Sounds silly, I know... but that's my latest theory and epiphany from this weekend. Potentially a pretty powerful concept. Everything I've gone over up until this point has been pretty basic and straightforward.. but this is actually 'good stuff.' A lot exists in the public domain already, going back to the 80's or 90's. Despite that, I'm not giving too much of this away.
A hint, is an epiphany from a couple years ago... that you really have to keep in mind the relative importance of roll response and yaw response when it comes to handling. Who gives a shit about roll response for the sake of roll response? Turn-in is about yaw. You can throw all the spring and bar, bump stop and coil bind you want at a vehicle... and at some point it doesn't do anything for you and you still have a car that's lazy. That point can come earlier than you might think.
Anyway, here's the gist of things... the more I learn about this crap, the less interested in what I used to think of in terms of transient vehicle simulation. Lap sims tell you a little bit about your car, but it's easy to completely miss a lot of the insight to why the car drives the way it does. A lot is really analogous to a spring-mass-damper system. I could have a S/M/D sitting on my desk, perfectly still. I don't need to poke it or run a simulation of it... so long as I know three basic constants (time invariant) I can completely characterize and understand the dynamic response of the thing... as it sits there in its static or quasi-static state.
I'll have to put some numbers to it... but this would make a lot of sense as to why you could have two vehicles with similar kinematics and setups... even the same instantaneous understeer gradient of neutral or even understeer... but one still feels loose and unpredictable relative to the other.
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Interesting. Not sure I agree with the SMD concept being invalid, at least when looking at yaw/steering response for a 'linearized' system in a small operating window.. be it on-center or off-center. In any event, the example was to illustrate that 'steady state' parameters like front and rear cornering stiffness are good indicators of dynamic response.. not unlike knowing directional effects of increasing 'k' or 'm' terms for a SMD system.
Out of curiosity, what's your background?
I'd been thinking of the rear roll-steer thing though. Good to hear some extra justification of it. On a downforce car, maybe even something worth playing with progressive rate springs or bump stops.. so at high speed the roll motion of the chassis is small, and the roll-steer component small along with it.
Good points. Still, again.. the point isn't so much to say that yaw response is a purely '2nd order denominator' system.. as much as that the (more or less) 'time invariant' tire properties drive the dynamic response of the car... and that changes in derived 'time invariant' values point to directional changes in dynamic response. Similar to Mimuro's work.
The easiest analog to that system, for your average Joe Engineer, is a SMD system... where if you understand the thing you can roughly predict dynamic response by looking just at the steady-state terms.. ie without having to do an actual dynamic simulation. That's the point I'm trying to get at.
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