Hi,

If your geometry is modelled in centimetres then every parameter needs to be expressed in centimetres. I think the source of your problem is that you have been converting the centimetre dimensions of your car into metres.

Let’s say that your car is a box that is 100cm * 100cm * 100cm. The moi of the vehicle will be

[100*100 + 100*100, 100*100 + 100*100, 100*100 + 100*100]*mass/12

Similarly, if the wheel radius is 50cm then the wheel moi will be

0.5*50*50*wheelMass

The default values of the car assume that the metre will be the adopted length scale. If centimetres are used then many of the pararmeters need adjusted to reflect that they are expressed in inches or centimetres or furlongs or whatever the chosen length scale might be.

Let’s think about what can go wrong when the units end up being mixed. Imagine that the wheel center is offset from the rigid body center of mass by (100cm, 100cm, 100cm) but the rigid body moi is expressed in metres. If a suspension force F is generated at the wheel then a torque of (100,100,100) X F will be generated.

torque = (100,100,100) X (Fx, Fy, Fz).

The angular acceleration will be

angAccel = torque/moi.

If moi is expressed in metres and the torque is expressed in centimetres then angAccel will end up being orders of magnitude larger than is physically correct. However, if they are expressed with the same units of length then the length scale will cancel in the numerator and denominator, thereby generating the correct angular acceleration in radians per second. There are many parameters combinations in the vehicle code that end up with this kind of error if the units are mixed.

To re-express the vehicle in centimetres you need to go through the list of affected parameters and re-compute their values. Remember that vehicle parameters have been assigned default values under the assumption that metres will be the adopted scale. If centimetres are chosen instead then the default values are no longer correct and need re-computed.

Thanks,

Gordon