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"roll axis" and effects on mass moments of inertia

"roll axis" and effects on mass moments of inertia

"roll axis" and effects on mass moments of inertia

(OP)
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.

http://www.eng-tips.com/viewthread.cfm?qid=161490&page=3
"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
http://www.eng-tips.com/viewthread.cfm?qid=76824&page=7
"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.

RE: "roll axis" and effects on mass moments of inertia

The roll /axis/ is an outcome, a result of the interaction between the force matrix and the inertia matrix.

The roll /centres/ on the other hand are defined by the susension geometry, crudely.

I bet that hasn't helped a bit!

Cheers

Greg Locock

Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: "roll axis" and effects on mass moments of inertia

(OP)
Dam, i stuffed up. Where i said "roll axis" i meant "roll center", as in the geometric suspension related point. I hope im clear in what im trying to ask, im well know to muddle things up.

My question was do you add the inertia md^2 term from the roll center to the cog which some people do but doesn't seem mathematically correct?

Or more generally, does the suspension system/roll center alter the effective inertial properties of the sprung mass, or in other words is the frequency of oscillation dependent on the current geometric properties/roll center of the vehicle?

And even more generally, is this even significant, or is the roll center close enough to the roll axis to not be of concern?

I wish there was a text that dealt with this clearly. I really had contradictions :)

Thanks

RE: "roll axis" and effects on mass moments of inertia

"do you add the inertia md^2 term from the roll center to the cog which some people do but doesn't seem mathematically correct?"

I don't think so, as we seem to agree, the roll centre hasn't much to do with it.


"Or more generally, does the suspension system/roll center alter the effective inertial properties of the sprung mass,
or in other words is the frequency of oscillation dependent on the current geometric properties/roll center of the vehicle?"

YES - the effective spring rate in roll is changed

"And even more generally, is this even significant, or is the roll center close enough to the roll axis to not be of concern?"

If you increase the rear RCH by 50mm on a truck with a watts link the roll gradient (deg roll/g of lat acc) reduces, /and/ the roll frequency reduces very slightly. So it is not obvious what is going on. The roll stiffness goes up.



















Cheers

Greg Locock

Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: "roll axis" and effects on mass moments of inertia

Greg, since it was "very slightly," are you certain the measured reduction in roll frequency was real?

RE: "roll axis" and effects on mass moments of inertia

Calculated, in ADAMS. Certainly close enough to no change for me to believe that there was no real difference.

Cheers

Greg Locock

Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: "roll axis" and effects on mass moments of inertia

To back up Greg,
-the moment of inertia about the "roll axis" is NOT a quantity of interest.

-Raising the "Force-based" roll center reduces the roll moment on the sprung mass for a given lateral acceleration this is why the roll gradient is reduced....I would expect little change in the natural frequency in roll with a raised roll center since the suspension roll stiffness is unchanged.

RE: "roll axis" and effects on mass moments of inertia

One reason I like a Watt's link on a Hotchkiss is that at least you know where something that you might like to think of as the roll centre is.

That's about the end of the good news. Here's some runs I did messing about with the height of a Watt's link on the back of a truck. Some of the results are obvious... some less so.

http://members.optusnet.com.au/greglocock/rollcentre.zip

or failing that in the gallery at http://www.geocities.com/greglocock

You'll need a ps viewer eg ghostview to look at them.

Cheers

Greg Locock

Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: "roll axis" and effects on mass moments of inertia

(OP)
Interesting. I don't suppose you have the corresponding graphs for bounce and pitch frequency??

Thanks, Matt

RE: "roll axis" and effects on mass moments of inertia

No, how would bounce and pitch be affected by a Watts link?

Cheers

Greg Locock

Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: "roll axis" and effects on mass moments of inertia

(OP)
I have found increasing roll stiffness (which is what i assume the watts link is doing by changing the attachment points) effects the other modes of vibration due to the coupling of the vehicle rigid body modes, not by alot in my experience, maybe 1-2%, but thats why i was asking because im not that experienced smile

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