rec.autos.simulators

by **jonas echterhof** » Fri, 07 Sep 2001 21:54:18

sorry to keep you people on this list from more important stuff - each
question i ask raises five new questions ;-)

i kept hearing about roll centers a lot recently, and did some research
throughout the net, but i don't quite grasp it. at the moment i
calculate the torque vector a force applies on the car body as

torque=(attackPoint-CG) X force .

attackPoint is the point where the force is applied (usually road
contact point of a wheels). X is just a cross-product. now it is
dawning me that this is not correct, and that i should calculate torque
seperatly for the x,y and z axis. instead of using the CG for
calculating roll (z in my coord system) torque i should use a roll
center. is this correct?

but if this is so wouldn't there also have to be a pitch center and yaw
center? why does one never hear about those. and also where does the CG
come into the results, if i use roll centers instead of CG for torque
calculation? or have i misunderstood it all?

TIA

jonas

hmm.. has anyone ever thought about moving these discussions to a
rec.autos.simulators.physics ng?

by **Sebastien Tixie** » Sat, 08 Sep 2001 01:03:30

> sorry to keep you people on this list from more important stuff - each
> question i ask raises five new questions ;-)

> i kept hearing about roll centers a lot recently, and did some research
> throughout the net, but i don't quite grasp it. at the moment i
> calculate the torque vector a force applies on the car body as

> torque=(attackPoint-CG) X force .

If attackPoint , CG and force are in world coordinate
this will give the angular torque in car coordinate.

Then from the car orientation, the car angular momentum, the car local
inertia tensor you
can calculate the wolrd inertia tensor and the new car orientation.

Imagine a longitudinal force. If you aplly it BELOW the CG you have the car
dive correctly but if you apply it above the car will dive like you brake
instead of when like you accelerate !

--
Sebastien TIXIER - Game Developer
Dynamics and Car Physics
http://www.eden-studios.fr
GPLRank Normal:-42.98 Monster:-117.44

by **jonas echterhof** » Sat, 08 Sep 2001 03:41:24

> > sorry to keep you people on this list from more important stuff - each
> > question i ask raises five new questions ;-)

> > i kept hearing about roll centers a lot recently, and did some research
> > throughout the net, but i don't quite grasp it. at the moment i
> > calculate the torque vector a force applies on the car body as

> > torque=(attackPoint-CG) X force .

> If attackPoint , CG and force are in world coordinate
> this will give the angular torque in car coordinate.

> Then from the car orientation, the car angular momentum, the car local
> inertia tensor you
> can calculate the wolrd inertia tensor and the new car orientation.

> Imagine a longitudinal force. If you aplly it BELOW the CG you have the car
> dive correctly but if you apply it above the car will dive like you brake
> instead of when like you accelerate !

sure - all this works for me as you described. the question was where
do roll centers come into the equation.

jonas

by **Jonny Hodgso** » Sat, 08 Sep 2001 02:29:36

I believe a reasonable way to do it is to treat the sprung and
unsprung masses separately. For the sprung mass, lateral forces
generate a couple between the RC and the CG; for the unsprung mass,
it's between the RC and the contact patch (ground level).

In the steady state this only really affects sprung mass roll angle,
but in transients an important difference is that some force is
'absorbed' in rolling the body and thus doesn't all appear at the
tyres immediately. The other thing to note is that not all the weight
transfer appears at the springs - some goes directly through the
suspension linkage.

CG

The pitch centre does exist, although for horizontal / parallel
wishbone hinges it's at ground level. It becomes more relevant with
anti-squat/dive suspension geometry. Yaw centre I'm not sure about -
I /think/ that for one to exist, the wheels would have to move
significantly in the horizontal plane (relative to the chassis) during
the application of suspension loads.

See above... HTH.
Jonny

by **Dave Pollatse** » Sat, 08 Sep 2001 10:27:34

One quick trick I've used is to find the normal to the plane that contains
the instant axis and the contact point, and use that normal to split the
force vector into body and wheel components (If I recall correctly, the math
is something like
f_n = f dot n, f_wheel = n * f_n, f_body = f - f_wheel ) this can
give you anti-dive and other jacking effects for independent suspensions...
live-axle rears are more of a pain. you can start out by just pre-computing
a fixed normal in body space, and later on start perturbing it with
suspension travel. The rational behind this technique is that torques about
the instant axis will only be reacted to by the springs, shocks, and ARBs.
I tend to think of roll centers as being a secondary concept like engine
horsepower, used for analysis, and instant axis as being something you
simulate directly like engine torque. Unless, of course, you simulate the
links directly, in which case roll centers are tertiary and instant axis is
secondary (Note: this is all handwaving). I think every simulator seems to
use a different approach here, many of which are equivalent--it depends on
whether you model wheel hope and/or gyroscopic effects...
-Dave P.
MGI

by **Sebastien Tixie** » Sat, 08 Sep 2001 14:51:50

hooo... sorry, i didn't understand correctly the question... well, i still don't
understand your question , my english is not good enough :o(

regards,

--
Sebastien TIXIER - Game Developer
Dynamics and Car Physics
http://www.eden-studios.fr
GPLRank Normal:-42.98 Monster:-117.44

by **Gregor Vebl** » Sat, 08 Sep 2001 16:57:34

Wow, Dave, I was about to write something of the kind myself, but you've
worded it so nicely that I'll just add a 'what he said' :). I guess that
separates you from us mere mortals... ;).

-Gregor

> I tend to think of roll centers as being a secondary concept like engine
> horsepower, used for analysis, and instant axis as being something you
> simulate directly like engine torque. Unless, of course, you simulate the
> links directly, in which case roll centers are tertiary and instant axis is
> secondary (Note: this is all handwaving).

by **Doug Arna** » Sun, 09 Sep 2001 12:03:45

Simply, Lateral force is applied through the CG. However, the car rolls
about the roll centers - or roll axis . There is a torque arm that forms
from the distance between the two (the CG always being above the RC). This
torque arm has to be figured into your roll resistence formulas. This is why
changing a trackbar height on the back of a NASCAR will change the
over/understeer characteristics of the vehicle. It changes that ends
resistence to roll buy changing the rear RC height, which in turn changes
the "torque arm" thats resisting the roll - making that ends lateral force
more or less effective on that axles spring package.

As far as the formulas to do all that?............no clue :)

--
Doug Arnao
www.vehiclecraft.com

by **Gregor Vebl** » Tue, 11 Sep 2001 16:24:11

As Dave already mentioned, while the way you describe it is very good in
terms of understanding the car balance, it's not the best way to handle
car physics in calculations. It's best to use instant axes for wheel
travel and apply forces and torques per Physics 101, and the effects you
mention should be taken care of automatticaly.

There's usually a difference between how our mind percieves physics and
what's the most natural way to describe the phenomena. On the other
hand, describing phenomena in the way your mind isn't used to thinking
in has the effect of adjusting the way you see the problem, which is
half, wait, whom am I kidding, all the fun there is to physics ;).

-Gregor

> Simply, Lateral force is applied through the CG. However, the car rolls
> about the roll centers - or roll axis . There is a torque arm that forms
> from the distance between the two (the CG always being above the RC). This
> torque arm has to be figured into your roll resistence formulas. This is why
> changing a trackbar height on the back of a NASCAR will change the
> over/understeer characteristics of the vehicle. It changes that ends
> resistence to roll buy changing the rear RC height, which in turn changes
> the "torque arm" thats resisting the roll - making that ends lateral force
> more or less effective on that axles spring package.

> As far as the formulas to do all that?............no clue :)

> --
> Doug Arnao
> www.vehiclecraft.com

by **Doug Arna** » Tue, 11 Sep 2001 22:02:12

From sebastians message, I got the impression he wasn't sure how the RC and
CG interacted to make roll etc. I was just describing that
process.....understanding the "phenomenon" I would think makes the coding
process easier and more accurate?

As far as the coding or claculations........again I have little clue <g>.

--
Doug Arnao
www.vehiclecraft.com
1-800-845-5948

by **Gregor Vebl** » Tue, 11 Sep 2001 22:15:44

Hi Doug,

too much knowledge may in fact be detrimental <g>. The fact is, the
basic system isn't all that difficult to implement into equations to be
solved, but to analyze and understand the special solutions (e.g. steady
state) of such a system, concepts like roll center etc. are very helpful
and they follow from these basics, not the other way around.

-Gregor

> From sebastians message, I got the impression he wasn't sure how the RC and
> CG interacted to make roll etc. I was just describing that
> process.....understanding the "phenomenon" I would think makes the coding
> process easier and more accurate?

> As far as the coding or claculations........again I have little clue <g>.

by **Sebastien Tixie** » Tue, 11 Sep 2001 22:47:37

Well, my first problem is to translate r.a.s posts into french ;o)
I "think" i understand that rolling center and cg are not the same. the
simpliest exemple is a cube. if a cube is just touching the ground by 1 edge ,
this edge is the rolling axle of the cube. If the cube is touching by a corner,
this corner is the rolling center. Meanwhile the CG of the cube has never
changed ... the more the rolling center is far from the CG the easiest it will
be to rotate the cube. the more the force is far from the RC the more torque
you'll have.

If what i'm saying isn't true then i REALLY didnt' understand what rolling
center is ;o))) so just enlight me please !

regards,
--
Sebastien TIXIER - Game Developer
Dynamics and Car Physics
http://www.eden-studios.fr
GPLRank Normal:-42.98 Monster:-117.44

> From sebastians message, I got the impression he wasn't sure how the RC and
> CG interacted to make roll etc. I was just describing that
> process.....understanding the "phenomenon" I would think makes the coding
> process easier and more accurate?

> As far as the coding or claculations........again I have little clue <g>.

> --
> Doug Arnao
> www.vehiclecraft.com
> 1-800-845-5948

> > As Dave already mentioned, while the way you describe it is very good in
> > terms of understanding the car balance, it's not the best way to handle
> > car physics in calculations. It's best to use instant axes for wheel
> > travel and apply forces and torques per Physics 101, and the effects you
> > mention should be taken care of automatticaly.

> > There's usually a difference between how our mind percieves physics and
> > what's the most natural way to describe the phenomena. On the other
> > hand, describing phenomena in the way your mind isn't used to thinking
> > in has the effect of adjusting the way you see the problem, which is
> > half, wait, whom am I kidding, all the fun there is to physics ;).

> > -Gregor

by **Eric Cabr** » Wed, 12 Sep 2001 09:53:27

On Mon, 10 Sep 2001 15:47:37 +0200, in rec.autos.simulators Sebastien

>Well, my first problem is to translate r.a.s posts into french ;o)
>I "think" i understand that rolling center and cg are not the same. the
>simpliest exemple is a cube. if a cube is just touching the ground by 1 edge ,
>this edge is the rolling axle of the cube. If the cube is touching by a corner,
>this corner is the rolling center. Meanwhile the CG of the cube has never
>changed ... the more the rolling center is far from the CG the easiest it will
>be to rotate the cube. the more the force is far from the RC the more torque
>you'll have.

>If what i'm saying isn't true [...]

No problem, that's correct :-)

Eric

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