noob question about ride height

[Duke]

Well, I'm trying to search and see if I can find something that specifically says static ride height does not affect static weight distribution. I've found numerous articles saying that you can adjust static weight distribution with independent ride height adjustments (IE cornerweighting the car via a coilover system) but that method is out of our reach in GT4.

However, I did find a pretty good series of article on handling and suspension tuning. These may help anybody who needs more info on the concepts.

 

[rk]

...but hopefully we are not mincing semantics...

 

[KurtisGSXR]

I'm trying to remember, and I hope I've got my statics and equilibrium equations right...

Also I've never attached a pic, so I don't know where it will be visible.

First, we assume the car is static, and that it's mass in concentrated about a single point, mg. We assume that the wheels are the only two contact points at the ground, and have reaction forces R1 and R2. Also assume that there s no friction, and the contact is a point contact under the axle. All this is to simplify and generalise the system.

For calculation ease, and lack of trig, assume the car's wheelbase is 8 feet long, and the CoG at mg is 3 feet vertically above the line drawn between the wheels, giving us hypotenuses of 5 feet and an included angle of 36.87 degrees.

On the left is the horizontal calculation. The first equation is the vertical force equilibrium, showing that the grav force, mg, is equalled by the reaction forces (has to be or the car would float away or bury itself in the ground). The second equation takes moments (or torques) about the point 1 at the front wheel, as R1 acts through point 1, it can apply no torque and is eliminated. The rest of the derivation shows that the Reactions are equal and are half mg each. As expected.

The right hand side shows the car at 36.87 degrees (for easy numbers) And follows the same calculation route of equalising vertical forces, then taking moments about the point of the front axle. The final figure shows that the front reaction force is only 28% of that at the rear. This would change depending on angle of the ground, wheelbase and most importantly the height of the C of G.

This can be related to a car in roll going round a corner and weighting the outside wheels, although I think a roll angle of about 38 degrees means you've already lost it. Unless you drive an old range rover. Or a Volvo.

You could do the calc again, using a more complex geometrical drawing and frictional forces, but this is a simple set of calcs to give a general idea. The real effect of a slope of around 3-5 degrees and a low C of G of a race car would be very small. Indeed it would be zero if the C of G were in line with the contact points.

cheers,


Kurtis.

P.S. It's been a while since I've done this. So if I missed something I apologise.

 

[Duke]

Hi, Kurtis. I was wondering if you'd make it into the virtual engineering world here with us laymen.

So if I follow, what you're saying that ride height does affect static weight distribution? In other words, assuming that the initial balance is 50/50, raking the car fore or aft will bias the weight distribution in that direction?

If so, I owe doormeister an apology and myself some more study time.

 

[KurtisGSXR]

Hi Duke,

As far as I can see, it will, but only by changing the position of the C of G. In the extreme case in the diagram, it moves a lot, but with a racing car with a very low C of G it will be barely discernible. The ratio of CoG height to wheelbase and the shallow road angles mean it's a very minor factor.

I'm reasonably happy with the calc I did but I made a LOT of assumptions. I think it shows the correct basic tendency, but I have a feeling there may be frictional effects that can have an impact.

The way I was thinking about it when I was looking at it, was when I ride my BMX up a hill when I'm half cut on the way back from the pub it's really easy to wheelie as the weight is distributed to the back. I can testify to this as I commonly fall off. But it doesn't hurt when you've got your beer jacket on...

I ask for anyone who reads this to have a look at the diagram and offer an opinion, in case I'm wrong.

thanks,


Kurtis.

 

[rk]

Kurtis, thank you for your well reasoned and carefully explained diagram, I have said before and will re-iterate that it is the community of participants that makes GT Planet strong. Let me make it clear that I would gladly be found wrong in the name of discovery; that said, I would like to comment on your equations.

The first thing I notice is that your diagram shows the "test" car tilted, as if on a hillside, when in actuality, you should have (according to the problem discussed) placed a leg vertically under point 1. In keeping with your figures, the leg would be just slightly over 4.8 feet. However, the results, based on your calculations, appear to be unchanged. The new hypotenuse would be approx. 6.93 feet and the new wheelbase is 6.4 feet. Look familiar? 6.4 is almost exactly 8 times the cosine of 36.87, the rake angle of our test vehicle, which formerly was depicted as the tilt angle. Bottom line is that R1 still works out to be .28R2; not looking like good proof for we flat Earth theorists; nonetheless, I still have the nagging suspicion I am missing something...

Another point is that you used the illustration of a Volvo weighting the outside wheels while turning and I would have to believe that the chassis is trying to travel tangentally from any part of the turn the wheels are negotiating ( I think of the picture of a rock rotating on a string when the string is cut). By virtue of the suspension, The chassis being accelerated into the turn, hence the tilt. The force vectors are probably the same, but (for me) the illustration does not easily lend itself to one of static weight distribution, since the amount of roll can be so easily changed by adjusting the steering wheel.

I still have the gut feeling that the above calculation using the hypotenuse value of 5 as the horizontal distance does not apply until it becomes a horizontal displacement, i.e. the car is accelerating, but for now I will chalk the feeling up as stubborness.

 

[MADTJ]

ok, i'm somewhat confuse... but let me get this right, the reason is putting back heigh a little higher than front... because of downforce...

Like for ex: Minolta Toyota that have back downforce greater than front.. the back ride height is about 20 mm higher than front

while going high speed on Le Mans or nurb or any high speed track.. the downforce push the back down harder than the front, which cause both front and back ride height is same???

and SORRY For my english!!!!

 

[rk]

while going high speed on Le Mans or nurb or any high speed track.. the downforce push the back down harder than the front, which cause both front and back ride height is same???

The front and sides of the cars skirting forms a realtively tight seal to the road surface. The raked bottom of the car creates a partial vacuum, which makes downforce. In LMP style cars The rake is adjusted in increments of about 1/10 inch to control the level of vacuum (and presumably clearance).

 

[Ezz777]

just a little summary from what i can gather. that in the situation where the CoG lies on the same horizontal plane as the two points of contact - then the CoG remains consistent relative to the position of the contact points due to the shortening of the wheelbase. Thanks to RK for that side of things.

BUT - in the situation where the CoG is above (or below for that matter) the plane of contact with the wheels (blocks of wood or piano carrying men/pygmies/whatever), then there IS a shift in CoG towards one end DESPITE the shortening of the wheelbase. Thanks to the KurtisGSXR for that side - which seems more relevant to a car. Yet as he says - the lower the CoG (as in a race car) the less effect a change in height will have on the shift in CoG.

More important points to ponder - given the location of this forum - is how is this modeled in the GT4 physics engine?? I personally doubt that it is at all - hence this is all very interesting, yet a moot point in terms of the game. Has someone with way too much time on their hands tried to test this somehow?

While on the subject of 'things that may well be true in R/L that i doubt are modelled in GT4', has anyone managed to ascertain whether the rake - ground force - chassis downforce - whatever you want to call it - actually affects D/F in GT4. As i said - i suspect NOT - but someone may have proved otherwise.

 

[rk]

just a little summary from what i can gather. that in the situation where the CoG lies on the same horizontal plane as the two points of contact - then the CoG remains consistent relative to the position of the contact points due to the shortening of the wheelbase. Thanks to RK for that side of things.

BUT - in the situation where the CoG is above (or below for that matter) the plane of contact with the wheels (blocks of wood or piano carrying men/pygmies/whatever), then there IS a shift in CoG towards one end DESPITE the shortening of the wheelbase. Thanks to the KurtisGSXR for that side - which seems more relevant to a car. Yet as he says - the lower the CoG (as in a race car) the less effect a change in height will have on the shift in CoG.

More important points to ponder - given the location of this forum - is how is this modeled in the GT4 physics engine?? I personally doubt that it is at all - hence this is all very interesting, yet a moot point in terms of the game. Has someone with way too much time on their hands tried to test this somehow?

While on the subject of 'things that may well be true in R/L that i doubt are modelled in GT4', has anyone managed to ascertain whether the rake - ground force - chassis downforce - whatever you want to call it - actually affects D/F in GT4. As i said - i suspect NOT - but someone may have proved otherwise.

Actually, I think most will agree whether the shift occurs or not is moot. The real question is whether this shift changes the percentage of weight each end bears. Kurtis has presented a very convincing argument to explain that it does. I still cannot think of a way you could weight a tripod, given the limitation that the weight must remain within the perimeter of a line drawn between any two legs, that such weighting actually unweights a third, or in the extreme case, balances the tripod on two legs. Forgive my obstinance, but my mind rebels as the notion seems absurd; I would be relieved if someone could explain in what way this illustration does not apply to the question at hand.

As to the question of ground effect downforce and whether it is simulated in game, there have been many posts discussing it, as you might see upon running a search. Many members apply the concept to their posted tunes, however, I don't know of any proof of its veracity and a simple test I had devised actually proved inconclusive.

 

[MADTJ]

is anyone can test this one out? it's just my option, and i know it's stupid but let try...

grab some fast car (minolita, totyota gt1, etc) put back ride height by 50 mm up or highest, and soft the spring rate...

while go slow speed coner, the back is all the way up which creat more downforce for turn, but while go stright (nurb, le man, test course) the back is down about same ride height as front.. which less downforce.. less dragging.....

nah, i don't think it's will work

 

[Ezz777]

I still cannot think of a way you could weight a tripod, given the limitation that the weight must remain within the perimeter of a line drawn between any two legs, that such weighting actually unweights a third, or in the extreme case, balances the tripod on two legs.

Imagine the weight not actually being just a point on the top of your tripod - but instead a very tall weight - ie so that the weight's CoG is significantly above the point at which it contacts the tripod. Now imagine tilting (by making a leg longer - or adding a few pygmies) your tripod so that the weight's CoG is outside the perimeter of the line drawn between any two legs. What will happen???

Just remember that while most people conceptualize weight distribution as merely being a point on the plane between the two wheels - the truth is that it is a vertical translation of the CoG onto that plane between the front and back. I think you were limiting your arguments to a 2D plane.

As to the question of ground effect downforce and whether it is simulated in game, there have been many posts discussing it, as you might see upon running a search. Many members apply the concept to their posted tunes, however, I don't know of any proof of its veracity and a simple test I had devised actually proved inconclusive.

I'm well aware of the concept continually showing up in posts - but i am more interested in whether GT4 actaully models it. I suspect the main benefit of raising a car's bum is to prevent it from bottoming out on the bumpier bits - hence giving a less jittery ride - but not necessarily increasing downforce. I suspect GT4 models downforce as merely a function of the front and back values.

 

[rk]

Imagine the weight not actually being just a point on the top of your tripod - but instead a very tall weight - ie so that the weight's CoG is significantly above the point at which it contacts the tripod. Now imagine tilting (by making a leg longer - or adding a few pygmies) your tripod so that the weight's CoG is outside the perimeter of the line drawn between any two legs. What will happen???


I'm well aware of the concept continually showing up in posts - but i am more interested in whether GT4 actaully models it. I suspect the main benefit of raising a car's bum is to prevent it from bottoming out on the bumpier bits - hence giving a less jittery ride - but not necessarily increasing downforce. I suspect GT4 models downforce as merely a function of the front and back values.

Well, the "rule" is that the weight must stay within said perimeter. How often can one jack or rake a car such that the cg actually even APPROACHES a similar line, but the point is moot as the illustration does not fit the situation, read on...

As for anyone's "suspicions" about chassis downforce, I have built up a strong body of empirical evidence (that means I have tested and tested and then tested some more) and it is the only way I could get cars like the Lotus Elan to drive faster than 125mph, but forget all that hard work; consider the circumstantial evidence: if one accepts that default (or midrange) ride height is optimal minimum at bumpy tracks like Nürburgring, then one may begin to see a pattern emerge. The F1 has exactly 10mm left for rear raking, ALL LMP cars have EXACTLY 25mm left for chassis rake, most if not all JGTC type cars have 30mm left, etc., etc.
Whether chassis downforce works, it seems like Polyphony went to some effort to allow provision for it.

So, as to what does fit the situation, what else? Empirical evidence, of course and I would have provided images, but decided most would accept my proof as it pretty much soundly refutes my previous assertions. Before you break out the lol smilies, consider I spent $20 in postal scales and will have to spend this Fourth of July weekend working to make up the time spent proving myself wrong, crow is not so bad unless you avoid meat like myself.

Well, enough about me, let's get to the procedure. I took Kurtis' 8 foot board and attached a 4.8 foot board at a right angle to one end. I then attached one of 3 small loops of wire to each "corner". From the ceiling of our warehouse I hung two more wires and affixed a Pelouze swing style postal scale to each. I had to drill the ends of the scales and hang 6" crescent wrenches from them to bring the scale under the needle(s) because of the weight of the boards, so they measure in polyphonygrams instead of grams, no big loss there. I first hung the board arrangement with the long leg dangling below the plane of the 8 footer, it seemed appropriate and I didn't want to add the "jacking post" because I felt by weight alone it would throw the experement. I managed to record a value of 1 (I think it was the ounce scale). the other side was, I think 3/4, but it didn't matter and I could not read the same scale in the next phase because I had to steady the right angle end (that was now hung by the loop at the end of the 4.8 foot board with the 8 footer raked at an approximation of 37 degrees) to get a reading, which shakily, but clearly, found the needle at 1/2 The "heavy" end had magically lost half its original reading just by becoming inverted, and that pretty much sums it up, but at least I got an answer to my first question: "Where do you people get this" would be some repository of sensibility I apparently have no access to.

Well, I would gratefully enjoy Kurtis or some other genius coming along and proving me wrong, err, right, but for now I will get back to something I am at least mediocre at and have a crack at Panjandrum's GT.

 

[eforer]

Does anyone know definitively how the GT physics model "raises" the ride height? Is it increasing the pre-load on the springs, changing the downstops? Is it changing the pickup points on the suspension? Are the springs on the model preloaded when the suspension is at full droop? Without knowing these facts, we can't really come to a conclusion as to what the benefits in the game are beyond seat of the pants evaluation and testing. I can tell you this, if you put a real race car on scales to set the tweak, weight jacking whatever you want to call it. Preloading one half the car or one spring will change the value on the other scales. Trust me I along with many others do it as a routine part of chassis setup.

 

[rk]

Does anyone know definitively how the GT physics model "raises" the ride height? Is it increasing the pre-load on the springs, changing the downstops? Is it changing the pickup points on the suspension? Are the springs on the model preloaded when the suspension is at full droop? Without knowing these facts, we can't really come to a conclusion as to what the benefits in the game are beyond seat of the pants evaluation and testing. I can tell you this, if you put a real race car on scales to set the tweak, weight jacking whatever you want to call it. Preloading one half the car or one spring will change the value on the other scales. Trust me I along with many others do it as a routine part of chassis setup.

All springs must be pre-loaded to some extent or they will bounce out of their seats upon unweighting. The main effect of pre-load adjustment is to change the portion of the useable range which is reserved for initial re-bound (what happens when the road surface suddenly drops away from the tire, like a pothole) and the main reason is to compensate for changes in sprung weight, think of a passenger on the pillion of a Kawasaki. The fact that such adjustment alters ride height is a secondary side effect, and, to my knowlege, is only employed to such end, for example, by using aftermarket spring clips or spacers, both of which eat up useable suspension range - the clips can actually cause you to drop a coil.
You may have other fully valid reasons for adjusting pre-load, for example it is not easy to move the pivot points, so you compromise and get more travel "under" the midrange in exchange (in the event you are increasing), but it is not very likely it is how ride height adjustments are modeled in-game.
I used to roadrace motorcycles and had much fun learning how useless it was to jack the collars on my Koni's (my best were Ohlïns which had 200psl nitrogen bladders and NO pre-load adjustment) to try and stiffen my suspension. Sure it worked, but...there's always a compromise, it seems.

 

[Duke]

I can tell you this, if you put a real race car on scales to set the tweak, weight jacking whatever you want to call it. Preloading one half the car or one spring will change the value on the other scales. Trust me I along with many others do it as a routine part of chassis setup.

Eforer:

You're 110% correct that weight can be redistributed around the car by corner jacking. Raising the ride height of the left rear wheel WILL increase the load on the right front wheel, etc.

But we can't do that in GT4. The big question is: if we raise the ride height of BOTH rear wheels the same amount, will it increase the load on BOTH front wheels.

My stance is NO. Corner jacking works because the diagonal wheels that you AREN'T adjusting (ie, the right rear and left front wheel) form a fulcrum across which you can tip the load. But if you're raising both the rear wheels (or, for the sake of argument, both wheels on one side) there is no geometrical fulcrum generated and the car simply rotates without changing the weight distribution.

Theoretically, if the CG is high enough above the axis of pitch, a small amount of weight shift might occur, but I maintain that this is negligible under any real circumstances and is in fact probably smaller than the margin of error of the scales themselves.

I used to be friends with a racer and chassis engineer for a major automobile company. I'm going to see if I can get a definitive answer on this. Great discussion, guys!

 

[doormeister]

O.k. I'm going to throw out another idea that I take for granted, but I haven't seen discussed . Cool?

Since I haven't gotten to the point of racing the formula cars, many of the street legal cars I've driven have been top heavy. And this is where we may be agreeing that there is more of a weight shift, though of an argueable amount, in cars with CoG significantly above the axles.

I really think that in the instant of take-off, the important point is the contact patch of the rear tires (rear drive example only...). Saying so (go with me for a sec on this) means that the least shift in CoG occurs when the CoG is about at tarmac level, and increases the farther away from the tarmac the CoG is.

Now imagine that car drawing you've seen with the CoG located a couple of feet off the ground and somewhere between the wheels. There is a force arrow drawn straight down from the CoG when the car is at rest. This is the force of mass x acceleration(gravity). Now when the car is accelerated, the acceleration will shift the direction of the force toward the rear of the car. You can imagine that under light acceleration the arrow would rotate a little, and under heavy acceleration, it could rotate a lot. To check my self, I often take a concept to the extreme: infinite acceleration would point the force arrow straight back, and be of infinite length.

So I'm thinking that at the green light, and when the tires are first starting to grab, I want that combined force to be intersecting the tarmac at or as close to the leading edge of the tire contact patch. If it gets to the rear (center?) of the tire contact patch, then the car will wheelie. Placing it on the contact patch just before wheelie condition provides the most traction.

So if you and I race a 400m with our high CoG family FR sedans, and you tilt your car forward, and I tilt my car all the way back, I'm figuring that under the first instant of acceleration, I'm starting with my mass x acceleration force intersecting the pavement maybe 4" or 5" closer to the tire contact point than you are. Yea, I'm guessing at the numbers, but they are not unrealistic, I think. If both cars rotate back at the same rate, roughly, then my final position will put my 'intersection' about the same distance (4" or 5"?) closer to the contact patch of the tire. So I started out closer and ended up closer to the contact patch.

Seeing as how the original posters question was answered correctly with a response about down-force and ground-effect, I can see where I threw things off a bit. Thanks for the messages.

 

[rk]

So if you and I race a 400m with our high CoG family FR sedans, and you tilt your car forward, and I tilt my car all the way back, I'm figuring that under the first instant of acceleration, I'm starting with my mass x acceleration force intersecting the pavement maybe 4" or 5" closer to the tire contact point than you are. Yea, I'm guessing at the numbers, but they are not unrealistic, I think. If both cars rotate back at the same rate, roughly, then my final position will put my 'intersection' about the same distance (4" or 5"?) closer to the contact patch of the tire. So I started out closer and ended up closer to the contact patch.

Very, VERY interesting, my friend (and supervisor) is pulling low 1'14's at Monaco (Côte d'Azur) in his F1 and he SWEARS by his reverse rake, he completely discounts my downforce theory and says traction is improved by raising the front height more than the rear. Now I know that it would qualify as one of the "low" cg cars, yet I, nor anyone else in our circle, can touch him...yet.

 

[Wild Cobra Z28]

Wow... you guys are getting a bit serious here. Well, I have enough experience with tuning to know these minor changes can make a huge difference.

First of all, a slight change in CG (center of gravity) may seem insignificant. It may howevermake a 5-10MPH difference in maximum cornering speed. The rake of the car not only makes minor brake balance and acceleration differences, but acts like a spoiler for cars that don't have one.

Now consider this for brake balance. We can use the balance controller to apply front and rear brakes differently. Normally, cars are set high enough that the front does most the braking. For racing, we normally lower the cars stance. We do this so the CG is lower, allowing us to corner faster. It effects braking the same way. For cornering, instead of nearly all the force being on the outside tires, more is kept on the inside tires, allowing more grip. This can be disastorous in braking, because now the front is braking harder than the traction can maintain, and we need to either change the brake balance, or raise the rear, and usually both. If we change the balance controller only to keep the front from skidding, we cannot brake as fast. Keeping the rear a little higher helps maintain the designed distribution in proportion so both front and rear brakes can be utilized to the maximum. Note that this does not really benifit cars designed as a race car, as they are meant to be low already, with the suspension and braking geometry for that purpose. The rake mainly is benificial to regular production and tuner cars. I gave the Buick Special a slight rake and it made all the difference in the world for staying on the road for braking and high speed. With no rake, that car becomes very unstable at high speed.

 

[djaft3rb3ats]

but ur talking about higher rear and rk is tlaking about his friedn who has a higher front...is it me or does this not make sense?

 

[eforer]

Well if your car has incurable understeer by anyother means. Raising the front will create a more dramatic weight transfer to the outside tire during cornering increasing front "side bite." This would come at the expense of the chassis ability to direction change and turn in hard. Not knowing much about how the GT physics engine is put together, its very hard to speculate on what these changes are doing to the models suspension geometry. I agree with the previous posts obviously about the real world practical drawbacks to things like preload collars etc, but I wonder what is actually changing on the GT suspension geometry when you screw with ride height. I suspect its a down stop setting type of deal, which would mean that the chassis would roll more and transfer more weight beyond just the height increase of the cg. As far as the undercar areo, who knows how far polyphony went with the areo models. I suspect that the game isn't capable of accelerating the air under the car to create chassis genereated downforce.

 

[djaft3rb3ats]

PD included that i think or else thered be no other reason for eide hieght than to get over curbs, and yeah........they simulate it, but its the front and back weight distribution through the springs that is confusing a lot of us


djaft3rb3ats

 

[rk]

As far as the undercar areo, who knows how far polyphony went with the areo models. I suspect that the game isn't capable of accelerating the air under the car to create chassis genereated downforce.

I learned the effectiveness of the phenomenon I am calling chassis downforce with small sporstcars at Nürburgring. I had good driveable tunes for these cars in that their nimbleness allowed their lap times at tracks like Deep Forest to approach those of more powerful cars.
When I took these cars out on the high speed sections of Nürburgring, their tunes seemed to fall apart and I was confounded, the cars would inevitably veer of the track as if they were "crown sensetive" from too much toe in. This is the relatively straight section that runs between Quiddelbacher Höne and Adenauer Forest, you should be able to run the Auto Union V-16 flat out through that segment and that car barely turns.
I sequentially tried every "logical" fix. Nose weight was helpful, but not enough. Lowering didn't help and was disasterous in other sections and it would still happen with max stabilizers. I did notice that the few small cars that allowed wings were less prone to the syndrome.
While reading an article about the Mercedes GTR that flipped, I decided to try chassis rake, which solved the problem completely on every one of the "problem" cars. Now one may come up with a perfectly valid reason why raked weight transfer works where ballast positioning didn't and I would enjoy reading it. For now I believe ground effect downforce is indeed modeled; although other factors, like the "corner bite" described above, may trump chassis DF.

 

[RenesisEvo]

While reading an article about the Mercedes GTR that flipped, I decided to try chassis rake, which solved the problem completely on every one of the "problem" cars. Now one may come up with a perfectly valid reason why raked weight transfer works where ballast positioning didn't and I would enjoy reading it. For now I believe ground effect downforce is indeed modeled; although other factors, like the "corner bite" described above, may trump chassis DF.

http://www.mulsannescorner.com/techarticle1.htm <-- this article??

This thread makes for heavy reading. And I haven't seen any mention of a venturi yet. By running a car close to the ground, front lower that the rear, the air travelling underneath the car is forced to accelerate under it to occupy the greater space behind. Higher pressure at the very front, and lower pressure underneath. If the pressure under the car reaches sub-ambient levels, the resultant force acts downward - downforce.

As for weight shifting about.... well, normally, a lower ride height would require stiffer springs to prevent wheels rubbing on arches/bottoming out. The stiffer springs, the more they resist weight being shifted in that direction. have a car accelerate from standing with really soft rear springs, and the front will raise up. Have really hard springs on the rear, and there will be little shift in the car's attitude. So a lower front end implies stiffer front suspension, which implies softer rear, which means that under acceleration, the weight can more easily shift to the rear, giving more grip (since the grip is directly proportional to the weight, by F- (mu) x R) to the rear wheels. So not recommended for anything front-wheel drive.

That's my take on things. I see a lot of static stuff flying about - erm, surely dynamics is where we should be focusing our efforts?

 

[rk]

http://www.mulsannescorner.com/techarticle1.htm <-- this article??

Yes

This thread makes for heavy reading. And I haven't seen any mention of a venturi yet. By running a car close to the ground, front lower that the rear, the air travelling underneath the car is forced to accelerate under it to occupy the greater space behind. Higher pressure at the very front, and lower pressure underneath. If the pressure under the car reaches sub-ambient levels, the resultant force acts downward - downforce.

I wrote this not quite 1 month ago:
https://www.gtplanet.net/forum/showpost.php?p=1686969&postcount=5

I am clearly the strongest advocate for chassis DF in this forum as any search will show, and I am starting to consider recanting. (get it re"cant"ing )

That's my take on things. I see a lot of static stuff flying about - erm, surely dynamics is where we should be focusing our efforts?

This was the post that started this thread:

almost everyone's setting for ride heights are always back is HIGHER than front...

for ex: FRONT / BACK
ride height: 90 / 98

Can someone tell me why and how it can help?

In this regard, discussion of static states can simplify understanding of dynamic states, imo.

 

[Zardoz]

...By running a car close to the ground, front lower that the rear, the air travelling underneath the car is forced to accelerate under it to occupy the greater space behind. Higher pressure at the very front, and lower pressure underneath. If the pressure under the car reaches sub-ambient levels, the resultant force acts downward - downforce...

Certainly true, but I wonder if PD has also modelled the effect on the top of the car of raising the tail and lowering the nose? Stands to reason that if you point the whole machine down, you'll see an increase in downforce because you've turned the whole car into a "spoiler", so to speak. Question is, has PD coded that in as well, along with the underbody effect?

EDIT: Oh, ya think they've modelled for top-of-car effect? How about this:

I took a Pesky '03 to the test track with stock power, 2.00 final, and Autoset at 1. I tried it with a ride height setting of 80/80 and hit 242 MPH. I changed it to 55/105 and hit just 229!

Now I have to ask: Has PD modelled for underbody effect, top-of-car-only, or both?

 

[rk]

Now I have to ask: Has PD modelled for underbody effect, top-of-car-only, or both?

Well just as soon as someone makes a car which has a body that articulates like a bellows, it will matter. In the meantime, every car that rakes pan also rakes bonnet and the only way to seperate the two is to raise the overall height such that the ground seal is broken, which brings us to our next point:

I took a Pesky '03 to the test track with stock power, 2.00 final, and Autoset at 1. I tried it with a ride height setting of 80/80 and hit 242 MPH. I changed it to 55/105 and hit just 229!

If you re-read this article
http://www.mulsannescorner.com/techarticle1.htm
you will see that chassis rake is adjusted in small multiples of an inch. Following Doormeister's lead by carrying things to extremes, you surely can picture what would happen if you stood your "Peskie" on its nose, you would have completely broken the ground effect seal, not to mention the large airdam you have made of your body top spoiler. (Picture the Mercedes/McLaren when the brake lights are on. In case you use cockpit view, it has a braking airdam.) I suspect 50mm of rake on an LMP comes a lot closer to doing that than to providing increased traction. Furthermore, chassis downforce as mentioned above is a venturi effect, which loosely means a non linear return on effort. The linear effort is that of pushing the raked body through the air. the linear return is the partial vacuum generated underneath. The non linear return is the augmenting of this vacuum, through the use of the skirting and under chassis fences that causes the slipstream rushing past to draw out yet more air, making more vacuum. Take a careful look at the rear skirting on the BMW.Mclaren F1. You will see that it actually flares away from the tires as it approaches the rear, which, if effective, would draw even more air away from the rear and bottom of the craft, further strengthening the vacuum, if only in visual effect. Try your "pesk" with a maximum of 25mm and tell us how it did.

 

[Zardoz]

...you surely can picture what would happen if you stood your "Peskie" on its nose, you would have completely broken the ground effect seal, not to mention the large airdam you have made of your body top spoiler...I suspect 50mm of rake on an LMP comes a lot closer to doing that than to providing increased traction...

No doubt about that. That's why I was so amazed at how much rake Duck uses.

The fact that extreme rake slows the car down so much means the game models increased drag from increased frontal area for sure. Does it also model a broken "ground effect seal"? For that matter, with no ground effect skirts lowering themselves (like your bellows-car analogy), are we getting any ground effects at all, or is it all body-top-spoiler-effect being applied?

And does it even matter? The end result is the same: More rake, more downforce. Whether we call it "chassis downforce", "ground effects", or "body rake downforce" doesn't really matter, I guess.

 

[rk]

For that matter, with no ground effect skirts lowering themselves (like your bellows-car analogy), are we getting any ground effects at all, or is it all body-top-spoiler-effect being applied?

And does it even matter? The end result is the same: More rake, more downforce. Whether we call it "chassis downforce", "ground effects", or "body rake downforce" doesn't really matter, I guess.

The results I found using raked sports cars as detailed above in post 53 strongly indicate to me that some sort of venturi effect is being applied. Remember that ALL wings are indeed wing sections, inverted albeit to provide "lift" downwards through the chassis, allowing, again, a non linear return on effort. Most spoilers, airdams and the like simply redirect airflow and provide little, if any lift. The fact that the little four bangers were able in some cases to reach speeds of 175mph leads me to believe the drag was disproportionate to the amount of downforce provided. Granted the data are vague, but I tend to believe the physics engine is modeling vacuum rather than "push" and I am very open adjusting my premise based on addional precise observations or tests like the Pescarolo experement.
Duck, and myself, came to the conclusion that the programmers gave us a clue in the pattern of available rake after setting "optimum" ride height. Although I don't feel we were actually misled, I do intend to appraise him of recent discoveries soon.

 

[Zardoz]

...I am very open adjusting my premise based on addional precise observations or tests like the Pescarolo experement...

A "downforce gauge" would sure help, wouldn't it? Failing that, all you can do is test and experiment.

I'll re-run my Pesky test with 25mm of rake like you said, soon as I turn on the PS2 again.