Wobbly back wheel (Shimano Nexus SG-8C31 internally geared hub)

Question

I am new here and have little experience in repairing bicycles. I have an about 15-year-old bicycle with an 8 gear Shimano Nexus internally geared hub (SG-8C31). Unfortunately, the back wheel is not tight and stationary, but can be pushed a few mm to the sides. I wonder whether this is a capital damage or if it can be repaired. I don't know what parts are responsible for the wobbliness nor how they are named so it is hard to find information.

Here is an image depicting the issue:

Edit #1

Just unscrewed the wheel. The coaster brake reaction arm wobbles relative to the wheel axis. Is this supposed to be? Is it a question of tightening this 6-angle thing here?

Edit #2

I think the ring on the second image is the cause of the problem. If I tighten it, the wheel stops wobbling. If I tighten it too hard, it also stops spinning. Are there things to consider when thightening this or is it just about finding the sweet spot between wobbly wheel and fix wheel?

Answer 1

You are correct that the nut you've identified needs to be tightened down. That is the left side locknut. The reaction arm engages with the left side cone and acts as a wrench on it for adjustment purposes (whereas most other cup and cone hubs have a similar arrangement, but no reaction arm). The cone and the locknut have to be tightened against one another to secure the adjustment.

Doing the above will likely be able to take the play out of the wheel, but you still need to ask why this happened to such an extreme amount. The likely choices are:

• The bike was ridden without the reaction arm properly secured, which can cause the left side axle hardware to loosen and/or the threads become damaged.
• The bike has been ridden for a long time without any internal maintenance to the rear hub, and the looseness come from wear to the main bearing surfaces.

If the hub has had its periodic dunk/overhaul procedure done recently, just doing a simple adjustment could make some sense. Otherwise it should have it, and while the core is pulled the threads and bearing surfaces can all be inspected. That is the appropriate response to that kind of looseness developing on such a relatively expensive and hassle-to-replace part.