Yeah, you need something to keep them from sliding. My buddy used to bolt his ramps to a long piece of thin(ish) wood so the car tires were on the wood before hitting the ramp. That keeps them from sliding out from under or pushing as you try to go up.
I rarely list on there because the barrage of bullshit just isn't worth it. I've often been told, if you can deal with a months worth of pain for a day of joy, CL is for you.