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Sometime ago, I bought a cheap bike from eBay for my daily commutes which turned out to be a single-speed bike. It works fine but I am curious as to how I am able to have a shifter that simulates gear changes.

It's obviously because of the box indicated in red in the first picture below. But after a through search I wasn't able to find a clue how that works let alone what it is.

So, what is it and more importantly how does is it work?

Also, what would happen if I just disconnect the cable going to the black box below but still keep the box attached on the bike? You know, just to enjoy a proper single-speed bike from time to time.

Thanks.

enter image description here

enter image description here

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    What happens when you twist the shifter on the handlebars? Internally-geared hubs like this will shift while stationary (which is a neat party trick)
    – Criggie ♦
    Commented Dec 27, 2021 at 7:10
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    What do you mean by "simulate" gear changes? Do you have gear changes or not, and do you want them or not?
    – mkrieger1
    Commented Dec 27, 2021 at 20:12
  • Are internally geared hubs so rare where you are?
    – gerrit
    Commented Dec 28, 2021 at 10:16
  • @gerrit Seems they aren't. It's just that I never saw this tech before.
    – Can Sürmeli
    Commented Dec 29, 2021 at 13:07
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    @Criggie I can't remember the exact behaviour but as mentioned in a post below, I gotta pedal smoother for a shift to happen. Can checkout later when my new tyres arrive as I have a bad puncture at the moment.
    – Can Sürmeli
    Commented Dec 29, 2021 at 13:10

4 Answers 4

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You do not have a single-speed. The rear wheel on your bike has a 7-speed internally geared hub. This uses a couple of sets of planetary gears, each of which can be engaged in two different ways, plus a direct-drive gear. If you hold up the rear wheel and pedal, you'll probably see that in each gear, the wheel turns at a different speed than the rear sprocket.

Internally geared hubs are ingenious, complex, compact mechanisms. Here's a video of one very similar to yours being disassembled.

The cable is probably spring-loaded on the back side; if you disconnected it, it would probably force the hub into whatever the detensioned state for that cable is (either top gear or bottom).

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This is not a single-speed bike. It's an internally-geared hub made by Sachs, later bought by SRAM, which later stopped production of the hubs. If you take a better photo of the label on the hub, we can identify the model.

These geared hubs are/were very common on city bikes in Europe. To be honest, it's pretty amazing to see that someone wouldn't recognize it!

If you disconnect the shifter cable, the hub will be stuck in the highest or lowest gear until you reconnect it.

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    Yeah, now that I think about it, I could've thought better but I haven't really spent too much time with city bikes. Let alone old technology like this. Thanks for your input
    – Can Sürmeli
    Commented Dec 27, 2021 at 8:25
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    They're still quite comon for city bikes, just bought one last spring
    – Hobbamok
    Commented Dec 27, 2021 at 11:07
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    @CanSürmeli It’s not really ‘old’ tech. Bikes are still made with these, and big name manufacturers still create new designs from time to time. Shimano, for example, makes no fewer than four different models of IGH currently (counting the 7-speed Nexus and Allfine as the same thing, because internally they are). They’re just a somewhat niche thing in a rather specific sector of the cycling world, so they’re not widely seen in parts of the world.
    – Austin Hemmelgarn
    Commented Dec 27, 2021 at 12:26
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    Generations in Britain have known about Sturmey-Archer hub gears. They traditionally made 3-speed hubs, but now they offer hubs up to 8 speeds.
    – Michael Harvey
    Commented Dec 27, 2021 at 14:49
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    There are even expensive high-end internally geared hubs (e.g. by Rohloff or Kindernay) found on new high-end touring, electric, gravel and even mountain bikes. So they're far from "just on cheap old city bikes".
    – Erlkoenig
    Commented Dec 27, 2021 at 18:59
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As other answerers have said, that bike has hub gears rather than being a single speed bike.

However, no-one so far has commented on what the differences with this style of gears are. Compared to derailleur gears, they have two main advantages: you can shift gears when stationary, and they require less maintenance since the mechanism is not exposed to the elements. These benefits make them suitable for city bikes where you will be stopping more often and are more likely to be required to stop unexpectedly, and want something that is a tool to get you from A to B with minimal fuss rather than a hobby or sport.

The downside is that they're heavier, have fewer gear options, and are less mechanically efficient than derailleurs. Again these differences are typically less important in low speed cycling and so the downsides aren't as pronounced for city bikes.

Another difference is that you should stop pedalling when shifting gears, since the mechanism doesn't work well when under strain, and you may also find that the hub gears have an integrated coaster brake that means that you can brake the bike by pedalling backwards.

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    "less mechanically efficient than (a well maintained) derailleur". Fix it for you. Again, for city bike usage, this is a plus, since city bikes can see fewer maintenance cycles compared to a road/mountain bike.
    – Aron
    Commented Dec 28, 2021 at 2:20
  • @Aron I actually wrote that in the first draft but removed it since I thought it overcomplicated that section.
    – Jack Aidley
    Commented Dec 28, 2021 at 9:02
  • IME, pedalling "lightly" (i.e. spinning the pedals but not so fast as to exert any actual torque on the wheel) also works fine, and has the advantage that it works well on both hub gear and derailleur bikes. Which makes it a useful habit to get into if you find yourself switching between the two types occasionally. (For example, the local city bikes here have hub gears, while my own bike has a derailleur. Sometimes it's more convenient to grab a city bike, e.g. if I took the bus somewhere and decide to bike back, or vice versa.)
    – Ilmari Karonen
    Commented Dec 28, 2021 at 23:12
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Ah, but this is not a single speed bike. The gears are within the hub and invisible to the naked eye. If you remove the cable, the hub will default to (probably) the highest/heaviest/hardest gear.

If you want a real single speed, set the gear to the hub's 1:1 ratio (a middle gear) or buy a cheap new singlespeed rear wheel that fits.

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