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RebekahsZ

Experimenting with rear spoilers

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RebekahsZ    106

I have drilled my rear deck to accept 2 different spoilers: a small version similar to the BRE/factory/432 style and a larger version that I think was manufactured by ZCC-JDM. I've got them set up so that I can swap them fairly fast (depending on how fast I can find each screw I drop in the grass or under my workbench). I wish I lived closer to the A2 wind tunnel, because I'd probably rent a session if money were the only restriction, but I'm also restricted by time and distance. So...I'm hoping for low-wind days at the GA 1/2-Mike and the Ohio Mile (and a consistent driver), so I can report back to the forum about how these spoilers (to include no-spoiler) affect top speed and subjective "stability." Here are some pics to whet your appetite.

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Edited by RebekahsZ

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Chickenman    4

I know you've increased your spring rate on the front, but what is it on the rear? You may want to think about increasing that as well. The larger wing should add a considerable amount of down force. At 170 MPH  possibly as much as 200lbs. Issue may be fender to tire clearance. Throw 200 lbs of weight in the back and see how the fender lip clearance is.

 

A great book on Aero is " Competition Car Downforce" by Simon McBeath. It explains a lot of complicated Aero subjects in laymens terms. Shows how you can make simple DIY changes that really work. ISBN 0-85429-977-7

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RebekahsZ    106

My understanding is that a wing causes downforce and that a spoiler reduces lift. Class rules forbid a wing. I wanna say that the old S30 wind tunnel data showed 70# of lift (on the rear axle) at 80mph. If the car weighs 2400 and half of that is on the back axle, without a wing, the most you can make the rear axle see is 1200#. Using the term downforce suggests that one could make the rear axle see more than 1200. I'm pessimistic. Rear springs are 400#. Feels good.

Edited by RebekahsZ

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260DET    25

Reducing lift/increasing downforce is a confusing way to consider rear aero, fact is both a properly designed spoiler and wing will induce downforce. If the downforce is sufficient to overcome lift is a whatever question, what really matters is the overall effect. Going on what gnosez said, a whaletail is the way to go if wings are verboten.

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Chickenman    4

The book I recommended has some fascinating info.  A simple rear spoiler can create a LOT of downforce....especially on a sloped Hatchback like a 240Z or Third Gen Camaro. ( I use to Hillclimb a 1986 Camaro   and we worked with some fairly simple Aero mods starting back in 1999. The results were amazing, in fact we started a huge Aero war in the Street Production Class. )  

 

Stock the 240Z has a low pressure area over most of the rear Hatch surface. That creates lift as noted. A properly designed rear spoiler ( Nascar style of at least 4" to 5" in height )  can generate a lot of Downforce. A rear spoiler changes that low pressure area ( acting on the rear Hatch ) to a High pressure area. And there are a heck of a lot of square inches of Hatch area to work with on a 240Z

 

My Camaro's Hillclimb spoiler was at the maximum height according to calculations taken from McBeath's book.On my car that gave us a flat rear  spoiler at 9" of overall height. I won the SP3 class at Knox Mountain from 2000 through 2005. And broke my own Track record every year. Here are some shots of the Aero on my car and some of my Competition.  car. Simple, but very, very effective. I designed the rear spoiler so that it would flex at high speed and create less drag.

 

Knox 2007:

 

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Front Splitter was VERY effective. Hard to see but note end fences and underneath strakes. The bottom side strakes directed air to the bottom feed radiator deflector. Note: A splitter does not have to be close to the ground to be effective. Not the same principle as an Airdam.

 

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620HP Trans-Am. 3,850 lbs, AC and freakin fast.  315/35x17 Kumho V710's  which it could haze with ease. Edit: Front splitter was made out of a honeycomb foam core aluminium sheet from Boeing Surplus. We tested the strength by having a couple of 200 lb guys stand on it. No deflection.

 

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One of my fave photos of my buddies bad ass Trans-Am. ( And I kicked his Ass :D )

 

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 A link to Photos of Knox 2011. Every Class winning car was running some for of down-force creating Aero.

 

https://www.flickr.com/photos/[email protected]/sets/72157626669500009/

 

My record setting run from 2005. Good weekend. Broke my own Hill record three times that weekend. Jump ahead to 1:21 to miss boring brake and rear tire warming.

 

https://www.youtube.com/watch?v=nollo7iTRtg

Edited by Chickenman

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JMortensen    235

For anyone interested in the book, Comp Car Downforce morphed into Competition Car Aerodynamics. It's had 3 updates and is quite a bit longer and has more tech info than Downforce did. Here's the latest ISBN, check for a 4th edition if you're ordering, the 3rd has been out for a year or two at this point: ISBN-13: 978-1845847760

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Chickenman    4

Thanks for the update. I'm ordering the updated version today. I wonder if McBeath did an update on Autocross and Formula SAE wings in the latest edition?  He wrote a few articles in Race Car engineering on the low speed evolution of wings and Aero use in Solo events.

Edited by Chickenman

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260DET    25
Posted (edited)

Where are we at here, RebekahsZ was in the middle of an interesting comparison of several enhanced bobtail rear spoilers and chickenman as usual had some useful info. I've decided with both the 280ZX and Z31 track cars not to use wings but rather stick with period correct devices at the back. gnosez mentioned the whale tail as being good, I'm thinking one with a Gurney lip may have a fair bit less drag than an enhanced bobtail. Will have to dig through my pics to see if there is one on a Z there.

 

EDIT: pic show what I had in mind.

 

 

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Edited by 260DET

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Chickenman    4
Posted (edited)

That whale tail may work a bit too good. You'll likely a big arse splitter up front to balance the downforce for Road Racing. 

 

Edit:  Deleted extraneous thread material that I'd already posted up in #6. 

Edited by Chickenman

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RebekahsZ    106

Tall Retrospec spoiler steadied the car. I'm now ready to go faster. No loss of top speed. Consistently 170mph with an out-lier of 173. Car ran that regardless of the spoiler I installed, including no spoiler. But with no spoiler or the short factory style the car was scary.

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mmendes    0

Can you point me to the sanctioning body or class rules you have to work with? Is there anything preventing you from installing a diffuser, or any other modifications to the bottom of the car? If it were me, I would look to address the front end, hood and mainly the underbody/diffuser since you already have a spolier with a known directional improvement. 

 

If you are looking to change the rear spoiler from the off the shelf parts you have tried I think you would want to make the spoiler closer to 90 deg to the surface of the hatch. IE The rear window and spoiler make a 90 degree angle between each other. You can also likely go a bit taller. My experience is with an underpowered road racer so we are less concerned with drag but IIRC in MCbeaths book they continue to see efficiency gains as they make the spoiler taller; past the point of anything you would install on a car or be legal.

 

One low hanging fruit thing you could do would be to block the front of the rear tires. 

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RebekahsZ    106

For class rules with ECTA, I may have to run without flares (which means my tires will be hanging out in the breeze a couple of inches). You can cut rear quarters for tire clearance but you can't add flares (even though ZG flares likely hurt aero). So I'm not gonna spend time there. I'm not gonna spend more time on aero until I go to forced induction. I've seen some ugly dirty bricks go really fast when enough hp is applied to the ground. If I get into the 190s and can't break thru, I will try to slick up more. Unless she gets squirrelly again.

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260DET    25

Tall Retrospec spoiler steadied the car. I'm now ready to go faster. No loss of top speed. Consistently 170mph with an out-lier of 173. Car ran that regardless of the spoiler I installed, including no spoiler. But with no spoiler or the short factory style the car was scary.

170 mph is pretty damn quick in a S30, I'm surprised that you have found no difference in apparent overall drag with your various test setups, I'm guessing that any extra drag resulting from the tallest option is cancelled by the air flow at the back of the car being tidied up and negative pressure around the tail light area being reduced. I'm assuming that the Retrospec spoiler you refer to is the one in the first pics in your OP.

 

My S130 should react about the same so for my next  run at the ultra fast Bathurst circuit later this year your findings will come in very handy, thanks.

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RebekahsZ    106
Posted (edited)

Still lots of vacuum and turbulence behind car. Enough to wrap my chute tether around the chute mount and cut it on deployment. Another obstacle to overcome. The photo that shows whole car is how tether hangs at rest; other photos show how it gathers up behind the car at speed. Chute launches fine, but I need to make a guide of some kind to make the tether stay organized at speed. Will pull out some Screamin' Eagle concepts from Ft. Campbell and some 1/4" cotton webbing-lol. Inside joke for the veterans.

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Edited by RebekahsZ

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RebekahsZ    106

Do any of you really understand how the flat spoilers work that the Mustang crowd uses? Just looks to me like the make the air think the car is longer... can't find any thoughtful discussions (on line) regarding how they supposedly work.

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