Delay - yaw velocity to lateral acceleration
Delay - yaw velocity to lateral acceleration
(OP)
I haven't checked Gillespie yet.
In steady state circular cornering lateral acceleration is directly related to the yaw velocity:
LatAcc=tangential velocity * YawVelocity
However, in dynamic events, eg slow sinusoidal sweep of steering wheel angle, there is a phase delay between the two. I cannot wrap my head around this - is it due to Coriolis acceleration? That is given by radialvelocity*yawvelocity*2. typically we are looking at slip angles of say 0.25 degrees, so that would be tangential velocity*0.25/57.3*yawvelocity*2, which is in the right ballpark, but I don't like it.
Or is it the intuitively appealing argument that the yaw has to 'build up' before the lateral acceleration can increase?
In steady state circular cornering lateral acceleration is directly related to the yaw velocity:
LatAcc=tangential velocity * YawVelocity
However, in dynamic events, eg slow sinusoidal sweep of steering wheel angle, there is a phase delay between the two. I cannot wrap my head around this - is it due to Coriolis acceleration? That is given by radialvelocity*yawvelocity*2. typically we are looking at slip angles of say 0.25 degrees, so that would be tangential velocity*0.25/57.3*yawvelocity*2, which is in the right ballpark, but I don't like it.
Or is it the intuitively appealing argument that the yaw has to 'build up' before the lateral acceleration can increase?
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.





RE: Delay - yaw velocity to lateral acceleration
Is it analagous to a slalom skier? While the skis are unweighted they are experiencing a high yaw rate, but lateral acceleration is negligble until the edges are carving the turn.
What is the 1/57.3 factor in the slip angle equation?
Jeff
RE: Delay - yaw velocity to lateral acceleration
I'm thinking along the lines of acceleration being dv/dt, and if v is a sine function, acceleration would be some cosine function, (or a sine function with a phase angle).
Or maybe I'm all wet . . .
Norm
RE: Delay - yaw velocity to lateral acceleration
RE: Delay - yaw velocity to lateral acceleration
Shanba gets a star for that!
Greg, the reason for the delay is compliance related. In a steady-state corner the compliance of the suspension is already near it's designed elastic operating limit. But when the car is in a slalom type situation it has to travel through the compliance before it can reach it's lateral limit, causing a delay. In laymen's terms: the bushings have already "set" during a constant radius, while they need time to "set" during a transition. The car is not a rigid body in an ideal situation, so it doesn't act as math would suggest. As Shanba mentioned, if you would fun the same tests on say an F1 car, the results would be MUCH different, as they basically only have the compliance of the tire,and localized rigidity of the cabon tub to deal with.
RE: Delay - yaw velocity to lateral acceleration
Jeff
RE: Delay - yaw velocity to lateral acceleration
RE: Delay - yaw velocity to lateral acceleration
RE: Delay - yaw velocity to lateral acceleration
RE: Delay - yaw velocity to lateral acceleration
RE: Delay - yaw velocity to lateral acceleration
Incidentally I'm a bit disappointed by statements like " it doesn't act as math would suggest"
I didn't make those equations up. Mr Newton did. If you measure the properties at a point, whether it is in the middle of a bowl of jelly, or a block of steel, they still apply.
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Delay - yaw velocity to lateral acceleration
I didn't make those equations up. Mr Newton did. If you measure the properties at a point, whether it is in the middle of a bowl of jelly, or a block of steel, they still apply."
I am saying that the system you are describing does not fit your mathmatical model you laid out. When the car is in a constant radius turn, it is basically at a steady state, therefor your model applies! BUT, when the car is in dynamic event, you have damping(from the compliance) that does not allow your model to hold true. So, factor in the yaw damping of the vehicle and it should then make more sense.
I can ask Gillespie if you really want a very direct answer, my friend works under his direction at Carsim.
RE: Delay - yaw velocity to lateral acceleration
RE: Delay - yaw velocity to lateral acceleration
Assuming that the contact patch is not slipping and is describing a sinusoidal motion, and that the phase shift is due to the elasticity of the tyres/suspension/driver's seat, is this not very similar to a vehicle driving over a series of sinusoidal humps?
Surely you suspension guys have got spreadsheets to cover this.
On the back wheels following the front wheels concept, if you drive a forklift, the initial lateral acceleration at the driver's seat is negative.
RE: Delay - yaw velocity to lateral acceleration
I don't think this experiment would hold true. A vehicle's dynamic behavior can be simplified as classical 2nd order dynamic system with a spring, mass, and damping term. Milliken's book has a derivation of the equations. In any case the phase lag between yaw and lateral acceleration is not a simple linear relationship with compliance. These equations can easily be programed into Matlab if you want to play around with them.
RE: Delay - yaw velocity to lateral acceleration