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-   -   Lateral force on wheels - friction (http://www.autobanter.com/showthread.php?t=72553)

Wrza August 17th 06 10:43 AM

Lateral force on wheels - friction
 
Hi!

This question is more about simulating physics than simulating exactly
cars. We have lateral force acting on a car's wheels. Let's assume the
car is moving sideway with the constant velocity so the fricton acting
on the wheels is very high and causes the car to slow down. As much as
I know this friction doesn't depend on car speed only on coefficient of
friction (which depend on side slip) and load on wheel. My problem is
that when the speed of the car is very low this friction force still
acts on the car and finaly applies acceleration that moves my car in
the opposite side it was moving before and then once again and the car
starts to oscillate.

The question is: how to simulate this lateral force on wheels on low
sppeds? How to limit this force on wheels and not let it act like it
was dynamic force (not friction).

Thanks.

Wrza


Remco Moedt August 21st 06 03:12 PM

Lateral force on wheels - friction
 
On 17 Aug 2006 02:43:11 -0700, "Wrza" > wrote:


>The question is: how to simulate this lateral force on wheels on low
>sppeds? How to limit this force on wheels and not let it act like it
>was dynamic force (not friction).



I'm no expert, but isn't friction more a damper then a force? Can't
you substract the friction 'force' from the dynamic force, but don't
allow the result become less then zero?

BTW, do you use the Pacejka formulea?

Cheers!


Remco



Jeff Reid August 22nd 06 01:00 AM

Lateral force on wheels - friction
 
> The question is: how to simulate this lateral force on wheels on low
> speeds? How to limit this force on wheels and not let it act like it
> was dynamic force (not friction).


Lateral force is related to downforce, grip factor, and slip angle.
Slip angle is the angle between the direction the tire is pointed, and
the direction the tire is actually moving.

At very low speeds, limits on the steering angle cause the slip angle to
be reduced. There's a minimum speed for a given maximum steering angle
to cause maximum lateral force, below this speed, the force decreases.



Todd Wasson August 30th 06 03:20 PM

Lateral force on wheels - friction
 
Wrza wrote:
> Hi!
>
> This question is more about simulating physics than simulating exactly
> cars. We have lateral force acting on a car's wheels. Let's assume the
> car is moving sideway with the constant velocity so the fricton acting
> on the wheels is very high and causes the car to slow down. As much as
> I know this friction doesn't depend on car speed only on coefficient of
> friction (which depend on side slip) and load on wheel. My problem is
> that when the speed of the car is very low this friction force still
> acts on the car and finaly applies acceleration that moves my car in
> the opposite side it was moving before and then once again and the car
> starts to oscillate.
>
> The question is: how to simulate this lateral force on wheels on low
> sppeds? How to limit this force on wheels and not let it act like it
> was dynamic force (not friction).
>
> Thanks.
>
> Wrza


Low speed tire modelling is a tricky thing indeed. I had the same
problem in Virtual RC Racing's development (or should I say, "have," as
the problem is still there). It's a bit trickier to solve with the
little cars than bigger ones, but the basic problem is the same. The
car is barely moving sideways, but the slip angle is still +90 degrees,
so you get a kick back the opposite direction, now it's -90 degrees
slip angle, so you get another opposite kick. Back and forth, back and
forth... Same thing in slip ratio. Either way, the result is that the
car floats around instead of stopping completely and will slide down
the most gentle hill even with the wheels locked.

If you haven't heard of slip angle/slip ratio and aren't using them,
the problem is the same with a coloumb friction model in that the force
acts opposite the velocity vector, which itself can change signs every
step. Big force to the left, then one to the right on the next step,
back and forth, back and forth... Unfortunately people usually
interpret this to mean the "physics are wrong" and anything that
happens over 1m/s must be equally screwy :-P Not true of course, but
they don't know that so we're stuck spending a bunch of time trying to
make a realistic "sitting in one spot" simulation that is convincing.
Oh well :-)

If you're using a slip angle/slip ratio based model this can be fixed
up by incorporating a relaxation length into the model. A more simple
way is to make the slip angle/slip ratio that you feed into your tire
model a state variable that progresses slowly toward the "real" slip
angle/slip ratio depending on wheel rotation and movement. Another way
would be to switch to a spring type of model at very low speeds.

You might try googling here at RAS for "Car physics" + "tire" and so
on. There have been plenty of discussions on this topic and many other
vehicle dynamics modelling topics here over the years. Should be
plenty to peruse :)

Todd Wasson
www.PerformanceSimulations.com
www.VirtualRC.com
http://www.PerformanceSimulations.co...ToddSim17a.wmv


Jeff Reid August 30th 06 11:24 PM

Lateral force on wheels - friction
 
> The car is barely moving sideways, but the slip angle is still +90 degrees,
> so you get a kick back the opposite direction, now it's -90 degrees
> slip angle, so you get another opposite kick. Back and forth, back and
> forth...


Seems a better model is needed. Maybe one based on strain, contact patch
displacement, to determine force generated by the tire. This approach would
be independent of speed.



Todd Wasson August 31st 06 05:51 PM

Lateral force on wheels - friction
 

Jeff Reid wrote:
> > The car is barely moving sideways, but the slip angle is still +90 degrees,
> > so you get a kick back the opposite direction, now it's -90 degrees
> > slip angle, so you get another opposite kick. Back and forth, back and
> > forth...

>
> Seems a better model is needed. Maybe one based on strain, contact patch
> displacement, to determine force generated by the tire. This approach would
> be independent of speed


Right, that's what the relaxation approach does. The slip angle
doesn't just go right to 90 even though literally, according to the
definition of slip angle, it is really 90 degrees. Instead, it has to
travel a little distance before the state variable version of the slip
angle hits it. In the mean time, the force builds up gradually until
you hit an equilibrium. That's the idea, anyway. It can be a bit
tough to really make it work in practice though because you still need
just the right amount of damping to get it to work right.

The displacement stuff you mentioned is indeed a way to do it as well.
A simpler way would be to use a spring model at low speeds or when
stopped. A long time back I modelled the tire as a torsion spring at
low speeds. This worked quite well actually in the longitudinal
direction. The tire would spring back just a touch after a quick stop.
That didn't do anything for the lateral direction, but it improved the
car a lot when trying to stop on hills/banks and so on.

Todd Wasson
www.PerformanceSimulations.com
www.VirtualRC.com
http://www.PerformanceSimulations.co...ToddSim17a.wmv



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