rec.autos.simulators

I'm trying to figure out how I should describe an anti-roll bar in my
sim.
Previously, in SCGT I saw a length value, in mm (millimeters). But
when I read about the rollbars, I see it as a spring, where you get
something like:

d1=suspension_compression_left   (relative to the rest position)
d2=suspension_compression_right  (...)

d=d1-d2
k=arb_spring_coefficient (arb=anti-roll bar)
F=k*d

Where the last force, F, is applied to the left suspension as
positive, and negative to the right suspension (or vice versa ;-).
Probably 'arb_spring_coefficient' should rather be named
'arb_torsion_stiffness', right?

What would the SCGT value describe? I read that the torsion stiffness
of the middle part of the ARB goes up proportionally with length^4, so
there's a link to SCGT's ARB-length, but there must then be a magic
coefficient again where:

torsion_stiffness=magic_coeff*arb_middle_length;

And ofcourse you get the eternal question 'what is a good value of
'magic_coeff'? I'll have a look at all the books to see if I can get
some realistic values for anti-rollbars. I must say in most literature
I've come across the anti-rollbar is quickly handled without too much
indepth information. I learned the most on a website
(http://www.racesimcentral.net/).

Thanks for any ideas on how to define anti-rollbars in an accepted
way,

Ruud van Gaal, GPL Rank +53.25
Pencil art    : http://www.racesimcentral.net/
Free car sim  : http://www.racesimcentral.net/

Ben

Since your driver will put in a roll bar "stiff enough"
for his application, the model you described above will
work (as long as the signs are correct :)
The goal is to provide additional suspension force when the
compression states are different on the two sides.

It's more like bar diameter to the 4th power, or
outer diameter to the 4th - inner diameter to the 4th power
if the bar is hollow (usual).  So you would also have to decide
on a plan to handle standard bar thicknesses :)
The stiffness would be inversely proportional to the length.
(the longer it is the weaker.)

Staniworth has a good section. Watch for units problems though.
Puhn gives equations for angled bars (and several
photographs of hardware.)

Try setting it up so that the effective spring rate is around half your
suspension rate to start. Note that because of the powers of 4 and 2
and the large torsional resisting moment of steel, etc.,
in the various equations and units conversions, it is easy to make a
huge mistake :)   So since you are selecting your bar geometry
somewhat arbitrarily (unless you go all the way to bar dimensions
and installation geometry, etc.) just make sure that the overall
effect was as you planned when you test.

"How to Make Your Car Handle," Fred Puhn, HPBooks
(The Berkley Publishing Group), N.Y. N.Y., 1981. ISBN 0-912656-46-8

"Competition Car Suspension, Design, Construction, Tuning,"
Allan Staniforth, 2nd Ed., Racecar Engineering / G.T. Foulis & Company /
Haynes Publishing, Sparkford, Near Yeovil, Somerset BA227JJ, England,
1994.
ISBN 0-85429-956-4

--
Matthew V. Jessick         Motorsims

Vehicle Dynamics Engineer  (972)910-8866 Ext.125, Fax: (972)910-8216

Thanks for the info,

Ruud van Gaal, GPL Rank +53.25
Pencil art    : http://www.marketgraph.nl/gallery/
Free car sim  : http://www.marketgraph.nl/gallery/racer/

...

Sounds more complex than what I need, which is like the wheel rate;
the effect on the suspension is what I need, and the less I define
geometrically, the better, I guess (for now).

Let's not. :) Didn't know the bar was often hollow, let alone know
about standard bar thicknesses. ;-)

Photographs do clarify things often; I'd never seen an anti-roll bar
before a month ago on a site, and it gave me a better idea of how this
thing actually worked.

More books! I just obtained 'Tires, Suspension and Handling' from John
Dixon, so I guess I'll be busy for a while. :)
I can't believe the amount of work that goes into a carsim, with all
this ever-increasing detailed theory. But it seems worth it. :)

Thanks,

Ruud van Gaal, GPL Rank +53.25
Pencil art    : http://www.racesimcentral.net/
Free car sim  : http://www.racesimcentral.net/

  Matt's post is excellent, I saved it because it's far beyond anything I've
tried yet ;-)

  If you want a simpler approximation to start with, the anti-roll bars (if I
remember correctly) are usually measured in units-force/degree-twist.  Try
taking an orientation vector across the front/rear two wheels, then get the
angle between that vector and the chassis roll vector.  The difference would be
the degrees of twist in the anti-roll bar.  Multiply that by the anti-roll bar
"torsion rate", which could probably be treated as a torque on the chassis,
then divided by the distance from the center of the car to each wheel to get a
vertical force at each wheel.  I tried this some time ago and it worked pretty
well, even though it doesn't anything else into account.  
  The thickness (if that's what it is) is just a way of calculating the spring
rate.  Perhaps it would be quicker and easier to bypass that.  Aside from that,
I'm about to rewrite a huge chunk of my engine, seeing as I *finally* got
matrices and inertia tensors working properly, so I won't be spending time on
anti-roll bars again for a little bit :-)  No quats yet, this is tough enough
as it is already ;-)  

  Anyway, maybe that's enough to get you going.  BTW, when picking anti-rollbar
rates with this approach, I tried using a soft spring/shock setting (not the
bars, but the regular springs), then raising the anti-roll bar rates to limit
roll to around 3 degrees or so.

Todd Wasson
---
Performance Simulations
Drag Racing and Top Speed Prediction
Software
http://PerformanceSimulations.Com

Ben

  Absolutely, the player doesn't need to see all the resulting "hoo-har" :-),
but just needs to have a couple of inputs to use to set up the anti-rollbars in
the first place like you said, such as diameter or length.  What I was
suggesting to Ruud is to skip that stuff at first, base his force/torque
calculations off the angle of twist between the wheels/axle-centerline and
chassis, then just set the spring rate directly to get some form of anti-roll
bars working in his model.  After that, if he'd like to look up equations that
produce the spring rate from other types of inputs (length, diameter, mounting
point/geometric stuff), it would be an option.  Then, he could just plug the
new calculated spring rate into the model he'd already written to handle data
in Newton-meters/degree anti-rollbar twist.  Then, it'd (probably) be an easy
little module to change if and when he wanted to add more "complexity" to it,
meaning more inputs for the bars, without having to change the underlying code
that really just runs off the spring rate anyway.  

  I haven't studied or thought about the actual kinematics of anti-roll bars
much at all yet, so at first I would suspect the length and geometry is
probably not arbitrary, given what little I've been learning lately about
suspension kinematics, but I'd check it out to make sure anyway.  However,
since Matt Jessick indicated that these are large factors, I'll take his word
for it, 'cause he knows a lot more than I do :-).  I'm not yet to the point
where I can hook up an anti-roll bar system and make accurate calculations
based on installation stuff like he is.

  Am I following what you were saying there?  Or am I missing your point ?  :-)
 I do that sometimes.... :-P

Todd Wasson
---
Performance Simulations
Drag Racing and Top Speed Prediction
Software
http://PerformanceSimulations.Com

I'm not sure I had a point! 8^()

I only know what my own car has, which is pretty basic:  a pushrod halfway
up the lower wishbone acting on the most basic bar.  This is 1964 technology
at its finest.  I haven't seen any system that would contribute any
different forces...though I haven't really looked into it either.  There's a
whole world you guys are delving into that I haven't really looked at beyond
posts here and magazines.  Maybe some day....or maybe not!

Cheers,

Ben