Roll Center Migration

[ajb81]

What are the effects of lateral roll center migration? From what I have read, I generally understand that significant lateral migration of the roll centers is a bad thing. Is this true? Why? I can’t seem to find much information on the effects of roll center migration. Does it matter if the rear moves farther than the front, or the front farther than the rear? Thanks for any insight!

 

[GregLocock]

No it is not a bad thing. In fact according to the SAE it is not even a thing at all.

First you have to figure out how to calculate it, and then you have to consider what its migration by your defintion would affect.

The method I prefer to use to establish RCH,

d(Fz)/dFy*semitrack

in a lateral force at the contact patch test, gives a RCH only.

However people who like geometrical constructions can find a lateral position for it.





Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[ajb81]

Greg, thanks for your response. I realize that many people do not put much stock into the geometric roll center. I haven’t done enough research/thought through it enough to decide what I think of the geometric roll center (it’s an ongoing process for me, as I am somewhat new to vehicle dynamics). However, the tools I am currently working with do take it into account. I am currently working with the software package OptimumK, which outputs a RC lateral position. Making the assumption that the lateral position of the RC can be defined, what are some potential ramifications of its lateral migration?

 

[hemipanter]

Rc is a tricky story. If I may mention the name of Racecar Engineering so they have just had two numbers in which cower this issue. There is a man behind the program Carsim explaining his views on the whole, and then Mark Ortiz who enters an appearance with a slightly different view. I myself have done my best to find out how it is. I would say that Rc is an important issue, while at the same time beeing less important. Actually, it is not so sensitive so that the car must behaves strange about that Rc is located in slightly different places, but it is important to understand the connections if we are to interpret the behavior of the car and take the right steps to get it in balance.
Goran

 

[GregLocock]

My point was that until you have defined what you mean by it then you can hardly be expected to predict the effect of moving it.



Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[ajb81]

Again, I realize I’m talking to people who know much more than me, so thanks for your patience!

If you defined the roll center to be the intersection of the force lines from each tire and defined the roll axis to be the axis about which the car rolls, then what sort of effects would its lateral migration result in?

 

[hemipanter]

First let me say that the axis the car actually roll about is not like a hinge. It is possible to roll the car around a center located within a large area. Therefore, Rc axis position becomes a product of the sum of the car's motion, inertia, mass, geometry, suspension and dampers. Independent (pretty much) from the intersection of the FL. This is what we can say about the actual movement of the sprung mass.

Then the angles of the FL determines how much geometric weight transfer we will get for suspension of each individual wheel. This is quite independent from the previously described movement of roll.

We have now four FL where the force goes straight into the ground without affecting the suspension system. The greater the angle of a single FL are, the more power goes geometrically through the suspension in question.

If we now go back to the place where the FL intersect, but do not call it Rc, so we'll see what significance this has.

If the FL intersecting the middle between the wheels all FL will get the same angle. In a curve the load of the outside wheel FL will produce a lift of the car, while the FL of the inside wheel will press the car down. Since in this case, the intersection of FL is located in the cente of car (the same slope on all fours) so would these forces become equal. But now the outer wheel receives more load and the result is that we get a greater lifting force than lowering. So by altering the FL angle, and thereby altering the spot of FL intersection, during suspension movements we may be able to lessen the lifting (jacking) effect produced by the outside wheel. This is done by creating a geometry that moves the intersection of the FL to the inside wheel during roll.

Of course, there are a number of other things that happen if altering the FL angle, like camber effect during roll and car balance, but this is a to big an issue to bring up here.

Goran

 

[ajb81]

Thanks both of you for your posts. You've both given me alot to think about. Goran, thanks for pointing me to those two Racecar Engineering articles. I just read them both, and I'll probably re-read them five or six more times... I really appreciate being able to get help from people who are so much more experienced than me!