Friday, March 26, 2010
Still gonna be slow around here...
Giving a race tire data seminar in Florida tomorrow of all things.. then catching up on a million agenda items when I'm back. Sadly the racecar side project has taken a back seat to the whole day job thing...
Sunday, March 7, 2010
There's a fine line between steady-state and transient vehicle dynamics
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.
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.
Anyone familiar with wind tunnel data?
The whole thing of stability & control derivatives and such has made me think about this... since at 100+ mph sideforce and aerodynamic yaw moment are probably non-trivial. I don't suppose anyone out there is familiar with typical wind tunnel yaw sweep data?
What I'm wondering is this... with tire data, there is a linear range for both Fy and Mz which usually only lasts for a few degrees of yaw, before saturating, or peaking and dropping off significantly. See below (grabbed off Google but illustrates the concept)
Does aero data have a similar trend? Within a realistic range for chassis side slip angle (say 6 degrees?), are aerodynamic Fy and Mz pretty linear? Do they saturate? Peak? Do something wild?
What I'm wondering is this... with tire data, there is a linear range for both Fy and Mz which usually only lasts for a few degrees of yaw, before saturating, or peaking and dropping off significantly. See below (grabbed off Google but illustrates the concept)
Does aero data have a similar trend? Within a realistic range for chassis side slip angle (say 6 degrees?), are aerodynamic Fy and Mz pretty linear? Do they saturate? Peak? Do something wild?
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