Quote from Shotglass :

what the hell is an lbs of pressure ? per what ?

...per square inch (PSI). What unit of pressure do you use? I've never known anyone use anything other than PSI for tire pressures.

Quote from ajp71 :

...per square inch (PSI). What unit of pressure do you use? I've never known anyone use anything other than PSI for tire pressures.

bar ... like everyone else in germany and probably the entire rest of europe does

or technically i think its actually bar - 1

Quote from Shotglass :

bar ... like everyone else in germany and probably the entire rest of europe does

or technically i think its actually bar - 1

That depends whether you're talking about absolute pressure or gauge pressure
Atmospheric pressure is 1 bar absolute, 0 bar gauge. Your tyre pressures will almost certainly be gauge pressures, which is the pressure above atmospheric.

Speaking of strange units, I work in the automotive emissions sector and we measure catalyst metal loading in g/ft^3 !

Quote from Shotglass :

seriously ? never heard of that

Indeed. I forget the exact figures, but I'm talking pretty small changes in yaw. It doesn't seem logical to me, since less surface area is exposed to the direction of flow but nonetheless it's true.

Quote :

what the hell is an lbs of pressure ? per what ?

Per anything, it's a flexible unit unlike a bar

...Really bad habit typing lbs instead of PSI :hide:

:hihi:

Quote from StewartFisher :

g/ft^3 !

Quote from Shotglass :

what the hell is an lbs of pressure ? per what ?

1 Bar is pressure that is in your personal intake (nose) during breathing in as you probably are naturally aspired as most of us, naturally 2 bars is pressure doubled, very practical unit as 1 bar is ~1 atmosphere at sea level so put 2 bars absolute pressure (1bar of boost) and you get double power, right? (for those that don't know of my perversions, I'm joking a bit again :razz

Oh, and sorry that my quote is all f*ckd up, that was about BBT's bar thing that I meant to quote, but I'm newbie

Quote from StewartFisher :

That depends whether you're talking about absolute pressure or gauge pressure
Atmospheric pressure is 1 bar absolute, 0 bar gauge. Your tyre pressures will almost certainly be gauge pressures, which is the pressure above atmospheric.

i know i know but we were talking about tyre pressures which round here are usually measured in bar - 1 ... or actually bar - atmospheric not that the changes of atmosheric pressure matter but anyway

Quote from Ball Bearing Turbo :

Indeed. I forget the exact figures, but I'm talking pretty small changes in yaw. It doesn't seem logical to me, since less surface area is exposed to the direction of flow but nonetheless it's true.

are you sure this is a general effect and not just something with that particular wing profile in the test youre refering to which might improve its perfomance if you look at the profile cut slightly diagonally ?

how much of an increase is it anyway ?

Quote from JTbo :

1 Bar is pressure that is in your personal intake (nose) during breathing in as you probably are naturally aspired as most of us

no i breathe turbo air fueled from my farts

Quote from StewartFisher :

That depends whether you're talking about absolute pressure or gauge pressure
Atmospheric pressure is 1 bar absolute, 0 bar gauge. Your tyre pressures will almost certainly be gauge pressures, which is the pressure above atmospheric.

Speaking of strange units, I work in the automotive emissions sector and we measure catalyst metal loading in g/ft^3 !

That would make sense for the tire pressures to be in gauge pressure, since they would be way too low otherwise.

Here's two of Todd's posts from RSC (http://forum.rscnet.org/showthread.php?t=269177&page=5), they're great reading - so I have no problem posting them here:

At Shotglass: looks like he was mainly talking about spoilers, not wings per se - shame on me!

Quote :

For a quick mental exercise I'd like to point out something that might get the gears churning a bit when people are thinking about force curves. Again, I'm not going to just lay it out for people because I get paid for this type of thing and am a greedy pig that way, which I hope is forgiveable so long as this post is a little thought provoking and gets folks to research this area more. I found tire data that went up to 45 degree slip angle in about 5 minutes of Googling. You guys can too, so please, let's not spend 10 minutes writing a post chastizing me for not fixing the curves up. The info is out there and freely available so just go find it

In the contact patch of a tire that's operating in pure side slip where you have only slip angle, pure cornering with no acceleration or braking at all, you essentially have two zones of operation. This has been discussed at rsc before by several people but I'd like to touch on it again because it's important. At low slip angles the majority of the contact patch is stuck to the ground. The rubber is being pulled sideways on its way towards the rear. At some point in the rear of the patch, however, the vertical load tends toward zero and suddenly the rubber slips back to it's undeformed shape.

There's a portion of the tire that's stuck to the road and another portion at the rear that is sliding. The important part here is that some percentage of the contact patch is always sliding whenever you have any slip angle at all, no matter how small. As you increase the slip angle further and further, the point where the sliding in the contact patch begins moves forward in the patch. At some slip angle only the rear 20% of the patch is slipping. At a higher slip angle the rear 50% might be sliding. At a still higher slip angle the rear 70% is sliding. Etc..

As this is occuring the lateral force is going up and up. This is the part of the curves you see for GTR2 and all these sims where the force is rising as you move to the right on the graph (where slip angle is increasing).

Now, it just so happens that when you've increased the slip angle to the point where the entire contact patch is sliding, where the slip point has moved all the way to the front of the tire, is also the point where you have the maximum lateral force (generally speaking, with high hysteresis tires this isn't strictly true, but it's still close within a couple of % usually).

With me so far? The peak of the lateral force curve is right where the entire contact patch is now sliding. I.e., not 50% or 70%, but 100% sliding. If you increase the slip angle even further then you still have 100% sliding. There is not a point where the tire is gripping or stuck to the road and then suddenly "breaks free" into a slide. There is no "static" versus "dynamic" friction coefficient difference at play here.

Now, let's imagine looking upwards through a glass plate at the contact patch. There's an axis that the tire spins around, right? This sticks out straight to the left/right of the tire through the center. The part of the contact patch that is sliding is sliding right along that same axis. I.e., the slipping is in a direction perpendicular (at a 90 degree angle) to the direction the tire is pointing. The rubber is just sliding to the left or right relative to the tire.

At our force peak this means that the entire contact patch is sliding along this line. It's just sliding to the left or right. What happens if we increase slip angle beyond this? The sliding is still to the left or right. I.e., if you take a look at one little spot in the rubber as it travels through the contact patch there's really no difference in the sliding direction whether the tire is running at 10 degrees slip angle, 20 degrees, 30, 50, or even 90 degrees. The only thing that's different there is the speed at which the rubber is sliding across the surface is increasing as you increase the slip angle beyond the peak.

Imagine a block of rubber sliding across a road surface. The block of rubber has absolutely no idea what the slip angle of the tire is. It's just sliding to the left or right.

Now, can anyone give a logical reason why the force curves would suddenly drop off after the peak if the rubber has no idea what the slip angle is, but only knows its sliding speed is changing?

Quote :

Aero can have a very large effect on what happens. However, the tire curves are strictly the lateral/longitudinal forces that you get at a given load, slip angle, slip ratio, etc.. I.e., the aerodynamic stuff changes the load on the tire which changes the tire force. So the aerodynamic effects are completely separate from the tire data. What we see them doing, however, is adjusting the tire curves in order to achieve the effect. Sorry Simbin/Blimey, I love your sims and enjoy them very much, but if that's what happened there, yuck

In one of Milliken's books, "Race Car Vehicle Dynamics," page 494, there is a chart for how the coefficient of lift (downforce) at the rear axle varies with yaw angle (+/- 15 degrees) on a GT car model (it looks like a GT40, but am not sure exactly what car it really is measured from). This shows several curves for different spoiler heights. With no spoiler at all, at 0 yaw you have lift instead of downforce, which then turns into downforce at about 7 degrees. The downforce then keeps increasing out to 15 degrees. After that I don't know of course because the data only goes to 15 degrees of yaw.

There is one spoiler height where the downforce is just barely effected by yaw. There's a little tiny wobble towards less downforce at about 4 degrees, but it's still quite flat. At the highest spoiler height tested you see a fairly sizeable increase in downforce at 4 degrees and then it drops off quite a lot on the way to 10 degrees yaw, then stays pretty flat out to 15 degrees. The difference between the highest and lowest downforce in that case is about 30%, so it's a lot.


It would be interesting to see data past 15 degrees of course, but I've never seen it so don't know what happens out there. Keep in mind that this is a spoiler, or a flat plat sticking up at the back of the car rather than a proper wing. It wouldn't surprise me at all to see a wing lose a lot of downforce as the yaw angle increased, though.

There are some aerodynamic textbooks out there that have a LOT of wing data. Wing data isn't all secret stuff like you see with tires. It's widely published so you can probably find that info online or at the library if you want it badly enough.

That was an awesome reading, now I want to know more about this subjet.

well i guess for a spoiler is might make sense if you consider the spoiler at 0° yaw is usually in the low pressure area behind the driver compartment and as you increase yaw it will stick out more and more from it
a simple test for this theory would be to see if the downforce is different on both rear tyres

Quote from Shotglass :

no i breathe turbo air fueled from my farts

Omg, CH4 boosted beast, I never try to beat you in running, that is really a badass setup you got

I tend to believe what Todd says as it really has shown dramatic improvement to that known game mod handling, even it is surprising that many books show wrong graphs, but that could really be so.
Luckily I don't have time to test everything thoroughly, so I can just believe what I'm told

Quote from RiGun :

That was an awesome reading, now I want to know more about this subjet.

Get "Race Car Vehicle Dynamics"

It's a great book. A little tough to read, but it's good.

Quote from Ball Bearing Turbo :

At Shotglass: looks like he was mainly talking about spoilers, not wings per se - shame on me!

The only "concrete" info (in quotes because I don't recall the numbers) I can recall on wings and how downforce is effected by yaw angle was from an SAE conference I went to last October. One of the presentations I sat in on was a CFD (computational fluid dynamics) deal where they were showing the latest aerodynamic modelling on a formula car. Very impressive stuff to be sure!

Anyway, you know those little vertical fins that you see somewhat outside of the center on the front wing of a lot of formula cars? The thing I remember most was a CFD comparison before and after adding a pair of those little fins. What tended to happen when yaw angle was increased to 4 degrees, which is right at the peak of the lateral force curve (tire slip angles would be about 4 degrees then too, pretty much), was that a set of low pressure "bubbles" would come off the outside half of the wing and stream into/around the outside pod. It was neat seeing it in CFD. When he showed the fins the bubbles coalesced and more or less disappeared, restoring pressure in that area.

The result was a slight increase in downforce. However, I don't know if there was an increase from 0 degrees yaw or if the fins just increased downforce at 4 degrees when compared to 4 degrees and no fins. I want to say there was indeed an increase from 0 degrees yaw, but don't really remember for sure. The impression I got was that the tiny, extra downforce (14 or 18 lb comes to mind, but I could be remembering wrong) wasn't generated at the wing, per se, but as a result of how the airstream went around the side pod afterwards, slightly forward of the car's center of gravity and thereby increased front downforce. A slight change in pressure on any top or bottom surface anywhere on the car changes downforce, so changing the front wing can actually effect more than just that part. It changes the airflow at the rear too. I don't know how much though. I've got quite a lot of aero data on one formula car and they don't specify a change in rear downforce when the front wing flaps are adjusted, so perhaps it's not much or enough to worry about.

In conclusion, I don't know how much yaw angle effects downforce on wings. Without end plates I'd tend to think the downforce would decrease with any yaw angle at all, but don't really know for sure. Remember that the airflow around the rest of the car is also influencing downforce, and that is changing with yaw too. With end plates I wouldn't be surprised to see it go either way over a small yaw angle range. Indycars on super speedways tend to spin in a real hurry when you get the slip angle much past the peak, so it looks to me like downforce plummets pretty quickly after a point. That's at the rear though where the airflow has gone all crazy over the body compared to the front wing which still has clean air. So it's a body/wing interaction. There could very well be a small region from 0-5 degrees or so where there's a slight increase, but I don't know for sure. If there is, I doubt it's much though (1-2% or less if I had to take a wild guess). Measured data has gone contrary to my intuition before, so take that with a grain of salt

BTW., the CFD simulation covered probably just a couple of seconds of simulated time. IIRC, the computation took 120,000 CPU hours, which would take over 13.5 years on one PC! They ran it on a massive CPU cluster and it took a week straight, 24 hours a day. All for a few seconds of simulated time. The result was sure pretty to look at, although I don't know how practical that really is.

EDIT: If each CPU did 1.3 billion calculations per second (my P4 3.6Ghz does that), 120,000 CPU hours would be 561,600,000,000,000,000 calculations (561.6 quadrillion!)

Here's a copy of the graph charting yaw angle against coefficient of lift for various rear spoiler configurations that Todd and BBT are talking about.

http://i9.tinypic.com/4pvajhg.jpg

In the third config, the lift coeff rises very slightly at small yaw angles.

As Todd mentioned, this graph is taken from 'Race Car Vehicle Dynamics' by Milliken and Milliken, pp 494.

It's amazing how threads can change subjects so fast. This has got to be one of the most informative online forum threads ever. LFS, tire physics, FOV, tire pressure, units of measurement, and now aero stuff... What's next I wonder?

Don't forget humans running on F/I methane! (although incidentally not all humans produce methane apparantly)

Quote from Ball Bearing Turbo :

Don't forget humans running on F/I methane! (although incidentally not all humans produce methane apparantly)

Yes, some produce only bad smelling gases that actually decrease performance and may cause even dizzy feeling, but few quality persons do have proper F/I methane setup and we mere normals are no match for them

You find it hard to believe? Well, how did you think some can do those incredible WR times, by practising? LOL, no way you could get that fast with practise

The experts? You mean the people who write books on racing tyres?

Have they actually done REAL research, or are they just talking from experience and what they've read? I suspect that your average 'tyre expert' has never seen any actual data themselves. Todd has. Todd is going to events that even F1 teams find enlightening. You'll probably find several senior tyre company people in the audience too, learning about the things they make.

Quote from Whitmore :

Its true that books on racing tyres don't agree with Todd's analysis. I prefer to believe the experts personally

Todd was also specifically addressing these books, and the general consensus is that the first authors maybe took own driving experiences or reports from race drivers of the "cars letting go" once you go "over the limit". That way those funky curves with huge dropoffs after the peak were created. However, feeling a change in force, like when the car starts sliding and lateral force is replaced by rotational one, has nothing to do with what happens at the tyres and how much traction they provide - these curves describe more what the driver feels, rather than what the tyres actually do. Newer racing books then, instead of doing own research, more or less just copied this author's fantasy curves which in the end created the myth of tyres that lose huge amounts of grip when they slide.

Quote from Whitmore :

Its true that books on racing tyres don't agree with Todd's analysis. I prefer to believe the experts personally

I have nothing to add what Tristan and AndroiXP replied, but tell me have you seen how much of real measured data from tires? Have you raced IRL?

Specificaly?

Quote from Whitmore :

The books I'm referring to were written by people who have properly researched the topic and have seen actual data

i doubt it because if those guys had actually spent a second thinking about what those curves would mean for car handling (especially countersteering) they would never have put them into their books
which goes to show how much or rather little they must know about physics

Quote from Shotglass :

i doubt it because if those guys had actually spent a second thinking about what those curves would mean for car handling (especially countersteering) they would never have put them into their books
which goes to show how much or rather little they must know about physics

But you can t disagree that there is always a moment with a dropoff in the curves,i can t imagine a car who would be catchable in all situations(angle).

I m pretty sure that is the point they try to explain with the sudden dropoff.
You are going sideways at 30deg,then 40..........and more and more and poufff,you lost the ass(that is what i mean with violent griploose).Can you really say it doesn t happend to you one time.

Quote from GHOSTRACER1 :

But you can t disagree that there is always a moment with a dropoff in the curves,i can t imagine a car who would be catchable in all situations(angle).

I m pretty sure that is the point they try to explain with the sudden dropoff.
You are going sideways at 30deg,then 40..........and more and more and poufff,you lost the ass(that is what i mean with violent griploose).Can you really say it doesn t happend to you one time.

You are talking from illusion that led to this misbelief of dropoff.

Does rubber know what is it's slip angle and does it turn ice after certain angle is exceeded? No, it does not, what happens is that sideways movement velocity becomes greater than amount of grip can handle and car is sliding and this can lead even to loss of control when velocity is great enough.
Also when tire slides it starts to heat up and when tire overheats this reduces grip, but you don't put this to tire curves, it is own modifier that modifies grip and tire curve according to tire temp. Also there are few other modifiers that cause that there really is less grip when sliding, but those are not to be included to tire curve itself.

Todd told once that reason books have such curves is that books are for racing drivers and intention is to illustrate to drive how there is less grip when sliding. Correct me if my memory is inaccurate, but that is how I remember it was.

Also I don't know all fancy words and such, I know just basic principle and that is enough for me, I don't have time to study it deeply enough to know all words and proof things with numbers etc.