Mounting loads behind the rear axle can affect how the motorcycle turns and brakes. It can also cause a wobble. An uneven load can cause the motorcycle to drift to one side. Elastic cords with more than one attachment point per side are more secure. A tight load won't catch in the wheel or chain, causing it to lock up and skid.
Rope tends to stretch and knots come loose, permitting the load to shift or fall.
If you ride with others, do it in a way that promotes safety and doesn't interfere with the flow of traffic. Keep the Group Small Small groups make it easier and safer for car drivers who need to get around them. A small number isn't separated as easily by traffic or red lights. Riders won't always be hurrying to catch up. If your group is larger than four or five riders, divide it up into two or more smaller groups.