Hi Danny,

You up for a discussion about the stability index you mentioned in your video blog? Just to clarify, I'm speaking of the equation used by yourself and Milliken:

SI = (dN/aAy)(1/mL)

I've been stuffing about with the stability index calculations with a mix of FSAE and non FSAE data in both simulation and from track data. I thought it would be appropriate to discuss here and hopefully it can develop into a useful discussion for everyone.

So... Milliken (if I understand correctly) describes the SI as a representation of the static stability moment (dN/dß)ß and the yaw damping moment (N_r x r) for a constant control moment (dN/dδ)δ.

Myself and another aquaintance are trying to get our heads around two things:

1. Theory: how applicable is this linear, steady state calculation to non linear/transient track data

2. Practice: how do you filter/calculate this without ending up with nothing but noise?

On my first point, I'm a bit skeptical about how valid it is to calculate dN/dAy from track data considering that its definition comes from a condition of constant steering angle (not to mention trimmed steady state).

For me, the fact that the driver is continuously correcting the steering on track means that the yaw acceleration (and therefore yaw torque) is not a function only of the static stability moment and yaw damping moment but also the control moment. In fact I'd guess that most of the yaw dynamics of the car in a turn are coming from the driver's steering inputs rather than the yaw damping and static stability moment.

My second point is about implementing this on track data. With dAy in the denominator of the formula, the resulting signal is constantly going to +-infinity. When I've put the first equation above into my track data, the result looks like white noise. I don't want to filter out high frequencies because I feel that these higher frequency (ca. 10Hz) movements are exactly what we are trying to measure. Am I going about this the right way?

I hope I've explained myself properly. Would be interested to hear anyone elses comments on this...

T

PS, this forum needs an equation editor!!