Snowboarding

using UnityEngine;

public class Snowboarding : MonoBehaviour
{
    private Rigidbody rb => GetComponent<Rigidbody>();

    [SerializeField] private float maxSpeed = 5;

    RaycastHit slopeHit;

    private void FixedUpdate()
    {
        if(OnSlope())
        {
            rb.velocity = GetSlopeDirection() * rb.velocity.magnitude;
        }

        else rb.velocity = new Vector3(rb.velocity.x, rb.velocity.y, rb.velocity.z);

        //Limit velocity speed
        float clampSpeed = Mathf.Clamp(rb.velocity.magnitude, 0f, maxSpeed);
        rb.velocity = rb.velocity.normalized * clampSpeed;
    }

    private bool OnSlope()
    {
        if(Physics.Raycast(transform.position, Vector3.down, out slopeHit, 1))
        {
            float angle = Vector3.Angle(Vector3.up, slopeHit.normal);
            return angle < 45 && angle != 0;
        }

        return false;
    }

    private Vector3 GetSlopeDirection()
    {
        return Vector3.ProjectOnPlane(rb.transform.forward, slopeHit.normal).normalized;
    }
}