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Old 03-16-2008, 04:58 PM   #1
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Default aerodynamics and the touring car body

I found a discussion on rear wings but I have always been puzzled as to how much air actually gets to the rear wing with all the holes cut in the front, rear and side windows.

I was getting ready to cut some holes in the rear and side glass and decided to hold off until I got some input.

I understand the basics of aero and the need for the air to stay attached to the body going over the roof and onto the rear wing. With all the holes, this airstream has to be extremely "dirty" to say the least.

Does a more open rear glass area allow more air to pass through the body and attach to the rear wing...or are less holes better, possibly allowing the air to pass over the roof staying better attached to the rear wing?

One tidbit on the rear wing placement. I know you can move them rearward and forward to gain more or less rear downforce. When you move it rearward it gives more rear downforce but this technically is not proper as it also leverages the rear of the car and lessens the front grip. The wing should really be raised or lowered for more or less rear downforce although I am aware in come cases rules may not allow this adjustment. Some of my big race car experience

Any other aero tricks anyone would like to share?
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Old 03-16-2008, 05:37 PM   #2
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I cut my rear windshield out completely. Air traveling through the cooling hole has to exit somewhere. If you left the rear windshield intact, the air would have to exit out underneeth the car. This would mean the air would hit the windshield and then push the body up as the air moves down. If you cut the windshield out, the air has an easy exit out. I've seen oval race bodies that have the entire rear of the car cut out to increase rear downforce. Thats why there are rules to enforce maximum allowable heights from the track surface to the rear bumper cutout.

It also improves cooling. Lets say you had a box with two holes on opposite sides and you're blowing air into one hole. You cover up one hole and all the airflow in the box stops since there is pressure in the box now. The air blowing in is now moving around the box, thus not carrying away any engine heat with it.

I cut my rear windshield out completely. If you dont want to do that, the next best thing is to align the entrance hole in the front windshield with a exit hole in the rear windshield. This way there is little resistance for the air to out.
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Old 03-16-2008, 05:49 PM   #3
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I cut my rear window out completely as well, but am a noob and a local racer. I have read many of Carroll Smith's Books and have a Fred Puhn book anda Herb Adams boook that are meant for big car racing (I plan to start running American Iron, soon) but I find most of it applies. Your comment on wing placement forward and rearward are valid but remember you are ultimately just effecting balance, sometimes that's all you need. I was thinking about making a front splitter since I just run a WRX STi body. The first thing I did when installing the rear wing was to make vertical supports for the rear wing. As stock it was too flimsy to tranfer much force onto the rear wheels, now I can totally bottom out the car pushing on the rear wing without it deforming at all. All I did was make two "Z" shapes from extra lexan and viola, one sturdy wing...
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Old 03-16-2008, 05:53 PM   #4
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Hi genesisG4

I think this is one area that has had very little R&D work done in respect of the rc car.
Anyhow. You are correct in thinking that with the 'standard' holes that are put in a nitro sedan body cause the air that does reach the rear wing to be very dirty and far from ideal. Airflow to the wing area can be significantly improved by not cutting the holes (ALA electric car body)

Unfortunatley. There are other, higher priorities which must be thought of first, engine temps being the main one. Which really vertually mandates the use of cooling vents (holes) in the sedan bodies.

There have been recent 'tricks' incorporated into the newer bodies seen now. Namely. If you look at the area at the front edge of the rear wing.You will notice a slight upturn, a 'bump' between the two wing mounts. This is an attempt to increase the velocity of the airflow under the wing, to create a lower pressure in that area.

Also. The actual wing mounts have been re-shaped to 'invite' more air to go under the wing too.

As to actual placement of the wing front to back. This can be used only really as a fine tuning aid as the restriction's placed on it's position restricts greatly the amount of benefit that could be gained.

My tip:
If you are a driver that tends not to crash much......... then use the side dams supplied in the body 'kit'. Make them the largest the rules allow. to keep the air going under or over the wing in the effective area as much and as long as possible.

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Old 03-16-2008, 06:12 PM   #5
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It makes sense to open up the rear glass....
What about the side glass...I am not sure I see where that would help at all with airflow to the head/motor...and the air would surely be cleaner going around the body?
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Old 03-16-2008, 06:26 PM   #6
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I like this question because you can get a real feel for the answer. I own a 4 door pick up. When you open the rear slider window and the front window, the pressure inside the cab decreases as airflow increases. The cabin pressure is equal to the outside. When I first got into pickups back in the day, that was called redneck airconditioning. When it is applied to the RC arena, I have not seen any ill effects on just about every body that I have observed with the rear window cut out. Where I run, the back straight, 10th scale cars routinely run 55mph+ on the back straight. All of the guys run with the rear widows cut out without any problems. I have hours of video showing no deforming of the bodies at speed. The real aerodynamics on these cars is primarily from the front. You can increase rear downforce by increasing the height of your rear wing. You have to check with you race rules for that one.
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Old 03-16-2008, 08:24 PM   #7
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Yes the rear wing is a must, if you don't think so try to run the car without it. I learned along time ago when I first started running 1/10 pan cans couldn't get the car to hook up cause it didn't have a rear wing soon after I added a rear wing the car was on rails. Hope this helps
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Old 03-17-2008, 12:12 PM   #8
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so how about the side windows....open 'em up or leave 'em closed.
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Old 03-17-2008, 12:26 PM   #9
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One other note is you want to minimize the pressure differential between the top of the body and the underside of the body. Cutting the rear window insures the pressure inside the body is minimal. Any pressure that can build up under the body creates lift(not ideal). That is why nitro bodies bumpers are cut as high as possible and the rear fenders are angled outward to aid in the exhaust of air that passes thru the body.


Race you later!!!
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Old 03-17-2008, 02:23 PM   #10
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Quote:
Originally Posted by genesisG4 View Post
so how about the side windows....open 'em up or leave 'em closed.
yes cut them
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Old 03-17-2008, 05:11 PM   #11
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I cut out the front window, drivers side window and rear window. I keep the rest for structural rigidity.
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Old 03-17-2008, 06:00 PM   #12
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Do you cut the side windows out to decrease pressure under the body, or for additional motor cooling?
With exception of the front end and the wing for downforce, pretty much everything else goes against conventional big car aerodynamic wisdom. Of course we have issues to deal with that big cars do not...primarily air getting into and under the body for motor cooling.
It would sure be interesting to be able to do some testing in a scale wind tunnel, there has to be some things that could be done to improve cooling without all the additional high pressure under the body?? I know this is highly unlikely, but have any of the body manufacturers done any studies?
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Old 03-17-2008, 06:08 PM   #13
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I have always kept my side windows in. I guess they could help reduce the required size of the front hole or help cool the car while you're spinning out.

Why don't many people cut out the grill and replace the lexan with screen mesh? On most bodies, the Front grill is much closer to the height of the head. My head barely sticks up into the windshield opening on my WRX body. I used to cut the grills out on some of the bodies I did, but never really took head temps, bad science but decent theory.

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Old 03-17-2008, 06:42 PM   #14
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Quote:
Originally Posted by genesisG4 View Post
Do you cut the side windows out to decrease pressure under the body, or for additional motor cooling?
With exception of the front end and the wing for downforce, pretty much everything else goes against conventional big car aerodynamic wisdom. Of course we have issues to deal with that big cars do not...primarily air getting into and under the body for motor cooling.
It would sure be interesting to be able to do some testing in a scale wind tunnel, there has to be some things that could be done to improve cooling without all the additional high pressure under the body?? I know this is highly unlikely, but have any of the body manufacturers done any studies?
I cut both of my windows out for extra cooling but I cut my bumper as high as possible (within roar rules)to relieve pressure from under the body. I don't believe roar rules allow you to cut any hole in the front grill. this also hurts your front down force.
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Old 03-17-2008, 06:48 PM   #15
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I cut out the side window for cooling and sometimes it is more convienent to grab and or fuel the car (depending on your pit man). I even cut the window out of my truggy. Again, that is for my pit man to have options when fueling and tuneing.
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