A bike with steering problems has staying-upright problems. These simple safety checks should keep you rubber side down.
While you're most likely to encounter steering problems on a bike that's new to you, they can develop on your own bike. Sometimes it's sudden: a jolt knocks the handlebar loose. Sometimes it's gradual: the headset works loose over weeks. Sometimes it's the result of a maintenance mistake: you fail to put the wheel back on properly. So it's worth checking your own bike periodically.
These fixes are for common, easily-solvable problems. If the bike has a steering fault that you can't fix, do not ride it. Take it to the bike shop.
Problem: The handlebar turns independently of the fork. You can't steer!
Check: Stand in front of the bike with the front wheel between your knees. Try to turn handlebar left and right with some force.
Solution: Modern threadless stems clamp the outside of the fork's steerer tube, via one or two bolts on the side of the stem. Tighten these. Note that adjusting a threadless stem often requires headset adjustment too (see below), as the headset bearings and the stem are both held in compression by the stem top-cap. Old-style 'quill' stems clamp inside the steerer with an expander bolt. There are no bolts on the side, only a large one in the top. Tighten this firmly.
Problem: The handlebar can rotate in the stem, sending you lurching forward if you hit a bump.
Check: Stand over the bike and try to rotate the bar.
Solution: Tighten firmly the bolt or bolts in the end of the stem, where it holds the handlebar. If there's more than one bolt, work incrementally, tightening one bolt and then the bolt opposite – diagonally opposite if there are four bolts.
Problem: The grips can rotate or slide off completely.
Check: Twist them. See if they'll come off without undue force.
Solution: Lock-on grips have a metal collar at one or both ends, with a small Allen bolt in the collar. Tighten this. Ordinary grips are friction-fit. If such a grip is loose, remove it. Wrap electrical insulation tape one layer thick tightly around the bar where the grip goes. Spray lightly with hairspray; it provides hold! Refit the grip. Leave it for 10 minutes, then re-check. Alternatively, buy and fit new grips.
Problem: The fork's steerer tube doesn't just rotate, it moves back and forth, clunking over bumps.
Check: Stand over the bike. Wrap your left hand around the top of the headset. Apply the front brake with your right hand and rock the bike back and forth. Is there any play?
Solution: If your bike has a threadless headset, loosen the bolts on the side of the stem and then tighten the top-cap bolt. Tighten the top cap just enough to stop any play in the headset. If you tighten it too much, the steering will feel stiff and rough. If you're not able to tighten it enough to prevent play, the top-cap is probably bottoming out against the top of the steerer; add an extra headset spacer. Finally, tighten the stem's side bolt(s).
If your bike has an old-style threaded headset, you'll need two big headset spanners. Undo the lockring (the top ring with flat faces) and screw down the adjusting cup (the bottom ring with flats) just enough to prevent fore-aft play. Then tighten the lockring while holding the adjustable cup in position. Note that either kind of headset adjustment requires trial and error.
Problem: The wheel isn't fastened firmly in the fork, so it can wobble or even fall out.
Check: Standing beside the bike and holding it upright, rock the wheel from side to side with your free hand. Any looseness? Then lift the front of the bike up and hit the top of wheel with the heel of your hand, trying to knock it out of the fork. (Small lips on the fork should prevent the wheel falling out if it is only a bit loose.)
Solution: Close the quick-release lever firmly. You should start to feel resistance when the lever is halfway closed, and be left with an imprint in your palm after it's fully closed. Axle nuts need screwing firmly with a spanner. If the wheel still feels loose after this, the bearings are loose. Go to the bike shop.
Problem: The wheel wobbles from side to side.
Check: Lift the front of the bike and spin the wheel, sighting along the wheel rim to spot deviations. If it wobbles more than 1-2mm, it needs to be corrected.
Solution: True the wheel
Tyre not seated properly
Problem: There's a bulge or flat spot on the tyre that makes the wheel bump. The tyre's 'bead', its rubber-coated wire or kevlar edge, is not sitting on the rim correctly.
Check: Lift the front of the bike up and spin the front wheel, looking for distortion at the tyre rather than the rim.
Solution: Deflate the tyre. Put your hand over it and squeeze the two sides in towards the centre. Work your way all around the wheel doing this. Reinflate the tyre fully. It should pop into place on the rim.
Problem: The tyres are so soft that they squirm when you go around corners.
Check: Squeeze the tyre between finger(s) and thumb. It should feel firm like an apple not squashable like a satsuma. If you've got gauge, check the pressure against the range stamped on the sidewall of the tyre.
Solution: Get pumping! It's much easier with a floor pump.