I have a carbon fork on my Dulce and have had her up to 43.5 mph/69.6 kph, but haven't had a shimmy. I do loosely grip the bars (was in the drops for the 43.5 mph) and tuck my knees into the tube, though. That's an interesting situation, runawaymum. Could it have even been the road surface you were on? Stupid question, I know, but I know on chip seal, I feel as if the front wheel wants to vibrate right out from under me!