Lets talk aerodynamics

[TiredBirds]

So we run a 1984 Trans Am, and of course we ditched the rear window (heavy and glass) there has been some discussion about adding some lexan or not to add a lexan rear window. What would be better for aero, for places like Daytona and VIR? A. run like it is w/ less rear spoiler?  B. Run w/ window, side leaxn and less spoiler or perhaps something in between like C, where you have some sort of Lexan on the sides that directs the air coming thru the cabin over the rear deck lid and spoiler. 

[Kentite]

12 minutes ago, TiredBirds said:

So we run a 1984 Trans Am, and of course we ditched the rear window (heavy and glass) there has been some discussion about adding some lexan or not to add a lexan rear window. What would be better for aero, for places like Daytona and VIR? A. run like it is w/ less rear spoiler?  B. Run w/ window, side leaxn and less spoiler or perhaps something in between like C, where you have some sort of Lexan on the sides that directs the air coming thru the cabin over the rear deck lid and spoiler. 

Run the lexan. It will help establish the air pocket that the spoiler is trying to create.

The spoiler actually doesn’t crater too much drag once that air is going where it is supposed to. A rear diffuser can help create more downforce as well. This may help 

https://oppositelock.kinja.com/wings-spoilers-youre-probably-doing-it-wrong-1665312667

[TiredBirds]

Thanks, we were also looking at a diffuser... now that we have points to burn

[TiredBirds]

15 minutes ago, Kentite said:

Run the lexan. It will help establish the air pocket that the spoiler is trying to create.

The spoiler actually doesn’t crater too much drag once that air is going where it is supposed to. A rear diffuser can help create more downforce as well. This may help 

https://oppositelock.kinja.com/wings-spoilers-youre-probably-doing-it-wrong-1665312667

The Chrysler engineers designed the rear wing in a wind tunnel...additionally they had to make it that high so one could open the trunk. Maybe I should inform the guy? 

[Kentite]

1 hour ago, TiredBirds said:

Thanks, we were also looking at a diffuser... now that we have points to burn

Going to start mocking one up this weekend. Probably use ABS plastic sheeting for flexibility and strength. Worked well for our air dam and splitter tunnels 

[wvumtnbkr]

I think your car would benefit from a lexan rear window and NO spoiler.  I would lower that rearend and I bet your rear stability issues go away.  Look at the "B" pic you posted.  "B" is about the correct ride height for an F body.

[hotchkis23]

I agree with Rob.  Get that ride height down.

[TiredBirds]

8 minutes ago, wvumtnbkr said:

I think your car would benefit from a lexan rear window and NO spoiler.  I would lower that rearend and I bet your rear stability issues go away.  Look at the "B" pic you posted.  "B" is about the correct ride height for an F body.

it is a tad lower now and we don't have any rear stability issues currently. I was thinking more about making some high speed passes at Daytona. If we cut the springs anymore we will have to cut and re-weld the rear panhard bar.  The current "discussion is about air flow. As we have an open "green house" one thinking is the air passes thru the cabin and across the spoiler, if you add a rear window it will collect in the cabin. I would think the rear window would help direct the flow down off the roof and towards the spoiler. 

[Xph]

You could use a truck topper, no lift created there, saw it on TV so it must be true  

[Team Infiniti]

1 minute ago, TiredBirds said:

if you add a rear window it will collect in the cabin.

Far from a aero expert here but my understanding is the cabin will become a bubble that air (roughly)flows around

[Xph]

You also might want to consider a two part door, like they use in the trans-am series for cars with small windows...  they have a removable upper section of the door, keep the OE bodyline for aero, but still have that part removable for ingress and egress.

 

From an aero perspective, all you are trying to do is create that "truck topper" air path, by trapping a pocket of air...  you need to keep that air low energy (ie not a fast rotating vortex)...  and then it doesn't matter much what your rear window looks like.

 

Aerodynamically its the difference between a slow ramp down keeping the flow attached, and into a WING, where you are using the underside of the airfoil to create downforce, and a SPOILER, whose job is to create that ball of air to prevent lift, but not necessarily create downforce.

[TiredBirds]

15 minutes ago, Team Infiniti said:

Far from a aero expert here but my understanding is the cabin will become a bubble that air (roughly)flows around

That is what I thought. the other thought is all the air passing thru will end up on the deck lid. I'd think it would get jacked up from the air coming over the roof.

12 minutes ago, Xph said:

You also might want to consider a two part door, like they use in the trans-am series for cars with small windows...  they have a removable upper section of the door, keep the OE bodyline for aero, but still have that part removable for ingress and egress.

 

From an aero perspective, all you are trying to do is create that "truck topper" air path, by trapping a pocket of air...  you need to keep that air low energy (ie not a fast rotating vortex)...  and then it doesn't matter much what your rear window looks like.

 

Aerodynamically its the difference between a slow ramp down keeping the flow attached, and into a WING, where you are using the underside of the airfoil to create downforce, and a SPOILER, whose job is to create that ball of air to prevent lift, but not necessarily create downforce.

We could go with a "window" and a wing. The spoiler on there now is more for NJMP, Summit point and VIR south. 

[ross2004]

Not to say this is right or wrong, but here's our 3rd gen. Have fun forming the rear window...

[wvumtnbkr]

59 minutes ago, TiredBirds said:

it is a tad lower now and we don't have any rear stability issues currently. I was thinking more about making some high speed passes at Daytona. If we cut the springs anymore we will have to cut and re-weld the rear panhard bar.  The current "discussion is about air flow. As we have an open "green house" one thinking is the air passes thru the cabin and across the spoiler, if you add a rear window it will collect in the cabin. I would think the rear window would help direct the flow down off the roof and towards the spoiler. 

My comment is ENTIRELY about aero.

 

Lower the rear and get rid of the spoiler.  Lowering the rear is going to help your aero tremendously while at the same time removing the requirement of needing a spoiler (possibly).  if you don't have rear stability issues, why run the spoiler?

 

If you want the spoiler, you SHOULD have a rear window.  The rear window is what the high pressure zone created in front of the spoiler will act upon.

 

Once you close in the rear window, you will get about zero airflow through the car.  You MAY need to add air ducting to the driver (we had to with our rear window installed).

 

 

Lower the car for Aero, use a spoiler IF you need it.  Add the rear window.  Get better handling.

 

Its a win-win!

[enginerd]

8 minutes ago, wvumtnbkr said:

Lowering the rear is going to help your aero tremendously while at the same time removing the requirement of needing a spoiler (possibly).

I dunno... the redbull cars in Formula 1 always run a lot of rake and the announcers talk about it being really good.

[pintodave]

Will not build pressure in the car with a rear window... our car has insanely little airflow inside. You want a back window.

[TiredBirds]

2 minutes ago, enginerd said:

I dunno... the redbull cars in Formula 1 always run a lot of rake and the announcers talk about it being really good.

The one team member with the most track experience ( a lot of circle track)  is the one that set the car up. He likes the way it handles now, claims the rank is correct for how we race. it is squattier when driving hard.  It does handled very well as is.  He also read up a bunch before creating the rear spoiler. We will not be using that spolier for Daytona. It will be a smaller spoiler or a wing. 

[TiredBirds]

1 minute ago, pintodave said:

Will not build pressure in the car with a rear window... our car has insanely little airflow inside. You want a back window.

That is what I am thinking.

[RandomTask]

We run VIR w/o rear glass (... or a hood. )

 

This year, we'll be running a hood, but still no rear glass; I'm curious, do you guys run into buffeting issues on your 2 doors w/ rear glass? 

[TiredBirds]

1 minute ago, RandomTask said:

We run VIR w/o rear glass (... or a hood. )

 

This year, we'll be running a hood, but still no rear glass; I'm curious, do you guys run into buffeting issues on your 2 doors w/ rear glass? 

not sure what you mean? our doors are now just skins welded to the body. 

[enginerd]

21 minutes ago, RandomTask said:

I'm curious, do you guys run into buffeting issues on your 2 doors w/ rear glass? 

You mean like when you’re in a car on the highway and open one of the rear windows and you cringe and feel like your ears are exploding and then open one more window and it’s fine??

 

No, we don’t have that issue in the race car. 

[TiredBirds]

3 minutes ago, enginerd said:

You mean like when you’re in a car on the highway and open one of the rear windows and you cringe and feel like your ears are exploding and then open one more window and it’s fine??

 

No, we don’t have that issue in the race car. 

ohhhh that. I don't think that will be an issues as we have no gasktes left in the car. Since it wouldn't be "sealed" you wouldn't get that "effect". 

[pintodave]

We do not experience any odd or annoying buffering either. 

Do notice the top/middle of the back window bowing inwards at speed, which is exactly what I would expect. Net outside pressure on roof/window/decklid is greater than inside the car. Yay downforce! 

[TiredBirds]

7 minutes ago, pintodave said:

We do not experience any odd or annoying buffering either. 

Do notice the top/middle of the back window bowing inwards at speed, which is exactly what I would expect. Net outside pressure on roof/window/decklid is greater than inside the car. Yay downforce! 

did you make yours or oder it pre-fabbed

[pintodave]

I was in a hurry before Daytona last year so I ordered a 5 star, but it's probably curved gently enough you could just buy a sheet and make it yourself by tracing over a stock window