Physics: inertias numbers

when trying to create a car simulation you sure come across a lot of numbers you don't find in any car specs. not only does it seem impossible to find pacekja constants on the net, i'm also having a hard time finding values for the various inertias in a car. sure - these are hard to determine experimentally but someone must have vague numbers. if anyone has values for inertial moments of car engines or drivetrains, or, more difficult to measure, PMI (Polar Moment of Inertia) of the whole car body around x/y/z axis, please share them with the world. i don't think i really need very accurate numbers for specific car types (would be nice though), but an idea of the dimension of these values would be a good start. TIA, jonas

Hi, 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. jonas echterhoff wrote:

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> when trying to create a car simulation you sure come across a lot of > numbers you don't find in any car specs. not only does it seem > impossible to find pacekja constants on the net, i'm also having a hard > time finding values for the various inertias in a car. sure - these are > hard to determine experimentally but someone must have vague numbers. > if anyone has values for inertial moments of car engines or > drivetrains, or, more difficult to measure, PMI (Polar Moment of > Inertia) of the whole car body around x/y/z axis, please share them > with the world. i don't think i really need very accurate numbers for > specific car types (would be nice though), but an idea of the dimension > of these values would be a good start. > > TIA, > jonas

Does anyone else's head hurt when they read these type of posts? :) jonas echterhoff wrote:

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> when trying to create a car simulation you sure come across a lot of > numbers you don't find in any car specs. not only does it seem > impossible to find pacekja constants on the net, i'm also having a hard > time finding values for the various inertias in a car. sure - these are > hard to determine experimentally but someone must have vague numbers. > if anyone has values for inertial moments of car engines or > drivetrains, or, more difficult to measure, PMI (Polar Moment of > Inertia) of the whole car body around x/y/z axis, please share them > with the world. i don't think i really need very accurate numbers for > specific car types (would be nice though), but an idea of the dimension > of these values would be a good start. > > TIA, > jonas >

There are rules of thumb for various dynamical parameters. For example, the value (wheelbase / yaw radius of gyration) squared varies from 3 for a station wagon to 7.5 for a formula 1 car. Higher numbers are obviously more responsive. 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. jonas echterhoff wrote:

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> > when trying to create a car simulation you sure come across a lot of > numbers you don't find in any car specs. not only does it seem > impossible to find pacekja constants on the net, i'm also having a hard > time finding values for the various inertias in a car. sure - these are > hard to determine experimentally but someone must have vague numbers. > if anyone has values for inertial moments of car engines or > drivetrains, or, more difficult to measure, PMI (Polar Moment of > Inertia) of the whole car body around x/y/z axis, please share them > with the world. i don't think i really need very accurate numbers for > specific car types (would be nice though), but an idea of the dimension > of these values would be a good start. > > TIA, > jonas

-- Matthew V. Jessick Motorsims [email hidden] http://www.moto1.net Vehicle Dynamics Engineer (972)910-8866 Ext.125, Fax: (972)910-8216

Huh? We talk about this stuff over lunch and dinner all the time. In-between meals we work on real problems<grin>. On Tue, 21 Aug 2001, Rob Swindells wrote:

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> Does anyone else's head hurt when they read these type of posts? > > :) > > jonas echterhoff wrote: > > > when trying to create a car simulation you sure come across a lot of > > numbers you don't find in any car specs. not only does it seem > > impossible to find pacekja constants on the net, i'm also having a hard > > time finding values for the various inertias in a car. sure - these are > > hard to determine experimentally but someone must have vague numbers. > > if anyone has values for inertial moments of car engines or > > drivetrains, or, more difficult to measure, PMI (Polar Moment of > > Inertia) of the whole car body around x/y/z axis, please share them > > with the world. i don't think i really need very accurate numbers for > > specific car types (would be nice though), but an idea of the dimension > > of these values would be a good start. > > > > TIA, > > jonas > > > > >

In article <[email hidden]>, mjessick-Motorsims <[email hidden]> wrote: > There are rules of thumb for various dynamical > parameters. For example, the value > (wheelbase / yaw radius of gyration) squared varies > from 3 for a station wagon to 7.5 for a formula 1 car. > Higher numbers are obviously more responsive. > thanks, this information is quite helpful.

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> 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. >

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

jonas echterhoff wrote:

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> > 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. > > > 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 ;-))

My associate who is on vacation had it locked in his office ;) "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 [email hidden] http://www.moto1.net Vehicle Dynamics Engineer (972)910-8866 Ext.125, Fax: (972)910-8216