STEM & HEADSET: How to troubleshoot a loose stem and handlebars
These are the common causes of loose handlebars:
1. A properly tightened stem bolt should be between 10-12Nm (Newton meters are a metric of torque). In most cases of loose handlebars, we find that the stem bolt is not properly tightened, thus causing the stem to become undone mid-ride.
If your handlebars are rotating at all, this is a sign they need to be tightened. Try removing the stem and making sure the line on the stem is flush with the line on the frame and then tighten it back down with a torque wrench if you have one available. If you do not have an extended 5mm Allen key, a visit to the bicycle shop may be needed to achieve the recommended 10-12Nm.
2. A properly functioning handlebar should include a clean steerer tube free of any grease and/or debris. If the steerer tube is dirty, the grease and/or debris can often be removed with a bit of soap and damp cloth.
3. Are the bolts/threads damaged? If the bolts and/or threads are damaged in the stem, achieving the proper torque will not be possible, and the bolts or stem might need to be replaced.