Modal dynamics, base motion issue

[ptsal]

Hi guys, first of all thanks for spending time reading this.

I have a model of a Bus and I'm trying to do a modal dynamic analysis (the simulation finish with no problem), however I'm getting huge displacements.
I'll explain my problem:

I have a bus with a gravity applied all over the model and I'm trying to simulate a bump through secondary base motion, type=displacement.

The thing is, If I encastre the rear wheels and apply the secondary base motion(type=displacement/acceleration) to the front wheels they move as much as I want and I dont have any problem.

However in reality the rear wheels are able to move and rotate freely except in Z-direction.

When I try this second case(or enable rotation around the wheel axle), base motion displacement in z-direction with a given amplitude(the same as before), I get huge displacements and accelerations. I cant understand why this is happening.
If anyone could point me to the right direction I would be grateful


I tried this way, but if anyone has an alternative to simulate a bump I would appreciate the suggestion.

Thanks in advance.