Rear Wind Deflector/Blocker

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Rear Wind Deflector/Blocker

Postby WestieWestfield » Sat Jul 10, 2010 6:56 pm

Hi,

As the weather turns cooler, I have been having thoughts about a rear wind deflector/blocker.
Before the usual responses come back, have already considered scarf, neck warmer and hardening up pills!

Have had experience in a friends MX5 with and without and it seems to make a difference.

In addition, have noticed in research that a few brands do versions of them:
BMW
http://www.bimmerfest.com/forums/showthread.php?t=116056
Mini
http://new.minimania.com/web/Item/nma7638/invDetail.cfm

So figure there must be some merit.

Would it improve the aero dynamics of the car (or reduce it)?
Has anyone had experiences with them (including with a 7)?

Cheers,
Si
1991 Westfield SEi
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Re: Rear Wind Deflector/Blocker

Postby Bigred » Sat Jul 10, 2010 7:08 pm

Hi Si

You could test out the theory by taping a bit of cardboard inside your roll bar, or stretch wrap if you can get some, and see if it makes a difference.
If you use cardboard you'll have to be careful it doesn't detach.
I seem to recall a car with perspex fitted inside the roll bar to reduce the wind. Might have been on Oz clubbies.

Cheers
Cliff
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Re: Rear Wind Deflector/Blocker

Postby WestieWestfield » Sat Jul 10, 2010 7:20 pm

Hey Cliff,

Most of the manufacturer developed models are mesh construction. Some airflow seems to be important.
Not sure I could be bothered with poking that many holes in the cardboard and would be concerned about it's integrity at speed if I did!

Si
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Re: Rear Wind Deflector/Blocker

Postby Super Seven » Sat Jul 10, 2010 8:11 pm

I've got one of these fitted to my Caterham.

You can see them at http://www.softbitsforsevens.co.uk they help reduce the buffeting but don't entirely eliminate it. The wind still whips around the sidescreens. The photo below shows it fitted, it is held in place by the head restraints on the seats.

Another alternative is this http://www.users.waitrose.com/~mickburrell/forsale/index.html which is essentially an extended sidescreen. They apparently are pretty effective.

I've tried using a barrier attached to the rollbar (actually a spare windscreen) and it made things far worse. The mesh helps reduce things a bit but ultimately you are trying to fight the non-aerodynamics of the car.

Image

Mike
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Re: Rear Wind Deflector/Blocker

Postby ibreadmore » Sat Jul 10, 2010 9:24 pm

A wind deflector is apparently a finely calculated scientfic and aerodynamic placed item, the effectivness is related to the distance of the deflector relative to the height of the screen, its angle of deflection and the eddy currents coming from the top of the screen. Mostly any deflector will work but will sometimes affect the aerodynamics if placed incorrectly.

The purpose is only to stop the rush of air you get from behind your head in a convertible, causing those with long hair to get their hair blown into their face.

personally, i think they look cool.

Cheers Iain.
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Re: Rear Wind Deflector/Blocker

Postby WestieWestfield » Tue Jul 13, 2010 6:27 pm

Thanks for the tips and links lads.
Some good food for thought there.

Si
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Re: Rear Wind Deflector/Blocker

Postby davew » Wed Jul 14, 2010 2:50 am

Here what I posted on USA7s a while back.
Reducing Buffeting in a Seven
I have experimented with reducing wind buffeting on my Locost for several years now. The high vacuum “ low-pressure area” behind the front windshield is worst then if you are on a bike at the same relative road speed. I have tried several approaches to reduce the buffeting. Some worked, others did not, I have no way of measuring buffeting but these are my general findings for the various modifications.
Winglets will reduce buffeting around 10 to 15% depending on size. The attachment should allow some air to pass between the screen and the winglet.
Laying back the front windshield an addition 5* reduce buffeting by about 10%. It helped mostly at lower speeds and no change at higher speeds.
A solid Plexiglas wind screen behind the seats, attached across the full width of the roll bar and at head height did not help. I also experimented with cutting down the screen with a gap on the top, gap at the bottom, and both top and bottom gaps to allow some air to fill the low-pressure air behind the windshield. Using solid Plexiglas did not help, It actually made it worst.
Side doors, without the top. My doors are a lot longer then most. They run from the front of the windshield all the way to the roll bar. They reduce buffeting a good 40 to 50% when compared to an open cockpit.
A Bikini top, you still have high vacuum area, but a high percentage of the air flow is behind your head. It reduced buffeting in the 30 to 40% range.
Fixed sun visors that help defect the wind up over the top of the cockpit did help about 10%, but the visors probably need to 6 to 8” wide. My visors are only 4 ¼” wide, and worked the best when set about 20* from horizontal.
Make up air, I have a defroster, and a 5 speed heater fan, but I can not feel any difference in buffeting with the system running. I would venture to say that it would take a lot more air volume then a typical heater fan could put out. For the 5% of the Sevens that use bikini tops, I just installed a rear window that attached across the full width of the roll bar just a couple inches behind your head. The car was tested with Winglets and a bikini top installed. With a full width windscreen i.e. rear window, installed it increased the wind buffeting.” This is without doors” and the results where basically the same that I had with a solid wind screen tested without the top installed. But with the bikini top installed the air flow is back and forth alternating from the left side to the right side. Air flow was verified by using a single turf hanging from the roof bow. I design the rear window with center sliding section this allowed me to adjust the center opening. Adjusting the rear opening 1 or 2 inches wide in the center section had almost no effect on the buffeting. I next tested the windscreen with about 20 to 25% of the total rear window area opened in the center section to allow air to suck out into the low-pressure area behind the windscreen. A moderate 20-25% area opening actually reduces the total wind buffeting probably in the 10 to 15% range Vs a completely open rear area. It was more effective the higher the speeds. It was tested to 100+ mph and it prove to be even more effective at reducing the buffeting at those speeds when compared to a completely open rear area. Using an opening in the center area of the rear wind screen appears to work much better then trimming off the top and bottom of the full width Plexiglas windscreen and warrants further testing without a top installed or the used of an open mesh type wind screen which allows air flow. Your results may vary; Dave W
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Re: Rear Wind Deflector/Blocker

Postby Wayno » Sat Jul 24, 2010 8:13 am

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Re: Rear Wind Deflector/Blocker

Postby WestieWestfield » Sat Jul 24, 2010 2:52 pm

Cheers for sharing your testing results Dave and the link Wayno.

Some good ideas for either making or buying (at that price would probably go for a performance upgrade instead).

Si
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