Restrict the rigidbody to be affected by only 1 joint

so I'm testing a motorcycle game where all the action is happening in the ZY plane. I have a ragdoll sitting on my bike. his hands are bound to the handlbar by spring joints, the feet to the pedals. now I want it to lean forward/backwards. for this I have created another rigidbody object that is bound to his butt with the spring joint and moves from the back of the motorcycle to the front. the butt follows it OK, but the problem is that it also drags the motorcycle along with it. so how can I restrict the ragdoll from draggin the bike?

in real life the ragdoll would apply a counter-force with his feet on the bike and the bike would stay static... but I don't have an access to the forces the rigidbody is experiencing.

screenshot:

thank you!