Shimano Nexus in the cold

I've been a committed utility biker for at around 15 years now, probably every one of those years with freezing temperatures.  Freezing or sluggish shifter cables on gear hubs are a recurring nuisance that even 15 years of experience hasn't fully eliminated.  It is, by the way, always the cable that freezes.  If you have water in your hub such that it can freeze, its going to end badly.

The old hubs with shift boxes sticking out from the axle end are hopeless, IMO.  (For example the old SRAM P5 or S7, great hubs with shifters totally unsuited to winter.)  There are also some hubs that survive it all pretty easily, for example Rohloff, in my experience.  Most hubs though are from Shimano, or shift in a similar way, with one cable which you pull one way, and the hub pulls the other.  A component on the drive side of the hub rotates according to what the cable demands (and what the return spring can achieve).  These are the shifters that are worth writing about.

I have nailed down the ritual of getting water out of the cable housing.  Ideally a person has to do this just once in a year, or less.  Take the bike (or trike!) inside and melt it off (ideally overnight).  Disconnect the cable from the hub (doesn't take long if you know the motions), pull the cable out of the "cassette joint", take some toilet paper around the cable end, and hold this bundle in the end of a vacuum cleaner hose (running).  This pulls water out pretty well after a short time, which can be easily demonstrated with a bit of before-and-after testing in the cold outdoors (if you have time for playing around).  I used to use the vacuum to pull through some tri-flow oil, but in the end I don't think that achieved anything, might have even made things gummy.  I have also greased the cable as it is being inserted into the liner, again without any clear results.  If there is a rubber accordion at the cable end, I cut off the tip and pack it with grease.  (Those accordions not only fail to keep out water, but they potentially get wrapped around the cable pulley and throw off the shift points.)  I pack grease in everywhere as the cable is re-assembled, and then re-attached.

To clarify a bit with photos, here is the "cassette joint" for 4 hubs.  This component holds the end of the cable housing, so when the cable is pulled through the housing, a component on the side of the hub is compelled to rotate.

  CJ-8S40 on Nexus 8 SG-C6001-8C 
19t sprocket

  CJ-8S40 on Nexus 8 SG-8C31 
24t SRAM sprocket

 

 

  CJ-NX40 on Nexus 7 SG-C3000-7C 
24t SRAM sprocket

  CJ-8S40 on Nexus 8 SG-C6010-8R 
19t sprocket

 

note the accordion on the cable

Here we have some photos of one of the hubs where the rubber accordion has for some reason stuck to the cable in such a way that it pulled around the rotating component, and resulted in less rotation for a given cable movement.  This then resulted in the hub not quite finishing shifts correctly in the higher gears.

the accordion should certainly
not get pulled like this

A cable prepared in this way (vacuum and grease-packing) does shift between lowest and highest gear at temps well below freezing, the only trouble is that the spring in the hub may still return rather slowly.  Maybe one second behind the shifter motion.  This slow return sets in gradually as the temperature drops, which seems like grease or oil thickening, but not all my hubs show it.  In addition to the cable, there are internal hub components that need to move, as well as external surfaces that need to rotate past each other.  These surfaces may be exposed to for example chain lube, road salt, grit, and all that the outdoors can offer.  It may therefore be a combination of thickening lubricants, dirt, and moisture.

There is a test that can be performed to discover the approximate location of whatever shifting problem you have.  Its simple: while your cycle is standing outside nice and frozen, you take a 2mm hex key, plug it into a little hole (made for this purpose) on the cassette joint as shown below, and rotate the thing in the opposite direction that the cable pulls.  (You can try both ways, it works only in one direction.)  If it rotates easily (understand that there is a spring at work) then you know the cable is your problem.

One note before proceeding: most of these Shimano hubs are "default low" which is to say the spring pulls to the lowest gear, and without any cable at all, they go to 1st gear.  However apparently at least some Alfine models are the opposite... "default high".  In the photo above, on a regular Nexus 8, this means you can rotate the hex key downwards, and not upwards.  The following text assumes the hub pulls towards 1st gear, as most do.

Separate from the exact cause, its possible for the hub to simply refuse to shift down to 1st gear.  In this case, you can stop and use a thumb to manually push the rotating component forward, pushing towards the top edge.  The cable, running along the bottom edge, is then pulled through the housing to the position indicated by the shifter.  This is a good idea if you are about to start a serious 1st-gear climb, and want to be sure things are actually engaged.  (This is of course annoying, so good to fix.)

When downshifting is sluggish, a rider who is aware of it, and disposed to care about mechanical things, will give the shift a moment to complete, and do fine.  Sometimes downshifting twice, and then once back, will provide good feedback about shift success (or the lack thereof).  An experienced rider can often feel in the shifter if the resistance and travel suggests a good upshift.  Shifts to higher gears are (generally) reliable, because your hand overcomes the resistance better than the return spring does.  When downshifting, if the return spring isn't strong enough, you end up pushing the cable towards the hub, which works somewhat better than pushing rope, but not a lot better.  So its the downshifts that are most likely to give problems.

As you might have guessed already, a Shimano gear hub requires correct instructions from the shifter, or it works poorly.  The indexing for gears is all in the shifter.  The rotation of the cable-pulled rotating component on the hub is perfectly smooth, without any sign of what positions are valid.  The shifter can therefore show numbers that are not entirely in agreement with what the hub is experiencing, and cold weather is a big culprit when it happens.  There is an adjustment mark to check shifter-hub agreement, but that is only one point in the cable travel.  Therefore, in cold weather, if your hub has difficulties holding a gear, suspect that not all is well with the cable.