Hi

Does that mean you want to compare different bodies with a fixed underbody? If so, can't you consider the underbody rigid in your software? If not, can you ask your question more clearly?

Hello,

Yes, I want to compare different upper bodies (torsional stiffness) with a fixed underbody. Only each underbody has a different torsional stiffness value. So to have a fixed under body, I want to modify the Young's modulus of each of the under bodies so as to have torsion values ​​that are the same for different underbodies

But the problem is that there are underbody that have different lengths, thicknesses, inertia that others so i don't know what to do for this problem.

The comparison is only meaningful if all the upper bodies are subjected to same external torsion and boundary condition (in this case implies the same contact or reaction forces with the under body, which is impossible due to dissimilar under body's shape or stiffness). This is despite the modification in Young Modulus. The upper body torsion stiffness 'works' only when it is coupled with the matching under body to function as a complete torsion system.

They are all subject to the same boundary condition and the upper body is associated and matching with his under body.

Each upper body is associated with its under body. And the boundary conditions are the same, there is only the Young modulus of the under body that you want to change to make the comparison. For example if we puts a Young's Modulus at 200,000 only in the under body and finds a stiffness of 0.70 and for another a young modulus at 100,000 and finds a stiffness of 0,70 we can compare two upper bodies because the under bodies will have the same stiffness.

Jiann Haur Tang Are you saying it's impossible to compare two upper body to each other in terms of torsional stiffness? Why can we compare the value of torsional stiffness of two cars together, but not two upper bodies together? Complete cars (upper body + under body) also have dissimilar geometry.