There was some good reading on the "experiments in tire physics" thread, but I wanted to report some testing of my own. I was on the skidpad, basically I removed all of my setups and started over. I did the UFGTR first, since it's probably the car I use the most, and made a really great setup for it. Ok, on to a RWD car, better start easy, used the LX4. So I make a "baseline" setup, and take it for a spin on the skidpad. Basically I was heating up the tires evenly, (burnouts, full front wheel lock, etc..) then going around in a circle to see what happens and measure my max sustained G's around the turn. When I noticed something strange. I held the car at about 1.25 G's or so, then pushed on the gas just a little more and the car did a 180 on me. Hmm. Did the same thing again, only this time in 5th gear (somewhere around 2000 rpms) at about 40 mph, held at 1.10 or so G's, then pushed on the throttle again. Once again, around she came. What the heck? I know the car doesnt have enough power at 2000 rpms in 5th to spin the wheels. I did the same test with the same setup, and used about 20 different diff settings, all with identical results. Ok, time to switch cars. Used the XRG non turbo, surely it is low power enough, it barely spins the wheels in 1st!! Did the same basic test, got it near 1.0 G's, put it in 5th, pushed on the gas, around it came. I think this is what makes the RWD cars so tricky to drive. Really I think the tire physics are done quite well. In the FWD cars, they do feel pretty darn good, especially the RB4, it feels great. On dirt too, the tires feel quite good- maybe a little too "slippy"- almost like wet grass than gravel, but the car reacts the way it should. This makes me think that the tire physics are quite close, but it's the weight transfer that is off.

I ended up running a new PB at a few tracks with the LX6 with a new setup. And suprisingly enough, it has 0 front swaybar and about 10 rear swaybar. I found that the front end will not turn as long as the rears have traction until I took the swaybar setting WAY down. Diff settings didnt matter a whole lot either. Now, the setup is a bit twitchy, but I can actually hold the car in a semi-drift if it gets too loose instead of snapping around like usual. Only problem now, the fronts will not heat up. This setup follows most of my standard setup rules in real life, keep the lower control arms slightly above parallel with the ground and run stiff enough springs to keep them that way. My new UFR setup is done the same way, zero front bar, and only about 15 rear but VERY stiff springs. Seems to work for me as I beat my BL1 PB by about 1.5 seconds by the third lap.


Anyhow, try out these experiments for yourselves- I think the weight transfer is what is causing the RWD cars so much grief. Sure, the tire physics arent perfect but I think they are a lot better than most people give them credit for.

Brendan

Quote from Lord_Verminaard :

Basically I was heating up the tires evenly, (burnouts, full front wheel lock, etc..)

locking up the front wheels doesn't heat them evenly (maybe it does in the game but it still puts flatspots). it heats up one spot and makes it flat. braking without locking will heat the tires. all they need is load put on them, not to be melted down to square blocks.

Quote from Lord_Verminaard :

When I noticed something strange. I held the car at about 1.25 G's or so, then pushed on the gas just a little more and the car did a 180 on me. Hmm. Did the same thing again, only this time in 5th gear (somewhere around 2000 rpms) at about 40 mph, held at 1.10 or so G's, then pushed on the throttle again. Once again, around she came. What the heck?

i wouldn't call that strange. i'd call it "oversteer" or "loose". if as you're creeping up on the limt of a car's turning ability the rear wheels break loose first, it's oversteer. if the front wheels lose traction first it's understeer.

speedfreak227

Quote from speedfreak227 :

locking up the front wheels doesn't heat them evenly (maybe it does in the game but it still puts flatspots).

i think he was talking about full steering wheel lock but i might be wrong after all

Quote :

i wouldn't call that strange. i'd call it "oversteer" or "loose". if as you're creeping up on the limt of a car's turning ability the rear wheels break loose first, it's oversteer. if the front wheels lose traction first it's understeer.

you should try that yourself ... iirc its not a predictable oversteer but the tail really snaps round and there is nothing you can do about it

Quote from Shotglass :

i think he was talking about full steering wheel lock but i might be wrong after all



you should try that yourself ... iirc its not a predictable oversteer but the tail really snaps round and there is nothing you can do about it

i'll happily give it a shot but what is "iirc"?

i'm old school and we type out words rather than abreviate everything.
same sentance for the new school: iosawtowrtae

;o)

speedfreak227

If I recall correctly, IIRC stands for if I recall correctly

Quote from speedfreak227 :

i'll happily give it a shot but what is "iirc"?

i'm old school and we type out words rather than abreviate everything.
same sentance for the new school: iosawtowrtae

if i recall correctly ... and its a rather common acronym on the internets

Quote from Batterypark :

If I recall correctly, IIRC stands for if I recall correctly

I wonder why you think the problem is weight transfer?

I think the reason for the behaviour of the cars in your experiments is due to the way the lateral and longitudal forces of the tyre relate to each other, i.e. when you are cornering hard there is not much grip left in the tyre to deal with the extra torque coming from the engine when you increase the throttle.

This effect is often described as friction circle theory. In reality there seem to be quite a lot of different approaches to this problem, examples can be found on the Racer homepage and in Brian Beckman's PHORS series. I don't really know what the reason for all the differant approaches is, wether it's because tyres aren't understood well enough or CPU power isn't high enough, or test data simply isn't availible.

Another problem in discussing these issues is that no sim developer will tell you a lot about how his tyre model works as this is the most important part of any sim.

Quote from J.B. :

This effect is often described as friction circle theory. In reality there seem to be quite a lot of different approaches to this problem, examples can be found on the Racer homepage and in Brian Beckman's PHORS series. I don't really know what the reason for all the differant approaches is, wether it's because tyres aren't understood well enough or CPU power isn't high enough, or test data simply isn't availible.

The problem typically is a lack of test data. With Pacejka's commonly used Magic Tire Model, you'll typically have a set of constants that produces the amount of lateral force given slip angle, load, and so on, but another set for longitudinal force from slip ratio. I.e., you've basically got a graph of lateral force and another one for longitudinal. However, when you're sitting at a certain slip angle and then increase slip ratio, you don't know what the combined effect is. I.e., the longitudinal force will influence the lateral force, even when you're not at the limits of the friction circle.

What developers typically do is take that lateral force and the longitudinal force separately from the two formulae, then if the vector sum goes outside the friction circle they'll either trim the resultant down to fit, or give precedence to the longitudinal force. I.e., you let the longitudinal force determine how much is left for lateral force to fit it. I tried both in Virtual RC Racing and the second method was closer to reality. However, then you don't get the variations of lateral force with slip ratio that occur inside the friction circle when you're not at the limit. This can make a sim lack quite a lot of feel and warning when approaching the limit. And even at the limit, the results aren't really right either. As soon as you hit 0.10-0.15 slip ratio or whatever, you have zero lateral force capacity. This isn't how it works in reality. Sims are harder to drive than real cars in my opinion primarily because of the commonly faulty approach used to model tires. Much more so than anything due to lack of feel in my opinion.

There are ways to deal with it. For instance, the Milliken/Radt nondimensional method described in "Race Car Vehicle Dynamics." However, this method requires even more extensive data on top of the Magic Tire Model, so you're really back to just guessing again unless you have really deep pockets.

I've figured out how to beat this though, and no, I'm not telling

Quote :

Only problem now, the fronts will not heat up

Use induced understeer to heat them up. Take a look at the start of this hot lap world record.

http://www.lfsworld.net/get_spr.php?file=124&w=1

For a race, just use a bit of induced understeer until the fronts get warmed up.

Quote :

LX6 with a new setup. 0 front swaybar and about 10 rear swaybar

Can you post this setup? If not, can you mention what camber settings you used, as these affect cars in LFS about the same as swaybars.

Quote from Lord_Verminaard :

I did the same test with the same setup, and used about 20 different diff settings, all with identical results.

Did you try the open diff? Since that would let the inside wheel spin to hell but let you keep some traction on the outside wheel. While this will result in oversteer it's a much slower more controllable slide, and gives you some warning to ease off the power.

Quote from jtw62074 :

And even at the limit, the results aren't really right either. As soon as you hit 0.10-0.15 slip ratio or whatever, you have zero lateral force capacity. This isn't how it works in reality. Sims are harder to drive than real cars in my opinion primarily because of the commonly faulty approach used to model tires.

Yeah, after reading your post a few times it makes sense to me. Logically that really seems like the only way to explain what's going on. I've noticed when driving the LX6 on a track, sometimes I will keep the car in a higher gear until out of the turn before I downshift, but even then, the car could be at oh, -.55 or so lateral G's in a corner, and pushing the throttle even a little in a higher gear will cause the car to snap around.

I've posted the setup that I made here too. Made for left-hand drive, which will even out the camber all around. I've used about 40 different combinations of diffs on this setup on a lot of the tracks, and really the only difference I notice is that when using the locked diff, it helps the car remain stable under braking or trail braking, and it seems like it helps catch the car if you get in a spin, at least for me it does. :P The times I tried the open diff, it just made the car harder to launch and it would still snap oversteer long before the inside rear wheel would spin. Good in theory, but not in LFS world.

Quote from jtw62074 :

I've figured out how to beat this though, and no, I'm not telling

Yeah, I'm curious. Have you released any type of playable demo of that project you are working on yet? The vids looked pretty great.

Now that I think about it, this behaviour of slip ratio limit vs. lateral force capacity does show up in the FWD cars as well but for some reason not as severe. (besides the obvious, when your fronts are slipping at least the car is still pointed in the right general direction) My real life experience tells me when I had a somewhat lightweight, high torque V6 FWD car, when coming out of a turn and flooring it, the wheels would spin some, the car would tend to understeer a little, but for the most part, would still go in the right direction. Only in extreme situations, like snow, would the car just go off in a straight line. But this is what the LFS cars seem to do, even with the slightest amount of wheelspin, the cars will violently at times jerk back and fourth between grip/slip. Now, the good side is that diff settings in the front really make a difference here, unlike the RWD cars. I've noticed too on rally tracks, with a stiff front diff and the right setup, the FWD cars can actually oversteer a little under power in a turn, just like my VW Scirocco does on dirt roads.

So maybe it's not so much weight transfer. Although I know it still feels funny launching the cars, the suspension does not compress as much as I think it should and there is way too much wheelspin going on, FWD or RWD.

Just stating the obvious here, I'm not complaining about the game or whining about anything, I just really enjoy hearing intelligent conversations about what's going on in this game and why it does the things it does.

Brendan

I'm enjoying these conversations lately too, and am a pretty big fan of LFS as well

About the weight transfer issue, it's not likely that it's done incorrectly. 3D rigid body dynamics in itself is pretty straightforward. Essentially you have a set of equations that will rotate and move a body (a car for instance) given the forces acting on it and what the angular and linear momenta/velocities were the last time you calculated everything. These equations follow Newtons laws very closely and have been used in sims since, well, probably GPL for starters

What you're typically doing when simulating a car, basically, is finding the amount of spring compression and using that to find the vertical force at each corner of the car. Once you have all those forces and add them at the appropriate locations, you then run through the rigid body dynamics stuff to rotate and move the body. On the next cycle the spring compressions have all changed (and therefore the forces that result from them). You run through the rigid body stuff again, and repeat. If you were in a steady state corner with a certain amount of throttle/braking or whatever, the car would eventually come to an equilibrium with all the forces in check and the weight transfer exactly what it should be. I.e., the vertical forces at each tire would eventually balance all the cornering/acceleration/aero drag and whatever other forces you have. So really, the weight transfer itself is very likely just fine. You don't typically calculate weight transfer in a 3D sim, it just happens as a result of how the body moves and how the springs are compressing.

So... You can't really get the weight transfer wrong if you follow the basic 3D dynamics approach. If weight transfer doesn't feel right, there could be a lot of causes. If the weight transfer is actually "wrong," that would most likely be due to a vastly goofy center of gravity height. I doubt very much that's the case with LFS though. And of course since the cars are their inventions, they can have any CG they want

Quote from jtw62074 :

If the weight transfer is actually "wrong," that would most likely be due to a vastly goofy center of gravity height. I doubt very much that's the case with LFS though.

Hmm, I was playing around with the "slickmod" and made the mistake of putting slicks on the UF1000. First turn in FEClub and the car flipped right over! Noticed that the GTI would do the same thing. Does that mean that if I put slicks on my Scirocco it would flip? And I dont even think it's that the slicks have too much grip- I think I saw a max of 1.3G's at the moment of "flippage". I know of a Race-prepped Scirocco with slicks that can turn 1.2 G's in a turn and it doesnt flip. Not the best comparison but it's all I got right now.

On another note, I put slicks on the LX6 and boy what a ride. Also makes me think that the physics on slicks are better than they are on the road tires. I know there was a bit of placebo effect because of the grip but I found that I had a lot more warning when I was getting out of line before it started to slide. Also, I never got into a slide that I couldnt get myself out of. Arent slicks supposed to generally brake free a lot more suddenly than street tires? I dont know because I've never driven anything with slicks before in real life.

So I guess that is another factor to ponder. I just hope there is a GTR LX4 with slicks and some aero parts for S3.

Brendan

Quote from J.B. :

I wonder why you think the problem is weight transfer?

I think the reason for the behaviour of the cars in your experiments is due to
the way the lateral and longitudal forces of the tyre relate to each other, i.e.
when you are cornering hard there is not much grip left in the tyre to deal
with the extra torque coming from the engine when you increase the throttle.

This effect is often described as friction circle theory. In reality there seem to
be quite a lot of different approaches to this problem, examples can be found
on the Racer homepage and in Brian Beckman's PHORS series. I don't really
know what the reason for all the differant approaches is, wether it's because
tyres aren't understood well enough or CPU power isn't high enough, or test
data simply isn't availible.

Another problem in discussing these issues is that no sim developer will tell
you a lot about how his tyre model works as this is the most important part of
any sim.

Bingo! The fact that the LX4 losses rear grip at maximum G if you accelerate
some more is because there is not enough traction left to sustain the torque
at 2000rpm, even longitudinally.

I find these discussions very interesting, but more importantly, very
constructive and i'm sure Scawen himself must enjoy them Reading the
other posts i'm starting to think the relation between the lateral and
longitudinal grip must be the issue. Typically i find the lateral model to be
lacking and i'm curious as to what LFS is using as a model. Does it have
an x/y 'traction circle' or is it more like 2 seperate data charts ?! I know
it's mentionned somewhere what model it's based on, but don't remember
if it was indeed Pajecka. Or was it confirmed what it DIDN'T use ?!

Another factor in the lack of data also seems to have to do with 'trade
secret'. A lot of tire manufacturers might not want their data to be publicly
available. Any competitor could easily use them. Racing simulators are the
same. I'd imagine we'd see some more older tires though seeing how modern
tires have evolved a lot in the last 20-30 years. A good old Pirelli P600 would
be grrrrrrreat! They had them on Rabbit GTi and other VWs back in 83-84 and
i've had them myself. Surely their technology is no secret but they are good
tires and could provide a great example for simulators to use. Then again, how
DO you get data out of a tire?! Isn't the fact that they are not fully
understood a reason why tires have evolved in the first place ?

Or is this a result of available technology and manucfacturing methods ?
Anyone ever imagine how cars would be today if it wasn't for the WWI and
WWII ? They have create immense industrial growth in many countries and
most of todays manufacturing methods and capabilities were established
during those times.

I think i'll go take a drive on that skidpad a bit myself

Quote from Lord_Verminaard :

I held the car at about 1.25 G's or so, then pushed on the gas just a little more and the car did a 180 on me. Hmm. Did the same thing again, only this time in 5th gear (somewhere around 2000 rpms) at about 40 mph, held at 1.10 or so G's, then pushed on the throttle again. Once again, around she came. What the heck? I know the car doesnt have enough power at 2000 rpms in 5th to spin the wheels.

Did it come around right at 1.10Gs, or after a your foot had been down a while?

I always attributed that to torque building up because at first you can put your foot down a lot. That is going to stress the tire more longitudinaly (izzat a word?), spending more of your traction capacity. Then, you hit a point in the power curve where the engine puts out more. With your foot down that far, you really don't know how much you're giving. You're going just as fast, or faster, but it doesn't seem like it because the acceleration is not a quick and the engine sounds like it's not working as hard. So, higher cornering speed + more foot + power coming on when you don't expect + too much foot = snap oversteer. I feel much more comfortable "on the pipe" in most corners. The tires just seem to be more sensitive to the foot that way.

But maybe I'm just making up excuses to rationalize my translation so I can get around lap after lap.

Traction Circle.

No one has the right to even comtemplate discussing tyre physics without knowing what this is!

Basically, imagine Grip is a circle. If you are using Full throttle, you are at the top of the circle. If you move to the left (steer left) then you would now go outside the circle (and therefore be outside Grip, hence slide).

If you are using the full lateral capacity of your tyres, then the tiniest increase in power will cause the tyres to lose grip. And of course, tyres don't regain grip as easily as they lose it (weird dynamic/static friction variables and what not).

If you want a better understanding, Google Tyre Traction Circle (probably)

If you remember Tristan, Todd's tyre model doesn't use any traction circle theory (or so he says) and apparently works very well (so he says). So perhaps you don't need to know what it is (although the underlying principals are important).

Just a thought.

Quote from Lord_Verminaard :

Hmm, I was playing around with the "slickmod" and made the mistake of putting slicks on the UF1000. First turn in FEClub and the car flipped right over! Noticed that the GTI would do the same thing. Does that mean that if I put slicks on my Scirocco it would flip? And I dont even think it's that the slicks have too much grip- I think I saw a max of 1.3G's at the moment of "flippage". I know of a Race-prepped Scirocco with slicks that can turn 1.2 G's in a turn and it doesnt flip. Not the best comparison but it's all I got right now.

Go here http://www.rqriley.com/suspensn.htm

And scroll down to Rollover Threshold. There is a very useful table that shows lateral G rollover threshold for typical types of vehicle. Compact car = 1.1-1.5. So as you can imagine, a skinny wheelbased car like a UF1 could easily be expected to rollover at 1.3G.

Quote from colcob :

Compact car = 1.1-1.5. So as you can imagine, a skinny wheelbased car like a UF1 could easily be expected to rollover at 1.3G.

but then again the UF is a Mini clone which is known to have an extremely
low cog so it would still remain flat i say. Today's 'compact cars' aren't exactly
minis. Ok, now time to go make that UF flip over on the skidpad Finally
a use for the convertible pink car full of girlies. Hang on girls !!!

Quote from tristancliffe :

Traction Circle.

No one has the right to even comtemplate discussing tyre physics without knowing what this is!

If that was in response to my post, I suggest you reread it assuming that I understand the traction circle. You may come away with a different meaning. If not, sorry.

Quote :

Basically, imagine Grip is a circle. If you are using Full throttle, you are at the top of the circle. If you move to the left (steer left) then you would now go outside the circle (and therefore be outside Grip, hence slide).

Not the best explaination. Let's start over:

Basically, imagine Grip is a circle. The point in the middle is 0% of your tractive potential. The perimiter is 100%. Outside the circle is where tractions starts falling off again. Down and up represent forward/rearward traction. Left and right, lateral. If you are using Full throttle in *exactly* a straight line, you are at the top of the circle, or beyond it if in an FO8, or closer to the middle in a weak car. If you move to the left (steer left) then you would now go towards the side (not neccesarily outside) the circle (and therefore be outside Grip, hence slide).

But I do get your point.

Quote from Slartibartfast :

Did it come around right at 1.10Gs, or after a your foot had been down a while?

Came right around immediately. And notice if I kept the G's a little lower than what the car's maximum was (close to 1.25) which means it should not have a problem accelerating (albeit slowly at 2k in 5th gear) or making the front end wash out due to weight transfer.

I understand the "traction circle" perfectly, but I think there is a problem with what the "traction circle" looks like in LFS, maybe more like a "traction hexagon". :P Also, traction circles for different tire types will look different than others. For example, put some drag slicks on your car and try to run an autocross track with it- probably wont work too well. Likewise, the tires that they use in Nascar probably arent the best for drag racing. Heck, they even have special tires that they use for drifting now a days. illepall

But in thinking about it more, it makes me think that LFS uses a traction circle type of model for the tire dynamics, but really I think having separate models for longitudinal vs. lateral grip might work a little better. I just know that I've had my big brick of a Camaro (which are known to be "tail happy" anyway) with crappy street tires on the edge of lateral traction many times, and pushing the throttle will not snap the car around. In fact, most of the time the car will push understeer a little. I really have to jerk the car hard or use an excessive amount of throttle going into the turn to bring it sideways. Now, if I'm on rain or snow, that is a COMPLETELY different story.....

Brendan

hmmm... very interesting. You realize I now have to go to the skid pad and waste more time.

I like the hexagon. I use that on some mental level. It seems like LFS has more braking realism than accelerating realism. I can't really tell where weight transfer would be wrong though. It seems more like tires at this point. I feel like I'm reading the wieght transfer well enough, just not the foot transfer.

So.... it's an RS is it? I've owned more than a few IROCs and Z-28s, and you can certainly get the rear around at any point. For a brick they sure work great, eh? Not the ideal sports car, but as fun as American front engine gets. I was getting pretty good at rolling 360s in my 92 before I sold it.

Yeah but those camaros are basically pickup trucks with a car body They
ARE easy to slide and do it well, my 240sx was a bit like that too. Tires also
play a part in the behavior at the limit obviously, but i still feel the problem
is in the lateral grip, i don't have much problem with the 'traction', but i do
find that when at the limit, lateral grip seems icy, like driving on rain.

Let's get some graphs man, i think with Gmeter i could make a plugin to output
data...thats what it does after all, just need a graph, but that eh, would be
an app in itself, ehem. Just unsure how to get usefull data, how do you seperate
lat/long ? Either that or a way to measure both individually. using skidpad and
drag strip. Ideally, both ways should be used to confirm i guess. Anyone have
some ideas or some other ways to measure this? RAF outputs or something,
i'm not familiar much with that myself. Colcob, surely you must have an idea

Quote from Bob Smith :

If you remember Tristan, Todd's tyre model doesn't use any traction circle theory (or so he says) and apparently works very well (so he says). So perhaps you don't need to know what it is (although the underlying principals are important).

Just a thought.

My model doesn't explicitily use the traction circle, but it indeed does work out to follow the theory anyway. I.e., there is a maximum force vector and adding longitudinal traction/braking force will influence the side force according to friction circle theory, although the code does not directly apply the theory. Because of the approach I'm using, the theory "just happens" rather than specifically coding for it as in my previous models (like the one in VRC).

Whether or not it's nice, you don't have to take my word for it of course. Check out what Gregor said about it today

http://www.west-racing.com/for ... amp;st=25&#entry54854

Anyway, it's obvious that LFS does indeed follow friction circle theory at the limit. You can feel it and also see it happening in the (F)orces view. Watch the forward/side forces. As you get a bunch of side force and then increase longitudinal force (either traction or braking) the side force will indeed drop. If the resultant vector was drawn it would be easier to see how it works, but basically yeah, it's obviously working according to the theory too.