Quote from colcob :

Sorry about this, but you've been wasting your time. There is no way from the data we have to determine the unsprung mass distributions or the sprung mass distributions.

Sprung mass distribution may be obtained from raf (vertical loads) provided that there is a moment when the car is standing still on a flat surface. Total mass distribution may be taken from garage view (with driver and fuel). I hope the two distributions are not equal. The calculation from the first post seems reasonable, though I'll check it one more time.

Car's accelaration is second time derivative of (X, Y, Z) vector. You can normalize right-vector and forward-vector (their length should be 1). You can also construct up-vector as their cross product. Then lateral accelaration is dot product of accelaration vector and normalized right-vector etc.

Quote from axus :

as the wheel goes along an arc...? Atleast your X and Y co-ordinates will be inaccurate

Yes, but it is a very small error, especially when the position is then used to calculate accelaration.

Quote :

but is the CofG position (ie, height, distance from front axel and rear axel) in the RAF files?

Yes, it's all there, but maybe it's better to think of it the other way around. X, Y and Z in static wheel info is a position of a given wheel relative to car's cog. So Y is really the distance from a given wheel's axle, X is track for a given wheel's axle, height of cog would be equal to - Z - suspension deflect (with no car movement) + wheel radius.

Quote from tristancliffe :

To find wheel accels (well, actually suspension accels but for the time being it will do). I took the difference in suspension positions (and converted it from mm to m) and divided that by 0.01 seconds. I then took the difference from each of these suspension speeds and divided them by 0.01 seconds.

Suspension deflect (RAF) is already in meters, so there is no need to convert them. But in this way you get wheel acceleration relative to car body. I think you should rather calculate wheel acceleration in track (world) coordinates, because in fact to find tyre normal load we use F = m * a equation so we must be in an inertial system. We consider only linear movement of the wheel so we take into account all the forces acting through the suspension and forces at tyre contact patch. Through the suspension there are vertical load, X force and Y force (all in RAF dynamic wheel info). I believe these forces are aligned with car body, not road surface. So in fact when there is body roll (cornering even on completely flat surface) part of X force affects tyre normal load. Similarly for body pitch (braking or acceleration). BTW, now I see I have to improve RAFTyreExtract a little. The other part of forces, one acting at the contact patch, which we want to calculate, is usually expressed in 'tyre-road' coordinate system and thus divided into tyre normal load, tyre longitudinal force and lateral force.
For now remaining part is calculating wheel positions in world coordinates, necessary to get linear wheel accelarations. We start at X, Y, Z (RAF data block) which is the position of car's CoG. Also in RAF data blocks there are right-vector and forward-vector, which define car orientation in world coordinates. We can also get up-vector, which is cross product of right-vector and forward-vector. They must be all normalized (their lengths must be equal to 1). Then to CoG of car we add:
X (static wheel info) * right-vector
Y * forward-vector
(Z + suspension deflect) * up-vector
The result should be the position of a given wheel in world coordinates at a given time step.

Last edited by w126, Fri 13 Jan 2006, 11:33.

I see things progressed a little and there are more questions. I'll try to answer wherever I can.

Quote from tristancliffe :

And does the suspension ratio have an effect (I guess it does), because 1cm of wheel travel won't necessarily be 1cm of suspension travel.

I assumed suspension deflect (RAF) is wheel travel, I don't remember why now. Maybe because spring constant (RAF) is wheel rate and not spring rate. I'm not sure, it must be still verified.

Masses are in RAF header. For wheel accelerations you have car positions for time steps, neutral relative positions of wheel centers, suspension deflections for time steps, body orientation for time steps (right and forward vectors). From these you can calculate positions of wheel centers in world (circuit) coordinates for time steps and then calculate its 2nd derivative and take its vertical part. As you can see it can get interesting.

Quote from axus :

How did you get the actual vertical tyre load? We need it to make the programme work - we will be doing the tests on the skidpan which means it is flat anyway.

It's basically what Scawen said. To the load from RAF I add wheel mass (1/4 of (mass - sprung mass)) * 9,81. I also add wheel acceleration in the vertical direction multiplied by its mass (these oscillations), but maybe it's not needed for flat surface.

Last edited by w126, Thu 12 Jan 2006, 20:23.

Vertical load from RAF is not really tyre normal load but rather vertical part of the force acting between the suspension (including arb) and the wheel. This part is probably vertical in car coordinate system, so it's not always perpendicular to road surface.

See the attachment for comparison. It contains rear tyre normal loads from the same wr lap and was generated with RAFTyreExtract. Of course this part is calculated, not taken directly from RAF. It looks reasonable, so I hope it is correct.

After reading this thread http://www.lfsforum.net/showthread.php?t=3567 I did some tests comparing fps with sound switched on and off on my computer. I used replays of one lap from in-car view and logged framerate with Fraps. I turned out that when there was only one car on the track switching the sound on caused decrease of minimum fps of about 1,5 % only. But with 12 cars the decrease of minimum fps with sound switched on was almost 20 %. The results are similar when comparing average fps. It looks like this sound overhead grows almost linearly with number of cars.

The reason for such large framerate drop with sound on and many cars is probably my sound hardware being unefficient (SoundStorm) or maybe outdated drivers, rather than LFS using so much processing power to generate sound effects. Anyway, whatever the reason, there always may be people with unefficient sound hardware and implementing this simple idea may help them enjoy LFS even more.

I'd like to propose adding the ability to set maximum number of cars (closest to player's view) for which sound effects are generated.

For example, with 12 cars one has 35 fps min. without sound and 28 fps min. with sound. One could set maximum number of car sounds to 3 (hearing only player's own car and 2 closest other cars) and have over 33 fps min, provided that it is indeed a linear relation.

My mistake. I edited the link to the original image in the first post.

Yes, the graph for LFS is reasonable. The only strange thing is that for slip angles over 14 green, black and blue points become very close.

Quote from axus :

Not to mention suspension dynamics which would have a huge effect on this...

Influence of suspension is largely eliminated, because data for the graphs are measured in standardized (SAE) 'tyre-road' coordinate system, forces are forces acting on tyres, some parameters already account for suspension influence (tyre to road camber).

Last edited by w126, Wed 14 Feb 2007, 09:52.

Yes, you are right. These two graphs are definitely for different tyres with different sizes and pressures. I forgot to add that I didn't mean the exact comparison of the graphs, but rather treating the first graph as a typical example of load sensitivity data and comparing only the basic properties of the two graphs, like if the curves converge or not.

Let's compare two graphs. One is here http://www.carbibles.com/tyre_bible.html just after 'tyre load sensitivity' phrase (use 'Find in this page' browser function). It shows tyre load sensitivity for lateral forces. The other graph (see the attachment) shows the same but for LFS road super tyres (RB4). The data for it are extracted and derived from RAF. More info and tools are in this thread http://www.lfsforum.net/showthread.php?t=3667. I hope the data are correct and accurate enough.

The first graph ends at around 7 degrees of slip angle, but nevertheless it's hard to imagine that the curves plotted for different load values converge for larger slip angles. However in the second graph (LFS data) the curves seem to converge for larger slip angles, as if load sensitivity effect disappeared then.

How can this influence car handling? I think load sensitivity may help recovering from drifts, because less lateral weight transfer (achieved by slowing down or increasing turn radius) means more effective grip for both wheels of the same axis taken together. Now imagine rear wheels at slip angles over 14 degrees in LFS. It seems we lose this positive effect of load sensitivity in that case.

I may be wrong, in fact it's very likely. However as I'm posting it in Suggestions subforum I just wanted to say that maybe it is worth analysing this aspect of tyre behaviour one more time. Good luck!

Last edited by w126, Wed 14 Feb 2007, 09:51.

Low speeds are tricky. You can study the forces at low speeds, but some way is needed of verifying if they are correct at low speeds. It may be convenient to look at lateral grip vs slip angle plots, but there may be different ways of calculating slip angle (and also slip ratio) than one used in RAFTyreExtract. At the moment RAFTyreExtract outputs 'classic' slip ratio and slip angle. Probably for low speeds it's better to use another definition of slip ratio and slip angle, one with relaxation length.
The attachment contains an example. There are strange looking points there (red colour), which are at speeds below 36 km/h. I'm not sure why they look like that. It may be a bug in RAFTyreExtract, maybe it will look completely different when slip angle with relaxation length is used. I checked the replay and these strange red points correspond to very tight turning after a spin out.

Grip is actually only added for convenience, as it's very simple once you have the forces. I am not even sure if 'grip' is a good name for what it means here.
Here are the definitions:
tyre longitudinal grip = (tyre longitudinal force) / (tyre normal load)
tyre lateral grip = (tyre lateral force) / (tyre normal load)

Last edited by w126, Tue 13 Dec 2005, 09:17.

Hello,
I wrote a program that extracts some data from RAF files, adds some calculations and produces csv output that may be used to analyse (at least to some extent) the characteristics of LFS tyres. The program is called RAFTyreExtract.

There is also another simple program (MultiDim) that may be used for plotting data from RAFTyreExtract, unless you have a better tool to do it that you are already familiar with.

See the attached picture for an example of what is possible with these programs. It shows FZ50 rear tyre lateral grip vs slip angle for various tyre normal loads.

I hope there will be some people here who may find RAFTyreExtract useful and help me verify if its ouput is correct, especially parts which are calculated and not taken directly from RAF file.