Ground Clearance vs Aerodynamic Drag

[Xist]

So, I should raise my Civic to six inches?
I think that is my Forester height!

[skyking]

I measured the truck as it sits. 20" is my starting point on both the side skirts and front air dam. I must say, DAM! that is high.
I can get maybe 1.6" with a tire change, and I could get another 0.75" by lowering it as much a I dare. There is only 2.25" to the bump stop now, so a drastic lowering would be just that, drastic.
The tire change is interesting, it is 7% too fast now and probably optimized for 70+ freeway speeds. When I tow I go 60 or less. Reducing tire diameter should help and not hurt economy. Buying tires hurts economy big time though.

[pgfpro]

Originally Posted by MetroMPG

Quantified:

0.34 - original Cd

-.08 - lowering (that's massive! and it's the only time I've seen it quantified)
-.025 - full grille block
-.015 - mirrors deletion (doesn't mention wipers though)
-.01 - full belly pan

I suspect they couldn't mod the door handles or add skirts in order to keep within the "stock" definition for bodywork. Probably a class thing.

EDIT: the slide isn't exactly comprehensive. It doesn't address the decklid extension or the smooth wheel covers...

http://ecomodder.com/forum/showthrea...34-0-a-85.html
Wow that is more then I would ever guessed?

[aerohead]

Originally Posted by BabyDiesel

Aerohead, what effect do you guesstimate the decklid extension had on Cd reduction? I find it odd that it was not included in the report Cd posted in post #3.

I have no conclusions,only thoughts.A number of LSR cars have run them,and no reported findings so far.
*In Hucho's book he talked of 'saturation',at which,say,when the decklid was long 'enough',any additional length would return zero benefit.
*The extension would affect the downwash off the backlight allowing it to exit the car parallel to the ground which is preferred by some designers.
*The extension also has capping plates which could allow the device to act like a half-box cavity,which would help with pressure recovery and higher base pressure.
*The capping plates themselves move the center of pressure rearwards,helping to keep the car straight on the track.
*Also,the wake is underneath the decklid,giving a vertical force vector,which could help mitigate wheelspin which could cause a wreck at high speed.
*Finally,the improved flow would aid drag chute deployment.
-------------------------------------------------------------------------
*The USFRA and SCTA don't allow drag reducing mods in non-Lakester/Streamliner classes,so it implies that the decklid mod is about safety,like the deep airdams and air fences.

[aerohead]

Originally Posted by Xist

So, I should raise my Civic to six inches?
I think that is my Forester height!

My thought right now,is that you'd have to take the car to a wind tunnel,with different suspension links to simulate springs,for a range of heights,and see where the lowest drag comes in at.
You'd also want to experiment with inclination (rake).
Raising,pushes the center of gravity higher (not good) and increases roll moment (not good).
It also exposes more tire to the air,increasing frontal area (typically not good).
It is true that raising the car up out of ground effect will cut the body drag in half,but you'd lose all practicality.(A wing-inspired solar racer on a closed race course yes,any other scenario,no).

[aerohead]

Originally Posted by pgfpro

http://ecomodder.com/forum/showthrea...34-0-a-85.html
Wow that is more then I would ever guessed?

I suspect that Gary Eaker of the A2 Wind Tunnel could tell us many things about these.However,he's also probably sworn to secrecy about certain projects unless the client volunteers to share the info.

[aerohead]

NeilBlanchard mentioned the pilbug and I've started a workup on it up at the 1st post below the PROBE-IV.

[skyking]

Originally Posted by aerohead

My thought right now,is that you'd have to take the car to a wind tunnel,with different suspension links to simulate springs,for a range of heights,and see where the lowest drag comes in at.
You'd also want to experiment with inclination (rake).
Raising,pushes the center of gravity higher (not good) and increases roll moment (not good).
It also exposes more tire to the air,increasing frontal area (typically not good).
It is true that raising the car up out of ground effect will cut the body drag in half,but you'd lose all practicality.(A wing-inspired solar racer on a closed race course yes,any other scenario,no).

blocks of wood would do it. That is the easy part, coming up with the full size tunnel is tough.

[Cd]

Originally Posted by skyking

blocks of wood would do it. That is the easy part, coming up with the full size tunnel is tough.

Or some bottle jacks just behind the tires ( out of the airflow )

The higher end A2 tunnel* is able to raise and lower the ride height on the cars that they test without any need to go back in the tunnel.

* They operate two tunnels.

[BabyDiesel]

Originally Posted by aerohead

I have no conclusions,only thoughts.A number of LSR cars have run them,and no reported findings so far.
*In Hucho's book he talked of 'saturation',at which,say,when the decklid was long 'enough',any additional length would return zero benefit.
*The extension would affect the downwash off the backlight allowing it to exit the car parallel to the ground which is preferred by some designers.
*The extension also has capping plates which could allow the device to act like a half-box cavity,which would help with pressure recovery and higher base pressure.
*The capping plates themselves move the center of pressure rearwards,helping to keep the car straight on the track.
*Also,the wake is underneath the decklid,giving a vertical force vector,which could help mitigate wheelspin which could cause a wreck at high speed.
*Finally,the improved flow would aid drag chute deployment.
-------------------------------------------------------------------------
*The USFRA and SCTA don't allow drag reducing mods in non-Lakester/Streamliner classes,so it implies that the decklid mod is about safety,like the deep airdams and air fences.

Incredible info, aerohead! Much appreciated, I need to read into Hucho's works.

Thinking out loud here... What IF the length were optimized according to the Aerodynamic Template, and the end of the spoiler continued downward following the template?

To reduce the amount of lift under the decklid, extend the belly pan up at the 10° angle to meet the downward curvature.

At 45 mph, no need to worry about correct parchutes deployature. Wait, are we not trying to get rid of the natural parachute our cars create?

Could this be a boat tail without the kammback section, perhaps??