Can longitudinal distance from CG to axle be negative in 3DOF trailer body model

Hi, guys, I want to move CG backwards and forwards to observer vehicle motions under different weight distribution, but I came across following three questions

1. Are the distances between each axles fixed ?
2. Can a/b/c < 0 , which means CG is behind of the axle ?
3. Is the trailer total lenght be (dh_f + c) ? There's no rear hitch, so no dh_r.

I can not find specific algorithm or discriptions, but I tried several times, I believe the distances is not fixed, and distance can be negative.

My observations:

• There is obviously no check on a/b/c values, because I can basiclly type whatever value I want, for example, a = 1 and b = 1, which means a and b axle are in the same location, the simulation worked successfully.
• I don't know why parameter df_h's influence is so dramatic,
• set df_h = 3.18m, the tractor-trailer can be stable under sine wave steer input,
• set to 3.28m, the vehicle lost its stability,
• set to 3.08m, vehicle won't response to steer input.

Thanks in advance.

Answer 1

[idris]

The author needs to update the question for clarity: state what you had done before and what challenge being observed clearly.

[明明]

Thanks for your answer.

I have updated my question, do you have some insights for my question?