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Old 08-08-2019, 06:19 PM   #11 (permalink)
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Old 08-10-2019, 12:32 AM   #12 (permalink)
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Quote:
Originally Posted by rob.e View Post
has anyone fitted a DIY diffuser and seen improvements?

its something i'm considering for my car. it has pretty good /flat floor already but some parts at the rear bumper could be better if there was a smooth panel there.

that part of the underfloor already slopes upwards so easy to fit a "boy racer" ebay flat panel with straighteners. Do you think it'll make a noticable difference?


I'm thinking about something like this:
https://www.ebay.co.uk/itm/1X-Abs-Un...9/303179537634


they got it on ebay US considering the exchange rate you still win

https://www.ebay.com/itm/4-Fins-Car-...EAAOSwWrxcMypk
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Old 08-10-2019, 12:20 PM   #13 (permalink)
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benefit

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Originally Posted by fastflyer View Post
Yes, with a smooth undertray it works.
But with a normal car is there any benefit?
without paneling upstream of the diffuser,you'd just have turbulence coming at it,for zero gain.
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Old 08-12-2019, 03:44 AM   #14 (permalink)
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Quote:
Originally Posted by aerohead View Post
without paneling upstream of the diffuser,you'd just have turbulence coming at it,for zero gain.
yep, thats the plan. the honda has pretty good underfloor aero already from the factory, but yes i'll be looking to see if there are any areas that could be improved.


From the honda press blurb:
"Aerodynamic efficiency is the very basis of the Civic’s design and is crucial to achieving exceptional high-speed stability and efficiency. As well as a low drag mono-form design, the Civic also uses a full-length bumper-to-bumper undertray with flared leading edges to manage the flow of air under the car. Together with larger covers on the rear suspension, this undertray adds to stability, particularly when overtaking long vehicles on multi-lane roads"

Mine has something like a bodykit fitted which at the rear, seems to jut into the underfloor flow, dealer fitted honda option i believe. I either need to pull this part off (which i expect will leave holes and give me paintwork issues to resolve) or use something like that ebay diffuser to bridge the step and make it smooth.

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Old 08-12-2019, 01:41 PM   #15 (permalink)
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Only way you'll truly benefit from one of these (without doing a ton of testing) is to have it extend forward of the rear suspension so that your rear bumper isn't acting like a parachute. Other than that, as has been said, you need to smooth out all the airflow ahead of it first. If you already have the full undertray there likely won't be much improvement.
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Old 08-12-2019, 06:17 PM   #16 (permalink)
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One example of undercar airflow treatments specifically called out by Hucho in Ch. 4 (I don't have the book in front of me, so I apologize for not citing the section number) is on the 1996 Opel Calibra. There's an illustration that shows the results of air dam testing, etc. and part (d) has a figure of the rear diffuser panel. It stops short of the rear bumper cover, with a pretty substantial gap between them, but the text states that the diffuser's effectiveness is still increased by removing the center section of the front airdam and allowing more airflow under the car--both of these struck me because they run counter to conventional aerodynamic wisdom. Things don't necessarily have to be perfectly smooth to get some benefit; and, I think people tend to overestimate the "parachute effect" without any real information to back it up because we "feel" like it should be significant. It might not be, though--you'll have to test to find out.

I'm reminded also of the Audi A2 study from 2011; a completely smooth floor showed a 20-count drag reduction in a tunnel with no moving ground; that was reduced to just a 6-count reduction in CD in a tunnel with moving ground and rolling wheels. The stock A2 has a fair amount of paneling underneath, but with a substantial central opening for the exhaust and openings around the wheel housings, all of which were covered to get that -0.006 CD delta:



We're all really just shooting from the hip here. If you want to know if that eBay diffuser does something, the only way to find out for certain is to buy one and test it. Of course, then you'll run into the issue of noise in your results and how to reduce it so you can ascertain anything meaningful, which will likely be very difficult.
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Old 08-13-2019, 09:56 AM   #17 (permalink)
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Quote:
Originally Posted by Vman455 View Post
It stops short of the rear bumper cover, with a pretty substantial gap between them, but the text states that the diffuser's effectiveness is still increased by removing the center section of the front airdam and allowing more airflow under the car...... I think people tend to overestimate the "parachute effect" without any real information to back it up because we "feel" like it should be significant.
In my mind it's similar to the bed of a pickup.
A simple wing extension off the cab, while still having a significant gap to the tailgate, will show improvements, but a full aeroshell over the bed will be the best.
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Old 08-14-2019, 11:23 AM   #18 (permalink)
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parachute

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Originally Posted by Joenavy85 View Post
Only way you'll truly benefit from one of these (without doing a ton of testing) is to have it extend forward of the rear suspension so that your rear bumper isn't acting like a parachute. Other than that, as has been said, you need to smooth out all the airflow ahead of it first. If you already have the full undertray there likely won't be much improvement.
I've seen this argument through the years,and I believe it's based in urban myth or something.
A 'parachute' requires a 'clean',unobstructed flow source,which a rear bumper would not have available to it, unless a car was fully paneled,with laminar flow all the way underneath to the bumper,and the bumper extended below the elevation of the belly,into the laminar flow.
Without an operable diffuser,the leading,hollow face of the bumper would,simply be in train of a captured pool of turbulent air,traveling along with the vehicle.
I have seen some Ford cars whose rear bumper fascias 'quake' on the highway,but it's not do to 'parachuting,but rather due to the turbulent environment it's residing within.This should have been caught inside the aero-acoustic chamber.A true design oversight.Some webbing added to theinjection molds could have 'tuned' the natural frequency away from that turbulent signature,and any harmonic of it.
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Old 08-14-2019, 11:31 AM   #19 (permalink)
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pickup bed

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Originally Posted by Joenavy85 View Post
In my mind it's similar to the bed of a pickup.
A simple wing extension off the cab, while still having a significant gap to the tailgate, will show improvements, but a full aeroshell over the bed will be the best.
It is similar.It would be an inverted,rear-facing step,with reattachment,and a captured vortex if properly designed.You're losing some pressure recovery potential due to the vorticity,however,it's way out ahead of nothing.
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Old 08-14-2019, 01:43 PM   #20 (permalink)
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Quote:
Originally Posted by aerohead View Post
I've seen this argument through the years,and I believe it's based in urban myth or something.
A 'parachute' requires a 'clean',unobstructed flow source,which a rear bumper would not have available to it, unless a car was fully paneled,with laminar flow all the way underneath to the bumper,and the bumper extended below the elevation of the belly,into the laminar flow.
Without an operable diffuser,the leading,hollow face of the bumper would,simply be in train of a captured pool of turbulent air,traveling along with the vehicle.
I have seen some Ford cars whose rear bumper fascias 'quake' on the highway,but it's not do to 'parachuting,but rather due to the turbulent environment it's residing within.This should have been caught inside the aero-acoustic chamber.A true design oversight.Some webbing added to theinjection molds could have 'tuned' the natural frequency away from that turbulent signature,and any harmonic of it.
So the turbulence is similar to what you get in the bed of a pickup with the tailgate up, one big oval cushion of swirling air?

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