Front skirt redesign on The Chaparral 2J

[alj]

Hi All,

I have been heavily researching ground effects, I am in no way an expert unlike many on this forum, and I have come up with a conundrum. I see what Chaparral originally did with his front skirt of the 2J:



Notice the double skirt goes from left to right behind the front wheel (like a air/mud flap), then the main skirt behind that actually attempts to seal the underbody gap to the road.. This skirt sees the full frontal windage and, move up and down with the road surface.

What I am hoping is that I can make the front skirt more efficient by doing a 'V' shape instead of a straight skirt from left to right, but what I cant get past is how to effectively design a V without having a mass air gap where the two points meet. OR if some one else has a brilliant design so that the front wing removes the air from the skirt sufficiently so as not effect the skirt then that would be awesome also!

Perhaps the exhaust air from the twin downforce fans could blow forwards into the middle area, where the skirts meet? I'm not sure on the resistance calculations would be to cover the area (probably an area of roughly 100mm).

[bill shoe]

Anyone who is pondering Chaparral cars from West Texas has my attention. Have you seen the car in its Midland museum home, or somewhere else in person?

The Lotus 88 twin chassis concept might offer some useful advice. Suspending the skirts from the main chassis is probably not ideal. You may have better luck suspending them on their own "chassis", effectively mounting them to the wishbones (semi-unsprung) or corners (fully-unsprung) of the car instead of the main chassis mass.

There have been other practical sucker cars over the years. I think one car in the Grassroots Motorsports Challenge, and one autocross car, and a couple others like that-- what did they do?

Your V-concept has some merit. You worry about a gap where the V comes to a point, but there are always gaps or difficult areas like that-- your picture shows a corner gap between the front and side seal.

I wonder if you are being too conservative to think in terms of keeping the same basic seal system but just changing the shape.

What about a double-perimeter seal? This creates two zones of low pressure: 1. the main, inner zone, and 2. the zone in between the two perimeter skirts. You could suck each zone independently, maybe this would create more total suction under the car. Or maybe if you just suck the intermediate zone in between the two perimeter seals, this would create optimum overall downforce.

You still have issues of maintaining suction over severe bumps and chassis movement, that kind of thing. How can you make the system more robust/safe/confidence-inspiring for the driver? How do you make the suspension ride-height-compensating for different levels of downforce?-- So you can maintain relatively soft main suspension even when the car is making a gajillion pounds of downforce?

Again I think lots of great areas for improvement, maybe you are being too conservative. But easy for me to say since I'm just giving you more questions to answer.

[alj]

I would love to go and see the Chaparral cars in person. I've only heard of them travelling to the 'festival of speed' I haven't heard of them coming to Australia. I'd be the first through the door when/if they do!

I am opening my mind to many ideas, in fact it's hard to sleep. hahaha! But, I know I enjoy a topic when I find myself not sleeping because of it! My mind says 'it's all been done before', but I cant find too many in-depth discussions on it. I did see the 24hours lemons sucker car you are referring too, and I have read up on the 88 twin chassis design. I am wondering if I am trying too hard to get the design perfect. After all Chaparral himself didn't have it perfect and he blew the competition away!

Now that you mention it, going over bumps and changes in elevation of the road should, in fact. be the main focus. The local course I have has many changes in elevation and angles, which the skirt would have a reduced seal to the road surface.

I think a little trial and error is needed... in miniature model format. See if the effects of elevation change can be reduced by running longer or twin seals (or both). I need to keep my head in the practical application as well, alas I do not have superior funds for this project.

[NOT A TA]

Have you considered using a V shaped or U shaped air dam to direct flow (reducing pressure on the seal) in front of the front seals? Then front seal could go straight across the front attached to a low suspension point on each side reducing the leakage when cresting a hill.

Are there large run off areas on the sides of the courses you plan to run the car at? This is to me the biggest concern with a vacuum car. If you loose the vacuum due to a mechanical failure of some type while cornering (and exceeding mechanical grip) it becomes a very big problem with an instant effect. It wasn't a concern for the Grassroots Corvette Challenge car because it was built for a one time use parking lot auto-X.

[alj]

I will be running a front and rear wing, the front wing will redirect some of the air. A second U shaped air dam that redirects the air in front of the skirt is a great idea. I really have to get a model to test, I wonder if I could build an example on a RC car and test with confidence? seems lkike the only way to reproduce a rolling road with cost effectiveness..

My test circuit will have runnoff, the circuit I want to set the record on does not... logging on the test circuit will be paramount!

[bill shoe]

Interesting. I sent private message.

[bill shoe]

I found a relatively recent Jim Hall interview that I thought was interesting. Some relates to the 2J fan car.

http://www.caranddriver.com/features/wh ... y-jim-hall

Parts of the interview were surprising to me--

C/D: When you debuted the 2J “fan car” in 1970, did it  feel historic?

JH: The car wasn’t ready. It was hard to drive because of all that grip—you’d arrive at a corner and hit the brakes too early.    And the brakes were not adequate, and the car was pretty heavy. We should have taken that whole year to work out the 2J. It was complex. It was like having two cars stacked on top of each other—the vacuum system and the car system. It was hard to work on.

C/D: In your career, what would you have done differently?

JH: I would’ve slowed down the pace of our development, given myself time to make sure I had a reliable product. In ’66, when we ran the 2E, that dramatic moment in aerodynamics, it wasn’t ready. If  we had worked on it, we would have won almost every race. In ’66 and ’67, GM put a lot of pressure on me to run an alloy big-block. Then I had 11 engine failures in one season. So my career—our career at Chaparral, I should say—was marred by me jumping in too fast, trying to do things outside our capabilities in manpower and time.

So Jim Hall's innovations were big advancements for auto racing, but less successful in the smaller sphere of "what was good for Jim Hall's personal race team". Interesting. And his comments surely have to be interpreted as a warning shot to anyone thinking of making a fan car or any other radical innovation.

[alj]

I hadn't read that one, what an amazing interview. I'd love to buy the man dinner and chat over drinks (I dont play golf..hahaha), just to hear some pit crew and development stories.

Here are some of his others:

http://www.insideracingtechnology.com/jimhall.htm
"... Jim soon included them in his words and gestures and we were just a bunch of racers listening to a pair of our heroes telling stories." I can imagine this!

http://www.eas.caltech.edu/engenious/ten/alumni_hall

What I find amusing is people always quote how many times someone won or how many times they took a championship. Over the years I've seen some really talented drivers on my local course. None of them have won a championship, many of them with raw talent. Some recognised like Webber, some busy just enjoying life..

[NOT A TA]

I've been thinking about this project and the possibilities.

Building a scale model of your car with a scale sized vacuum system including seals etc. that would come close to simulating the actual car and producing a corresponding amount of suction sounds almost impossible. Then, trying to recreate scale model road features with elevation changes and getting any test results that could be trusted makes me think the scale model idea just isn't gonna fly. (Unless you reverse the fans! ahahaha)

Have you considered compartmentalizing the suctioned area and using multiple fans that are stand alone? This way certain fans could suck certain sections and if each fan was a stand alone deal it would be unlikely that more than one fan/compartment(s) would fail at the same time. That would reduce the possibility of losing all suction if the fan/compartment fails. By compartmentalizing you might be able to keep the car relatively stable in the event of a fan system failure by "balancing" which compartments each fan draws from.

If a multiple fan compartmentalized system was used the areas of the track where mechanical grip wouldn't be sufficient should a single fan fail would/could probably be limited to only the highest lateral G points on the course or the crest of a hill where the car could get light. This would reduce the odds of having an off due to a fan system failure.

[alj]

I've been thinking about this project and the possibilities.

Building a scale model of your car with a scale sized vacuum system including seals etc. that would come close to simulating the actual car and producing a corresponding amount of suction sounds almost impossible. Then, trying to recreate scale model road features with elevation changes and getting any test results that could be trusted makes me think the scale model idea just isn't gonna fly. (Unless you reverse the fans! ahahaha)

Have you considered compartmentalizing the suctioned area and using multiple fans that are stand alone? This way certain fans could suck certain sections and if each fan was a stand alone deal it would be unlikely that more than one fan/compartment(s) would fail at the same time. That would reduce the possibility of losing all suction if the fan/compartment fails. By compartmentalizing you might be able to keep the car relatively stable in the event of a fan system failure by "balancing" which compartments each fan draws from.

If a multiple fan compartmentalized system was used the areas of the track where mechanical grip wouldn't be sufficient should a single fan fail would/could probably be limited to only the highest lateral G points on the course or the crest of a hill where the car could get light. This would reduce the odds of having an off due to a fan system failure.

You made me laugh SO hard with that "reverse the fans!" comment! hahaha!

You are 100% correct, I think the correlation between a model and reality will be far removed at best.

My initial thought was to have twin fans one either side of the main chassis. This was dues to space and flow more than anything. I'm not sure that is what you are thinking. I wasn't going to divide the area underneath the car. Were you thinking divide it or have the second fan "make up' for the pressure loss if the one side skirt loses seal.

[NOT A TA]

Since you're thinking a fan on each side I'd consider splitting the suctioned area with a seal in the center running front to rear of the suctioned area. This way if there's a problem with a fan or seal you'd still have half the suction and wouldn't loose all of it on both front or both rear wheels. Since it's a single seat open wheel car with a narrow body even if you lost one side the other side is close enough to center that the down force would still be applied to all four wheels even though the functioning side would have more than the broken side.

I think it would be safer if compartmentalized. If a fan breaks with both fans sucking a common area the one fan functioning would be trying to overcome the losses of the full perimeter of the suctioned area, at that time it would most likely barely contribute to any down force. If a seal breaks with both fans sucking a common area you'll loose almost all down force immediately. Should a fan malfunction or a perimeter seal break or somehow malfunction if it's compartmentalized you would only loose 1/2 the down force and although the suction/down force would be off center a bit it would probably keep you from crashing and allow you to stop safely. Although you might need a new fire suit ahahaha.

My thinking is to try to avoid any sudden complete loss of suction and reduce the amount of time while on course that a malfunction could occur while exceeding mechanical traction. Splitting the suctioned area would probably reduce the risk of going 4 off only to the turns with the highest lateral g forces and only if a malfunction occurred right before or during the apex.