rec.autos.simulators

by **J. Todd Wass** » Thu, 15 Nov 2001 07:34:23

>> I think one of Baraff's papers covers implicit integration, but I'm not
>sure
>> which one. Perhaps somebody else knows?

>This one:

>http://www.racesimcentral.net/~baraff/sigcourse/notese.pdf

>(been reading it just yesterday :) )

>-Gregor

Sorry for being a little off-topic here, but could this method be used to
keep the rotational tire motion stable at low speeds?? I'm modelling RC car
tires that have a grip coefficient around 3.5-4.0 and a truly miniscule moi.
Unfortunately, the notation in the paper is unreadable by me :-( Any ideas?

Todd Wasson
---
Performance Simulations
Drag Racing and Top Speed Prediction
Software
http://www.racesimcentral.net/

by **Stephen Ferguso** » Thu, 15 Nov 2001 17:33:45

Now the real fun starts when you add in contact surface prediction for
deformable structures, like human knee joints, and then allow those contact
surfaces to exchange fluid. Oh, and let a bit dribble through the
interface. That's my current Pandora's box. It's a bit far removed from
sim racing, I will admit, but it looks like we're all in the same boat when
it comes to battling *** code.

Stephen

by **Doug Millike** » Fri, 16 Nov 2001 16:18:13

SAE is our publisher:

<https://shop.sae.org/servlets/productDetail?PROD_TYP=BOOK&PROD_CD=R-146>

.....

by **Gregor Vebl** » Fri, 16 Nov 2001 18:18:18

Stephen,

as I am sure you know, there 'exists' a so called law of complexity
invariance, which states that no matter how you modify the approach to
your problem, the complexity still remains about the same if not harder.
However, it seems that the complexity is also translationally invariant
across the different fields :).

-Gregor

> Unconditionally stable, but I think still succeptible to false response and
> oscillations. At least, the implicit solvers I use for modelling fluid flow
> in human tissues have some tight criteria on the size of the integration
> step required to achieve an accurate solution. They will always solve, but
> if you ignore the step criteria, the solution can do some wild things. Is
> this also true in the dynamics problems?

> Stephen

by **Gregor Vebl** » Sat, 17 Nov 2001 17:51:59

Indeed it could be, although when the tyre gets out of the nonlinear
regime one might get into trouble. Also, if the tyre cannot be
considered independently but the torque comes via a differential, then
things get hairy as well (esp. with locking wheels).

The first thing to stabilize low speed behaviour is the relaxation
length approach, though, which makes the desired timestep constant and
not diverging at low speeds. Still, I can imagine ridiculously high
frequencies still being necessary with your data.

-Gregor

> Sorry for being a little off-topic here, but could this method be used to
> keep the rotational tire motion stable at low speeds?? I'm modelling RC car
> tires that have a grip coefficient around 3.5-4.0 and a truly miniscule moi.
> Unfortunately, the notation in the paper is unreadable by me :-( Any ideas?

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

by **J. Todd Wass** » Sun, 18 Nov 2001 09:16:24

Thanks. Yes, it's hairy when doing the diff stuff. Luckily, the 4WD systems
essentially use a locked center diff with or without a one way front diff. So
on coasting, it's disengaged. I don't have the locking wheels correct yet,
although it's difficult at this point to tell. I'm still trying to get my head
around a possible Lagrange type approach to non symmetrical inertias on an
axle... I see what you meant about how it really changes things... Getting
the braking torque at a locked wheel will have to come after that, I'm afraid.

I was having to run between 20,000 and 60,000 hz to run the small tires below
about 5-8 kph and an open diff (locked was fine), but I found out yesterday I'd
converted incorrectly (Mars lander crash?). I was off by a factor of 100 the
light way, so these frequencies can now be reduced considerably.

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

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