When I first got my bike, I found it pulled to the left. After pondering this tendency for a while, I measured the alignment of the wheels using a string from front to back wheels. I found that they were misaligned. Further investigation showed that the front wheel dropouts were not inserted into the forks the same amount. The result of this was to throw the wheel out of plumb so that at the point of contact with the road, the wheel was a cm or so to the left. This was easily fixed by filing the dropout with a round file. The geometry of the wheel is such that if the axle is off by, say one mm, the wheel at the rim will be off by about four mm. It took longer to think about the problem than to fix it.