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Rear spoilers, opinions and comments welcome


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I would like to get a rear spoiler for my 280z.  I like the look of the BRE style 1 piece spoilers.  I have a couple of questions:

 

1) I have read that if you have a rear spoiler without a front spoiler, the front end becomes light and squirrelly at speed.  Is this true, and what speed are we talking about?  This will be a street car with a few passes at the dragstrip per year.  Is the BRE spoiler short enough that this effect is minimal?  Car will have a Xenon type 1 urethane air dam with aluminum supports so it doesn't deform at speed, but no splitter per day.

 

2) Is the 'real/original' BRE spoiler a good choice? ( https://www.datsun-garage.com/products/the-original-bre-rear-spoiler-1970-78-240z-260z-280z?variant=31473838391380 ).  Anyone have any experience?  Any pictures?  Seems like the 6 mounting points is a good idea.

 

3) Any other good options?

 

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FWIW, my experience is that any of those "smaller" rear spoilers essentially do nothing for downforce.  (They can help in cleaning up the rear airflow of the car to reduce in-cabin exhaust fumes.)  I can't point to any specific aero testing; but IME you need something like a whaletail, or a tall wing, to measurably increase your aero downforce on the car.

 

Similarly, front airdams can help with engine cooling by creating a negative pressure cavity under the engine compartment.  They can also increase high-speed stability by eliminating pressure buildup under the front end of the car.

 

If your car is a "street car with a few passes at the dragstrip per year", I would get whatever you like aesthetically.  I've had the BRE spoilers front and rear on several of my cars, and the look is timeless IMHO.

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We don't need to guess here. The wind tunnel testing did the MSA spoiler (BRE spoiler) and a 5.25" tall spoiler. At 120mph, the BRE made 22 lbs downforce. The 5.25" one made 30 more lbs downforce. Without a spoiler at baseline, it made 55 lbs of lift.

The much bigger thing with regards to Z car aero is to do something in the front. Cover up that huge hole in the front and I expect you'd see some real gains at the drag strip. Stock, the test car made 320 lbs LIFT. That work that the air is doing is slowing you down. If you look at the wind tunnel results the stock test #2 the car had .471 CD. If you look at the last test with the car with the most aero mods was .407. The hp to drive the baseline car at 120 was 102. The hp to drive it with the front end mostly blocked off was 88. So closing up the front is like giving yourself another 14 hp at the fast end of the track.

Edited by JMortensen
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@JMortensen Those charts are barely legible even if you zoom in. It's one of my big frustrations with the forums after the major update some years back. Images uploaded directly to it all get compressed and you lose detail on some of this stuff.

I have personally experienced the lift that you're referring to and the front definitely makes the most difference. A lot of people don't realize how much of a parachute effect the front hood has to create lift. 

My coilovers were adjusted poorly before adding more of the interior and carrying some things in the rear deck which caused the back end to lower enough to cause problems. Before the front splitter was added, the lift on the front end was enough to make the back rub since it was already close. Over about 55 or 60mph the lightest blip of the throttle would cause one side to rub. Over 65 or 70 mph the front end lift was dramatic enough to cause constant rubbing.

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@JMortensen When you say cover-up the opening in the front, do you mean:

 

1) Air dam w/ very low clearance to ground.

2) reduce as much opening to rad intake as practical

 

or both? Greg uses the 280Z RFP bumper precisely because it is huge and helps direct air around to the sides of the car vs. slamming right into the rad intake void. I will have to look more closely at how much block-off I can get away with in the front before I run into trouble with the rules. 

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Both. Stock height is kind of ridiculously high for track/high speed stuff. The front of the car is wide open and just shovels air in under the hood where it gets trapped and ends up going under the car, creating lots of drag. 

If your rules allow, making a duct from the front to the radiator so that all the air that goes in the front has to go through the radiator will reduce the amount of air going in, as without a duct it goes down in front of the rad and up over the top, and through all the holes in the rad support. Also louvering the hood to get rid of that air under the hood without forcing it under the car is a good idea.

 

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Another idea depending on the rules would be to vent the inner fenders and put some large section screen on the holes so rocks don't get stuck on top of the manifold.  Usually the wheel wells are low pressure and should help manage the radiator air.  Spats or sides to the airdam will also create more low pressure.  Done right this will reduce the lift and make the car faster in a straight line.  if you stand in front of the car you shouldn't see any of the tire tread area.  If you do it needs to be covered.  

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Look at the way bjhines did his air dam support. He covered the whole space between the rad support and the air dam, preventing air from going under the rad support. That's a much better way than what you're showing here. Doesn't prevent the air from going up and over the top, but 1/2 the job done is better than none.

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Was a full frontal/engine bay undertray from the front valence to the firewall ever tested (similar to what the z432r had)? 

 

Most cars I've seen with front aero use an air dam or chin spoiler but omit even the splash tray. Or they run a splitter just to cover the area under the valence but don't bring it further back. I recall seeing some pictures of Nissan wind tunnel testing some part scale prototypes to develop the full frontal undertray for homologation purposes? I don't believe they paired that with a front splitter or air dam, but the part was actually produced and there are some repro's of them in Japan. 

 

Found the pictures. There are a bunch of them testing various aero for the 432r as well including the rear spoiler. Really interesting stuff

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Edited by Dat73z
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They did not test any paneled floor ideas at all and said Bob at the wind tunnel told them it wouldn't work. Mike Kelley who organized the first test said that the radiator ducting was a bad idea because Bob said so. :( I mentioned testing flat bottoms they were talking about doing a follow up wind tunnel test and pissed off the guys involved to the point where I think I single handedly killed the project. Double :( Then they went and put both ideas on their own cars. Mike and Matt did NOT accept suggestions very well, let alone criticisms. Quickly resorted to insults and took their ball and went home. 

I can tell you that I emailed Simon McBeath, the author of Competition Car Aerodynamics and he suggested paneling as much of the bottom side as possible. The things you need to look out for are getting air to the trans and diff for cooling (NACA ducts work to get some air in there), exhaust heat, and when I do mine I will go aluminum in the middle so that the paneling itself isn't flammable. 

Edited by JMortensen
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My day job is doing aero analysis and design for a defense company. A big thing when we design planes is that to reduce drag, you want to capture the absolute minumum amount of air possible in the inlet. For the purposes ot a car, the inlet would be the grill opening. It's pretty common knowledge now, but ducting the radiator and blocking everything else off has huge benefits. 

 

In my experience, a lot of people who work around certain things think they know a lot about it just because they're near it. Welders thinking they know structural design, contractors thinking they know architecture, and apparently Bob thinking he knows aerodynamics. 

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Probably not useful to dredge up the past, but my take on it at the time was that they probably asked Bob if a flat paneled floor would make downforce, Bob looked at a stock ride height Z with stock front end and said paneling the bottom wouldn't do anything for downforce. That's probably true. You'd need it a lot lower and with some rake to do much for downforce. Mike's take away was "undercar aero doesn't work on Z's" and this info came from on high. BOB said it. I mean, it was BOB, FFS. I don't know what Mike asked him about the radiator duct, but he specifically called me out and tried to make an example of me on that one. We already didn't get along, and hey, I rub a lot of people the wrong way and I don't make excuses for it, but that one really got on my nerves because he obviously hadn't thought to test a rad duct and then tried to make me look like an idiot for asking about it.

You can go back and read all the drama if you want, but I think the basic point was this: 

They put together a wind tunnel test, which I donated to and am eternally grateful for.
Mike said he read some books but in the end they didn't test anything outside of some simple bolt ons. 
I pissed Mike off really badly because he went all out on this and as soon as he posted the results I started asking about all the stuff I thought he should have done and didn't. 

The second time he asked for donations I said things like "Make a flat plywood floor that you can clamp or screw to the bottom to test" and Matt came back telling me how ridiculous it was to use plywood and how they were making all of their parts out of carbon fiber. Before testing to see if they worked. :confused: Then they got pissed and started throwing insults. 

Edited by JMortensen
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That is unfortunate. Sounds like a lot of strong personalities. 

 

What we do know is a frontal undertray was wind tunnel tested and produced at some point in time by Nissan for racing/homologation purposes. I would think there would be some benefit otherwise the modification wouldn't have made it into production. I am curious why it wasn't paired with other aero but unless we find the original engineers from the 60s we may never know. 

 

It would've been nice to quantify those benefits, and how those benefits stacked or detracted with other aero but perhaps we'll get another go at it someday in the future...

 

Edit: maybe next go around show the wind tunnel people the actual Nissan racing homologation papers and wind tunnel test pics for the parts. There must be a reason why they went through all of the trouble right? 😁

 

MjlEaP.jpg

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Edited by Dat73z
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It's a no brainer that the tray would help reduce drag, although we know through 50 years of experience that making it flat, incorporating a splitter in front and having it lower at the front than the back will make it better than the original design. Some of the stuff they did back in the day was pretty shady. Look at their homologated roll bar. Terrible. And now we'll get Alan Thomas in here being an ass because I criticized the factory roll bar. BTW, you better hope those aren't his pics. He's super generous about it when people use his pics to spread the general knowledge online. ;) 

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