Flat Deck vs Drop deck

[XRNone]

bloomquist is back to a drop deck in his new car. What are the advantages/disadvantages?

[TheJet-09]

Did you see that from an in-car camera at Bristol? I noticed it, but also thought the car looked old. The air filter was even bent up on it.

I have a hard time wrapping my head around aero stuff. I know back when you could have the area right behind the hood open, it would help pull air out from under the front of the car. Bloomquist even ran a "belly pan" under the entire nose back then (which I believe is also illegal now).

One thing we all know, is nothing he does is by accident. After the "window net" race at Eldora, I looked at his car in the pits and noticed he had the LF fender bent at the top in a way that created a Gurney Lip for the majority of its length. How that wasn't illegal is well beyond me, but I couldn't cite what rule it violates.

He himself said air can't follow more than a 7 degree pitch, so I've also wondered how disruptive a straight drop behind the hood could be beneficial.

[MasterSbilt_Racer]

So aero laws changed is why they went drop deck, flat, and now he's back to drop?

[Peanut Butter Falcon]

I saw that also. And did you notice the Warrior car Ryan King is driving had a dropped deck.

[Ghopper]

If you want insight into the pressures above or below the deck.... Nearly any hole in the deck causes air to go from outside to inside, reducing the downforce of the body. Oil coolers on the deck have flow from outside to inside, a very effective passive system to move air across the heat exchanger. However, it costs total downforce numbers. Drop deck removes volume under the body, that could help peak downforce, but if you cut a bunch of hole in the deck it doesn't matter.

https://www.totalsim.us/simulating-a-dirt-late-model-in-cfd/

https://www.totalsim.us/dirt-late-model-revisted-spoiler-or-rear-wing/

[TheJet-09]

I'm not sure who implied aero laws changed, as I'm the first to admit I don't know why he would do that. But I do feel he's the master of "red herrings" so maybe it's nothing more than that.

[MasterSbilt_Racer]

I'm not sure who implied aero laws changed, as I'm the first to admit I don't know why he would do that. But I do feel he's the master of "red herrings" so maybe it's nothing more than that.

No one did. But, I felt it was implied that Scott Bloomquist was always right.

[ZERO25]

No one did. But, I felt it was implied that Scott Bloomquist was always right.

He's never wrong........he was just testing! lol

[XRNone]

Now the 2 coulter car is at Bristol with a completely flat deck and windows like it’s 1999. I can’t keep up!

[JustAddDirt]

You guys do understand that Bloomquists father was and, and still is in the aviation industry. So that gives him a very good insight on the way air works, and the way that air moves around things. Drag, lift, forces of air around things. How many of you guys remember him racing the car he called “Airplane” at Eldora? The roll cage was made out of teardrop shaped moly tubing that was used in construction of airplane wings. As much as people hate to admit it, the guy is pretty smart!

[ZERO25]

He raced that car at Speedweeks. I was fortunate enough to have seen it!

At GIS, he even had airfoils made outa plexiglass riveted to the front a posts. He had competitors scratching their heads! lol

[JustAddDirt]

He was and still is good at making people scratch their heads. Sometimes in a good way, others not such a good way. Wander what ever became if that car?

[ZERO25]

If you want insight into the pressures above or below the deck.... Nearly any hole in the deck causes air to go from outside to inside, reducing the downforce of the body. Oil coolers on the deck have flow from outside to inside, a very effective passive system to move air across the heat exchanger. However, it costs total downforce numbers. Drop deck removes volume under the body, that could help peak downforce, but if you cut a bunch of hole in the deck it doesn't matter.

https://www.totalsim.us/simulating-a-dirt-late-model-in-cfd/

https://www.totalsim.us/dirt-late-model-revisted-spoiler-or-rear-wing/

After reading it a couple of times, I noticed the author said the flow onto the back of the deck could be improved.

Any suggestions?

[MachineMasters]

Would holes in the surface of the deck not help to suck air out from underneath the deck?
Faster air moving across the surface of the deck vs slower air underneath...?

I noticed the dropped deck that Zero had at Bristol had a long sideboard running down the rightside of the driver's compartment. Wouldn't put it past Scott to opt for the drop deck just for the additional ~3ft sideboard.

[billetbirdcage]

Would holes in the surface of the deck not help to suck air out from underneath the deck?
Faster air moving across the surface of the deck vs slower air underneath...?

I noticed the dropped deck that Zero had at Bristol had a long sideboard running down the rightside of the driver's compartment. Wouldn't put it past Scott to opt for the drop deck just for the additional ~3ft sideboard.

High pressure tends (lol) to flow to low pressure so high pressure on top of deck will flow to underneath. I suppose if the hole had a tube on it sticking up (to get to the high flow rate air) then it might but on the deck surface for the car to make down-force the top has to be moving slower then the bottom.

PS: Not an Aero engineer

[ZERO25]

I have always wondered if some well placed naca ducts would be an advantage.

Im sure technology has changed but a good friend of mine was a crew chief on a team that had an engineer as a consultant. This was back in the late 80's, but his cars always had rolled lips on the roof and a and b posts. He said the engineer had Cup connections and had found this made more rear downforce in the wind tunnel.

[Ltemodel]

I think if the oil cooler were close enough to the drop in the deck after the firewall, it would pull air out from under the deck. The further back the oil cooler is, the more likely the oil cooler would push air under the car.

I never understood why people put scoops on the back of the oil cooler to force air under the deck.

If someone were to put a bump shroud covering the first small portion of the cooler then leave the back open, the air racing over the front would vacuum air through the oil cooler and onto the top of the deck.

[Punisher88]

I have and always will face the scoop backwards. No idea why you'd force air under the car on purpose.

[Ghopper]

Remember the ground is "moving" at nearly the same speed under the car as the air moving above (hence rolling floor wind tunnels are much more accurate that static floor designs).

A cross section of the pressure distribution is visualized in a link above. This car was an XR1 with the oil cooler covered.

The easiest way to know where air is moving is to tape on a group of strings and watch where they go. Deck mount oil coolers suck the string from the deck through the oil cooler. ....Suck, as in high pressure above deck moving to low pressure below.

[Ghopper]

<- there is a constant Keyser ad for wireless data acquisition using a string potentiometer for length measurement. These are great for slow speed test-rigs (like pulldown types). They are not as accurate for high speed suspension movement as they rely on spring tension within the sensor; which can give misleading results. Get regular linear potentiometers for suspension position and velocity measurement.

[Burke1118]

Keyser made some overpriced gimmick that doesn't actually work as intended!? Say it aint so........"DATA ENGINEERING TECHNOLOGY ACQUISITION STRING BOX WILL INCREASE TRACTION AND WHEEL LOADS USING ENGINEERING AND DATA TECHNOLOGY"

[MasterSbilt_Racer]

Keyser made some overpriced gimmick that doesn't actually work as intended!? Say it aint so........"DATA ENGINEERING TECHNOLOGY ACQUISITION STRING BOX WILL INCREASE TRACTION AND WHEEL LOADS USING ENGINEERING AND DATA TECHNOLOGY"

It works. It just has frequency limitations as Ghopper mentioned. I haven't looked at price for those specially, but that technology is much cheaper than an lvdt.

In general, data measurements practice is very crude and not well done in our sport, where it is done at all.

[MasterSbilt_Racer]

Those expensive smashers that auto run are great to compare with a guy that has the exact same equipment as you, but are terrible for actually knowing what you have.

[Jking24]

Those expensive smashers that auto run are great to compare with a guy that has the exact same equipment as you, but are terrible for actually knowing what you have.I love when a guy just has to spend that 7k and then he says all my numbers are totally different with this machine or what's even better is the guys that aren't even smart enough to catch it they just assume it's the same and "right" because it was expensive and the big guys use them.

[MasterSbilt_Racer]

I love when a guy just has to spend that 7k and then he says all my numbers are totally different with this machine or what's even better is the guys that aren't even smart enough to catch it they just assume it's the same and "right" because it was expensive and the big guys use them.

"Can't even race a LM without one" I was told.

[MachineMasters]

Good discussion in this thread.

Thx for the info.

[ZERO25]

Remember the ground is "moving" at nearly the same speed under the car as the air moving above (hence rolling floor wind tunnels are much more accurate that static floor designs).

A cross section of the pressure distribution is visualized in a link above. This car was an XR1 with the oil cooler covered.

The easiest way to know where air is moving is to tape on a group of strings and watch where they go. Deck mount oil coolers suck the string from the deck through the oil cooler. ....Suck, as in high pressure above deck moving to low pressure below.

So theres no device that could be made/mounted onto the deck, that would create a venturi effect?

[Ghopper]

So theres no device that could be made/mounted onto the deck, that would create a venturi effect?

It can be argued that is the intent of cars with the oil cooler scoop facing the rear spoiler. I did not test to see if that works, or just creates low flow across the heat exchanger.

I put my cooler under the deck with a fan for the last ~13 races I have done since 2018 (smh). This years car is standard Mars supplied Keyser cooling package. I will test relative to my Fluidyne system.

[Ltemodel]

"Can't even race a LM without one" I was told.

Funny, I was told the same thing by a very prominent shock guy. It strange when people believe in all this voodo magic.

[ZERO25]

It can be argued that is the intent of cars with the oil cooler scoop facing the rear spoiler. I did not test to see if that works, or just creates low flow across the heat exchanger.

I put my cooler under the deck with a fan for the last ~13 races I have done since 2018 (smh). This years car is standard Mars supplied Keyser cooling package. I will test relative to my Fluidyne system.

What about rear facing louvers in certain areas of the deck?

[turtle1hp]

Rear facing louvers is something that I have thought about also. I'm not sure how to test the efficiency of the sizing or the number of louvers in an area. I would think they would need to be placed in an area with high velocity non-turbulent flow to get the best results.