The tang on one side of the axle has a tendency to break off. Once this happens, it puts stress on the larger part of the axle which can crack and then stress the twin tang on the opposite side of the axle. The job of the tangs are to hold the main sun gear in place. The small holes you see are for a C-clip that keep the sun gear from moving sideways. We have seen this failure several times and is a good example of why NOT to stand on the pedals on hills. The symptom you may have if this happens is the hub skipping or popping in the same gear.

In the upper left photo you are viewing the main sun gear from the right (drive) side of the bicycle. This gear is being forced in a backwards (anti-clockwise, as viewed) rotation. In fact, most IGH axles are being rotated toward the back in the high torque low gears. When you stand on the pedals it creates even more force. This is why it is important to have non-turn washers that precisely engage the drop-out and torque the axle nuts to 32-35 foot-pounds. Be sure to grease the threads to ensure proper torque. Read more here.

It is our opinoin that modern internally geared hubs (IGH) (with the exception of NuVinci and Rohloff) cannot generally handle the forces created by standing or climbing super steep hills. Hence our recommendation to remain seated or get off and push. The main reason for this recommendation is path dependency to the 3/8" (9.5-10.5mm) rear axle dimension. If IGH makers were to make a hub with a larger 14mm axle with flats that were 10mm wide (just like a BMX axle) then they could drastically increase the durability of the whole hub because the internal axle parts could be larger. The quest to save weight is another reason that hubs are not as durable as they could be. In our opinion, the benefits of IGH outweigh the slight increase in weight. Durability and low maintenance are the main reason people purchase an internally geared hub.

The pictured axles are common to the Shimano 8 speed hubs including but not limited to: Alfine SG-S501 (axle unit # Y-35Z-98040), Nexus SG-8C20 (axle unit # Y-35P-98050) and Nexus SG-8R20 (axle unit # Y-34R-98040). We stock this part!