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Self-Aligning Torque, or the lack thereof?
(22 posts, started )
Self-Aligning Torque, or the lack thereof?
I was just playing with the new nKPro demo, and figured I could test its tyre modelling by setting the castor to zero. Well, it was what I expected: pure self aligning torque coming through the FFB. But then I went back to LFS and put castor to minimum on the FBMW, I got nothing from the FFB! If LFS truly does honestly transmit tyre forces through FFB, it probably means that self-aligning torque is not in LFS? I thought this torque would come naturally with LFS's "physical" modelling of the tyres.

Running with castor of course works, but I'm a bit disappointed.
LFS's FFB is much better than NetkarPoo. You're making comparisions between different games and cars and in my opinion, that's not a fair comparison.
It was my understanding that zero castor actually should equal no straightening force. Positive angle equals straightening/stability, whereas a negative angle would be the opposite.

I wonder what the force straightening the steering is, in nKpro, if you set zero castor.

Of course I may well be wrong.
Quote from sinbad :It was my understanding that zero castor actually should equal no straightening force. Positive angle equals straightening/stability, whereas a negative angle would be the opposite.

I wonder what the force straightening the steering is, in nKpro, if you set zero castor.

Of course I may well be wrong.

There is aligning torque arising solely from the tyre's flexibility; that thing is self-aligning torque; that thing I feel in nKPro.

Says wiki:
"Self aligning torque, also known as the aligning torque, SAT or Mz, is the torque that a tire creates as it rolls along that tends to steer it, ie rotate it around its vertical axis. Even if the slip angle and camber are zero, and the road is flat, this torque may still be generated due to assymetries in the tire's construction. Typically for a producion tire this torque reaches a maximum at 2-4 degrees of slip (this figure is very dependent on a lot of things) and falls to zero as the tire reaches its maximum lateral force capability."

You don't feel anything in LFS that resembles "...this torque reaches a maximum at 2-4 degrees of slip and falls to zero as the tire reaches its maximum lateral force capability."
Zero caster will offer no self centering action.
Quote from yoyoML :You don't feel anything in LFS that resembles "...this torque reaches a maximum at 2-4 degrees of slip and falls to zero as the tire reaches its maximum lateral force capability."

It has bothered me that caster needs to be set so high to get good feel you could be right. As good as the tyres are I think they are still in a pretty basic form - but simple is often better than complex - I'm sure tyres will develop more down the road when LFS is more complete overall.
Isn't this one of those mechanical trail vs pneumatic trail thread? IIRC they usually end when someone point out PT is not yet in LFS
Quote from NightShift :Isn't this one of those mechanical trail vs pneumatic trail thread? IIRC they usually end when someone point out PT is not yet in LFS

Apparently so and I agree that this is what the op is seeing...

http://www.lfsforum.net/showthread.php?t=47581

I wonder if the simulation of pneumatic trail would have an impact on how FWD vs RWD feels currently in LFS?
Quote from Glenn67 :Apparently so and I agree that this is what the op is seeing...

http://www.lfsforum.net/showthread.php?t=47581

I wonder if the simulation of pneumatic trail would have an impact on how FWD vs RWD feels currently in LFS?

So it seems PT isn't there...

FWD should really make you feel through the pneumatic trail, where more engine torque means less trail. But of course braking should greatly increase the PT, including in RWD. I guess most obvious would be that lightening feel when understeering due to steering too much (instead of spinning or locking).
Quote from yoyoML :So it seems PT isn't there...

It also makes me wonder if the effects of PT might make the locked diff behave more relisticly on the FWD...
Well I also wonder if/when the issue with longitudal grip will be tackled.

Guess we'll have to wait for a long time before one of those question receives an answer
We could try to write petition-thread with request to Scawen for Tyre Physics Patch after Scirocco is done.
Maybe pointing all the tire related bugs and shortcomings in one place will give Scawen some thoughts about it.
Quote from yoyoML :I was just playing with the new nKPro demo, and figured I could test its tyre modelling by setting the castor to zero. Well, it was what I expected: pure self aligning torque coming through the FFB. But then I went back to LFS and put castor to minimum on the FBMW, I got nothing from the FFB! If LFS truly does honestly transmit tyre forces through FFB, it probably means that self-aligning torque is not in LFS? I thought this torque would come naturally with LFS's "physical" modelling of the tyres.

Running with castor of course works, but I'm a bit disappointed.

http://www.lfsforum.net/showthread.php?p=539489#post539489
#14 - senn
i don't know if this self aligning torque thing is related, but i have always found the FWD cars to be quite unrealistic with their FFB.

I used to find my mildly tweaked fwd corolla would torque steer (left if i remember correctly)like a pig (no power steering mind you) and it didn't even have 100hp at the wheels, i can't imagine what a 200+hp turbo fwd would be like torque steer wise, but i imagine worse, especially with fatter tyres.

The FWD's in LFS have always felt kind of "light" in the steering and u certainly don't get any of that fwd wheel hop/driveline chatter feel through the steering wheel.

Is this similar to self aligning torque, or have i ranted about something completely unrelated....
Quote from senn :i don't know if this self aligning torque thing is related, but i have always found the FWD cars to be quite unrealistic with their FFB.

I used to find my mildly tweaked fwd corolla would torque steer (left if i remember correctly)like a pig (no power steering mind you) and it didn't even have 100hp at the wheels, i can't imagine what a 200+hp turbo fwd would be like torque steer wise, but i imagine worse, especially with fatter tyres.

The FWD's in LFS have always felt kind of "light" in the steering and u certainly don't get any of that fwd wheel hop/driveline chatter feel through the steering wheel.

Is this similar to self aligning torque, or have i ranted about something completely unrelated....

This is indeed similar to self aligning torque. However, strictly speaking, "self aligning torque" is the torque that exists strictly in the tire's coordinate system around an axis going straight down through the top of the tire to the ground, dead center in the middle of the contact patch.

http://www.performancesimulations.com/files/sat4.JPG

Self aligning torque is illustrated by the left column of pictures. The little red circle is the center of the contact patch (or any point along that line that is sticking out of the screen at you). The lateral force (cornering force) is the red line. At different slip angles the center of force moves around a bit, which is why you get self aligning torque at the tire (as opposed to the steering wheel) that changes with the total force and slip angle. When you see results from a tire force test where (self) "aligning torque" is shown, you are looking essentially at the blue line multiplied by the red line. This is only part of the force feedback picture, however, and many sims miss this entirely (not all sim programmers are vehicle dynamics experts) which is largely why these discussions happen at all

What you're really talking about in your comment above is "steering torque," which is what you feel at the steering wheel. The "self aligning torque" is part of this, but not the entire picture. Generally it's only a small part of it, really. Many sims only seem to pay attention to the "self aligning torque" part. When you drive a sim and find that changing the caster angle has no effect on the "steering torque" (what you feel at the wheel; the actual force feedback), and more importantly, if you find that the steering torque gets stronger to a point and then suddenly drops off to nearly nothing as you get to the cornering force limit, you're most likely looking at a model that is using nothing but the self aligning torque (the part measured only at the tire). rFactor and NetKar seem to be good examples of this. However, to be fair, if the caster angle is 0 this is largely correct.

This isn't all as complicated as it might sound. All you really have in the end is a force at the tire (or you can consider them to be 3 components of the same force: lateral/right/left, longitudinal/forward/backward, and vertical), and a torque arm. When attached to a car, the tire doesn't really steer around the center of the tire contact patch like you see in the left column of the sat4.jpg diagram. Instead, it steers around an invisible axis in space between the upper and lower steering knuckles on the suspension. More programmer art:

http://www.performancesimulati ... m/files/steeringaxis2.JPG

The red lines are (supposed to be ) the upper and lower suspension arms. The black line connecting them is the steering axis. The tire steers around this. In the diagram on the right side, there is a longitudinal (forward or backward) force from the throttle/brakes. This force does indeed cause a "steering torque" since it's offset from that steering axis by some distance. In a RWD car you only get a force here generally when you're using the brakes, so it's not nearly as noticeable as it is in a FWD car. In the above diagram, it's not hard to imagine that if you pushed forward on the back of the tire it would try to steer to the left, and vice versa if you pulled backward on it.

This is the same diagram basically, but showing steering axis along with the kingpin and caster angle:

http://www.performancesimulations.com/files/steeringaxis.JPG

In the end, you just have a force at the tire and a distance to that steering axis, basically. This is where the "steering torque" comes from. There is also a bit more modification to this as a result of the steering linkages that connect the tire ultimately to the steering wheel through the tie rods and so on, but it isn't all that necessary to understand that in this discussion so far.

In the sat4.jpg picture, the steering axis is represented by the green circle in the picture on the right. For the sake of this discussion it would be good to draw a third column with the steering axis offset to the left or right some amount too. Then one could see how the forward/rearward forces also contribute to the "steering torque."
Quote from jtw62074 :
... rFactor and NetKar seem to be good examples of this. However, to be fair, if the caster angle is 0 this is largely correct.

Just for info, the recently released 1.0.3 patch of NetkarPro fixes this problem. Kunos explained that in previous releases the caster angle was always fixed at 0 and now is correctly set.
Quote from flyby3d :Just for info, the recently released 1.0.3 patch of NetkarPro fixes this problem. Kunos explained that in previous releases the caster angle was always fixed at 0 and now is correctly set.

I can confirm this. I've tried different caster angles, including 0 of course, and the steering forces are correct.
Good to hear, YoYo and Flyby. I don't have a working wheel at the moment so can't try it out. Does the steering torque drop to nothing around or passed the limit?
Quote from jtw62074 :Good to hear, YoYo and Flyby. I don't have a working wheel at the moment so can't try it out. Does the steering torque drop to nothing around or passed the limit?

I can only say, when the torque drops to nothing, there's quite little lateral force change already.
After reading this interesting thread I tried the nkPro 1.03 .I spent some time in tuning force feedback parameters, and I get the steering torque to behave the way Todd says: increasing strength up to a point, and then the steering feels weaker, but not a lot weak, only small drop in force. This decreasing point seem to be the maximum point of grip and also the sound of the skid is present.

So it should be very close to real, however I have not a real racer so i am not sure.

I have not tried differents caster values yet. If I understood well, the more caster the less drop in force feedback when maximum grip is reached, but more overall strength.
Quote from S14 DRIFT :LFS's FFB is much better than NetkarPoo. You're making comparisions between different games and cars and in my opinion, that's not a fair comparison.

How's it an unfair comparison? Comparing piloting a bicycle to a Boeing 747 is an unfair comparison. Comparing two supposedly high-fidelity racing sims using identical hardware ... couldn't get much fairer.

Sorry. Had to play the devil's advocate.

Self-Aligning Torque, or the lack thereof?
(22 posts, started )
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