I presume you've checked that everything is solid and secure? It sounds as if you're hitting the natural resonance frequency of the frame. It's a problem that can occur with heavily-laden touring bikes as well - basically, you've got a heavy weight (the battery) on a spring (the frame), and it's oscillating. There are several ways it can be solved: Move the weight - easy with luggage, probably harder with a Powabyke battery. Reduce the weight - ditto. Stiffen the frame - if the battery was on a rear carrier, then this is doable, but for a Powabyke probably not - though you could try wedging it more firmly if it's moving about. Reduce the source of vibration - this might help - by fitting suspension forks. Change the tyres. Fit some kind of steering damper. With touring bikes, unfortunately it seems that some designs and geometries are more prone to it than others, and there's very little that can be done.