Repeatedly breaking rear wheel bearing cups can be a frustrating issue, and there could be several reasons why this is happening. Here are some possible causes to consider:
Over-tightened bearings: One common cause of bearing cup failure is over-tightening the bearings. When the bearings are excessively tightened, they put extra pressure on the bearing cups, leading to premature wear and ultimately, failure. It's important to follow the manufacturer's recommended torque specifications when adjusting the bearings.
Improper bearing installation: If the bearings are not properly installed, it can lead to misalignment and uneven load distribution. This can put excessive stress on the bearing cups, causing them to break. Ensure that the bearings are correctly seated and aligned during installation.
Insufficient lubrication: Inadequate lubrication or using the wrong type of lubricant can cause increased friction and heat in the bearing assembly. This can accelerate wear and contribute to bearing cup failure. Make sure to use the appropriate lubricant recommended by the manufacturer and regularly inspect the lubrication levels.
Contamination: If dirt, debris, or water enters the bearing assembly, it can damage the bearings and eventually lead to cup failure. Ensure that the seals and dust covers are intact and functioning properly to protect the bearings from contamination.
Axle misalignment: If the rear axle is not aligned correctly, it can cause excessive stress on the bearing cups, leading to failure. Check the alignment of the axle and ensure it's properly positioned within the frame.
Axle damage: A damaged or bent axle can create uneven forces on the bearing cups, causing them to break. Inspect the axle for any signs of damage or bending and replace it if necessary.
Excessive load or abuse: If the bike is consistently subjected to heavy loads or rough riding conditions, it can put excessive stress on the bearings and cups, leading to failure. Consider if your riding style or usage might be contributing to the issue.
Given your experience in bike maintenance, it's worth reviewing each step of the bearing installation process to ensure everything is done correctly. If the problem persists, you may want to consult a professional bike mechanic or an experienced technician at a Royal Enfield service center. They can provide further insight, inspect the bike firsthand, and offer specific guidance based on the model and year of your bike.