[backpacker3]

Do you have an idea of the wheel/tire you'll be using?

The issue is that without sufficient weight on the wheel there won't be enough traction to keep it from slipping and just spinning around doing nothing however to get sufficient pressure on the wheel you'll have to use the weight of the vehicle and to do that requires the arm to slightly lift the car. Not enough to take the tires off the ground enough though to actually put weight on the wheel (think about jacking your car up you have a good amount of travel before the wheel leaves the ground) the problem then becomes that under an emergency or hard breaking that tiny bit of lift could be enough to get the back end of the car to want to fishtail or at least make it pretty squirrelly.

If you were going to do this you'll need to either make sure it won't activate above certain speeds or have a switch you can easily get at to instantly deactivate it in an emergency scenario.