Wicker on end of diffuser.

[openwheeler37]

Walking around the pits at Road America the other weekend I seen a large amount of cars rear diffusers with a wicker on the end. Is the idea of this to help pull the air out from the diffuser?

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[Nardi]

Basically. Helps with creating a low pressure area I believe.

[Purple Frog]

Yes, wickers on the trailing edge of a diffuser help create more downforce.
But, it is a science to know how much wicker.
The object is to aid the air going through the diffuser to go faster than the air going over the top of the diffuser.
Sort of like flaps on the rear of an aircraft wing. They increase downforce, but also drag. So, there is a nice compromise point.
Usually you will see around 1/4". Rarely more than 1/2".

[openwheeler37]

So if I wanted to look into adding one would I follow the guide lines for size and length as if it was for a wing element?

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[Purple Frog]

A wide wing. Remember the downforce is generated far forward where the kick up from the flat bottom occurs.

[phantomjock]

Dan Gurney is still a very clever man/driver/engineer/owner.


Generates more lift without requiring a multi-element planform (+), keeps area within regs (+); increases induced drag (-).
The "Filled Gurney Flap" results in a new airfoil section with a bluff trailing edge.
Full section width -- easy fabrication (+) and installation (+).


Cheers - Jim

[DaveW]

IMO, the main thing to address in a diffuser is whether the air underneath stays attached all the way to its end. If it doesn't, you are losing downforce and creating unnecessary drag.

The easiest way to diagnose the flow is to look at the flow pattern on the diffuser bottom after a rain race. If the flow streaks go to the end, so does the airflow. If the streaks stop before the end, some air-accelerator at the diffuser end, i.e., a "wicker" or other device, is needed.

[provamo]

Dan Gurney gets the credit but i believe Dan Fowler was the inspiration

[problemchild]

Effective diffusers get the crap beat out of them and need continual service and repair, even if the driver stays mostly on track. Not only does a heavy duty wicker help with aero, it can provide some integrity to the structure.

[formulasuper]

IMO, the main thing to address in a diffuser is whether the air underneath stays attached all the way to its end. If it doesn't, you are losing downforce and creating unnecessary drag.

The easiest way to diagnose the flow is to look at the flow pattern on the diffuser bottom after a rain race. If the flow streaks go to the end, so does the airflow. If the streaks stop before the end, some air-accelerator at the diffuser end, i.e., a "wicker" or other device, is needed.

A few drops of oil works also.

[Bob L.]

What/who is a source for pre-made wicker products? Reciever slot/wickers (various heights).
Don't want to "(re)invent" something... "non std"... when stuff is off shelf....
Thanks.

Bob L.

[openwheeler37]

What/who is a source for pre-made wicker products? Reciever slot/wickers (various heights).
Don't want to "(re)invent" something... "non std"... when stuff is off shelf....
Thanks.

Bob L.

I was just going to ask this very question!

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[Purple Frog]

Aluminum angle at the hardware store down the street and racing tape...

[Marty Nygard]

Something Lee Stohr used to use was the plastic angles that are sold to protect drywall corners taped to the trailing edge of the wing. Very easy to trim height with minimal tools.

[Bob L.]

Aluminum angle at the hardware store down the street and racing tape...

Please, don't say it's so; tell me this isn't how the big guys do it..... been down that path so much the vested associates, no longer follow me; rather turn and head for the break-room ....was hoping for something more elegant.

Enough tape and alum. now flopping in my wake.. ...wanted something "trick"...you know those slots with the little flaps on ends, so you can slip stuff in and out.....during our 2-5 sec. pit stops...

BTW, during the bigbox visits, some alum extrusions with rounded slots caught the eye (used for weather stripping rubber on bottom of doors)...just nothing stiff to slide in and out, still stand tall in high wind...don't need it folding lower with speed.... (Like those FWD tuner cars with rear spoilers? faster>less traction on drive end?) Besides, moveable areo devices are frowned on today, even on my vintage cars.

Bob L.

[Purple Frog]

Please, don't say it's so; tell me this isn't how the big guys do it.....

I've seen it done that way at every level of racing.
Its lighter
its faster to apply
its less expensive
does not require tools on pit lane

K.I.S.S.

[SEComposites]

Please, don't say it's so; tell me this isn't how the big guys do it.....

I've seen it done that way at every level of racing.
Its lighter
its faster to apply
its less expensive
does not require tools on pit lane

K.I.S.S.

At Williams we used brass gurneys bought from a local hobby store on the wind tunnel models.

[openwheeler37]

A wide wing. Remember the downforce is generated far forward where the kick up from the flat bottom occurs.

So if my diffuser is 37" from side to side I could use a 1/4 piece of aluminium (or plastic) and cut it to a length of 27.75" (75% of diffuser width). Correct?

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[B Farnham]

Reading up on this a bit I know that this is intended to increase down force but that it also can be used to reduce drag. Would this be something that could be used on the side pods of a Citation FV to reduce separation and thus drag.

Link to pic of sidepods

Thanks!
Brian

[phantomjock]

Yes the Fowler Flap was perhaps inspiring, but it was Gurney that riveted the angle on the Cobra Coupes! (I really liked the Zap Flap - 'cause of the sooo cool name - but no moving aero in Club Racing)

Gurneys have influence and appear in many locations. I'll offer 2.

If you are looking for a less "obtrusive" solution - maybe consider Vortex Generators.

For a replaceable - toss away diffuser fence there is always vinyl siding.

BTW - why would you make it narrower than the full wing span? --Or did I misread.

Cheers - Jim

[openwheeler37]

Yes the Fowler Flap was perhaps inspiring, but it was Gurney that riveted the angle on the Cobra Coupes! (I really liked the Zap Flap - 'cause of the sooo cool name - but no moving aero in Club Racing)

Gurneys have influence and appear in many locations. I'll offer 2.

If you are looking for a less "obtrusive" solution - maybe consider Vortex Generators.

For a replaceable - toss away diffuser fence there is always vinyl siding.

BTW - why would you make it narrower than the full wing span? --Or did I misread.

Cheers - Jim

From reading through several other posts and articles I've seen people talk about only running a 70-75% length gurney as there was no additional down force gain, just added drag, with running a full length one.

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[DaveW]

From reading through several other posts and articles I've seen people talk about only running a 70-75% length gurney as there was no additional down force gain, just added drag, with running a full length one.

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That may be true on a wing in relatively clean air, but a diffuser is a totally different animal. To start with, usually only the outer portions have significant air-flow over the top, so I would wicker all that is exposed to air-flow.

[openwheeler37]

That may be true on a wing in relatively clean air, but a diffuser is a totally different animal. To start with, usually only the outer portions have significant air-flow over the top, so I would wicker all that is exposed to air-flow.

Thank you for clarifying that!

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[cjsmith]

My understanding of the workings of the gurney lip (wicker bill) it inadvertently tricks the air to making it believe the wing cord or defuse is longer that it actually is. In the case of the defuser, air will want to migrate from high pressure to low pressure. You have low pressure on the underside of the diffuse, high on top. At the back edge of the diffuser, the air going over the rear lip and want to migrate down ward the low pressure area. The lip can make a turbulent or transient zone that delays the air from migrating into the low pressure area. This does a couple of things, by slowing or delaying the upper air from migrating down is decreases turbulence and reduces rear drag, and it increases the efficiency of the diffuser by keeping the low pressure area in tact longer, delaying the infusion of high pressure air, as well as slightly increasing the flow under the car, again helping with down force. In the case of a wing, the wicker allows you to trim out the wing, (lower drag), and still get the benefits of the pressure differential which gives down force on a car or lift on a plane. If you ever get a chance to see a CFD model of a front or rear wing you will see that the underside of the wing does the lion’s share of the work. The gurney lip increases the size of the low pressure area, hence a bigger differential. Down force or lift is all about the differentials in pressure between the upper and lower side of a given component. The gurney lip is one of the few aerodynamic devices that provide improved down force with a minimal drag penalty.

[openwheeler37]

Thanks cjsmith! An explanation I can understand!

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[EYERACE]

What I found at the local hardware store was aluminum 90 degree angle lengths that were something like 1/4" one side and about 1/2" on other.......used the tiniest screws/washers and cap nuts I could find.........drilled tiny holes through 1/2" portion and rear edge of wing and mounted with the nut portion on the bottom [but after reading all this will go reverse them]

[924RACR]

We've just used tape to secure ours...

[Neil_Roberts]

Gurneys have influence and appear in many locations. I'll offer 2.

Ouch! the photo of a splitter with an air dam at its trailing edge makes my brain hurt. Either one by itself will improve aero performance, but that combination is bad. The air dam creates high pressure ahead of it. That high pressure acts on the splitter area to create lift, and a lot of it.

If you are going to put an air dam on a splitter, put it at the leading edge of the splitter.