Why Is My Lawn Mower Vibrating Excessively?

A little vibration is normal for any engine-powered equipment, but if your mower is shaking more than usual, something is wrong — and ignoring it will make things worse.

The most common cause is a bent or unbalanced blade. If you've hit a rock, stump, or buried object, the blade may be bent even if it doesn't look like it. Even a slight bend creates an imbalance that causes vibration at high RPM. An unbalanced blade (one side heavier than the other, often from uneven sharpening) causes the same issue.

To check: remove the blade and lay it on a flat surface. If it doesn't lay flat or rocks back and forth, it's bent. To check balance, hang it on a nail through the center hole — if one side drops, it's heavier. A bent blade should be replaced, not straightened.

Other common causes include loose blade adapter or mounting bolt (the bolt that holds the blade to the engine shaft — should be checked periodically), a damaged flywheel key (a small metal key that aligns the flywheel — if sheared from impact, timing is off and the engine runs rough), worn or damaged engine mounts (rubber mounts that isolate engine vibration from the frame), and a bent crankshaft (the most serious cause — usually from hitting a solid object hard).

For riding mowers, additional causes include worn spindle bearings (the bearings that support each blade — they wear over time and develop play), loose deck components, and worn drive belts.

How to prioritize: start with the blade — it's the most common cause and easiest to check. If the blade is fine, check the mounting hardware. Beyond that, bring it in for professional diagnosis. A bent crankshaft, in particular, requires proper tools to assess and is often more expensive to repair than the mower is worth.