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I agree with you, it is important to have that particular rotation and it has actually been included. If you have a look at the final page there is a good picture of the hitch and rod end. The brackets themselves are fixed rigidly to the frame but you will see that the trailer arms attach with a rod end and there is a bolt that goes through the bracket and rod end. This does not hold it in place in a rigid fashion but is actually designed to allow the rotation in the vertical direction as you have described.So basically the vertical movement is allowed for at the hitch and the horizontal movement is allowed at the headtube on the trailer.
Thanks, I tried a few different styles before I settled with this. I thought the bob trailers were too long and they would be even longer if you redesigned them with a bigger wheel. If you have a good look the pannier bag actually sits a lot lower than it would if it were on the bicycle's rear rack. Also the bulk of the weight is in the bags (lets say 10+10kg) which is low down, the sleeping mat is negligible, the sleeping bag weighs about 0.7kg and the tent is about 2.3kg. If the bulk of the weight is around the line of the axle it is very stable, that is where the centre of balance is (think of a gyroscope)
Single Wheel Bicycle TrailerView Instructable »