I've just used the last of set of Rohloff oil change kits I bought with the bike nearly nine years ago. Over time a little of the oil evaporates or leaks out, so this time I put in 22ml of cleaning oil. I ran the motor to turn the rear wheel while changing between gears 3 and 5, which between them engage all the epicyclics, so spreading the oil on all surfaces, then let the box stand for an hour with the drainage plug downwards. I turned the hub briefly through forty-five degrees to insert the tube again without making a mess, then with the plug pointing downwards left the syringe attached for a couple of hours. 22ml of slightly soiled oil came out.
(http://www.coolmainpress.com/miscimage/andre_jute_rohloff_oil_change_2016_800pxh.jpg)
That leaves 14ml of thin winter/cleaning oil attached to the gears, right?
Next I added what was left in the bottle of all-seasons running oil, which was about 22ml. None of this was spilled. I put the new plug in, gave the wheel a good spin, and took a 15km ride, noting that the box ran a small bit more silently. The box has 8600km/a bit over 5000m on the clock, so I expect it is about as quiet as it will ever be; thus the small bit of silencing an oil-chance seems to make could easily be a psychosomatic artifact, something you hear because you really want to hear something for your effort. (Riding on smooth tarmac on the painted line, the whoosh of the tyres should totally drown out any noise a sound Rohloff gearbox makes. If your bike has more than 2000 miles on the box and it is still noisier than that, consult the seller.)
But this math is cockeyed. We have been told, repeatedly, that a Rohloff gearbox is fully serviced by the 14ml of oil that sticks to the gears. But in my box every time I service it, there is 14ml of slightly dirty (because mine is a low-annual mileage by the standards of some of the tourers here) mixed new winter/cleaning and oil all-seasons oil sticking to the gears, which you cannot and shouldn't remove, further mixed with 20-25ml of brand-new all-seasons oil. I make that something around 34-39ml of oil. Even if you put in only a carefully measured 14ml of all-seasons oil, that's a minimum of 28ml in the box.
Since my box doesn't leak oil, I conclude that's a proper amount of oil in the box.
Thoughts?
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In the Middle Ages, the committed would torture themselves to madness with this sort of metaphysical question, often in the form of, "How many angels can dance on the head of a pin? And, when you solve that one, how many angels can dance on the point of pin?"