bubbrubb]

Anybody know if there would be much if any benefit for making a smoother underside on our cars? Our cars aren't exactly the most aerodynamic things on the road, so I was wondering if there was something simple we could do to our cars to get a little bit cleaner aero. I was thinking of fabbing something like the picture below, but wanted to hear some opinions first. I know that it probably wouldn't do much until after 80mph or so (if at all), but sometimes the pedal gets stuck.

underside of a subie wrx with an aftermarket under tray

Rough photoshop of what I'm talking about on a friend's sn95

without:

any good pictures of a mustang underbody would be appreciated

 

[Venomous01]

Interesting.

 

[Steve2050]

Very good idea! Maybe road racing guys will chime in.

 

[bubbrubb]

^thanks, and hopefully! That's one of the reasons why I didn't put it in the interior/exterior section :rolling:

My major concerns are that it will disturb the airflow that the car was actually relying on for cooling, as well as the obvious potential for it to create a low pressure area underneath it and get pulled off of the car.

It would also be pretty interesting to see how far it could extend back and where it would bolt onto. I might take my car to a friend's shop to get a better glance at everything and bring my camera

 

[SicStang03]

Eh, seems a little useless for a street car. I would think it would be a pain to pull that out to change the oil

 

[svt2ner]

If anything I would think it would create more drag.

 

[bubbrubb]

Eh, seems a little useless for a street car. I would think it would be a pain to pull that out to change the oil

haha, that is something I didn't think of. But I don't put many miles on the car so that isn't a big deal. I'm more interested in the idea behind it than I am in the practicality of it.

Look at the underbody of the z06, a carrera GT or even the GTR. They're all pretty damn smooth under there. People always say how vettes have such an aerodynamic advantage at high speeds, and I realize the vast majority of that comes from their shape and lower frontal area, but I'm sure that there would be some benefits to having a smooth body for the air to flow over under our cars.

something crazy like this:

If anything I would think it would create more drag.

how so?

 

[steedafever]

hmmm

 

[HEMIHUNTER]

Good idea, needs more info/research.Sicstang has a good point about the oil change.

 

[black 10th vert]

Good idea, needs more info/research.Sicstang has a good point about the oil change.

The oil change wouldn't be a big deal, as you could just make a smaller, removable panel in the drain area. It could be attached with dzus(sp?) fasteners to make it quick to remove an re-install, like the ones on race car body panels. I just installed a lift in my garage this week, and this was one of the projects that I was going to look at over the winter. I was originally planning to make a rear diffuser, but it may be some benefit to skin the whole underside...;-)

 

[wheelhopper]

I have had my car at 145mph at VIR and it still felt very stable. I am not sure at what speed that the "undertray" would be beneficial, but it is probably faster than 99% will ever go. Probably not worth the cost.

 

[SlowSVT]

Most likely it would improve the aerodynamics but to what degree I cannot say. A smooth surface will allow the air to stay more laminar minimizing turbulent air created when it bounces off irregular surfaces. The trick is to minimizing any disturbance of the air as the car passes thru it. Perhaps someone can model the car and wash the surface data through a CFD program and get some numbers. Probabaly not a pretty picture :nonono:

 

[bubbrubb]

Most likely it would improve the aerodynamics but to what degree I cannot say. A smooth surface will allow the air to stay more laminar minimizing turbulent air created when it bounces off irregular surfaces. The trick is to minimizing any disturbance of the air as the car passes thru it. Perhaps someone can model the car and wash the surface data through a CFD program and get some numbers. Probabaly not a pretty picture :nonono:

You know what you're talking about :beer:

While I know that our cars feel relatively stable at speed, that doesn't mean they're the most slippery.

Here are some things that I've dug up that I think might be some useful tools in researching this.

some good pictures of what a guy did on his saturn
http://s75.photobucket.com/albums/i310/lovemysan/saturn/

I'm going to do a lot of reading in this later this weekend I think (cause I wanna save the environment LOL):
http://www.gassavers.org/forumdisplay.php?f=11
http://ecomodder.com/forum/aerodynamics.html

Seriously though, I think that if some of those cars with more eco friendly aerodynamics are getting gains in MPG by doing undertrays and stuff when driving the speed limit, I have no reason to doubt that this stuff would help at 120+mph - especially because when we're on the gas and the rear is squatting it exposes more of the parachutes that we have built into our cars.

I was told Race Bronco did something like this for the rear of his car, so I'm going to search that later.

 

[Coiled03]

If anything I would think it would create more drag.

Affirmative. More downforce = more drag at speeds where downforce actually comes into play. For a street car, it would be practically useless.

 

[black 10th vert]

I feel that it would most definitely help improve mpg, as well as overall aero efficiency. The only reason that these cars are so "rock solid" at high speeds isn't because they are aerodynamic, it's because of the weight - they are tanks!

 

[SlowSVT]

You know what you're talking about :beer:

While I know that our cars feel relatively stable at speed, that doesn't mean they're the most slippery.

Here are some things that I've dug up that I think might be some useful tools in researching this.

some good pictures of what a guy did on his saturn
http://s75.photobucket.com/albums/i310/lovemysan/saturn/

I'm going to do a lot of reading in this later this weekend I think (cause I wanna save the environment LOL):
http://www.gassavers.org/forumdisplay.php?f=11
http://ecomodder.com/forum/aerodynamics.html

Seriously though, I think that if some of those cars with more eco friendly aerodynamics are getting gains in MPG by doing undertrays and stuff when driving the speed limit, I have no reason to doubt that this stuff would help at 120+mph - especially because when we're on the gas and the rear is squatting it exposes more of the parachutes that we have built into our cars.

I was told Race Bronco did something like this for the rear of his car, so I'm going to search that later.

I think the SN95 CD measures in at .36 (the S197 is .37). That is not very good. A C6 measure around .28. My Subaru Outback is suppose to be .32 but that seem a bit optimistic (it’s a friggen station wagon with a roof rack) . The Fox4 chassis is pretty dirty aerodynamically speaking. I've had my Cobra up against the speed limiter and it didn't do anything particularly weird considering. Some guys have stated things start getting spooky at anything over 160.

The science of aerodynamics at time can yield results are quite unexpected and can be hard to predict.

I was considering ditching my rear spoiler but am a little reluctant. Supposedly it does provide some downforce. Ford did some wind tunnel testing on the Terminator but I think the purpose was to make sure there were nothing abnormally wrong or any whistling noise generated at highway speeds. Other then that very little was done to optimize the shape of the car which would have impacted every contour on the body. Nissan took the opposite approach with the new GTR and the results are not too esthetically pleasing but it is slippier then a Vette so they claim.

 

[bubbrubb]

Affirmative. More downforce = more drag at speeds where downforce actually comes into play. For a street car, it would be practically useless.

What confuses me is why you think less turbulence under the car would create downforce. Why? Also, more downforce doesn't always = more drag, I've been reading into it.

I think the SN95 CD measures in at .36 (the S197 is .37). That is not very good. A C6 measure around .28. My Subaru Outback is suppose to be .32 but that seem a bit optimistic (it’s a friggen station wagon with a roof rack) . The Fox4 chassis is pretty dirty aerodynamically speaking. I've had my Cobra up against the speed limiter and it didn't do anything particularly weird considering. Some guys have stated things start getting spooky at anything over 160.

The science of aerodynamics at time can yield results are quite unexpected and can be hard to predict.

I was considering ditching my rear spoiler but am a little reluctant. Supposedly it does provide some downforce. Ford did some wind tunnel testing on the Terminator but I think the purpose was to make sure there were nothing abnormally wrong or any whistling noise generated at highway speeds. Other then that very little was done to optimize the shape of the car which would have impacted every contour on the body. Nissan took the opposite approach with the new GTR and the results are not too esthetically pleasing but it is slippier then a Vette so they claim.

I've got something I think you'll like. I just started messing with it right now, and this is by no means what our cars actually look like as a 2-d diagram. I'll refine all that later, but for right now I thought this was a good start. My next ones will have more lead/trailing room and be higher resolution. I'm just starting to play with it now.

[ame="http://www.youtube.com/watch?v=GOLcuPrctks"]YouTube - fluid dynamics mock up of 03 cobra[/ame]

I'll modify the image so that it actually resembles the rough surface of our cars rather than just the side skirts and the exterior appearance.

Since you want to take off the rear spoiler, I'll mock one up just for you since you're helping me

 

[SlowSVT]

What confuses me is why you think less turbulence under the car would create downforce. Why? Also, more downforce doesn't always = more drag, I've been reading into it.


I've got something I think you'll like. I just started messing with it right now, and this is by no means what our cars actually look like as a 2-d diagram. I'll refine all that later, but for right now I thought this was a good start. My next ones will have more lead/trailing room and be higher resolution. I'm just starting to play with it now.

YouTube - fluid dynamics mock up of 03 cobra

I'll modify the image so that it actually resembles the rough surface of our cars rather than just the side skirts and the exterior appearance.

Since you want to take off the rear spoiler, I'll mock one up just for you since you're helping me

VelociTT

Cool, we got someone doing some FEA on our car. Can you run a 3D model through your program? I’m not very good with compound curves on a CAD systems so don’t ask me if you want it done right (my model of an SN95 would probably result in something that resembles an AMC Matador :nonono We were going to buy a copy of FloWorks CFD program at work but can't justify the price for how often we would use it (currently I package 80 GHz microwave transmission equipment and we would use it to optimize forced air cooling inside the enclosure and thermal transfer characteristics).

A friend of mine owns a flowbench for cylinder porting use. He was telling me once someone brought in a chevy head with this peculiar bump in the exhaust port. He stated that most engine builders would just grind the bump away thinking it would improve flow due to this apparent obstruction it presented in the port. He took "before and after" measurements with and without the "bump" and found without it he lost 25% flow volume through the port after the bump was ground away. My own experience in this area is that not everything seems to follow a logical outcome.

Keep us posted on your finding :beer:

 

[bubbrubb]

VelociTT

Cool, we got someone doing some FEA on our car. Can you run a 3D model through your program? I’m not very good with compound curves on a CAD systems so don’t ask me if you want it done right (my model of an SN95 would probably result in something that resembles an AMC Matador :nonono We were going to buy a copy of FloWorks CFD program at work but can't justify the price for how often we would use it (currently I package 80 GHz microwave transmission equipment and we would use it to optimize forced air cooling inside the enclosure and thermal transfer characteristics).

A friend of mine owns a flowbench for cylinder porting use. He was telling me once someone brought in a chevy head with this peculiar bump in the exhaust port. He stated that most engine builders would just grind the bump away thinking it would improve flow due to this apparent obstruction it presented in the port. He took "before and after" measurements with and without the "bump" and found without it he lost 25% flow volume through the port after the bump was ground away. My own experience in this area is that not everything seems to follow a logical outcome.

Keep us posted on your finding :beer:

Interesting note with the flowbench findings, although I don't believe that a car which is moving air around it and a chamber that is moving air through it will follow quite the same laws, but the that won't stop it from being more complex than logic would have us believe.

The program I'm using just deals with 2D BMP images unfortunately, and it seems to be more of an artistic expression of fluid dynamics than anything - but its free so imma use it anyway :burnout:

here is the link:
http://chernyshenko.sesnet.soton.ac.uk/FlowIllustrator.aspx

I'm having luck with numbers around 3,000,000 and .018, based off of what I've read here: http://ecomodder.com/forum/showthread.php/flow-illustrator-simple-windtunnel-simulator-3709.html

Here are some examples of what I'm messing around with right now:




(adding more as I go)
I realize that in order to make it more realistic I'd need to use the contours of the underbody rather than the side skirts, etc. I'll get around to that later once I find some better numbers to use in that program. :beer:

Let me know if you (or anyone) find anything helpful

 

[bubbrubb]

[ame]http://www.youtube.com/watch?v=ZJ1qEgWppXE[/ame]
[ame="http://www.youtube.com/watch?v=FSWWAtH1LyA"]YouTube - undertray mock up[/ame]
give em a minute to process.

Looks better with the undertray than without. I didn't think to make it so air would flow through the hole in the front bumper though. I'll mess around with it tomorrow or something, but I'm calling it a night for this stuff