@Littgull has Rohloffs on 2 of his bikes. I am often caught out by him sneaking a gear change in when he is riding in front of me. He either speeds up or slows down for no apparent reason, but it is because I hadn't noticed him changing gear! He can also change gear when stationary, which is very handy for standing starts on steep climbs.Really? A 14 geared rear hub! I Had no idea you could get such a multi-speed hub like that. With so many gears, I’m wondering why they don’t use them for racing. I’m curious on that one. Perhaps it’s because they don’t shift as fast on the fly.
There is a good review of the Rohloff HERE.
Disadvantages? Increased weight (probably not quite as much as people think though because the bike has a lot fewer parts fitted). Cost - they are very expensive, about £1,000 ($1,400-1,500?) but again - you save money on not having to buy other parts. Probably very slightly less efficient than a perfectly maintained derailleur gear system, but probably on a par with a worn/mucky one.
There is only one cog at the back and you can choose how many teeth there are the one fitted. If it has lots of teeth then you will have a low gear ratio which makes it much easier for going uphill, but a nightmare for going downhill! (You basically can't pedal fast enough and you don't have the option of freewheeling.) If you put a small cog on it would be fine for the flat and much better for going downhill but then you would really struggle on climbs. Inevitably, you would probably choose a compromise, somewhere in between the 2 extremes.So, one can make adjustments to the actual fixed gear hub? This is quite interesting. If that is so, can you simply stop at the side of the road to do that? I can see this as a great way to train, and improve fitness, like you mentioned.
You could change the cog at the roadside but nobody in their right mind would do that except for extremely long climbs or descents. You'd have to take the wheel off and use tools on the hub to get the cog off, slide another one on then reassemble everything - a right pain!
I built myself a singlespeed bike but it has a freehub so I avoid the spinning out on descents issue by simply freewheeling downhill. It is fun to ride but I have a 52/19 gear ratio on it which is too hard for tough climbs so I generally avoid anything steeper than 8%. I can do short stretches of 10-12% but they fry my legs so I don't like to encounter too many of them. It is a great gear ratio for 16-20 mph. I can pedal to about 25 mph but my legs are then doing about 120 rpm, which is a higher cadence than I am comfortable with.