exFSAE
03-30-2008, 07:56 PM
After 3.5 years I think I may actually have got this thing figured out. WT and roll axis inclination and all that jazz. So let me throw a few statements out here, tell me if you agree or think I'm nuts.
Using RC's as a good approximation of things, rather than FAPs..
There's some lateral force generated by the tires. The overall amount of weight transfer, steady state, is essentially only dependent on weight, CG height, and track width.
Sprung mass rolling weight transfer is elastic, dependent on roll moment arm from sprung mass CG to the neutral roll axis. It takes to transfer this load since it is dependent on spring displacement and damper velocities. The split of elastic weight transfer front to rear versus time is therefore a function of spring and damper force distribution.
The rest of that total weight transfer then must come from the non-rolling, overturning moment. Its split is governed by the relative heights of the roll centers and the fore/aft CG location. This weight transfer is (near) instantaneous (??). It is direct.
WT from the unsprung masses is likewise (near) instantaneous). It is direct.
If the above is true I would think that has some implications for transient response and linearity. Ie your TLLTD may be set for a real balanced car, but if the elastic and direct contributions are proportioned opposite, then you could have somethin crazy where the car initially doesn't bite but then is neutral when the chassis takes a set.. or turns in really well initially and comes to balance (or washes out.) True?
Does all the above also imply that if your car pitches forward or back that your geometric roll stiffness distribution changes, since the RC heights will be changing? For example under the brakes having the neutral roll axis pitching further forward.
Using RC's as a good approximation of things, rather than FAPs..
There's some lateral force generated by the tires. The overall amount of weight transfer, steady state, is essentially only dependent on weight, CG height, and track width.
Sprung mass rolling weight transfer is elastic, dependent on roll moment arm from sprung mass CG to the neutral roll axis. It takes to transfer this load since it is dependent on spring displacement and damper velocities. The split of elastic weight transfer front to rear versus time is therefore a function of spring and damper force distribution.
The rest of that total weight transfer then must come from the non-rolling, overturning moment. Its split is governed by the relative heights of the roll centers and the fore/aft CG location. This weight transfer is (near) instantaneous (??). It is direct.
WT from the unsprung masses is likewise (near) instantaneous). It is direct.
If the above is true I would think that has some implications for transient response and linearity. Ie your TLLTD may be set for a real balanced car, but if the elastic and direct contributions are proportioned opposite, then you could have somethin crazy where the car initially doesn't bite but then is neutral when the chassis takes a set.. or turns in really well initially and comes to balance (or washes out.) True?
Does all the above also imply that if your car pitches forward or back that your geometric roll stiffness distribution changes, since the RC heights will be changing? For example under the brakes having the neutral roll axis pitching further forward.