We liked our safari when the rhinos got frisky. Made it more exiting. From what I understand the animals are accustomed to the ride vehicles, and as mentioned, only challenge the vehicles when threatened.A few years ago the vehicle that I was in got stopped by a big rhino that stared us down for about 10 minutes. He wandered in front of our vehicle and stood still, and we just had to wait it out. It was pretty cool (and a little scary ) to have a big mean rhino stare at us like he was going to charge.
I think the chains are (also?) part of the cleaning system, to keep the vehicle tires from tracking stuff from one section of the park to another (that's what I've been told while doing the special giraffe-feeding safari at BG).