After bump, bicycle feel as though tyre is flat (it is not) I ride a 1970s steel-framed french racing bike. A couple of days ago I went over a small pothole whilst leaning on the front brakes at some speed, and although nothing happened it's since felt strange when riding in a straight line, almost tramlining. It's as if the front tyre is flat; there's an unnatural resistance to changing direction that there never used to be. Both tyres are pumped up, both wheels are true, and there's no discernible play anywhere that I can find.

I'd be grateful for any suggestions as to what this could be the cause of this!
One possibility is brinelling of the bearing race, a good impact in just the right orientation could cause brinelling. 

From Wiki:


  Brinelling /ˈbrɪnəlɪŋ/ is the permanent indentation of a hard surface. It is named after the Brinell scale of hardness, in which a small ball is pushed against a hard surface at a preset level of force, and the depth and diameter of the mark indicates the Brinell hardness of the surface.


Generally this occurs in old bearings that have been heavily used, but a hard impact could have helped it on the way, and now your bearings are settling in the indentation causing the resistance you are describing.

If this is indeed the case, replacing the headset, or at very least the bearings and races, should remedy the situation.