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Aero vs. Weight

Old 11-26-08, 11:20 AM
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Aero vs. Weight

So this is going to be a bit of a complicated question as there is no clear answer I know, but I'm just looking for some discussion and your thoughts.

I was just about to...once again....embark on building a proper splitter/undertray for my car. I was planning on using alumacore sheeting which is essentially two sheets of thin aluminum held together by a plastic honeycomb center to give it high rigidity.

As with any add ons even though I'm trying to build it as light as possible there will be weight associated with it. On a car with a measly 163 hp at what point will this added weight not be worth the added aero efficiency?

I know speedsource was making splitters out of plywood that have to had added a ton to those cars. On the other hand they also had weight restrictions to deal with that Idon't anymore. I am currently running a simple skirt diffuser up front that doesn't weigh too much and I plan on keeping it around to be able to test in between it and the splitter to see the effect on times, but just for arguments sake there needs to be a weight at which an element like a splitter/ diffuser will be handicapping the car more than it is assisting.

After spending so many countless hours trying to shave a pound here and a pound there I'm questioning if it makes sense to add 10+ pounds of splitter back on (not sure of the weight yet).

Any thoughts?
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Old 11-26-08, 03:18 PM
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Couple comments...

163rwhp? Come on! You can do alot better with a stock motor. You are missing at least 15rwhp somewhere.

I don't think a splitter is optional on our cars, I MUST have mine for cooling purposes. There is no way I could keep temps under control without one.

I used to have the one from Flatout made from fiberglass. Every off resulted in repairs that I got tired of making. Finally a really good off removed the splitter from the car and I never bothered to retrieve it.

Since then, I have done 2 homemade ones from 3/8" multi-ply Birch plywood. I am VERY happy with the results and it takes a beating. I do not know the weight.

Alumacore sounds cool, but I don't think it would take the beating wood will. If you like making spares it would be a good idea.

Here's some older pics of one I did.



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Old 11-26-08, 04:36 PM
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tommy used a piece of plastic he bought at home depot. its actually too strong an off bends the brackets that hold it on the car.

and when the car was the vertical bit of the T-bone, the splitter actually sliced the other one open.

http://www.k2rd.com/teammidnight/mid...y/IMG_2059.JPG
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Old 11-26-08, 08:38 PM
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Ultimately, 10lbs is nothing, and I'm sure you can do it with less weight. The aero benefit should vastly outweigh the weight penalty.
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Old 11-26-08, 10:27 PM
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LOL...yeah I know, thats the reason Im paying so much attention to weight and aero. 163hp is what I get by giving Mike at ISC the job of building me a decent engine. Im set on eventually going with a bridgeport engine, but Im just trying to buy myself some more time before it happens. The engine I have in is still fairly new and it would be a pitty to through it away as is. As far as knowing where to find those 15hp left on the table, at this point Im lost. As far as Im concerned I've pretty much done everything I can while in ITS spec.



Originally Posted by SCCAITS View Post
Couple comments...

163rwhp? Come on! You can do alot better with a stock motor. You are missing at least 15rwhp somewhere.

I don't think a splitter is optional on our cars, I MUST have mine for cooling purposes. There is no way I could keep temps under control without one.

I used to have the one from Flatout made from fiberglass. Every off resulted in repairs that I got tired of making. Finally a really good off removed the splitter from the car and I never bothered to retrieve it.

Since then, I have done 2 homemade ones from 3/8" multi-ply Birch plywood. I am VERY happy with the results and it takes a beating. I do not know the weight.

Alumacore sounds cool, but I don't think it would take the beating wood will. If you like making spares it would be a good idea.

Here's some older pics of one I did.



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Old 11-26-08, 11:58 PM
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I'm planning to fab up something more or less along the lines of this:

http://www.specialprojectsms.com/ind...?categoryID=30
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Old 11-27-08, 04:28 PM
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I built a splitter much like what he discussed. 1/4" ply with fiberglass top and bottom. I also added a few chunnks of aluminum angle iron for mounting points. It works quite well.

-Trent
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Old 11-27-08, 10:21 PM
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sccaits, how many points do u have to count for your splitter according to the PT NASA rules, it looks like the splitter does not pass the centerline of the front axle, so do you only count points for your chord width?
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Old 11-28-08, 06:24 PM
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Originally Posted by Jross427 View Post
sccaits, how many points do u have to count for your splitter according to the PT NASA rules, it looks like the splitter does not pass the centerline of the front axle, so do you only count points for your chord width?
I've had this discussion with Greg. It's probably the reason he added "(measured perpendicularly from the leading edge)" to the splitter rule. I tried to argue it should be measured at the "viewed from above" point but that's a no go. Mine is right at 3" so I take 2 points. I do not take air dam points although the picture might look like I have an air dam but it's really just designed to close up the gap between the top of the splitter and the bottom of the bumper because the bottom of my bumper was not completely flat. In the picture I had some plastic attached to the bumper, now I have a piece of aluminum attached to the splitter but same concept. The Flatout splitter does require air dam points as well and not sure on it's exact chord length.

Where do you get the part of not passing the centerline of the axle? I don't recall anything in the rules about that. In any case, my splitter extends back to the bottom of the radiator only. It's not an ITS legal splitter as it extends too far back.
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Old 11-29-08, 09:53 PM
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im asking because under rule five under aero in the pt rules it say "add or fabricate flat bottom/belly tray +5", but under the no point modifications # 41 it says "Undertray/belly pan foward of the centerline of the front axles", so i am assuming you only have to count points for the chord length. two other questions regarding that, the turbo II lip, do you think it will recieve points for being an "airdam"? also if i where to run the lip, do i measure the length from the edge of the lip, or the front bumber?
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Old 11-30-08, 10:44 AM
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Originally Posted by Jross427 View Post
im asking because under rule five under aero in the pt rules it say "add or fabricate flat bottom/belly tray +5", but under the no point modifications # 41 it says "Undertray/belly pan foward of the centerline of the front axles", so i am assuming you only have to count points for the chord length. two other questions regarding that, the turbo II lip, do you think it will recieve points for being an "airdam"? also if i where to run the lip, do i measure the length from the edge of the lip, or the front bumber?
Yes, chord length only, at least in my case as the splitter does not extend pass the centerline of the front axles. Yes, the Turbo II lip would receive points, it's not on the base model so you have to take points. Not sure exactly how many, email Greg a picture and he can tell you. Hop on the NASA PT forums to ask about points, you'll get more answers and opinions there.
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Old 12-04-08, 04:36 PM
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Weight vs Aero:

Downforce if very useful at fast tracks. If you are only running slow autocross, or very tight, dinky road race tracks, then it is hard to get much downforce. Some SCCA GT car racers say they can get as much as 300 lbf from a good splitter.

Back in August I ran an SCCA race in the Super Production Over class, which is an unlimited class. I knew my PP was down on power compared to the V-8s, so I added as much aero as I could bolt on. I stuck on a nice Liebeck profile GT car wing on the rear, and to balance the front I put on a 4.5" splitter. My 4.5" front splitter was made from 5/16" plywood, and it weighed in at 9 pounds. (After the splitter was an undertray diffuser made from sheet aluminum; very light. From previous experience, I knew that the splitters take an awful beating if I ever try and clip an apex and touch a curb, so I stick with cheap plywood. (A multi-million dollar Audi R-8 I studied at a race had a plywood splitter on it too; they are very durable.)
I was racing at tight Barber Motorsports Park. First lap out, the car felt good the first few lower speed turns. I then went into a 100 mph S turn, turned the steering wheel - and the car did not turn! The rear wing made so much downforce, that the splitter could not balance it out, the front end got too ligh, so the front wheels did not want to steer. Even a splitter that big could not match that wing. I went right back into the pits, and cut the wing angle of attack in half, and that made it driveable on the track.

The aero downforce felt so good; made the car feel so stable, that it was definitely worth the extra pounds of car weight too me. For you, it will all depend on your application, and how fast you are actually running your car in racing.
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Old 12-04-08, 09:00 PM
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If you have trouble with the thought of plywood on the car, then don't think of it as plywood, think of it as a natual carbon fiber composite.
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Old 12-05-08, 02:23 AM
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its under construction now, and allmost done. Should have some pics of the whole setup within the next couple of days. Its a bit more complicated than originally anticipated as it all needs to be removable to get it on the trailer. But its coming along nicely.
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Old 12-21-08, 02:24 AM
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glad someone asked that question because i was about to ask the same
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Old 12-21-08, 11:44 AM
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Do you guys have pictures of how exactly you mounted the splitter and wing?
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Old 12-21-08, 12:04 PM
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Originally Posted by gkmccready View Post
Do you guys have pictures of how exactly you mounted the splitter and wing?
I have more if needed. Real simple and works very well - aluminum angle bolted to the splitter. Aluminum plate bolted to frame. Quick release pins hold the two together. Also added two splitter support rods in front, also with quick release pins. This setup results in zero body damage in the event of an off, you may destroy your splitter but the bumper and fenders remain intact.

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Old 12-21-08, 12:32 PM
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Do you find the fasteners pull from the splitter, or do the pins shear? That does look simple and easy. Thanks!
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