rec.autos.simulators

TIA,
jonas

To estimate inertias, one needs to know the mass of the object and it's
dimension. Moment of inertia around a given axis is given as the mass of
the object, multiplied by the average square of the distance of the mass
distribution around that axis.

Take for example an F1 car. Let's try to estimate the inertia around the
nose to tail axis (in roll). It has a mass of 600kg, and is 2m wide.
This means that the effective radius is smaller than that, an probably
of about 0.5m (most likely even less). This will give you the inertia of

J_roll = m * r_effective^2 =600 kg * 0.25 m^2 = 150 kg m^2.

If you look at the inertia of the same car from the top, estimate that
it's about 4m long, which means that the effective radius should be
close to 1m, and the inertia around the yaw axis is about

J_yaw = 600 kg * 1 m^2 = 600 kg m^2.

The largest source of error in this estimates is the effective radius,
which is defined as the radius, for which the inertia is the same should
all the mass be concentrated at it, as the moment of inertia is
dependent on the square of that value, and you can easily make a mistake
of 50% in radius, which, however, more than doubles or halves the
resulting inertia.

I hope the above will get you going. Greetings,

-Gregor

P.S. Data for a (generic?) Champ car can be found in Millikens' book
Race Car Vehicle dynamic, and for the roll axis it is about J_roll=70 kg
m^2, while in yaw the value is about 1300 kg m^2. Not far from the above
estimates, which are just as good for a Champ car as well, given the
errors we made.

:)

When you set up your front andn rear tire models,
they work with the inertia to give a certain
undamped natural frequency for yaw oscillations.
This frequency needs to be in a range appropriate to
your class of vehicle. Formula one cars will be faster,
station wagons slower, etc.

Another possibility are SAE papers where researchers have
made measurements of large numbers of vehicles.
You could find a vehicle in a class similar to your
desired car and use variations off of that data.

Huh?  We talk about this stuff over lunch and dinner all the time.
In-between meals we work on real problems<grin>.

that would be great. do you have any references to such papers (i
browsed the SAE site a little, but i didn't come up with anything - so
many papers there ;-))

jonas

"Measured Vehicle Inertial Parameters - NHTSA's Data Through
November 1998", Heydinger, Bixel, Garrott, Pyne, Howe, Guenther,
SAE 1999-01-1336. International Congress and Exposition,
Detroit Michigan, March 1-4, 1999.

For example, yaw Moment of inertia for 1998 Dodge Neon = 2340 kg-m^2
It includes charts showing the correlation of various useful ratios
such as CG/roof height versus mass, etc.

This paper is new enough that it was prepared in electronic form
and is probably available for download for a fee from SAE.

--
Matthew V. Jessick         Motorsims

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