You should never tie the bounced weight of the car down without blocking. When the trailer hits a bump the car will also bounce. This causes the straps to slack and snap tight.
I have been towing cars long distance for a very long time . Always tie the unsprung weight of the car . Rear axle and the end of the control arms near the ball joint. Let the car bounce the straps stay tight.
You don't know how many times I see people pull the body down with straps. just to see them come off when getting slack on bumps
If you use k-member tie down hoops welded on . Put a block on top of the control arm bump stop and pull the car down until they are tight. Or stick a tall wooden block under the K-member. Never let the car have slack to bounce if tying the body down .
On a stock suspension car I use these straps pictured and wrap the axle and the control arms on all 4 corners of the car and X the ratchet straps. I Never had a car come loose or a strap rip .
They are called axle straps They have a cut resistant sleeve that can be position anywhere on the strap.
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