You're going to want to run it somewhere in between. You do not want it so tight you can not move it and you do not want it so loose it's jiggling loose. A nice, snug fit, with some play is perfect. Too tight may cause the chassis to bind and too loose may cause the chassis to over-flex. We test and run our chassis' with a snug rear bumper.Which is most prefered, run the rear bumper loose or tight and why.