Bakersb

Hi, I recently bought a used road bike off craigslist. The actual bike is a mid-range Cannondale, and the parts are entry-mid ranged, but still of pretty good quality. It runs a threaded 1" quill stem.

The problem is, I can easily knock my front wheel out of alignment from the handlebars with a hit, not a very hard hit either. I noticed this when I had a small fall which could not have truly damaged anything; I believe it came like this. I tightened the bolt on the quill stem, all the way so that I couldn't tighten it any more. But the problem persists, the wheel still gets knocked out of alignment with a hit (not too hard, not too soft). This is unlike my other bikes, where a much harder hit would not even budge, and that bike would stay aligned.

Are there any parts in particular that need replacing? The headset? Fork? Quill stem? Or is there a DIY fix/check I can do?

I plan to visit a lbs soon, but I wanted to see if I could fix it myself before I pay them a visit.

FBinNY 

Two possibilities, one benign and easily fixed and the other very dangerous.

The dangerous scenario is very rare but it does happen so I want you to check. It possible that the steerer tube is working loose on the fork drown and allowing the crown to move. This is a precursor to a face plant, so check by holding the front wheel between your legs and moving the bars sideways Watch the headset locknut and confirm that the locknut did not turn with the bar.

Assuming that the stem moved in the fork, which is the more likely and non serious issue, loosen the expander bolt until it rises about 1/4" and drive it in with a hammer to free the wedge. Pull up the stem, and check the condition of the expander bolt's threads and the wedge itself. Something is preventing the wedge from rising far enough to secure the stem, so tighten the bolt to draw the stem up and see if it goes far enough to bind. If not, find the cause and deal with it, ie. new expander bolt.

BTW- there's a third possibility, also rare. A prior owner might have chronically over-tightened the expander and bulged or split the steerer tube. Shine a flashlight down there and check that it still looks OK on the inside.

Kimmo

I'm thinking stripped thread on an ally expander nut, perhaps.

Bakersb

Thanks so much, I'll check it asap. Could have possibly saved my life

Andrew R Stewart 

Sometimes parts that need to bed into each other can devolop notching/trenching/indenting that will make these parts return to their notched position after having moved them slightly. I have found a major change to the relationship of the mating surfaces often means that the parts will hold this new position well. Andy.