cavedweller
Mechanical
- Jan 17, 2006
- 8
Just trying to get a better understanding of how the sprung mass modes are effected by suspension geometry. There is lots of dis-information out there and it has got me confused.
My understanding of the ""roll axis"" is that this is some arbitrary point defined by the intersection of the instant center lines based on the suspension geometry. The car does not roll about this axis, it is merely a point that defines how much load is transfered to the vehicle by the linkages and by the suspension springs. It does nothing but alter the angle of the sprung mass.
However, in some books, for example blundell, he states that the vehicle does not roll around this axis (sounds good to me), then proceeds to give examples later on with vehicle models rolling around the roll axis! Great! Gillispie, Millken also give differing definitions and various contradictions in their books.
Further, alot of papers i has read assume simplified vehicle models and state that the vehicle rolls around the "roll axis" and alter the mass moment of inertia by using the parallel axis theorem and adding the m.d^2 term from the roll axis to the sprung mass principal axis. This goes against what is stated, that the car does not roll about the axis?
I did a search here some and members said to add this term to the inertia. Sorry to pick on your Greg.
"The distance between the RC and sprung mass COG is critical. A higher RC is effectively a stiffer anti-
roll bar. It follows that the lower the roll centre, the more roll that occurs in a corner. The chnage in moment of
inertia about a given roll axis varies with the SQUARE of the distance between the roll axis and the cg of the body."
Then in another thread
"Perhaps because cars DON'T roll around the roll centre.
Or, more accurately, the instantaneous axis of rotation of the sprung mass rarely coincides with the "roll centre". "
So Im confused.
My questions are
1. How is the inertia of the vehicle effected by the "roll axis" if at all? By this i mean does the roll axis cause this extra m.d^2 term to be added to the roll inertia, causing the effective inertia to increase and the frequency of oscillation to decrease?
2. And if it does this would mean the mass moments of inertia would be entirely dependent on the current position of the suspension geometry and constantly changing.
3. And when would this occur, if there is no lateral acceleration (say driving forward, no acceleration hitting a pot hole) then would the roll center have any effect on the roll inertia?
Im getting the feeling that adding this inertia term is more a rule of thumb, seat of the pants thing then actually based on sound mathematics.
Thanks all.
My understanding of the ""roll axis"" is that this is some arbitrary point defined by the intersection of the instant center lines based on the suspension geometry. The car does not roll about this axis, it is merely a point that defines how much load is transfered to the vehicle by the linkages and by the suspension springs. It does nothing but alter the angle of the sprung mass.
However, in some books, for example blundell, he states that the vehicle does not roll around this axis (sounds good to me), then proceeds to give examples later on with vehicle models rolling around the roll axis! Great! Gillispie, Millken also give differing definitions and various contradictions in their books.
Further, alot of papers i has read assume simplified vehicle models and state that the vehicle rolls around the "roll axis" and alter the mass moment of inertia by using the parallel axis theorem and adding the m.d^2 term from the roll axis to the sprung mass principal axis. This goes against what is stated, that the car does not roll about the axis?
I did a search here some and members said to add this term to the inertia. Sorry to pick on your Greg.
"The distance between the RC and sprung mass COG is critical. A higher RC is effectively a stiffer anti-
roll bar. It follows that the lower the roll centre, the more roll that occurs in a corner. The chnage in moment of
inertia about a given roll axis varies with the SQUARE of the distance between the roll axis and the cg of the body."
Then in another thread
"Perhaps because cars DON'T roll around the roll centre.
Or, more accurately, the instantaneous axis of rotation of the sprung mass rarely coincides with the "roll centre". "
So Im confused.
My questions are
1. How is the inertia of the vehicle effected by the "roll axis" if at all? By this i mean does the roll axis cause this extra m.d^2 term to be added to the roll inertia, causing the effective inertia to increase and the frequency of oscillation to decrease?
2. And if it does this would mean the mass moments of inertia would be entirely dependent on the current position of the suspension geometry and constantly changing.
3. And when would this occur, if there is no lateral acceleration (say driving forward, no acceleration hitting a pot hole) then would the roll center have any effect on the roll inertia?
Im getting the feeling that adding this inertia term is more a rule of thumb, seat of the pants thing then actually based on sound mathematics.
Thanks all.
