Author Topic: Pan Skirts  (Read 6073 times)

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Offline Simspeed

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Pan Skirts
« on: April 23, 2019, 02:22:07 PM »
I've noticed what appears to be floor pan skirts on the latest Speed Demon.  You can see what I'm referring to at the 14:24 minute mark of the Youtube video here https://www.youtube.com/watch?v=-t0RSBnNLyQ.  I remember back when Jim Hall put Lexan skirts around the bottom of his quickly outlawed Can Am sucker car to help create a vacuum for downforce and traction.  That was one hell of a machine.  I was in high school in Odessa, Texas at that time and lucky enough a buddy of mine Rodney Rogers' dad was head mechanic at Jim Hall Motorsports.  We got to visit their shop pretty regular in those days and we watch that car come to life during its construction.  I was like a kid in a candy store watching and listening to those racers...fawn memories. 

So... is the Speed Demon crew using their skirts for the same reason...or just to limit aero-drag under the car?  I'm somewhat up to speed on floor tunnel aerodynamic theory but always looking to learn more.  Any inside info on the SD car or others using a floor tunnel for aero effect?  Thanks...

Offline manta22

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Re: Pan Skirts
« Reply #1 on: April 23, 2019, 04:40:11 PM »
Simspeed;

Hall also employed two JLO snowmobile engines to power fans that sucked air from under the car. I think this Speed Demon application isn't for that reason.

Regards, Neil  Tucson, AZ
Regards, Neil  Tucson, AZ

Offline Bratfink

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Re: Pan Skirts
« Reply #2 on: October 15, 2019, 08:14:13 AM »
Not knowing the design team I can only speculate their intentions. But in this instance I?d describe that part as an air dam. The shape of the vehicle kinda forces the air dam to become side skirts as well.

There are essentially 2 ways to manage underbody flow; use it and guide it, or prevent it. SD is of the latter camp. Vesco being of the former camp.

Typically the direction to airdam a car is driven by the testing methodology. I?m talking auto industry here. Typically cars developed in a static ground tunnel will benefit more from an airdam. Those developed in moving ground tend to find opportunities under the car not seen in a static ground tunnel.

Neither answer is more correct than the other.

Safety could be a deciding factor in the decision to let air under the car. On a flat bottom shape like SD you have to manage the pitch of the car quite carefully to avoid choking the diffuser and destabilizing the car. Sometimes it?s just smarter to not let it under in the first place.
 
Again knowing nothing of the design process of the team everything I say is speculation, but educated speculation.

Offline ggl205

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Re: Pan Skirts
« Reply #3 on: October 15, 2019, 12:53:06 PM »
Sim:

Those don?t look like skirts but rather body attachments. The car is low enough to keep air out from under the car but that can?t be actual ride height. Just too low for current surface conditions.

John

Offline Simspeed

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Re: Pan Skirts
« Reply #4 on: October 15, 2019, 01:12:18 PM »
Sim:

Those don?t look like skirts but rather body attachments. The car is low enough to keep air out from under the car but that can?t be actual ride height. Just too low for current surface conditions.

John
Hi John,
Yes, probably so.  I need to come to the salt and look at how other teams have dealt with the issue to have an accurate representation of how to deal with them.  Thanks... Terry

Offline Simspeed

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Re: Pan Skirts
« Reply #5 on: October 15, 2019, 01:14:58 PM »
Not knowing the design team I can only speculate their intentions. But in this instance I?d describe that part as an air dam. The shape of the vehicle kinda forces the air dam to become side skirts as well.

There are essentially 2 ways to manage underbody flow; use it and guide it, or prevent it. SD is of the latter camp. Vesco being of the former camp.

Typically the direction to airdam a car is driven by the testing methodology. I?m talking auto industry here. Typically cars developed in a static ground tunnel will benefit more from an airdam. Those developed in moving ground tend to find opportunities under the car not seen in a static ground tunnel.

Neither answer is more correct than the other.

Safety could be a deciding factor in the decision to let air under the car. On a flat bottom shape like SD you have to manage the pitch of the car quite carefully to avoid choking the diffuser and destabilizing the car. Sometimes it?s just smarter to not let it under in the first place.
 
Again knowing nothing of the design process of the team everything I say is speculation, but educated speculation.
Hi Batfink,

Thanks for the description.  Due to the chassis configuration diverting the air from under the car appears to be the better option in my uneducated opinion.  As I mentioned to John above i need to visit the salt to see how others have handled the problem.  Thanks for your input... Terry.

Offline tortoise

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Re: Pan Skirts
« Reply #6 on: October 15, 2019, 02:59:32 PM »
I remember back when Jim Hall put Lexan skirts around the bottom of his quickly outlawed Can Am sucker car to help create a vacuum for downforce and traction.
Given the low coefficient of friction of salt or dirt, and the shorter than optimal courses available in the US, sucker technology looks like a good idea for LSR. It might enable you to get by with 2wd, potentially reducing weight, complexity and drag.

Offline Peter Jack

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Re: Pan Skirts
« Reply #7 on: October 15, 2019, 04:30:18 PM »
Just a thought; what would be the affect on the somewhat fragile surface of sucker technology?

Pete

Offline kiwi belly tank

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Re: Pan Skirts
« Reply #8 on: October 15, 2019, 05:41:04 PM »
Tire growth is very relevant when it comes to designing the bottom of a liner. A 30" Mickey will be more than 2" bigger at about 400 & that will vary with the rim width.
For that very reason, the bottom of my liner is not flat, it's tunneled with a large radius on the outside edge to match the tire profile.
  Sid.

Offline Beef Stew

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Re: Pan Skirts
« Reply #9 on: October 15, 2019, 07:44:44 PM »
Just a thought; what would be the affect on the somewhat fragile surface of sucker technology?

Pete

The sucker-car blew dirt and gravel out of the fan, according to drivers that followed it  :? Probably salt would come flying out of the fan exhaust  :-)

Former record holder at RIR ½ mile drags, El Mirage and Bonneville.

Beef Stew doesn't have his head where the sun-don't-shine. His head is in SoCal where the unusual is an everyday happening.

Offline Simspeed

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Re: Pan Skirts
« Reply #10 on: October 16, 2019, 01:24:14 AM »
I remember back when Jim Hall put Lexan skirts around the bottom of his quickly outlawed Can Am sucker car to help create a vacuum for downforce and traction.
Given the low coefficient of friction of salt or dirt, and the shorter than optimal courses available in the US, sucker technology looks like a good idea for LSR. It might enable you to get by with 2wd, potentially reducing weight, complexity and drag.
That's an interesting idea I hadn't considered Tortoise.  I don't think it would be too hard to implement given that the aluminum wheels should't experience much diameter growth over the course of a run.  I wonder if anyone has tried it before?  Thanks... Terry.

Offline Simspeed

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Re: Pan Skirts
« Reply #11 on: October 16, 2019, 01:29:53 AM »
Just a thought; what would be the affect on the somewhat fragile surface of sucker technology?

Pete
Hi Pete,
I would imagine whatever is loose (or is made loose by the tires) on the surface that the car passes over will be sucked up and blown out the exhaust port to resettle back down onto the salt surface after the car passes by.  I doubt any harm would be done to the hard surface different than what the spinning wheels already impact.  The cloud of salt (or dust on the dry lake bed) would look any different than what we now already see.  Thanks... Terry

Offline Simspeed

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Re: Pan Skirts
« Reply #12 on: October 16, 2019, 01:31:12 AM »
Tire growth is very relevant when it comes to designing the bottom of a liner. A 30" Mickey will be more than 2" bigger at about 400 & that will vary with the rim width.
For that very reason, the bottom of my liner is not flat, it's tunneled with a large radius on the outside edge to match the tire profile.
  Sid.
Hi Sid,
As you may know the Simspeed design uses all aluminum wheels so I don't think tire growth would become a factor in this regard.  Thanks... Terry

Offline Simspeed

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Re: Pan Skirts
« Reply #13 on: October 16, 2019, 01:32:48 AM »
Just a thought; what would be the affect on the somewhat fragile surface of sucker technology?

Pete

The sucker-car blew dirt and gravel out of the fan, according to drivers that followed it  :? Probably salt would come flying out of the fan exhaust  :-)
Hi Pete,
Oh for sure we'd see a cloud of salt or dirt behind the car...but we already do so with a low mounted exhaust duct I don't think we'd be able to see any difference.  Thanks... Terry

Offline JimL

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Re: Pan Skirts
« Reply #14 on: October 16, 2019, 05:13:06 PM »
Dan and I skirted our mod roadster back in 1998-2002.  The skirting curved in and joined under the front axle centerline and was open at the rear of the body.  The first time I ran the car up past 175 it sucked the steel belly pan into the ground.  I had to pull it up and anchor it with motorcycle tiedowns to continue.   The skirts were very close to ground contact and we ran rigid axles.

Reinforcing the belly pan was the final fix.  That car had an opening in the hood, around our 2000cc 4-cylinder and so there was easily a big pressure differential occuring....something to think about I guess.

We never had a time slip over 186, but the pan held.