It is my position that the specific provision for diffusers in B.2. supersedes D.8.c.Originally Posted by Wren
It is my position that the specific provision for diffusers in B.2. supersedes D.8.c.
Stan Clayton
Stohr Cars
First off. My pet peeve, why in the hell would anybody read the FF rules to find out the legality of a diffuser in FC? or FB? queer.
I see Wren's point. (OMG, did i really type that?)
It says:
"in any horizontal section"
It does not say to measure only along the bottom.
If you cut a horizontal section out of Stan's diffuser two inches up from it's bottom, it does get wider as you move rearward of the axle centerline.
Good luck with that. FC rules specifically says that FC bodywork will meet the rules in D.8. D.8 draws a clear distinction between what rules apply to FC and FF and what rules only apply to FF. The rule against increasing width in a horizontal section is before the break between FF and FC rules. The paragraph even specifically references undertrays. A diffuser does not have to expand in width as well as height, it is still a diffuser if it expands only in height.
If you are right, you have figured out something that no other FC builder has ever figured out. I believe there is a reason that the diffusers you see on the cars now have a throat that stops before the rear axle centerline and then goes straight back without widening. Builders understood that the diffusers would be better if they could grow in width, they just didn't think it was legal.
I guess you are ok until someone protests it.
Maybe they just missed the date of the apocalypse by a couple of days?I see Wren's point. (OMG, did i really type that?)
Never really spent all that much time thinking about this particular rule, nor have I talked specifically about it with the original writers, but I would think that a reasonable interpretation would be that the rule only addresses increases in width from the axle centerline dimension, not a decease followed by an increase - as long as it did not exceed the axle centerline width.
In this particular case, Wren's interpretation might be correct - the phrase "in any horizontal section" can definitely be interpreted in the manner he wrote and Froggie agreed.
FC B.2 states nothing that supercedes that particular sentence in D.8.c. Diffusers, since they are an external panel licked by the airstream, are covered by the description of "Bodywork" as stated at the start of the Bodywork rules.
I have no clue as to how many cars might be in violation of this rule (IF that interpretation is valid).
Wren, Richard, and i agree on something... Scary.
I don't like that rule in FC, because I think it runs counter to the class philosophy. But, it is the rule as published on this date. And, I would have to rule accordingly.I would like to see it written so anything would be legal if it fit into a certain size "box".
I agree with this interpretation (obviously). Another sign of the end of days??
Therein lies the rub. The Pennon units would fall afoul of this interpretation for sure, though I am not sure of the stock Van Diemen unit off the top of my head. It would also seem to imply that diffuser size is essentially fixed aft of the rear axle centerline in all dimensions (if the diffuser height continues to grow, then the bodywork width at axle centerline is 0 - or whatever cladding goes 'round the gearbox - anything wider in that horizontal section aft of the axle centerline would be verboten).
Not to mention every single car with a rear wing assembly... those endplates sure do seem to be wider than the bodywork width at the axle centerline! Gurneys on your endplates?
Cheers,
Rennie
Are you sure that the pennon units would?
Linked from the Pennon website:
I don't see anything there that looks like it is increasing in width. What is illegal about that diffuser?
The stock VD diffuser looks similar.
Everyone understands that having the diffuser grow in both width and height would be better than just growing in height, they just knew there was a rule against it.
Why does it imply that diffuser size is fixed? The diffuser can grow in height, just not in width.It would also seem to imply that diffuser size is essentially fixed aft of the rear axle centerline in all dimensions (if the diffuser height continues to grow, then the bodywork width at axle centerline is 0 - or whatever cladding goes 'round the gearbox - anything wider in that horizontal section aft of the axle centerline would be verboten).
Interesting point. Feel free to protest the next person you see running gurneys in FC.Not to mention every single car with a rear wing assembly... those endplates sure do seem to be wider than the bodywork width at the axle centerline! Gurneys on your endplates?
It's a funky rule left over from a loooooong time ago, and probably needs to be addressed in light of current design realities.
Wren, Richard, and i agree on something... Scary.
I don't like that rule in FC, because I think it runs counter to the class philosophy. But, it is the rule as published on this date. And, I would have to rule accordingly.
It may be a funky, left over rule but changing it would change the game for almost anybody in FC that is trying hard.
A diffuser that grows in width and height is going to be better than one that just grows in height and is likely to do it with only an induced drag penalty (for the most part). That will be a real advantage and a lot of people will go buy new diffusers because of it.
Yes, I am quite sure. Add another dimension to your analysis, in light of the resulting horizontal section profile shape rendered by the stepped ceiling of the diffuser in your linked image. It is higher in the center - what will that do to the horizontal section profile? Draw it out.
No, it cannot grow in height past the axle centerline, because the width at the back end of the horizontal section will be wider than the width at the axle centerline. Again, draw it out.
You've missed my point, I'm afraid. The endplates themselves are wider than the engine cover bodywork, regardless of Gurneys.
Cheers,
Rennie
Here's an image of a diffuser that only "grows vertically" as Wren describes (similar to the Pennon, I believe).
And here's a horizontal section through that diffuser 50 mm above the bottom. Looks to me like the diffuser grows in width in that section.
Nathan
Last edited by nulrich; 01.06.15 at 4:16 PM.
That is interesting. I kind of doubt whoever penned that rule thought about it to the extent Rennie and Nathan have. I don't think most people would automatically "see" that without a lot of head scratching and Solidworksing.
"I see." said the blind man.
So they are all are illegal. How hard is it to run WGI without a diffuser? Like a FE...
I like the "fit in a box" idea for the diffuser/undertray rule in FC:
If i have a box; 100 cm deep, 95 cm wide, and 90 cm tall...
Then if your diffuser from the rear axle centerline back fits in that box, it is legal. If it doesn't fit, its illegal.
That is a rule I can enforce as a scrutineer.
I see what you are saying. I doubt that you would get very far with it. I remember something from the recent past about rules having intent.
Looking at Nathan's drawing, I disagree that it is growing in section, other than the fillet. There is a single width to the section view of the diffuser. If I draw a section on a piece of paper, only the "checkered" part (pink in Nathan's drawing) is part of the section, the paper is not part of the section view.
You won't have any trouble enforcing the existing rule either.That is a rule I can enforce as a scrutineer.
That slight curve at the corner of the turn-down for the side wall, when cut in horizontal section, creates a width change. Period. Illegal as written.
Only if there was a sharp 90 at that point would it section clean. I know it's picky, picky, picky. But you engineers drive us scrutineers in that direction.
I believe that at the time, diffuser undertrays were not thought about at all. Most likely they were thinking about DB1 type tails that got wider as you went backwards.
I'd say that "fit it in a box" is probably as good a wording as you are going to get.
Wren:
Nathan didn't draw in the vertical sides of the gearbox enclosure. If that horizontal section contained some of the gearbox enclosure forward of where it cuts through the diffuser, but did not also include the diffuser at that same point as the gearbox enclosure, then the bodywork is definitely "widening" in that horizontal section - unless you put a vertical fence out at the edge of the diffuser at that same point.
You are correct, thought, that if there is no bodywork that is being sectioned forward of the section through the diffuser, then there is no "widening" - you can't include something as being sectioned when it doesn't exist in that section.
Not sure if this literal an interpretation would hold up with the Stewards and COA in a protest, but it's an interesting exercise none the less.
True, the fillet makes it growing in width, but that is just on Nathan's example drawing he provided. That doesn't mean it is there on the actual diffusers. I have never looked at a VD diffuser for that kind of detail, so I admit that I don't know for sure. But, as a scrutineer you can still decide whether you accept the interpretation that disallows all diffusers in FC (where they are specifically allowed) or the interpretation that everyone else has used since they started putting diffusers on these cars.
I thought about that, but I didn't worry too much about it because almost no one has bodywork there anymore. If you look at Stan's original picture, there is no gearbox enclosure at all, so there isn't much of an issue there. That does seem to be the common setup these days.
Okay, you forced me to spend another five minutes in SolidWorks. Here is the same diffuser with some "bodywork" attached (yes, I know it's ugly, sorry).
Even without an engine cover or gearbox sides, and even if there was a sharp corner on the diffuser (which there isn't on any I've seen), the section increases in width. In the engineering world, a change in section from zero (in front of the diffuser) to a finite width is an increase.
By definition, there is no way to build a diffuser that does not increase in width in some horizontal section. Given that, I agree with Stan, the specific allowance for diffusers in the FC rules takes precedence over that section of the FF rules. But as we all know, my opinion does not always agree with that of the CoA.
Nathan
Last edited by nulrich; 01.06.15 at 4:16 PM.
No.
Note that the increase in width is as you go forward - as you go backwards, the radius causes the section to decrease in width - the rules reads as increasing in width as you go backwards from the axle centerline.
Still impressive for 5 minutes. I am solidworksless at the moment, so I can't do anything. I called my reseller this morning and they are supposed to fix it by tomorrow.
I completely disagree, you only get to look at what is actually in the section view, not on the piece of paper that the section view is drawn on. If the sides of the section view are parallel lines, then it absolutely does not increase in width. It is constant in width.
Even without an engine cover or gearbox sides, and even if there was a sharp corner on the diffuser (which there isn't on any I've seen), the section increases in width. In the engineering world, a change in section from zero (in front of the diffuser) to a finite width is an increase.
When I go to some old pictures I have of the radon VD diffuser, it looks compliant to the rules. Does your new car have a compliant diffuser on it?
We should all remember GCR 1.2.3 also. Trying to make the fillet on the outside of a diffuser the same thing as a diffuser that expands in width and height would certainly be "tortured."
Rennie-do you actually believe that your diffuser is compliant with GCR D.8.c?
There is absolutely no provision in the GCR for saying that something in the GCR is too hard to comply with or understand, so you get to ignore it.By definition, there is no way to build a diffuser that does not increase in width in some horizontal section. Given that, I agree with Stan, the specific allowance for diffusers in the FC rules takes precedence over that section of the FF rules. But as we all know, my opinion does not always agree with that of the CoA.
Not only that, Richard, but the Technical Glossary defines Bodywork as...
Since the (FC permitted) undertray is not part of the "bodywork", this sentence does not apply to FC.Body – All parts of the car licked by the air stream and situated above the belly / floor with exception of the roll bar or cage. For Formula and Sports Racing cars, further exceptions are those units definitely associated with the function of the engine or transmission.
Body Panel – A replaceable section of the body.
Bodywork – See Body.
Undertray (Belly Pan) – An attachment to the underside of a car intended to smooth airflow and/or to offer driver protection in this region of the car.
Stan Clayton
Stohr Cars
So now your position is that a paragraph that starts:
does not apply to the undertray?
Look, it is pretty obvious that you guys didn't catch this restriction when you built your diffuser. It's a shame, but it happens. Now you are looking for any way to torture the GCR in a way that would allow it. The diffuser you built will be completely legal for FB, just not for FC.
I am not sure how many more times something like this is going to happen until people realize that the first step in building parts/cars should be reading the GCR.
Um, Stan, the first sentence in the FF Bodywork section reads thusly:
D.8. Bodywork
For the purposes of this section, bodywork includes all panels external
to the chassis/frame and licked directly by the air stream. This includes
panels above or below the floor pan, and the bottoms of any side pods.
(my emphasis added)
This definition in the FF rules supercedes the general Glossary definition as per the Glossary introduction:
NOTE: Should any of the definitions contained in this Glossary appear to
be in conflict with a specific rule, then the GCR or Specification Book will
take precedence.
FC bodywork is specified as controlled by the FF rules as stated at the start of the both the FC and FF rules:
Start of the FC rules:
All newly constructed cars shall meet the 1986 construction rules for
Formula Ford cars as revised January 1, 2010, except as allowed in these
Formula Continental preparation rules.
Start of the FF rules:
"D. FORMULA F PREPARATION RULES
NOTE: Contained herein are the 1986 Formula F chassis construction
requirements (see D.6 and D.8)."
( The Club corrected the D.7 & D.8 reference at the start of the FF rules, but forgot about correcting the reference at the start of D.7. where it still refers to the pre-2010 GCR layout)
I think you can see why I've been trying to get the FF and FC rules fixed - it is too easy to miss some of this stuff.
Seems like another reason to disconnect the FF and FC rules. I did mine back in 2000 and didn't even think about reading the FF rules at the time. Idea came from the Mygale chassis. But I'd have to agree that since the FC rules define the diffuser much better than FF does, that the FC rule supersedes the FF side.
Just a minor correction to the neat CAD work by Nathan - the typical Van Diemen diffuser is a stepped affair, with the center section slightly raised compared to the outboard area. Attached are an isometric view, and a horizontal section of same.
Cheers,
Rennie
Rob,
I'm sorry, but the FC rules say to see the FF rules. There is no reason to not read the FF rules. I like that the rules are coupled together like this. It makes it much less likely that a FF rule would get updated and they would forget to update the same FC rule. It doesn't require someone to go out and buy a whole new chassis to change classes. There are a lot of people converting cars to FF that are glad the rules are the same. Disconnecting the rules doesn't do the members, constructors, suppliers, or prep shops any good.
BTW-your shock mounts should be in the mail this week
The FC rules don't define the diffuser at all. The only define the flat bottom of the car. The FC rules simply tell you that a diffuser is allowed and to go read the FF rules to find out what you are allowed to do with the diffuser.But I'd have to agree that since the FC rules define the diffuser much better than FF does, that the FC rule supersedes the FF side.
I guess you have something cool to take to the SOM and COA when you protest a VD diffuser.
There still is nothing stated in the FC bodywork rules that supercedes that particular sentence in the FF section, so that sentence still applied to FC as well. Sorry!
I agree there is no "must have" wing package. Aerodynamics is probably about one third as important as mechanical grip on these cars, so a good driver and a good setup will make more of a difference than any wing package.
I have a few minutes while waiting for something to finish, so I'll try to get some hard numbers for you. These values are from wind tunnel testing of an RF03 Van Diemen F2000 car. Of course, although it has boundary layer control, it's still a fixed floor wind tunnel, so it will not be absolutely accurate. I do think it's useful for comparison purposes.
It's very difficult to analyze wind tunnel data. You can't simply compare L/D, since it's easy to increase L/D by adding more downforce (formula cars have significant drag even trimmed out, so adding downforce will increase L/D up until the point you start stalling wings). The most useful visualization tool I've found is to plot lift against drag, but I'm not going to share all of our data! I did find some runs where drag numbers were comparable for the front wing packages we tested:
With the common Van Diemen parts most run, i.e. front aluminum wing and flaps, full width diffuser, aluminum rear lower wing, and two-element upper rear wing: C_D = .605, C_L = -1.002, -L/D = 1.66.
Same as above but with the Radon diffuser and Radon crush structure/wing mount and lower rear wing: C_D = .598, C_L = -1.028, -L/D = 1.72.
Then with a Radon front wing and flaps replacing the VD front wing assembly: C_D = .598, C_L = -1.115, -L/D = 1.86.
The Radon front wing was mounted 10 mm higher than the VD front wing. We have seen some pitch sensitivity with the Van Diemen, and our drivers (mostly masters) prefer a slightly higher front wing.
Including the upper rear wing, the Radon aero parts are consistently about 12.5% better than the next best parts we tested. This is actually slightly better than our CFD predictions. All the drivers who have tested them immediately notice the improvement. We lent a wing package to Justin Pritchard in 2009 and he finished second at the Runoffs. We've also won the masters championship in the F2000 series the last two years (Phil Lombardi in 2010, Tom Fatur in 2009).
We did not have a dihedral (moustache) front wing assembly at the last wind tunnel test, but from data I've studied, speed trap numbers, and observation at the track I believe that is the best setup (other than the Radon wings, of course!). This year in the F2000 pro series Remy Audette, Chris Livengood, and Jonathan Scarallo are all running the mustache wings, and they've consistently been the fastest in the field.
There were two other front wing main planes that we tested. One was slightly better than the "normal" aluminum VD front wing, although not as good as the Radon wing. The other was significantly worse. The Radon front wing profile is proprietary, and was developed about two years ago. Most wing profiles used in F2000 are at least a decade old, and many are based on "spec" profiles that aren't especially good (for example, the F3 mandated front wing profile does not perform very well).
If we were inclined to design more parts for the Van Diemen a new nose would probably be first. It would be a low nose, since the low front bulkhead of the Van Diemen chassis means the center section of the wing on a "raised" nose doesn't do anything. We could see that clearly with smoke visualization in the wind tunnel.
Nathan
Hi Nathan,
Thanks for sharing......very interesting data. Not many in this business are willing to put such information out for public consumption. A few questions:
Also, excellent point about the F3 airfoil sections. Although F3 cars are highly developed and quite fast, they use spec airfoil sections that are not necessarily ideal.
- How did you determine the reference area for your CL and CD calculations?
- Have you been able to compare your wind tunnel results to recorded on-track data? And if so, how did they compare?
- Was the pitch sensitivity you mentioned determined from driver reports or from data acquisition (or both)?
Hi Rick:
I measured the frontal area from the solid model I built of the Van Diemen. Of course, the wind tunnel instrumentation measures actual force, so the lift and drag coefficients are calculated.
We've seen similar results on track in a qualitative sense, but the best we can do quantitatively is to compare top speeds and speed in high speed corners. There are lots of other variables, of course, but we definitely see lap time improvements. We will do both wind tunnel and straight-line testing of the Rn.10. The test car has laser ride height sensors front and rear, as well as pushrod/pullrod load cells, and I think by adding a pitot tube and some aero sensors we can get generate good numbers, but I don't think it will be easy. I am looking forward to comparing CFD, wind tunnel, and straight line test results.
Pitch sensitivity is primarily as reported by experienced drivers, it's hard to see in the data. There is one front wing commonly used in FC which exhibits porpoising at certain ride heights. We did some CFD analysis of that profile, and it looks like the wing is actually oscillating in and out of stall! I think Rennie's concern about pitch sensitivity is well-founded, since I know for a fact that there are cars racing in the F2000 series that are very pitch sensitive (I've seen the evidence). I'm guessing the drivers are either good enough to compensate or unaware.
We chose the Radon front wing profile, in part, because it had the most driver-friendly characteristics in ground effect (and we've never seen pitch sensitivity on track). We have another front wing design that has better performance, but it would be significantly more expensive to tool and produce, and is more sensitive to ride height.
Nathan
Hi Nathan,
I am envious of the instrumentation on your car! With laser ride height sensors and load cells, it should be possible to develop a reasonably accurate C_L vs. ride height aero map for your car. I don't know if you had time to make wind tunnel runs at various ride heights, but if so, it would be very interesting to compare the data.
I suspect that it will be more difficult to obtain a reliable C_D map from track data. I have recently been searching for a practical way to do this, and think that I would probably use the method recommended by Danny Nowlan in his RCE articles and his new book (wherein the sum of the drag forces and of the thrust forces from the driven wheels is assumed to equal the vehicle momentum rate of change). This will only be as accurate as the thrust force estimate, but may allow for useful comparisons of the relative change in total vehicle drag vs. ride height. In other words, even though the measured C_D may not be 100% accurate, the variation of C_D with ride height might be mapped quite well. Comparing such data to wind tunnel results would be interesting, especially when a fixed floor tunnel was used.
I do not doubt you (or Rennie) regarding front wing pitch sensitivity. I was just curious if you could see this in the data, or if you were relying upon driver feedback. Does this behavior normally occur only under braking when front ride height is at a minimum?
I'm a little skeptical of Danny Nowlan. It is possible, with enough manipulation, to create a mathematical model to fit any observed data, but that doesn't mean the model is correct or that it can be generalized. That said, I believe we can get fairly accurate drag numbers using straight line aero testing.
Of course, at the end of the day, the numbers don't have to be absolutely accurate, they just have to help in setting up the car.
I have seen pitch sensitivity in two instances in F2000 data. One situation involved noticeable porpoising at the end of a long straightaway (consistently from lap to lap), and less noticeable pitch sensitivity under braking. The driver definitely noticed it, and it made him uncomfortable, but he wasn't sure what caused it. The other was only under braking. This was before we introduced our wing package and involved another (common) front wing used in F2000.
I have observed several cars from trackside that I would describe as pitch sensitive under braking. As I said earlier, I'm not sure if the drivers knew what was going on, but if I could see it, it can't be comfortable for them!
Nathan
Hmmm........skeptical of Nowlan's method of logging the aero data, or skeptical of his simulation and modeling methods (e.g. ChassisSim)? I have never used ChassisSim, but the techniques Nowlan recommends for logging the on-track aero data seem pretty reasonable to me.
Last edited by Rick Ross; 05.25.11 at 3:28 PM. Reason: grammar
Skeptical of Danny Nowlan's claims and of ChassisSim. I've slogged through several of his articles in Racecar Engineering, in many cases deriving the equations he uses on my own to validate, and I found some inconsistencies. I may be too hard on him, he seems like a good guy, but I remain cautious.
We started to evaluate ChassisSim, but a full-blown version that would be useful for design and development was just too expensive.
AeroLap, on the other hand, works quite well:
http://www.ansibledesign.com/aerolap.htm
I'd like to be able to afford it at some point!
Nathan
I understand that having a diffuser increase in vertical height takes the compressed, fast moving air and slows it down which creates a low pressure area that sucks the car down.
What effect does having the diffuser also widen at the rear create? Seems like you've already created the low pressure area. I also don't understand how going from wide to narrow back to wide would help. Low pressure- high pressure- low pressure??
Lastly, from a non-engineer's perspective, the Clayton's diffuser appears to get wider while the others pictured do not appear to widen. I'm not sure what everyone is looking at in the cross-section. I just don't see it widening.
Sean O'Connell
1996 RF96 FC
1996 RF96 FB
2004 Mygale SJ04 Zetec
Thanks, Nathan. I have not yet had the chance to play around with ChassisSim (or AeroLap), so I can not really comment on their accuracy or usefulness. Both are relatively expensive products, so I can understand why a certain degree of skepticism from a potential buyer might be in order. For now I will keep an open mind about the subject.
I will say that Nowlan's suggestions regarding how to log aero data make sense to me. As I understand his articles, he is essentially saying that if you can accurately log ride heights, aero forces, and dynamic pressure....then you should be able to collect the raw data necessary to create maps of C_L, C_D, and aero balance as a function of ride height. The absolute numbers may not be 100% accurate due to some of the assumptions required, but the variation of the parameters with ride height should be obtainable.
The real challenge probably lies in deciding how to filter and process the data into an accurate and useful product. I know that ChassisSim has an aero mapping function, and perhaps AeroLap does too. I also suspect that Matlab might be able to handle this task pretty well.
Race Car Aerodynamics, Designing for Speed, Joseph Katz
Sean,
I'd read Katz for basic underbody Pressure, Velocity and Diffuser Area airflow relationships.
I don't recall Katz mentioning anything, beneficial or not, about necking down a diffuser at the rear axle.
Rick
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