Problem with Tandems is that speed on the downhills is governed by the decibell rating. That in laymans terms is by how loud the stoker can scream. And no Drag brakes on my tandem- So the stoker has no chance.

Well done on the 100 mile ride. It will not be your last but get a metric in before the end of the year. In fact get two in. One to find out what they are like and the second to do in 4 hours. I am an 8 hour 100 miler but 4 hours on the metric so that is your standard to go for.