Scaff
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You know I was thinking about this center rotation point thing again alot while I was in the shower, I don't know why, I just was......anyways, to understand let me give you this visualization....... Imagine yourself from a birds eye view looking straight down at a car as it comes to the turn, it's a right hand turn, the car will understeer and respond slowly to the steering inputs due to the front tires being overloaded by turning the wheel too sharply, as the car slows down and begins to grip it will neutralize the understeer untill the car beins to go sideways due to a literall case of oversteer, then the driver will balance the car out and make the mistake of letting the rears suddenly regain traction sending the car into a spin to the left coming to a stop in the other lane facing the opposite direction.
Now think of how the car was rotating during all of this. The car will rotate at the EXACT MIDDLE as the car is understeering and even when it gets to a complete neutral state, will still be rotating in the exact center betwen the front and rear wheels. HOWEVER, once the car begins to oversteer the point of rotation will quickly make it's way forward. A tiny bit loose will have the point of rotation 3/4 of the way between the front and rear wheels toward the front. This is where the car is cornering the best, just a shade loose, anyways, the point of rotation will quickly make it's way to the exact front of car between the front wheels and will stay their as the car is drifting, however once the mistake of overcorrecting has occoured the point of rotation quickly moves to the rear of the car untill the front wheels grab more than the rears again and the car starts sliding the other way, moving the point of rotation to the front of the car again untill it stops.....then, obviously there is no point anymore.
This has 100% to do with the position and direction of the momentum of the car and that has alot to do with the grip levels at each tire. Momentum is created by grip level indifferences and grip level indifferences are created by momentum. All of this is very dynamic.
GT4 Does this all perfectly, As does EPR, all of the codemasters games fail miserably, this is why they float, this is why they have glitches and unrealistic things happen in them.
I have to say that if you got this far just from thinking it out then kudos to you, some of the basic here are right, but I do have to correct a few areas.
First you have correctly Identified a commonly overlooked area of sims, which is that of rotation, so lets have a better look at exactly what is involved in rotation.
Now the following diagram shows the three axis of rotation that a car encounters.
Now the principal one we are interested in here is that of yaw (Z Axis) and this will always occur at the vertical line through the Centre of Gravity for the car. Now the centre of gravity of a car will vary depending on the individual car, and while it will move during load transfer, this movement will generally be minimal. The point of rotation does not move around to the degree you have described and certainly under and oversteer does not depend on the location of the point of rotation.
So how exactly does it relate to under, over and neutral steer?
Well first of all every car will have a natural desire to yaw when its direction is changed, and every corner will have a every corner will have a yaw angle which depends on the speed the corner is taken at and the radius of the corner. The yaw angle is the angle between the direction a car is pointing versus the direction it is travelling in, as shown in the left hand diagram below.
The yaw is determined in part by the slip angles of the tyres themselves, as shown in the right hand picture above.
The following section is taken from Skip Barber's book Going Faster and explains these states excellently.
Redefining Attitude
Up to now we defined understeer and oversteer in terms of which end of the car 'slides' first. While this is an OK conceptual way of describing the sensation it not really true. A car does not need to slide to exhibit understeer or oversteer. A tyre does not have to be at its cornering limit to encounter slip angles.
Even at low speeds and cornering loads, cars develop slip angles at the front and rear tyres. Consequently the car as a whole develops a yaw angle. At low loads the slip and yaw angles are small, but they're there. A more accurate way of defining a cars cornering attitude at both low speeds and high is to compare the slip angles of the front and rear tyres.
Going Faster then goes on to use this to describe different attitudes a car can take.
A 100 foot radius arc is used, with a Yaw of 8 degrees required to travel the arc.
Understeer
Front Slip Angle - 14 degrees
Rear Slip Angle - 6 degrees
Yaw - 4 degrees
The yaw of 4 degrees is lower than that of the yaw required to travel the arc and the car will push forward and wide.
Oversteer
Front Slip Angle - 10 degrees
Rear Slip Angle - 16 degrees
Yaw - 14 degrees
The yaw of 14 degrees is greater that that required to travel the arc and the car will take a line that is too tight. If the Rear Slip Angle increases the Yaw will increase dramatically and the car will over rotate (Power Oversteer), however if the fronts have also exceeded their Slip Angle then the car will push wide while over rotating (Power Understeer).
Neutral
Front Slip Angle - 10 degrees
Rear Slip Angle - 10 degrees
Yaw - 8 degrees
The yaw of the car matches the yaw required to travel the arc.
As the above examples show, understeer and oversteer are characteristics that occur at any speed and both with and without a loss of grip. This means when we are looking at changing a setting that would increase or reduce oversteer, it does not mean that grip has to be lost to feel the effect; just that the balance between the front and rear slip angles of the tyres will change.
Conclusion
Now the above piece shows the direct relationship between yaw and tyre slip angle, during this the point of rotation (yaw) does not significantly move at all, rather we can use the balance between the front and rear slip angles of the tyres to increase, reduce or maintain yaw. Now how this applies to drifting is discussed in Going Faster when over-rotation occurs and is corrected.
Skip BarberCorrection
The first step is with the hands - dial in opposite lock, steering in the direction of that the rear end of the car is trying to go. This is the correction phase of skid recovery.
Settle the Rear
The second, almost simultaneous action should be to settle the rear of the car by transferring weight on to the rear tyres. You do this with throttle application - a little throttle application. A common mistake here would be to jump on the power to try and save the slide. if you did that you will have lost cornering traction at the rear by asking too much acceleration traction.
By Squeezing the power, applying 30% throttle or so, you move the load to the rear and cancel the engine braking effect of abruptly snapping off the brakes.
The rotation stops
The car begins to recover. The rotation around the centre slows, then stops from the steering wheel correction and throttle application. You are in the eye of the storm. The rotation has passed its peak. This is the pause phase, your cue to start taking out the opposite lock.
The Need for Recovery
Now the rotation starts back the other way, counterclockwise, reducing the car's overall yaw angle. If you don't take the steering out fast, you'll get caught with the car back at zero rotation, a lot of load on the right tyres and the steering wheel turned left. you will have saved the clockwise slide but got caught by the reaction of the rear rotating counterclockwise - you'll end up spinning back in the opposite direction.
This time you do it right and quickly get the wheel back to centre following the pause: you successfully dissipate the rear end's momentum back towards the right.
So what does GT4 and EPR get right that codies games get wrong? Well in my opinion its in the relationship between rotation and tyre slip angles, quite simply Codies games don't seem to both with the tyre side of things that much, relying on pure rotation.
Regards
Scaff