I put my front and rear bar on at the same time, so I can't say which one does what. I think this is the test for strut bars. Go down the interstate and get off on a cloverleaf. With no bar(s) the car will lean to the left before the tires break. With bar(s) the tires will break before the car rolls.

The top of the struts will want to flex as the car goes over uneven roads or around a curve. The strut bar simply ties them together to reduce chassis flex. As you stiffen the chassis, you make the car more solid. The downside to this is that if you have a suspension too tight, when you go around a curve and go over bumps, the car will want to bounce over them instead of absorbing the shock and keep all 4 tires in contact with the ground. We will never see this with our cars, as compared to a go kart. What you will notice is that you can take a turn about 5 mph faster with bars and still feel safe and in control.

My strut bars stopped some of the creaking sound when I drove over uneven surfaces, like driveways that have a gutter beside the street.

To make a long story short - go to EBay and get a strut bar for the front and back. You'll be happy. That's the most important thing.