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?
Tuesday, February 9, 2010
Wheelbase, stability and control derivatives, etc
There was a thread over on F1 Technical earlier discussing the merits of a short versus a long wheelbase (incidentally one of many parameters I haven't nailed down). When I was thinking of how to reply it dawned on me that I had little objective or rigorous data to back up what my initial thoughts were. As a start, some might think that long wheelbase implies stability, whereas short wheelbase implies being nimble.
As a generality, I'm not really convinced that's true, at least thinking about it to a degree in 'derivative notation.' Milliken goes over this idea around page 149 of RCVD. In a nutshell it wraps some basic tire and basic vehicle concepts together into a fairly powerful but simple way of describing vehicle dynamics.
All other things being equal, a longer wheelbase should imply higher 'static' directional stability; if you split the axles apart further it should require more torque to 'dislodge' the car from a particular attitude. It should feel planted and secure.. but that's not to mean unresponsive. At the same time as you increase the distance from the front axle to the CG you bump up the control moment derivative; the front tires have a larger moment arm with which to act about the CG, and create higher yaw moment for a given steer angle.
Given that most of my regulars here have FSAE experience, the following example may be enlightening. If there's a series that needs sharp, fast, predictable response almost all the time.. FSAE is it. If you were to test the Goodyear D2692 and D2696 back to back on identical cars, you'd find some big differences in how they drive. The '96 generally should have higher response rates (cornering stiffness) overall, while in the same construction, size, etc. As such, the static directional stability is higher. The car can feel more settled and planted. At the same time it is much more precise and responsive to steering inputs, since the front response is also up! The '92 by comparison, with lower control and stability derivatives, can feel both vague and lazy. (The '96 also has higher ultimate grip, comes in faster, and has unbelievably good wear. Pretty good improvement.)
On the other hand, while the control moment derivative increases linearly with "a" (distance from CG to front axle), if you think of that front axle as a point mass it's contribution to vehicle yaw inertia increases with a^2. In theory then you'd think a car with a short wheelbase would offer higher yaw acceleration capacity. My question is - how much yaw acceleration do you fuckin need? "More" of anything is not always better, except for beer and scallops. I may have to see if I can dig up some old DAQ to see just how much you need. If you already have 2x the acceleration potential that you could realistically need.. why add more when there may be benefit elsewhere?
I'll let you marinate on that.
As a generality, I'm not really convinced that's true, at least thinking about it to a degree in 'derivative notation.' Milliken goes over this idea around page 149 of RCVD. In a nutshell it wraps some basic tire and basic vehicle concepts together into a fairly powerful but simple way of describing vehicle dynamics.
All other things being equal, a longer wheelbase should imply higher 'static' directional stability; if you split the axles apart further it should require more torque to 'dislodge' the car from a particular attitude. It should feel planted and secure.. but that's not to mean unresponsive. At the same time as you increase the distance from the front axle to the CG you bump up the control moment derivative; the front tires have a larger moment arm with which to act about the CG, and create higher yaw moment for a given steer angle.
Given that most of my regulars here have FSAE experience, the following example may be enlightening. If there's a series that needs sharp, fast, predictable response almost all the time.. FSAE is it. If you were to test the Goodyear D2692 and D2696 back to back on identical cars, you'd find some big differences in how they drive. The '96 generally should have higher response rates (cornering stiffness) overall, while in the same construction, size, etc. As such, the static directional stability is higher. The car can feel more settled and planted. At the same time it is much more precise and responsive to steering inputs, since the front response is also up! The '92 by comparison, with lower control and stability derivatives, can feel both vague and lazy. (The '96 also has higher ultimate grip, comes in faster, and has unbelievably good wear. Pretty good improvement.)
On the other hand, while the control moment derivative increases linearly with "a" (distance from CG to front axle), if you think of that front axle as a point mass it's contribution to vehicle yaw inertia increases with a^2. In theory then you'd think a car with a short wheelbase would offer higher yaw acceleration capacity. My question is - how much yaw acceleration do you fuckin need? "More" of anything is not always better, except for beer and scallops. I may have to see if I can dig up some old DAQ to see just how much you need. If you already have 2x the acceleration potential that you could realistically need.. why add more when there may be benefit elsewhere?
I'll let you marinate on that.
New mini project
This will be a good one, if / when I get it to work. One of my better ideas recently.. as usual, spurred by a couple beers. If you want a refreshing, mentally inspirational taste that's as cold as the Rockies, reach for a frost-brewed Coors Light.
Anyway, the idea is.. it will work like OptimumK in reverse... and consequently much more practical for the design engineer.
More to follow if I put something together.
Anyway, the idea is.. it will work like OptimumK in reverse... and consequently much more practical for the design engineer.
More to follow if I put something together.
Sunday, January 3, 2010
Into the New Year
Sittin here at DIA waiting for my flight back to Akron, Ohio... after an amazing time in Denver and Boulder.
Been busy at the end of the year, so it's been quiet around here. When I'm back, things will pick up.
Would be nice if design can get wrapped up in 2010. We shall see! Many things I'd still like to do.
In other news.. up to 47 followers now! Very cool.
Been busy at the end of the year, so it's been quiet around here. When I'm back, things will pick up.
Would be nice if design can get wrapped up in 2010. We shall see! Many things I'd still like to do.
In other news.. up to 47 followers now! Very cool.
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