Relaxation length

A tire does not produce its steady-state force the instant slip appears. The carcass has to deflect, which takes about a third of a metre of rolling: the relaxation length. The core models this as a first-order lag on each slip quantity with time constant σ / V.

Two things follow:

1. Steering feels progressive rather than instant, and the lag grows at low speed.
2. At very low speed the time constant goes to infinity, which is right: the core integrates the contact-patch deflection itself, driven by the slip velocity and decaying at the rolling speed (ADR-0010). At speed that is the lag above; at standstill it is a spring between the contact patch and the road, so a parked car holds a static deflection on a slope instead of creeping. A damping term that fades out by lowSpeedDampingFade keeps that spring from ringing, and the deflection is capped at the force peak so a car pushed past its grip slides.

The transient slips (the deflections divided by the relaxation lengths) are part of the snapshot and appear in telemetry as SlipRatio_* and SlipAngle_*.