Ok, here's my little dissertation on why combined slip might be responsible for what's happening with the front wheel drive cars.
Here we have three brilliantly illustrated pictures of a car travelling forward toward the top of the page. The car is in a left hand turn and we're looking at the forces and components thereof at the right front tire.
Picture A is cornering with some throttle. The black line is the traction force due to slip ratio which is pointing in the same direction the tire is pointing. Steered a bit to the left. The green line is the lateral force that's pointed in the same direction as the spin axis of the tire. The red line is the sum of both forces. I.e., the red force is identical to the black and green lines acting together as separate forces. (The grey lines at the tip of the red line are supposed to be parallel to the black and green lines, but I'm not coordinated enough to do that perfectly by eye
Coders often make lousy artists
Now, what we'll do is increase the throttle (traction force) while keeping the steering the same. Two things can happen depending on how the tire model handles combined slip. These are represented by pictures B and C which are supposed to have the same traction force and direction (black line)
In picture B the lateral force does not change when you increase the throttle. This is the most frequent screw up that new sim developers do and is often the main reason why when you get a new guy on the sim dev block there is just something wrong with the handling that you can't quite put your finger on. With front wheel drive it becomes much more obvious, but with rear wheel drive it's a bit more subtle.
With rear wheel drive you end up with handling such that subtle changes in throttle don't change the attitude of the car in the way you'd expect, if at all. Increasing throttle increases rear weight transfer and therefore lateral force at the same time, which causes a reduction in rear slip angles (the car tends to understeer when you're under the limit). This is because the lateral force is not being reduced as the traction force is increasing as it does in a real tire.
With this type of approach, the lateral force doesn't begin dropping until, and this is very important, the combined force (the red line) hits the edge of the friction circle. Then, one or both forces are modified in order to fit friction circle theory, which says that the red line can only be so long. So if you calculate the length of the black line by using slip ratio, then calculate the length of the green line by using slip angle, the resultant (the red line, the "combined force") can be too long.
There are at least three main ways to deal with this in a Pacejka type or other strictly empirical model (which I think LFS does not use, correct?). One of them is fairly complex and I'm not entirely sure how it works, but is quite good and gives proper results. The other two are the more obvious things to try and kept me up thinking many nights in the early days
One way is to let the longitudinal force stay right where it was and then scale back the lateral force in order to bring the combined force back inside the traction circle. There's a very serious drawback to this method though and many of the sims you've driven over the years worked precisely this way. If your slip ratio hits 0.1 or so, which might be the peak of the traction force, the lateral force completely disappears no matter what the slip angle is. Yikes..
Ok, let's rewind just a little bit to illustrate what this does to the handling of the car. Imagine you're in the turn (still RWD, I've gone off on a tangent here). You're nowhere near the limit of traction. You're well inside the friction circle. Now, you start increasing throttle slowly towards full. If your tire model is calculating the length of the green line (lateral force) strictly as a function of slip angle, it will not change length at all. The lateral force at the rear of the car stays the same in the absence of any rear weight transfer. With the weight transfer the lateral force increases and the slip angle reduces. I.e., as you begin feeding in the throttle you get more and more understeer.
At some point the combined force (the red line again) finally hits the traction circle and you rather suddenly begin scaling back the lateral force. Suddenly, you are getting oversteer and even a little bit more throttle might put the traction force right over the limit. The lateral force plummets to near 0 very quickly.
Sudden, uncontrollable, snap oversteer with no warning whatsoever. Sound familiar?
This is precisely to me how LFS felt until last April's patch (Q?) and is what I was going on about with the combined forces being way, way wrong. After that it was *drastically* improved and as a result the cars felt much easier to drive. And of course, many people cried "arcade" when in actuality LFS had taken a massive leap forward in the realism department.
On to the second of the three main ways to deal with combined forces in an empirical type of model: Instead of leaving the longitudinal force right where it was and scaling back the combined force (red line) to fit the traction circle, you just scale the combined force back. The direction of the force doesn't change then, but the size of it does. The traction and lateral force (black and green) simply scale back and remain proportional to each other.
EDIT: The above paragraph should read:
On to the second of the three main ways to deal with combined forces in an empirical type of model: Instead of leaving the longitudinal force right where it was and scaling back the lateral force (green line) to make the combined force (red line) fit the traction circle, you just scale the combined force back. The direction of the force doesn't change then, but the size of it does. The traction and lateral force (black and green) simply scale back and remain proportional to each other.
This produces much nicer results without the sudden snap oversteer following understeer with slowly increasing throttle in a RWD car. This is precisely how Virtual RC Racing's combined slip model works (the public version that is, the new version in development is totally different). This is wrong too, however. Again, you have a situation where the lateral force is not changing as you feed in more traction force at all until you hit the friction circle. Then, it suddenly begins scaling back, but it does it more subtly and in combination with the traction force.
Both approaches are wrong at the friction circle limit and even more so when operating below the limit, as increasing traction force in that area does not change the lateral force at all. You get increased rear weight transfer which reduces the rear slip angles (increasing understeer), then suddenly at the limit the tires let go as the lateral force suddenly begins plummeting. You can't really steer with the throttle properly by coaxing the rear end out when under the limit like you can in reality in many cars.
When it's done properly, the lateral force will drop with increasing traction regardless of the slip angle. Even when you're way under the limit. As you approach the limit it winds up very smoothly transitioning into limit behavior. As such, you can steer with the throttle quite nicely. It's all very predictable and drifting actually becomes quite a lot easier. Suddenly you find yourself steering as much or more with the throttle than the steering wheel, even when you're below the traction limit. I've yet to see that anywhere except in Gregor Veble's model for Racing Legends and my own.
Ok, back to the FWD (that was a long tangent):
Starting at figure A, we are cornering pretty hard, a bit under the limit of the front tires, with some throttle. We then increase throttle. If our combined slip model works as either method described above, we can easily find ourselves in situation B. The lateral force stays the same and the combined force (red line) gets bigger and changes direction.
The blue line: This is the yaw component on the car that is trying to twist it to the left. If that gets longer we tend to get oversteer. See picture B? We increase throttle, the blue line gets longer, and we go away from understeer or might even get oversteer if it's long enough.
With a proper combined slip model, we should wind up with something more like figure C. We increase forward tractive force, the lateral force (green) drops, and the combined (red) force grows and changes direction. My illustration shows the combined force being shorter, but really I meant it to grow instead. However, the result is that the blue line gets shorter, meaning less lateral force or yaw torque in the car's coordinate system, and we get understeer instead of oversteer.
The combined slip model in LFS seems quite good to me. At least as good if not better than any other sim I've tried. Before patch Q it was, umm.., not so good, but now it's quite super. However, there is probably room for improvement here and what we see with the front wheel drives is likely proof that something is just a little bit amiss there still.
It's still my favorite sim to drive though, aside from my own of course
The new sounds are just fantastic and now that I've got a G25 wheel I can finally use FFB without getting all the rocking of the wheel around the center. The FFB is very good in LFS. FFB has kind of ruined some of the other sims for me in comparison, quite frankly