Anyone mounted skirts or fins to the side sills?

[MvM]

For the last couple of days I have been reading up on car aerodynamics in general.
One of the things you come across is the mouting of side skirts on racing cars to prevent air from flowing to the underside of the car from the side.
Now, of course, road cars are not as low as true racing cars but even then you can try to prevent any air from getting under the car. And every little bit helps in this matter.
Lowering your car is one step, getting a good front air dam and/or closing up the underside of the front bumper also helps.

However, I've been thinking about adding some L-shaped fins, preferably made of some kind of flexible rubber along the underside of the side sills between the front and rear wheels.
If these would be, let's say 1" (1 inch/2.54cm) in height, it would provide an additional barier to air from flowing under the car while driving. At the same time, being made of rubber, they would not be damaged by the occasional speed bump and could also easily be removed if necessary.

Does anyone have any ideas on if this would actually improve the aerodynamics of the NSX?

 

[pgilliam1]

Porsche thinks so. Take a look at their new GT3 RS 4.0:

http://www.caranddriver.com/reviews/car/11q2/2012_porsche_911_gt3_rs_4.0-first_drive_review

 

[greenberet]

On this web page, Honda states that the flatter side sills of the 2002 NSXs reduce turbulence along the sides of the car (you can translate the page with Babelfish). They didn’t mention whether they reduce lift, but reducing turbulence reduces drag. So 2002 side sills are an aerodynamic improvement over the 1991-2001 side sills.

For rubber fins under the car to be most effective, they should probably not just prevent air from getting too far under the car, but even from getting in front of the rear wheels. Due to the rounded side sills of the 1991-2001 NSX’s, the skirts would have to stick back out towards the sides of the car, kind of like on a Ferrari 458. If the skirts are set in from the sides by a few inches and you have a smooth underbody, they may still help with downforce, but they probably wouldn’t have that big an impact on drag.

Unfortunately, I like the looks of the rounded side sills on pop-up headlight NSXs even if they’re aerodynamically flawed.

 

[MvM]

On this web page, Honda states that the flatter side sills of the 2002 NSXs reduce turbulence along the sides of the car (you can translate the page with Babelfish). They didn’t mention whether they reduce lift, but reducing turbulence reduces drag. So 2002 side sills are an aerodynamic improvement over the 1991-2001 side sills.

For rubber fins under the car to be most effective, they should probably not just prevent air from getting too far under the car, but even from getting in front of the rear wheels. Due to the rounded side sills of the 1991-2001 NSX’s, the skirts would have to stick back out towards the sides of the car, kind of like on a Ferrari 458. If the skirts are set in from the sides by a few inches and you have a smooth underbody, they may still help with downforce, but they probably wouldn’t have that big an impact on drag.

Unfortunately, I like the looks of the rounded side sills on pop-up headlight NSXs even if they’re aerodynamically flawed.

Thanks for your thoughts...

The idea is of course that with adding some rubberized/flexible skirts to the underside of the windows sills I can get much lower towards the road surface than would ever be possible with a set of complete side skirts like the 2002+ skirts, MargaHills, Stacy etc.
Reducing turbulence will always reduce drag as well of course.

My idea was, since I have added a NSX-R styled VIS-hood, a bumper and battery tray undercover and lowered my car, the amount of air getting under the car has already been reduced. Normally speaking, this would lead to a (slight) underpressure area under the car which causes air from the side to want to rush in.
Adding some downwars pointing fins should/would be able to minimise this effect, therewith maintaining the low(er) pressure area as long and as much as possible, reducing drag, increase downforce and roadholding and ultimately increase the performance envelope of the NSX.

Which is of course what we all want.... :smile:

 

[greenberet]

If you mount the rubber fins at the outermost line at which the underbody is still flat they will be set in from the sides of the car since the side sills are rounded. Like that, I don’t know how much drag reduction you would achieve but they should still help with downforce if the front air dam is deep enough or if the underbody is shaped properly. If you mount the fins so that they are at the sides of the car they could help with both lift and drag but then they’ll be really noticeable because they’ll be sticking out from underneath the side sills.

As I understand it, you can get low pressure underneath a car if you have:

1. a deep front air dam, deep side sills, and a pretty much open rear end so that the low pressure zone behind the car tries to pull air out of the underbody faster than it can get in. NASCARs use this method.
2. an underbody shaped so that the air flows faster underneath the car than around it. For this to work, you need to have enough air getting in under the nose of the car, a smooth underbody that doesn’t slow the air down, and a diffuser at the rear. Modern Ferraris use this method.

In either of those cases, you need deep side sills to keep air from rushing into the low-pressure zone beneath the car. A stock NSX’s underbody is very jagged in the front, bumpy underneath the passenger compartment, and then jagged again at the rear. I don’t know how much downforce you’re going to generate due to the underbody’s shape unless you build a completely smooth and properly shaped undertray from front to rear bumper. If you don’t do that, you could mount a deep front air dam, deep side sills, and vent the cooling air over the top of the car instead of under it. If you don’t have a deep front air dam or a completely smooth underbody, I don’t know whether you’re really going to develop a low-pressure zone beneath the car.

Normal cars without deep front air dams or completely smooth underbodies develop a high pressure zone beneath them while driving. The air then flows out from under the car beneath the side sills. Preventing that from happening would actually increase lift. Smoothing the underbody of a regular car may not get you real downforce in terms of a low pressure zone but at least it reduces the high pressure zone. If the pressure below the car is higher than along the sides, deep side skirts that prevent the air from escaping will likely increase lift.

With a differential pressure gauge you could measure the air pressure beneath the passenger compartment of your NSX to see whether it is higher or lower than ambient. Based on that you would have a feel for whether deep rubber fins underneath your sills are likely to increase lift or increase downforce. But deeper sills that channel air away from the rear wheels should reduce drag, in any case.

.02¢

 

[Vega$ NSX]

Simple solution. Go to Lowes or Home Depot. Get a ton of that weatherstrip used to seal doors and window, glue it to the outside perimeter of the NSX. Bam! Done!

 

[MvM]

.... you need deep side sills to keep air from rushing into the low-pressure zone beneath the car. A stock NSX’s underbody is very jagged in the front, bumpy underneath the passenger compartment, and then jagged again at the rear. I don’t know how much downforce you’re going to generate due to the underbody’s shape unless you build a completely smooth and properly shaped undertray from front to rear bumper. If you don’t do that, you could mount a deep front air dam, deep side sills, and vent the cooling air over the top of the car instead of under it. If you don’t have a deep front air dam or a completely smooth underbody, I don’t know whether you’re really going to develop a low-pressure zone beneath the car.

Normal cars without deep front air dams or completely smooth underbodies develop a high pressure zone beneath them while driving. The air then flows out from under the car beneath the side sills. Preventing that from happening would actually increase lift. Smoothing the underbody of a regular car may not get you real downforce in terms of a low pressure zone but at least it reduces the high pressure zone. If the pressure below the car is higher than along the sides, deep side skirts that prevent the air from escaping will likely increase lift.

With a differential pressure gauge you could measure the air pressure beneath the passenger compartment of your NSX to see whether it is higher or lower than ambient. Based on that you would have a feel for whether deep rubber fins underneath your sills are likely to increase lift or increase downforce. But deeper sills that channel air away from the rear wheels should reduce drag, in any case.

.02¢

Thanks for your input.
What I first would have to do then is to see how the air flows along and beneath the side sills. After that, I (we) would have an idea if the fins as I propose would acutally have a beneficial effect or not.
After all, we wouldn't to have a car with MORE lift now would we... :biggrin: