Secured to your trailer

I always tie to the suspension. That way it is solid to the trailer. Let the
car bounce if it wants to. That way you won't have the straps getting loose.
Been hauling cars on my open trailers for 20 years. Never had one loosen up.
Just got back from getting my car, had no shocks in it and the tires were soft. Had my car straps to the axle and the lower control arms, but the car was getting air born due to the angle of the pull. Glad I bought my bike tie downs with me, went around the k-frame and to the shackle, no more hopping on the trailer. I will never again tow another vehicle with no shocks and soft tires.