slip rate
- sims rarely provide tire slip telemetry
- deducing slip from generally available SimHub properties is problematic.
Denying access to tire slip information makes sense for online competitions,
since it enables cheating by stability control, which is a well-explored,
thanks to e.g. FMVSS 126.
Arguably, the bible and evangelist for computational vehicle dynamics are
“The Multibody Systems Approach to Vehicle Dynamics” and Damian Harty,
whose widely repeated quote took me far too long to appreciate:
“Most drivers are acutely sensitive to the rate of change of body slip angle.”
- while tire slip angle directly influences grip, drivers do not directly sense it
- better to provide proxy feedback for sensed body slip angle rate
- SlipAngleRate calculation relatively is simple…
Small angle approximation
Tire slip angles greater than about 6 degrees are doomed.
For angles less than 6 degrees, tan(y/x) is nearly y/x in radians.
Basics:
A rail car rounding a curve has no side slip:
- its lateral acceleration
LA = Speed * Speed / R - it also has a yaw rate (change in yaw with time)
YawRate = Speed / R - then required lateral acceleration
LA = Speed * YawRate
Vehicles circulate skid pads typically with some slip,
their actual lateral acceleration differing from Speed * YawVelocity.
Difference between Speed * YawVelocity and actual lateral acceleration is called body slip rate;
body slip rate divided by Speed is body slip angle rate.
SimHub properties from which to calculate body slip angle rate:
- AccelerationHeave (AKA lateral acceleration)
- OrientationYawVelocity
- SpeedKmh
Then SimHub slip rate = ff * SpeedKmh * OrientationYawVelocity - AccelerationHeave
… and slip angle rate = ff * OrientationYawVelocity - AccelerationHeave / SpeedKmh
… where fudge factor ff = 1 for property values with compatible units of measure.
For games and cars tested in SimHub, fudge factor ff ~ 0.00485:
SlipAngleRate = 0.00485 * OrientationYawVelocity - AccelerationHeave / SpeedKmh