Roll center

[KSaiyu]

I can't find a definitive answer to this question (which I thought would be simple ) anywhere on the web or in books:

Would a higher roll center mean LESS roll and LESS weight transfer accross during roll, or LESS roll and MORE weight transfer accross during roll?

I've seen so many explanations, some saying that it means more roll resistance meaning more weight transfer, and some saying that is reduces roll so there is less weight transfer.

EDIT: Also, I read in my skip barber book that a higher ride height would raise the roll center, and that it has the same affect of stiffening the springs/anti roll bars. If this is true, then wouldn't this mean more weight would be transferred?

 

[halfracedrift]

I think the term centre of gravity is what you're looking for...

Higher CG means the car is higher, more weight is distributed to the top, thus, the car is more prone to rolling. That's why you can't turn many SUV's sharply, because they can tip *being so high*

Also the reason why race cars are so low and wide to the ground. Lower CG in racing is better, but lower your car too much and you might not be able to manuever *sp* over some bumps. The stronger shocks help counter this problem though.

Does this help?

 

[KSaiyu]

I understand the CG, but it's the roll center that's confusing. I mean, when they say high do they mean in relation to the ground, or nearer to the CG, and I'm confused on which would transfer weight more

 

[bengee]

The roll center is the center of mass/ gravity/ and for most application the centriod of the object as long as suspension is not considered...This is the point where all moment sums are performed... The reason y the higher your cm the lower g your car can take is due to the fact that you can moel your car like a point particle at the centroid... when you turn you have a force into the corner... the higher your cm the greater the moment that this force produces because M=F times x... so for the same force the greater the distance the greater the resulting moment....The way i have it set up shows that the moment created by the turn forces the wheels to balance it with their friction... thus with the more moment created by a higher vehicle than a lower one on the same turning radius... the lower car will be able to hold more g's because the same force will create a smaller moment which in turn means the tires have to do use less traction to balance the moment created while turning...

Rolling motion can be thought of in a reverse manner... for a given force pointing in to the corner on each tire (which keeps the car from sliding on a turn)... there is a verticle distance from the tire contact patch to the centroid... the greater this distance the greater the moment delivered to this point and because nothing exist (if we do not consider suspension) to balance this moment the car will roll (becuase moment induces rotation)... Therefor for the same forces at the contact patch... the higher the cm is the more rolling/rotation will occur... or the more moment the suspention will have to provide to counteract this movement... i hope this helps...

weigh transfer is good because it puts weight on the outside wheels which allows for greater turning forces because a greater friction force is available... weight transfer is a basic that ever real racer whould understand in some extent... even gt3 racers... weight transfer will occur when braking accelerating and turning... or any combination of the above... It does not matter if your car rolls or not... weight will still be transfered becasue a moment about the cm will exist... If however you put some sway bars in which counter the moment produced in a turn... you will decrease with weight transfer vastly... which is good only for high speeds and tight corners... to do certain things you need weight transfer... things like 4 wheel drift and real speed in low hp cars...

 

[Jugend]

No offense bengee, but I only briefed your post, mabye you said what I'm about to say..

Roll center and center of gravity are two different things. Center of gravity you all know I think but roll center is decided from the suspension geometry. From what I understand from the link below the trick is to make the RC and GC work together as efficient as possible to make the car handle well.

http://www.nyracer.com/rollcenter.htm

 

[bengee]

No offense taken... i was trying to give a more intuitive sense of the roll center... My first sentence is merely a statement that applies to general systems... not all hehe... for mine it worked fine... I left out suspension on purpose so that i could explain it in a simple manner... you are correct that the roll center depends upon susension geometry... and is one of the many purposes of complete allignment... Anyways... my explanation is simply an easy way to think about the physcis of it... when suspension is considered things get a lot more complicated than that site makes it... and by things i mean roll center... suspension can be designed so that roll centers change and stuff... great stuff

 

[KSaiyu]

thanks for the link, it explains how to find it clearly, but I still can't find what a different height means for the car.

I mean, what if you CG was very near to the floor, but the roll center was below ground; would you still roll a lot because of the distance between the 2??

 

[Jugend]

This is just a guess with logics(?).. The GC should be as near as the RC as possible. The RC is the body's physical axis right? and the CG creates a momentum when the car is rolling, that momentum (the actual roll) should be dependent on the distance between RC and GC, like how a simple lever works. This is just a guess but it sounds correct to me.

 

[KSaiyu]

That's what I came up with, but the part I'm getting confused with is how a higher or lower roll center changes the characteristics, and now I've read something about roll center affecting weight jacking, whatever that is

 

[Jugend]

..? Man, I had no idea car physics was this complex. I leave you here Ksaiyu in your quest for knowledge. Good luck!

 

[bengee]

car physics is vastly complex... i myself have actually started becoming interested in it... and done some analysis and research... i do not know the correct terms for everything but what gt is saying is correct... you want your gc and rc to be extremely close to minimize roll... but you still want some roll... esp when cornering at hte limits... a gc that is closer to the ground is also better... a gc above a roll center is bad because it is less stable... think of a pendulum that can rotate freely about a point... if the pendulum is above the rotation point is is unstable... if it is below it is more stable... and infact most stable... all this stuff has to do with energy and a similar ana\olgy can be made with cars...

GT when i say moment i am not reffering to momentum which depends upon velocity and mass... i am talking about a force multiplied by a distance... engineers usually call this moment in the US... it is also referred to as torque (but torque is usually reserved for things that take or create power)... hope this kinda helps... if it doesnt get into complete allignment and such...

 

[KSaiyu]

This page seems quite good:

http://www.teamassociated.com/racerhub/techhelp/marc/Handling.4.html#19483

You can see from equation 7 how the roll center affects the weight transfer

 

[bengee]

wow that site does seem useful
i will read it more carefully later
good find