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JMortensen

DIY Lexan spoiler - autox style

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JMortensen    235

I finally got around to building a spoiler. Was considering aluminum or Lexan, ended up with 1/4" Lexan, now I think AL might have been the better choice. I made the spoiler adjustable so I can move the supports between 3 holes, 1" apart, and even at the steeper setting an AL spoiler would barely cut any visibility at all. Anyway with some help from FB, I decided to use a piano hinge across the back of the hatch. Could have gone the full width of the car, but sticking with the hatch width is easier from the standpoint of building the thing, so that's what I did. Tom Holt had my trying to cut aluminum rod and thread it to mount the stays. Screwed around with that for an hour or so and said screw it. Went and got some fuel line, pinched the ends and bolted it to the brackets. Much easier, but not as pretty.

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JMortensen    235

If I can get to a autox test day I'll be able to do back to back runs and see if the angle makes a noticeable difference. I kinda doubt it will. Once I get a wing on there, then I'll use oil to make sure the flow stays attached and adjust the spoiler accordingly.

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gira    11

If you feel the effect at an autocross at those speeds, I'd imagine it would have to be way up in the air, huh? I'm looking for a rear deck spoiler to run on my vintage car that the sanctioning body is going to group me with all these factory built Porsche RSR's. The vintage guys don't like Daatsun winning too much.  I have seen a sort of overgrown BRE type spoiler that looks like it might work. At one point in SCCA E Production, the allowed us to use the BRE spoiler for one single race before that can of worms went viral. The RX7 guys had all kinds of spoilers their cars "came" with, so SCCA canned that rule.

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JMortensen    235

I've heard things like 50 lbs downforce at 50 mph, and apparently good drivers with cars that are well sorted can feel the difference at an autox. If it didn't work you wouldn't see it on every high level car that can run one. My class is so open because of the motor swap that I can do a lot on aero (except a diffuser), although I'm not likely to be able to compete with the Lotus that cleans up every year in X Prepared. 

 

Mark Belrose runs (ran? he was trying to sell it a few weeks ago) a VARA CP3 car in CA, he has a spoiler. Very fast from what I hear. Thing is built, 50mm carbs, button clutch, etc. Maybe that's the class you're trying to avoid, but it can be done from what I've seen.

Edited by JMortensen

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RebekahsZ    106

Here's the spoiler I just installed. I can plan to test only with top speed over a 1-mile course. I will compare it to 1) no spoiler, 2) small BRE spoiler, and the pictures spoiler. The rear hatch is pre-drill to accept all 3 conditions. I will be using o-rings on the strut rods to measure maximum suspension compression, and I understand that I will not be able to eliminate SO many variables like density altitude, winds, and driver error. "Stability" assessment will be totally a matter of driver impression. I wish A2 was cheaper, but I totally understand why it is not.

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Edited by RebekahsZ

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260DET    25

If it has to be flat then what about the bird bath style the first 280ZX racers used? RX7's had them too?

 

For a low speed wing profile I'd be looking at various NASA profiles for a STOL type. 

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Tony D    143

Our LSR Effie showed 3mph in top speed reduction running the small BRE style spoiler versus 309# of lead shot in the spare tire well.

Before adding the lead shot we had one 2mph faster run, and then lost traction on our third run of the day and managed to half-spin at 140mph...pulling the chute to prevent anything more hairy and gaining the distinction to be one of only a handful of people who have managed to run over their own chute leads and break them while on-course...

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JMortensen    235

Without the fences I think you lose a lot of effectiveness with it laid flat. The wind tunnel testing results from the whale tail really surprised me. I wrote Simon McBeath at the time and asked what he thought was causing the downforce that was shown by the whale tail and how it could possibly be as efficient as it was, and that email got posted somewhere on this forum. He basically said he didn't know. I have the feeling - and it sounds like you do too - that the airflow around the back of the car and underneath it is significantly altered by the whale tail. Since the whale tail isn't allowed, I figured hanging that spoiler as far as possible out the back might get some of that effect. My intuition tells me that it's delaying the meeting of the airflow on top and above the car that causes the majority of the change. Since this one is on a hinge I can lay it flat if I wanted to, but I'm thinking that having it still operate at an angle will be more beneficial. Again, intuitively I'm thinking that a spoiler like this at a flatter angle with the wing on top might be the best combo that I can get out of this ruleset.

 

This guy's youtube channel has a bunch of tests with different scale models in water. He doesn't calculate downforce, he looks at the streamlines over the shapes, and uses very simple analyses to evaluate different shapes, spoilers, etc. Things like the circulation of bubbles show wake drag, the overall direction of the flow off the back of the car indicates lift or downforce, etc. When I first saw these I thought it was way too simplified but the more I looked the more I think he's giving good info, even if you can't translate it to XXX lbs downforce at XXX mph.

 

EDIT: Something is going wrong when I'm trying to link to this guy's channel or videos. Search Gray's Garage and you should fine it on youtube.

Edited by JMortensen

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tube80z    17

If it has to be flat then what about the bird bath style the first 280ZX racers used? RX7's had them too?

 

For a low speed wing profile I'd be looking at various NASA profiles for a STOL type. 

 

There are parts listed in the rule group that are excepted that are contoured.  Everything else needs to be a flat plate.

 

Cary

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