Further in depth information
The IS has the ability to handle very well but historically possesses an unstable chassis in particular the front camber positions.
As the camber creeps toward the negative it also reduces the castor angle which is vital for high speed stability. Here is an explanation taken from wim-web.
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Tilt State Phenomena
In the chapter dealing with the camber angle of the wheels, it can be seen that the camber angle varies according to the positioning or length of the steering arms under the effect of the compression and release of the suspension,
This effect is very useful when steering around bends, when the centrifugal force deforming the parallelograms formed by the suspension units gives the outer wheel on a bend a negative camber and the inner wheel a positive camber,
The same result is obtained on the wheels with the castor angle, but this is generated on the basis of the steering width and the vertical movement of the suspension arms, so the following conclusions can be drawn: when the wheel turns about a kingpin with a positive caster angle, if it is in the outer position on a bend it will take on a negative camber that increases with the strength of the steering action, and will thus act against the overturning of the vehicle, if on the other hand, it is in the inner position on the bend it will take on a positive camber which follows and assists the turn,
Consequently, when the vehicle takes a wide bend (small steering action) at a very high speed, it will be the same centrifugal force which, by acting on the deformability of the suspension parallelograms, will create the advantageous position for the wheels: however, when the bend taken is very tight (strong steering action) and the speed is moderate, it will be the castor angle that will create the advantageous wheel camber angle,
Other factors influencing the castor angle, low-pressure tyres, in modern vehicles, contribute to reducing the castor angle given to the kingpin during construction, in fact, when under the influence of active or braking thrust, the tyres deform and tend to increase the longitudinal offset by shifting their point of contact with the ground,
The castor angle also varies with the variation in the weight distribution of the vehicle, If the load is shifted to the rear axis this will increase the positive castor angle, and if it is shifted to the front axis it will decrease it,
True vertical Kingpin axis extension
Theoretical pivot point
Trail distance
Longitudinal castor angle/ set as positive, the kingpin axis falls in front of the theoretical pivot point generating the trail distance, this gyroscopic affect guarantees stability, and can be defined as the castor angle.