# Aerodynamics on B13



## dho (May 15, 2002)

What can be done to make an aerodynamic sentra? I'd like stuff that looks and performs like the mathermotorsports front airdam and the shield thing. www.mathermotorsports.com How much would this cost? Also, how do they make those for the other sentras like:


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## bahearn (Jul 15, 2002)

Hmmm. He has a '93-'94 front bumper cover and a *home-made* airdam. Looks like the vinyl edging material you can get at Home Depot. Make some 90º brackets from the metal of your choice, bolt them to the bottom of bumper cover (after drilling the holes), them mount the air dam material to the brackets. Easy-peasy-japanesey. The bumper cover was also cut open above the existing slot and screen was riveted over it.

Another option is a splitter: a flat plate attached horizontally under the bumper extending both forward and rearward of the existing air dam. It limits the amount of air going under the car by forcing the air above the splitter lip to go over the car while simultaneously creating a low-pressure area under the engine, assisting the marginal radiator equipping all Sentras.

The air dam will do this to some extent but creates a larger low-pressure area with less differential pressure. For downforce purposes, they should be rougly equal but the splitter should work better for cooling.


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## dho (May 15, 2002)

I do not follow you when you talk about the splitter. I have a front skirt but it's made out of really flexible rubber/vinyl. Would this work if I was able to reinforce it somehow? How much of an aerodynamic difference would there be? My car is a street car so normal 70mph driving on the highway. Just whatever it takes to make it handle better at those speeds. (Occasional high winds, rain, avoiding road debris... ) Here's a picture of it:


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## bahearn (Jul 15, 2002)

You should post a photo taken outdoors; there's not enough contrast between your black airdam and the dark garage floor for me to see what you have.

As for Sentra aerodynamics, there's not much you can do for street driving that you haven't already done, except lower the car. If you have the rear spoiler, it's for looks. It's too narrow in chord and too close to the rear window to create much downforce. If you really want an aerodynamic Sentra, get an NX2000.

Also, go to the Road Race forum and read the Overheating thread. There are links to two photos of a B13 with a splitter.


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## dho (May 15, 2002)

How is that splitter designed? Is it one long piece that extends evenly under the car or is it just like a U? Here's some other pics of my car:


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## bahearn (Jul 15, 2002)

The splitter is indeed a large plate that extends behind the airdam as well as forward. I haven't built one yet but it could be shaped in any way imaginable to enhance airflow out of the engine compartment.

Downforce is mainly determine by the amount of plate area ahead of the airdam.

BTW, the extra pictures still don't show your airdam extension worth a darn. You need to place the back of the car toward the light source and photogragh from low in the front with a flash, i.e. back-lit with flash fill. You want the ground underneath the car to reflect light and you want a flash to make the under-bumber area bright. This will show the black extension to good effect.


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## dho (May 15, 2002)

I've been wanting to get another picture taken but I can't seem to get enough lighting. It's been cloudy here in texas. I found this one pic but I'm not sure if it's okay to post it here. It's one of the sentra owners from sentra.net or somewhere else. I've been looking for his name so I could ask his permission to post it. It's a lot brighter than my pictures.

Is it okay to post?? If not I'll edit and take it off.


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## bahearn (Jul 15, 2002)

Ah, a much better picture. Looks all right. If I can't get the splitter to work right, I'll probably do what Mathers did.

As an aside, one thing to note about airdams is that vertical devices reduce drag with little-to-no downforce while angled devices increase downforce at the expense of drag. A flat splitter is at the extreme for angle.


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## dho (May 15, 2002)

Do the wheels and tires extending out mess up the aerodynamics? If so, the mather motorsports idea would work better? So with this flexible skirt, is it best just to add reinforcement parts to make it functional? After it's made functional (when I get $$$) I should add a flat splitter below it? How far back under the center of the bumper should it extend? I would like it to help with the cooling but I'm not sure what exactly helps it get the air to the radiator. Is it the part that extends out the front or under the bumper? I don't want my car to be pushing tremendously for 60mph, I just want it to stick to the road better. You say that an angled device increases the downforce at the expense of drag, so what angle would be best? Flat? Slightly up in the front or slightly down in the front? How much of a gap should there be if any? I'm sorry if I'm not following everything easily. I get confused easily but once I understand, I'll eventually get it and do more research to amplify the positive effects of the aerodynamics.

How low to the ground does it need to be? Does it need to be the same distance off the ground as the front fender edge is to the tip of the tire? Or just as low as the front suspension techniques anti-sway bar that you can somewhat see on my pics? (you can see a little light/reflection above it on the right side of the picture (drivers side)... you can also see the bolt-nut that come down from the a frame..)










Why are there some splitters or add ons to some bumpers that look the opposite of touring wings? They're flat down the middle then go up on the ends like those MD-11 air planes or those DC 10's (forgive me if I'm wrong on the exact model).


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## bahearn (Jul 15, 2002)

> Do the wheels and tires extending out mess up the aerodynamics?


Yes.



> If so, the mather motorsports idea would work better?


Not necessarily. For a given frontal area, longer bodies have better coefficient of drag (CD) due to the extra time turbulent air has to smooth out. Front tires don't hurt CD that much. Also, as I mentioned, lift reduction via splitter and it's high induced drag may be more appropriate than simple air dam designed to reduce turbulence.

Of course, both a smooth surface and high downforce would be nice. In your case, being a street-driven car, don't do any more than you have; you'll get tired of repairing the nose everytime you crunch it on a steep driveway or road hazard. You can't get that Sentra fast enought to generate much downforce, anyway.

The cooling issue comes up for me because the SE-R water pump supposedly cavitates over 6,500 rpm. Air bubbles SUCK at cooling. I did overheat on the track until I installed the Nissan Motorsports radiator fitted to the NX2000. So, again, for your street duty, a splitter is of little benefit since there is a simple, proven cure. The splitter can increase cooling by managing air flow OUT of the engine compartment. Look at how close the engine and exhaust sit to the radiator and then imagine trying to get air out between them. A splitter can partially seal the under-engine area and create faster air under the car. Fast air = low pressure, helping to suck the hot air out of the engine compartment.

Downforce happens because the splitter forces air to go up, like a plane does to a piece of wood, or a razor does to your peach fuzz. Send air over the top that normally goes under and you create a slight vacuum. Force in this case is area * differential pressure (dP). The splitter will be a big flat plate (pretty much) under the car giving lots of surface for suction to act upon. Being a track-only car, I can put that splitter quite close to the ground for higher dP, so that decent downforce can be generated at relative low speeds. Road hazards are of little concern.

I don't understand your last question. Please paint me a painfully clear picture.


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## dho (May 15, 2002)

bahearn said:


> *I don't understand your last question. Please paint me a painfully clear picture. *


Thank you for explaining those cool details about the splitter, now I understand it a lot better. The last question better explained is:

Why do some of the splitters or front lips have little upward sides / tips? Imagine taking a touring wing that has the two ends pointing downward, now take that wing and put it upside down. The two ends are now pointing upwards / vertically. I know that rudders on air planes can direct the horizontal rotation of the plane, do these act the same way on splitters installed on race cars? Do these help with the stability of the front end at high speeds?


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## bahearn (Jul 15, 2002)

Clear as mud. Got a link? How about a particular photo in SCC or Car&Driver? Those are the only two car rags I read.

Here's a free but potent demonstration of a moving liquid (did you know that air is a liquid?) and differential pressure.

Start your kitchen tap flowing, flow rate is unimportant. Suspend a spoon by the end of its handle, concave side toward you, between your finger and thumb. It must be loosely held so that it can swing. Now, gently touch the back (convex) side into the stream of water; it should pull the spoon even farther into the stream of water. Despite the much higher density of liquid water versus air, the moving water exerts lower pressure so that the still air pushes the spoon farther into the moving water. You've demonstrated one aspect of Bernoulli's Principle.

Aerodynamics is really a small part of the broader studies known as hydrodynamics. Air is just a liquid medium with very low density.


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## dho (May 15, 2002)

Here's another pic that I took but I still didn't get enough lighting:


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