how to change the velocity of a rigid body based on a forward

hello, i'm trying to make a vehicle controller based on a rigid body.

with the steering i'm hoping to change the players velocity direction when turning. so let's say the player turns right, it'll preform the operation;

playerRb.velocity = (playerRb.transform.forward + (turnLeftForward * steeringAmnt));

-"playerRb" being the rigidbody used to control the movement -"turnLeftForward" being the new desired direction -"steeringAmnt" being the joystick input, used to difference between the current and the desired left should be calculated

the issue i'm having with this line is that when i attempt to use it, all the velocity the vehicle had is lost basically instantly, and preserves none of the momentum it once had.

can someone please help me understand where i'm going wrong with my logic? please and thank you.