What are the visible signs of excessive heating on a wheel bearing?

The visible signs of excessive heating on a wheel bearing primarily manifest through discoloration of the bearing surface. When a bearing experiences excessive heat, it undergoes a change in its material properties, which often leads to changes in color. This discoloration can be indicative of overheating that may have caused alterations in the microstructure of the metal, which can compromise the integrity and performance of the bearing.

Other signs of overheating, though they may be present, are less definitive than discoloration. For example, while cracking of the bearing surface can occur if the temperature has been excessively high, it usually indicates a more significant level of damage and is not as directly associated with heat as discoloration. Increased resistance when rotating can also occur, but it may not be visually identifiable and can result from factors other than heat, such as improper lubrication. The presence of smoke during operation could suggest severe overheating, but it's not typically a visible characteristic of the bearing itself; instead, it might indicate other issues elsewhere in the assembly, such as lubrication failure.

Therefore, discoloration stands out as the most direct and visually detectable sign of overheating in wheel bearings.