Undertrays and Old Cars

[MikeP]

I need some free advice...

I'm a newbie running a Swift DB-3 in the SW Division. Open FC class only, no CFC. I'm getting stomped by the newer cars on a regular basis and if I wasn't so damn cocky (just ask my in-laws) I might even get depressed...

Two questions...

I saw a post from "the frog" some time ago that correlated the CFC class performance versus the newer cars in the SE. It was something like 1.5 -2.5 seconds per mile that the older cars were giving up to the late models. Can you throw that number out again? It will either inspire me or depress me. I'm willing to take the risk...

My car has never had a diffuser of any kind. It is my feeling that this could substantially assist in my efforts to climb the ranks here in the SW. Does anyone sell them, build them, or should I make one myself? Aluminum or composite? Most of the new cars have composite trays, but I've seen aluminum trays as well. (As one who tends to do a bit of agricultural driving, aluminum seems like the way to go) What type of set up changes will be required by adding the diffuser? Do the CFC rules in the SE allow for a diffusor? (My folks live in Hotlanta so I'm toying with the idea of making the tow for a late summer race)

All advice is appreciated. Accurate advice is appreciated the most...

Mike P.

[Dave]

Well I'll be a newbie in an '84 Reynard starting in May. Rex also has a DB-3 so there will be at least 3 of us with old iron.

See you then.

[andyllc]

Hello
Yes, DB-3's can have difuser's and they are legal in CFC. I put one on my car, now Richard Matthews' car and I could tell a difference. It was not an Earth shattering difference but I would say it helped. I believe difuser's have a bigger difference on a new car than on an old car. New cars were designed with difusers while older cars weren't, so that is one reason why I think they make a bigger difference on a new car than an old one (I'm sorry, it's early and I don't know if that made sense or not but just go with me on it). With mine the car felt much more stable in medium to high-speed turns. I did not notice much of an effect in low-speed turns. That is not to say it didn't help there, just harder to notice. Chris Harrison made mine for me and I love it. It is made of a composite material. Chris Harrison owns Harrison Auto Dynamics in Phoenix, AZ. He is a really good guy to go to when you need something made. I do not have his number with me but I am sure someone on the forum does.

[size="1"][ April 01, 2002, 08:35 AM: Message edited by: bre86 ][/size]

[Mike B]

Mike,
We've made diffusers for Van Diemens, Reynards, and even a Lola S2. We have also had requests for DB-3s. If I can get some info on mounting, we can make one to fit just about any car. Our prices aren't too bad either

[BrooksHall]

Mike P I am waiting for one of Mike B's diffusers to arrive for my 89 Reynard. I plan on testing in late April or early May and I will let you know how the car feels.

[Adair]

I recently received a Gyrodynamics diffuser for my 88 Reynard. An outstanding piece! and the guys are a pleasure to work with.

(still haven't got the dif mounted yet... but working on it as we speak)

[BrooksHall]

Adair let me know of any tricks you come up with to help the install along.

[montfort]

I have an 88 also. Let me know how it goes, and if I ever get anything figured out I will let you know.

[mikeep]

A diffuser is one of the few aero devices that can work. However, there are lots of different designs out there and many only add drag.

We put an undertray/diffuser on our 1986 Crossle 63F and achieved two results - one was to change the balance of the car running through mid speed and high speed turns. The car seemed more balanced and more planted. In low speed corners - the value is low - wings don't do much until you hit 40 MPH or so. The second result was the ability to take the rear wing flap down from 40 degrees to 15 degrees - which lead to higher top speeds at the end of the two long straights at Waterford Hills. This lead me to the conclusion that that design had little or no impact on drag while adding downforce. The question is where does the down force come from...?

Our first diffuser was in a modified delta plan and made of aluminum by Averill Racing as an experiment. the diffuser ended right at the just behind the rear axles. The first one generated so much downforce that it ripped its mounting brackets out almost immediately. We guessed it was doing something... We then radically strengthened the mounts and it still distorted based upon pictures we shot. However, we were faster and more comfortable. I believe that this device is worth a second or more. Working in aluminum for our car made sense because there are not to many Crossle 63F's our there and we could then experiment for small $$. This year we had another diffuser made by Averill out of composites and re-engineered the mounting even more. We look forward to seeing/feeling the results. Hopefully, this plus a few other updates will even the playing field a bit.

If anyone is interested in pictures or more information etc - contact me at mikeep@concentric.net

David Keep
Crossle 63F
WHRRI #63

[bburke]

Assuming the diffusers work, will the springs in the rear require a higher rate? Info appreciated as my 84 Reynard currently has no diffuser. Is it a safe assumption the the previously mentioned diffuser for the 89 Reynard might fit an 84?

[mikeep]

Brian Burke,

A red and white 84 reynard...? The answer is yes we did incrase the spring rates as part of the whole package. However part of the reason for uprated springs was using Averills higher downforce front wings as well.

[Steve J.]

Dave,

To answer your question on where the downforce comes from, that's fairly easy: A reduction in the pressure below the car resluts in a normal (down) force because the relative pressure on the top of the car (body, sidebods, floor) is basically atmospheric.

The hard part is to efficiently move the air from under the car without drastically increasing drag. If designed properly, the diffuser will be the best drag to downforce component of your aerodynamic package. In other words, it's the best bang for the buck of any single componnet.

When I started looking at flow patterns and simulations for most of the more popular diffusers for sale out there, I was amazed at how poorly they were designed, and how little value they presented to the user. That was all before I made the first one for Mike.

[Richard Pare]

Well, you are at least partly right.

The air flowing across the top of the car is BELOW atmospheric pressure for 2 reasons: it is moving (relative to the car), which ALWAYS results in a reduction of pressure; and it is flowing up and over a curved surface, making it move even faster. This lower than atmospheric pressure is creating lift.

The air BELOW the car is moving even faster than the air above, and is creating an even lower pressure area, with the net gain ideally being downforce. ALL cars exhibit this - it's a function of the phenomena known as"ground effects" - but how well it does the job of creating net dowmforce depends on a ton of factors -exact proximity to the ground, rake, smoothness of the airflow, spillage in from the sides, etc, etc, etc.

A diffusers job is to allow the air that is emerging from the underside to slow back down to the same speed as the rest of the air flowing off of the body, AND do it in such a manner that turbulance behind the car is reduced.

The point of lowest pressure is at the "break" from the flat bottom to the slope of the diffuser roof, with the next lowest point being the at the front of the car where the air enters the underside.

Properly done, a diffuser will actually REDUCE the overall drag of the car (compared to a car without a diffuser).

For some further insight into diffusers, go to www.mulsannescorner.com

[Steve J.]

The air over the top of the car is below atmospheric to be sure. For discussions here, I wanted to keep it easy. I guess that was my fault.

The term 'ground effect' was coined in the early days of flight, and it referred to the floating pilots experienced when the wing was within one span of the ground. The air tends to 'pillow up' due to it's consrtictions, and additional lift is created. Dr. Lippish toyed with ground effect craft back in the '60's and found that the l/d ratio went up when certain conditions were met, so the craft could become very efficient. I'm in the process of building a one person Surface Effect Ship (ie a wing in ground effect) based on Dr. L's studies. It's been about 7 years since it first appeared on my CAD, but now it's actually starting to look like something. In my opinion, using the term 'ground effect' to talk about the accelerated flow over a vehicle body is a misuse of the term, but the same could be said for Army Intelligence. The Russians have several very large ground effect craft designed for high speed troop and logistics transport.

The real job of the diffuser is to accelerate the air under the bellypan, and make the pressure as low as possible. This is best done with laminar flow, not the quick break you see so often. The only reason the pressure (as measured by their pressure port) shows lower (localized) at the break is because of the seperation of the flow at that point. The fluid simulation programs show it best, but the model takes a long time to develop so it correlates to real world data.

The point of the diffuser merging the air to 'slow' it down to the same speed, I have to flat out disagree with. Years ago, there was a lot of misinformation on the flow at the rear of, and under the car, and even until recently, many high profile teams did not spend enough development time on the diffuser itself, and that shows in the market.

The diverging nature of a good diffuser lowers the pressure in the nozzle. The lower pressure in this area is what draws additional flow into the diffused section, and accelerates the flow under the car as the air from the bellypan tries to fill the low pressure area.

If you want to talk about the turbulent wake, it's going to be a long discussion, because behind the FC it's a real mess. There will be some turbulent wake reduction from a properly designed diffuser, but one that has flow seperation simply cannot contribute to turbulent wake reduction. Even with laminar flow out of the diffuser, there is still a lot of garbage hanging out in the wind (A-arms, push rods, roll hoop, etc) and each produces currents and eddies, adding to the problem.

[ehaiss]

Am I the only one lost here???? And I have a mechanical engineering degree! You guys are wayyyyyy to deep for us technical guys.

[BrooksHall]

Steve what exactly is a "quick break". Much is of this is over my head but I find it realy interesting. Also does the lack of diffuser on a FM make the air behind them so dirty? Following a FC I felt much less buffeting than compared to a FM.

[ennis bragg]

It seems to me that you would have to accept a higher level of exit turbulence in order to achieve any downforce. If you slow down the air underneath to match exit speeds, wouldn't you be increasing pressure underneath (at some point) and thus taking away downforce? Correct me if I'm wrong but this sounds like a classic case of design compromise.

[Bullitt]

Steve J,

Regarding correlating a model to real world data. Whose model and whose real world data? I am not aware of either existing for FC underbodies/diffusers. However, should you have a model proven to correlate with a current CART or F1 car, then I'm all ears and would like to talk about application to a new FC diffuser design. Items of particular interest include: sensitivity to side radii of the diffuser channels, interaction with rear tire wake, pumping the diffuser, and a rule of thumb for diffuser angle as related to underbody length forward of the diffuser, etc. (probably best to use e-mail/phone if you would like to talk).

Regarding the flow from a diffuser (I agree that it is supposed to speed up the underbody flow), it certainly is complex. One interesting real world example (ah sh*t) that I experienced racing in England was in Vauxhall Supersports. The team forgot to tell me to never follow a Phantom (car) closely through a high-speed corner. Apparently, what came out of their diffuser was so confused that a a following car's front wing (or even diffuser nose) could not function in it -- massive aero understeer which the guys behind me thought was hilarious both in impound and the track pub after the race ("welcome to the class, Yank"). All other car/diffuser designs could be tailed fairly closely, just not the Phantom.

As to the flow behind a FM being more disturbed than a FC (Brooks Hall Question), I believe it is more related to that barn door wing and its endplates rather than lack of a diffuser.

Bob Layman

[size="1"][ April 09, 2002, 04:50 PM: Message edited by: Bullitt ][/size]

[Aiken Racing]

I'm getting a headache.
Time to turn SPEED back on.
I just wish I was smart enough to understand what you guys are talking about.

[Purple Frog]

I'm keeping up... Don't get into private phone calls, Keep posting.

[montfort]

The buffeting you feel behind a FM is due to the swelling of the driver's head.

[BrooksHall]

WOW

[Purple Frog]

yeah...And I thought doing the sketches on a Hooters napkin was a step up from using chalk on the shop floor...

All this really is helping, because I'm building one for my 88 Reynard. Let me see... no more than 7 degrees at the break, extend at least back to the rear of the tires, don't worry about pumping it, the transition at the walls doesn't need to be rounded... see I am listening.

What about vortex generators just behind the break to maintain contact?

[Steve J.]

Man, I take a day off and I miss all the good stuff. I apologize in advance for the length of this post. At least there is no "per character" charge from Doug.

I don’t really know too much about the FM’s wake, but maybe I’ll wander over and take a gander the next time I see one at the track. One additional guess is that they have larger tires than an FC, true or not? About 1/3 of the total drag of your car comes from the tires, so that could be a big factor, but I’m sure the wing and back end has a lot to do with it too.

Brooks Hall: A quick break would be a sudden change in the angle of the surface. Usually, if the air is in laminar flow, a deviation of more than about 7 degrees will cause separation of the flow from the surface and turbulence results. Now, having said that, there are a lot of factors that influence the separation point, like if there is a radius at the break, or the surface roughness factor, etc. The turbulent zone is really an area of stagnated (and random) flow, and remember, the slower the flow the higher the pressure. You’ve seen the "spoilers" of the back of the NASCAR cars (my kids call them taxi cabs), have you ever wondered how they work? Here it is. If the air was moving quickly over the top of the body, the pressure is lower, tending to lift the car (like Richard said) and making the car more and more squirely the faster it goes. If we put a big plate (spoiler - the term is borrowed from aircraft) the flow over the trunk at a high angle (almost 90 degrees) to the flow, you can stagnate a big zone of air over the rear window and trunk. The slow flow is higher pressure than the fast air, so we eliminate some of the lift. Now the tradeoff is that there is a drag penalty to sticking that big piece of metal in the wind; but the lower the angle the less the drag. I have always wondered if the pressure in the cockpit of a NASCAR is more or less that that over the trunk. I’ll bet the cockpit is lower, and if that’s true there would be a bleed off of pressure (through the holes for the suspension adjustment) from the slow air over the window, decreasing the downforce. When I asked Spencer’s boys about it they had no answer for me; maybe that’s why they run where they do.

So what makes you go faster, more downforce or less drag? The answer lies in testing, but my money is on downforce for most (but not all, I’m sure) tracks. Remember at the F1 race in Indy when Eddie’s boys kept bringing the yellow car into the pits and then sending it back out? They were dialing in more and more downforce, and they were going faster and faster, so they has guessed wrong with the initial setup. Now Ennis, you call the cards correctly, almost everything is a compromise, but in the case of the FC, my opinion (and that’s all it is) is that downforce will get you bigger gains than the resulting drag penalty. If I remember correctly, the lap record for an FC at Elkhart Lake was run with all the downforce the car could muster. Somebody out there must know if that’s true or not. It really takes track time to learn how your car reacts to downforce increases, and see what works for your car at that particular track. Record and plot the data on a graph, you might be surprised at what you find.

Now, to make a short story long, I’ll consolidate years into a few paragraphs because Bob asks a good question, how do we correlate simulations to what we see on the track. My interest in all of this started many many years ago when I became interested in Lippish's work on the Surface Effect Ships. (I promise this will all come together) These craft exhibited reduced drag and increased lift when operating in ground effect. Since I had access to mainframe programming that could simulate flow, I began to play around with various conditions and sections just to see what was happening. I had no social life to speak of anyway.

While doing this 'work' I hooked up with some other guys doing some simulations for race car wings, correlating the results to a 1/12th scale model of an F1 car in their wind tunnel. I have to admit, their model was nothing like the awesome scale models you see today, but at the time it was great. We began to work together, but since all my wings faced "curvy side up" I had a lot to learn about all kinds of things. To answer Bob's question, I've never had personal unlimited access to a CART or F1 car, but I do have contacts at places that give me 'hints' without giving anything away. If you get a chance, go through the museum at Indy, and look at the evolution of each component over time. All the sims we did were to predict the behavior of the model when changes were made. Once we could predict what would happen, we knew we were representing real world (or model) situations. Now that I look back we were not radical enough with the computer models. I never even thought of trying a raised nose on the front of the car. I stayed within the box that was defined by the knowledge limits of the time. So, sorry Bob, but nothing current. But airflow hasn’t changed much, so some of those hints and secrets are still valid.

Now (Bob) to be a little specific, I have always had an issue with a radii at the profile to the outer fence of the diffuser, and that comes from aircraft design. Take a look at the wing root (or control surface) to fuselage intersection. Some are faired with a small radius, but most are at or near a right angle. There isn’t any separation problem in this area, so I’m not convinced a radius is needed. I’m waiting for a software package to be updated (to handle multiple elements) and then I may be able to run some sims (starting from scratch again) with a radius at the corners. I know F1 has a rather small radius, almost to the point of a sharp corner, but that is a very different ball game, with different rules.

Rear tire wake. If it weren’t for that rule that limits the width of the sidepods, FC would be a lot more interesting. The ‘turning vanes’ you see in CART or F1 at the front of the tires are basically barge boards to put the flow where they want it, almost forcing it into place. Remember that the tires in those series are very wide, so the stagnation area is large and the amount of air that needs to be moved is greater. With the tires in FC, and where they are located, it’s not pointless to try to ‘coax’ the air around the tire, but don’t bother doing it with a diffuser, you’ll need a board about 60% of the tire height to do anything substantial. Is it worth the drag? I should be able to tell you in a few months, as I’ve already started down that path. As for the flow at the rear of the tire, it is advantageous to end the diffuser "in the region" of the rear of the rear tire, since some of the flow there is lateral trying to fill the void left by the tire, which will have an influence on the flow at the end of the diffuser nozzle.

Diffuser angle to underbody length. The real point here is to move air under the car, basically as fast as you can, while also controlling the amount of air that is allowed to enter the underbody region. If you want to play with the angle of the diffuser, that will determine the amount of divergence you see and therefore the volume that the underbody air has to try to fill, which all adds up to how fast the air under the belly pan will move. There really isn’t anything to my knowledge that should specifically correlate these two, however the effective area of the diffuser is limited, but mostly due to the fact that air is sneaking in from the sides to the low pressure region. I would give it as much angle as you can, taking into account chassis clearance limitations, and don’t let the flow go turbulent if you can avoid it (tends to choke the flow). If you are making your own, keep the angle below seven degrees for the initial break.

Pumping the diffuser. If you mean running the exhaust into the diffuser, I have to strongly advise against it. When running flat out the exhaust velocity should be higher that the flow in the nozzle. However, when you let off the throttle for the corner, the opposite is true, and bad things happen. A really effective diffuser will augment the flow by taking advantage of the flow around the lower element. I don’t have that one worked out yet, but stay tuned.

Other stuff I don’t know if anybody is interested in. I did have a sim run of the new VD front wing as a function of height, just to see how much additional downforce was generated. I'm sure I still have the spreadsheet somewhere (since I never seem to throw anything out) and as I recall, the height did make a notable difference, but they also seemed to have a good compromise between the downforce, drag, and pitch sensitivity. Nobody wants to fly off the track because they lost the downforce they thought they had.

Back to "spoilers" or at least what most people think of when they use the term. If you have flown in a commercial airplane, and upon landing you see the big plates pivot out of the top of the wing? Those are spoilers, because they "spoil" the lift of the wing, and the pilot can brake harder without skidding, reducing his landing roll. It is similar to the NASCAR spoiler, but whereas they only stagnate the flow over the trunk, the spoiler on the wing creates a ‘high’ pressure zone in front of it due to the slow or stopped flow, and it also delivers only turbulent air to the rear of the wing so it cannot impart any downward momentum to the flow. The overall result is a nose down pitching moment and an overall increase in the normal force at the wheels. The big ‘spoiler’ (wing sections excluded) on that Honda that the neighbor kid drives? It does nothing, but don’t bother telling him that, just let him wax it over and over. And tell him the chromed coffee can on his exhaust looks 'fat.'

Bob, one more thing on the Phantom. It may have been the case where there was a lot of upward inertia imparted to the flow over the car due to the rear wing. You could have been caught (basically) in an updraft, throwing all your hard work to pot. Just a bit of speculation on my part. Next time you’re watching F1 and somebody (a Minardi) goes into the gravel, watch the tape over in slow motion, and pay attention to the upward velocity of the dusty air at the back of the car. It’s much more noticeable when the triple element wings are used, but whatever the case it’s neat to see the flow.

OK, now I HAVE to finish my CAD drawing before bed.

[Raj Nair]

Good stuff! I wonder if we should move this thread to the Aero Tech Forum?

[Bullitt]

Steve J,

Thanks for the interesting post/reply. Regarding radii, I am very curious about this, as although not in a tunnel, the wings of almost all the world speed record airplanes of the 1930's had very significant fairings (essentially large radii) at the wing/fuselage intersection. There is a blurb somewhere in the National Aerospace Museum (Smithsonian) that indicates the fairing was responsible for a big speed jump on one challenger (or maybe it was a P51 -- I'm getting old). Naturally, I wonder how this would apply to diffusers where the issue is laminar/turbulent flow and also more directly our FC front wings where that issue may play out mainly as drag. When I find more time, a search of old NACA(?) literature might turn up something interesting. If you already have done this, please consider providing a bibliography. (P.S. some of the fairings I saw at the Smithsonian looked like they were done by the proverbial "shade tree" mechanic, but. . . . . .)

Second point is that I could/should have been more specific about pumping the diffuser. I was thinking about the F1 practice (common 2 years ago, I believe) of introducing "free" air from between the body and rear tire into the rear of the central diffuser, not engine exhaust and its problems.

Diffuser angle relative to underbody length. About three years ago, I found a paper on this subject on the web. Unfortunately, the need to go to a meeting at my day job intervened so I signed off halfway through it and without copying it. When I tried to get it the next day, it was gone. Apparently, the author had been hired by a F1 team and pulled the paper and closed his website. At any rate he had done a lot of wind tunnel work and simulations. He found, as you know, that a lot of testing/tweaking is necessary to maximize downforce and minimize pitch and ride height sensitivity. But, the really interesting note was that he found a good correlation between underbody length in front of the diffuser and the angle of the diffuser for best results/starting point.

Regarding the updraft possibility, you probably are correct as it did seem that the Phantom had pretty good stick so the diffuser must have been working -- therefore, its flow couldn't have been too confused. BTW -- have you been at the US Grand Prix at Indy for the first cars at speed on the track. They suck the dust right out of the track and you get the most fantastic visual of rear/diffuser airflow courtesy the dust trail. Way cool.

Later,
Bob Layman

[ehaiss]

Steve J.,

What about using vertical walls longitudinally (sp?) in the diffusor (spaced every 6-8")? I can't remember the name of them, but I've been told they are a must have. My F500 does not have them, and I was considering adding them. Thanks for your input.

[Adair]

OK all of you Reynard fans....

A Gyrodynamics diffuser has successfully been installed on my '88 Reynard, with the generous help of Mike and Steve.

There was some cutting that had to be done to fit the piece around the bell housing, and, as the diffuser extends substantially farther aft than the original Reynard piece, slight modifications had to me made to get it to fit around the lower gearbox bosses. This process took about an hour to complete, and anyone who has carved a pumpkin could make the mods. As it seems all cars are different, others may have to make different modifications, but I wouldn't let that stop you. It is a cake walk.

The only other work that had to be done was to make angled extensions from the foward edges of the dif to the chassis and side pods. The Gyrodynamics dif is similar to a VD, and thus does not taper foward to the frame like a Reynard dif. No big deal though. Nothing a 2x2 sheet of oak and some flat black paint can't solve.

I will give you an update after we run the car at IRP April 19,20,21.

[Steve J.]

Vortex generators. Once upon a time I ran a sim of vortex generators verses a leading edge slat. I remember that the generators were sensitive to spacing and had to be positioned correctly along the upper (for aircraft) surface. I don’t recall the exact numbers or which "won out" but I do recall being very surprised at the results from the generators. One problem I see, though, is that the generator has to be in clean air flow (ahead of the break) to initiate the vortex effectively. That would mean it would have to stick down below the pan of the car, and I don’t think they would last very long. Don’t forget about the 1" deviation limit ahead of the rear axle line. I know of a number of homebuilt airplanes that significantly reduced the take off roll after the addition of the generators. The Osprey (not the military one) used them to help keep good air into the prop, since it was a pusher configuration with a pretty wide cockpit.

Now, extending to the rear of the rear tires may be an issue. Remember that on an FC the tires are pretty far away form the maximum allowable width of any body work (is it 95 cm?) or aerodynamic device. There will be spanwise flow in the region behind the rear tires, but it will diminish quickly with distance. I’d bet it’s a case where the trouble is more than the advantage it gives you. Test and learn, test and learn.

Now about the break. Stick to seven degrees yes, but remember that once the air is now following the angled path, you could pull it up another seven degrees, then another, and another. Get the picture? From what I have found, a properly chosen airfoil section works best. Be sure not to stall it though.

If you’re thinking of making your own, check with Mike B. @ Gyrodynamics. I know for a fact he has made a couple of diffusers for Renards, and I’m sure he’d be able to hook you up with the owners of those chassis if you want their opinions. If you want to send him an email: gyrodynamics@new.rr.com

Back to pumping the diffuser. Now that I’m on the right sheet of music, I did see that there was some work done to ‘link’ (if you will) the high velocity air flowing around the inside of the tire to the outlet of the diffuser. I believe the idea was to help drag more air out of the nozzle, and I think it is still used, so it must have some benefit. With the way things are situated on the FC, I don’t think it would be practical, but I’ve been wrong before. Maybe it could be done with barge boards, but it seems like the drag would be fairly substantial.

Interesting point about the ratio of body length to diffuser angle. I’ve never come across that thought before this discussion, so I’d be interested if someone found some info on it. Since the size of the diffuser is regulated for just about everybody, the investigation may lose some of it’s importance, but who knows. Anyone have the direct number to Woking?

[BrooksHall]

Adair I just recieved my diffuser from Mike B. and when I compared it to my old one the mounting was quite a bit different. The cut out on my old one mesures 22.5" wide at the front x 10.5" wide in the rear and 24" deep. How does that compare to yours and did you cut a simular opening in the Gyrodynamic diffuser? I you wouldn't mind sending me your phone number so I can call you I would like to compare mounting proceedures.

f2kracer@yahoo.com

[Barry Haynie]

I have back-fitted late model Van Diemen diffusers to Reynards and Van Diemens. I recommend going with the 98 or later design. I have an instructional article posted on my web site about fitting these to older cars. Check it out at www.formularacingservices.com

[Mike B]

Our diffuser's mounting is the same as the '98 and newer Van Diemen, so Barry's article is very helpful. It also describes some of the steps needed to install it on an older ('96 and earlier) Van Diemen, except you shouldn't need to modify the diffuser (I didn't.)

[Adair]

Michael (or anyone else interested),

Feel free to give me a call and we can discuss what mods I had to make. Basically anytime in the evening (except tonight) would be good for me.

317-295-1120.

Brandon

[BrooksHall]

Brandon I'll give you a call tomorrow night (Sunday). I took a look at the article and it all makes sense now my only problem seem to be where to get aircraft quality plywood.

[harshda]

Michael - you can find aircraft plywood and about anything else you might need from Aircraft Spruce and Specialty at Corona Airport. They have a website (www.aircraftspruce.com) and an extensive catalog that is free. Their phone no. is 1-877-4-SPRUCE.

Daryl Harsha

[Steve J.]

Ehaiss:

The fences you refer to can be helpful, depending on how the flow from the belly of the car is. On the FC cars I've looked at, there is an opening in the bell-housing for flywheel clearance. Not only does this allow air to bleed to the underside and curb some of your hard earned downforce, it introduces a lot of noise (turbulence) into the flow. To limit how wide the turbulent area can spread, a fence is needed to physically contain the vortex. You'll see them on aircraft wings, usually to control spanwise flow, or to help contain the turbulence from a stalled wing root, and keep it from spreading to the tip or vice versa.

Since the fences are so thin (no core to them) they don't hamper flow substantially. Just make sure they are smooth. I'm not really up on most of the 500 chassis, but I can guess that there would be a benefit to adding fences (parallel to the relative flow!) with little danger of any negative effects.

For FC cars, do not try to curve the fences like you may have seen on some out there. If you look at the flow trace from the dirt on the underside of a diffuser with curved vanes, you’ll see an area just behind the curved portion where the dirt just sits and spins. I think they tried to curve the vane to make it look like F1 or CART, but the FC (I'll have to look at a 500 someday) tires are too far away for that. All it does is add drag in that area, and tends to choke the flow. If the relative flow in that area was different, it would be a whole new ball game. Maybe Rock will let me study his 500 for a while.

Now, about using aircraft grade plywood on the Renard chassis extensions. Why? I use aircraft grade plywood on aircraft, but some BCX or CDX will work fine with some black spray paint. The pieces will be full of scrapes and dings by the end of the year, and you’ll probably make new ones, so longevity isn’t an issue. I’d save the money and head to Home Depot.

Steve