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I'm crafting a small 3 speed "gearbox" for a project of mine and I'm aiming for simplicity in the first place since I'll have to create it myself and I'm not particularly gifted when it comes to using my hands.

Initially the idea was to buy proper cogs but when I discovered how their prices go I resorted to using bicycle sprockets from a cassette like this one which go for ~$10.

Basically I'll take it apart and have 2 shafts with sprockets on both sides matching my calculated gear ratios, one side will be on bearings, all 3 pairs, respectively for my 3 gears, connected with a chain piece of their own and I'll have selectors sliding on the bearing side engaging different gears - pretty close to how a conventional gearbox works just a poor man's version.

My problem is I can't figure out what type of chain goes on a typical mountain bike you would see sold commonly in any casual bike store. There are a bunch of different claims I found which lead me to believe that it's either 25H or T8F but I couldn't determine with enough confidence to order.

The maximum force that will need to be exerted in order to propel my vehicle on the steepest hill I've designed it for is about 650N, which multiplied by wheel size comes out to about 150Nm on the output shaft. That's roughly about 60kg I'm assuming the chain and sprockets will have no problem handling that.

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  • Why not just use bicycle chain that is compatible with the cassette?
    – ojs
    Oct 3, 2019 at 10:19
  • @ojs basically I can't figure out what it's compatible with and unfortunately I'll need to purchase separate sprockets from different places because I need uneven numbers of say 11T ones and it makes no sense to order a whole cassette for just 1 sprocket. I'm trying to figure it out and buy sprockets that are compatible with the same chain type and it would be useful if I knew what type of chain the mass produced mtb's use since I may hope to find some used cassettes for cheap locally.
    – php_nub_qq
    Oct 3, 2019 at 10:30
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    Btw, you can also use single-speed sprockets, they come in different sizes as well. Cassette sprockets are designed to allow the chain to jump off/on the sprocket. To this end they have short teeth that barely grab into the middle of the chain, and they have special cutouts that help guide the chain during shifting. Both of these are anti-features for you. Single-speed sprockets, on the other hand, have longer teeth which are fully symmetrical around the sprocket. It is physically impossible for a well-tensioned chain to jump off a single speed sprocket, or to skip over the teeth.
    – cmaster - reinstate monica
    Oct 4, 2019 at 8:21

2 Answers 2

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There are several different sizes of chain based on the number of sprockets used in the rear cluster (e.g. 8-speed, 11-speed). These are 3/32" nominal width. Single sprockets for these chains are available, but can be hard to find and/or expensive. In your application you may be able to get away with a slight mismatch, i.e. using a wider chain than your sprockets would suggest (an 8-speed chain on 10-speed sprockets, for example). This wouldn't be a good idea on an actual bike, and it could get fiddly if you mix and match too much. Here are the relevant spacings.

The alternative is singlespeed components - they use 1/8" chain and sprockets are always sold singly.

If you're going to pick up cheap used cassettes, I suggest you pick one type and stick to it. All 8-speeds should work well together, or all 7-speeds, but less so for higher counts. 7- and 8-speed parts are cheap if you want to buy some new, while 5- and 6-speed can be hard to find and 10-speed or higher will get expensive.

As for the strength, consider the forces involved in hard pedalling - let's say a 100kg rider standing on a 175mm crank with the chain on a 22-tooth chainring.

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    @Swifty, thanks for the spot. I posted slightly hastily so I've got a couple of other bits to look at
    – Chris H
    Oct 3, 2019 at 12:15
  • Thanks for adding the link!
    – php_nub_qq
    Oct 3, 2019 at 13:13
  • BTW, largest chain tensions are generated from smallest chainrings. This is why anyone on MTB with 22-tooth ring puts more tension on the chain than 200 pound sprinter on track bike.
    – ojs
    Oct 3, 2019 at 14:04
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    Single speed cogs for thread-on fittings will be ideal - a good turner should be able to put a suitable thread on your shafts. The cogs rescued from a cassette will expect a slotted interface.
    – Criggie ♦
    Oct 3, 2019 at 19:41
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    @php_nub_qq Welding seems to work for motorbikes, but makes it a lot harder to change things.
    – Criggie ♦
    Oct 4, 2019 at 11:58
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A bike's crank has roughly twice the radius as the chainring. All bike chains are designed to handle a 100kg rider standing on the forward pedal. That puts a force of roughly 2kN on the chain. (This is a low boundary estimate, I think I remember reading something about 8kN breaking force on a chain once. Chains are one of the few parts of a bike that are really robust.)

Your usage is very well within the specs of any bike chain.

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